The spectral and basic properties of some aromatic carbonyl and amino compounds

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1 Loughborough University Institutional Repository The spectral and basic properties of some aromatic carbonyl and amino compounds This item was submitted to Loughborough University's Institutional Repository by the/an author. Additional Information: A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy at Loughborough University. Metadata Record: Publisher: c Peter Tickle Rights: This work is made available according to the conditions of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0) licence. Full details of this licence are available at: Please cite the published version.


3 THE SPECTRAL AND, B\3IC PROPERTIES OF SO~m AR0!1ATIC CAROONYL AND AMINO CO..'\fi'OUNDS l?e'.mr TICKIB, B,Tech, Grad. R.I.C. Supervisor: MR. J, M, WILSO!l, This ~esis is presented in partial fulfilment of the requirements for the Degree o= Doctor of Ph1loso~. ' Department of Chemistry, Loughborough University of Tochnology, August 1968.

4 Loughboreugh Ummsity Of T.,hno'osy ltbrary Date Class ~-'' A«No 0 ~9 '\1 "1/ot I

5 "It 1e a capital mistake to theorise before one has data". Sir Art.hur Conan Doyle )0.

6 'l'hel pk 4 valul!s of a series of progrese1vel7 't'leal-..e%' pril!llu':t nitro an111m bases, 1n aqu~ous sulr.hw;-io acid solutions, have been determin::>d b1 a spectre lilotometrio mthod over the 0 0 ter.~perature rtul...~ c. therefore they enable the ac1d1t;r function of th'! aqueous sulihuric acid solutions to bo calculated over the B!IIM ranse of temperature. 'nlis aoid1t;r function data ttas then used to dete%'!lline the p.'>: 8 values of a series of metl17l subst1 tuted sterically hindered benzal acctophenones over the te~perat~ range 150-YJOC 'lb'! the:r-.odym,.-rl.o quanti ties l:l G 0, l:l!i an:! l:l S 0 for protonstion ot tho nitro anillnes a.."1t1 tho be=~l acetolflenortes were calcula~d. 'lbese quant1t1eo wcro used as a measure of basic stren.,.-th!n each series of oor.1pounds. related, on a :eetl:l. qua."lt1tativs bad h to thoir rolacular structure. Suoosquently, b1 Uuc~l mole-cular orbital calculations, the bas1o strengths were related quantitat1vol;r to tho el6ctron1o 1nd1ces of MOlecular orbital theory. 1.

7 ACKNOI'ILEDGEMENT. I would like! to express 1111 gratitude to 1111 supervisor Mr. J.M. Wilson for his guidance and encouragement throughout this work. I wish also to acknowledge the help of Mr. J.E. Sawbridge who wrote the comp.lter programs and Dr. A.G. Fogg for his helpf'ul criticism in the writing of this thesis. '!'hanks are also due to Professor R.F. Brl.llips for his continued encouragement and for providing the necessary facilities and. the Aut!1oritiee of IJ:lughborough University of Technology for a Research Scholarship. 11.




11 t'!an;v' organic compounds aro weak bases and protonate 1n strong acid solutions to give their conjugate acids. A knowledge of the strength of weak bases is important in the stud:r of acid reactions and 1n the prediction of the site of protonation in complex molecules. Basic! ty constants are also important in structural comparisons of compounds. Arnett (l) has expressed the hope that the measurement of the strengths of weak bases coupled with the knowledge of strong bases will give new insight into the effect of structure on reactivity, Hamn:ett (2) has shown that solutions of much greater ac1d1 ty than those l~i thin the usual ph rang<! are required if weak bases are to be protonated. 'nle measure!mlnt of the acidity of such solutions posed a difficult problem which Hammett attempted to overeorne by introducing his acidity function concept. A measure of the acidity of a particullr solution is obtained by means of a basic indicator, which under the conditions of tho acidity of the solution is partially protonated. The indicator is added 1n very small amounts so that it does not alter the acidity of the solutions and indeed so that the ratio of its protonated and unprotonated forms will be dictated by the solution acidity, 'nle ti-ro forms of the indicator must have distinctly different spectra so that the ratio of the concentrations in the solution can be determined. Consider the protonat1cn of such a basic indicator B -2-

12 by a strong acid, B+H+ ~ Bt (l-l) The thermodynamic equilibrium constant of the reaction. Ka can oo expressed by the equation (l-2) or (l-2a) 1mere activities. activity coefficients and concentrations are expressed by a. f, and [ ] respectively. The concentrations terms in eqn, (l-2a) are readily measured spectrophotooetrioally but it is impossible to measure separately all the activity or activity coefficient terms 1n the equation, The ratio ~+fr/fbl however haa been shown to have a >ijysical significance (3). Thia ratio is given the aymbol ho Therefore eqn. (l-2a) can oo rearranged to eqn, (l-3). Ka "' h 0 [B] - (l-3) [Bl] Hammett and Deyrup (2) made the assumption that the activity coefficient term trffb_tis independent of structure for indicators of the same charce type and in any given solution it is a constant for all basesj this is termed the Hammett activity coefficient postulate. Thus ho serves as a unique def1n1tion of the acidity of the medium. It is also possible to relate the acidity, the dissociation constant of the conjugate acid (Bl) and the indicator ratio by taldng negative logarithms and rearran:;ins eqn. (1-3).

13 + lllta.. los:j.oho + los:j.o f:j 1 (1-4) Hamrnett and Deyrup (2) replaced the tem (-los:j_ 0 h 0 ) by I'fo, the Hammett acidity function and therefore + lllta Ho +log [Call In this study the acidity function data for sul.rnur:to acid at 25 C has been redeterm:tnad and data has been obtained alsg over a :t'lulge of temperatures The acidity function scale is referred to the standard state 1n water under which conditions all activity coefficients become un1 ty. Thus equation (1 4) can oo expressed as or + pkaa log ~1 + +log [~JJ pk ph log t ;1 a B It is apparent that the Ho scale can be regarded as an (l-5) extension of the Ji1 scale into the ranee of concentrated acid solutions. A aingl.e indicator can be used to masure the acidity function of a medium over approxir.ately 2-' Ho units. To measure the acidity function over a tdder ranee a series of progressively weaker indicators must be used. The basis or the indicator method of' masuring Ho consists in measuring + pk 8 and log [EH J/[B] values of the indicator in n:edia of U!lknotm acidity. The pka valus of' a relatively strong base, which is protonated 1n ver:r dilute acid solutions where the Ho scale can be equ.j.ted to Ji!, is also neaessar:r and this base is accepted as the 1n1 tial indicator or the Harmlett acidity

14 function scale, The widely accepted initial indicator for the acidity function scale is 4 nitro aniline which can be referred to the standard state 1n water, The extension of' the acidity function scale outside the r~ of dilute acid solutions is effected b,1 the stepwise comparison of progressively weaker + bases. ibe value of' the pka and log [m 1/[B] for the initial indicator (B) can be determined 1n dilute solutions. It then the ionisation ratios of (B) and also of' a slis:ltly weaker base (C) are measured 1n the same sl1s;ltly oore acidic med1u:n thent -log~+ pka(b)-log[eh+]-log fbh+ pk 8 (C)-log[CH+]. log f'c! 1 +(l-6) m r;- rcr ~ or pk 8 (B)-pK 8 (C) log[bh+]-log[ch+]+log fbh+ fc m rcr rbrch+ Applying the Hammett activity coefficient - postulate that rb.. re tm+ tcit (l-6a) then pk (B) - pk (C)=olog [Rt]-log [CH+] a a TBJ rct (l-7) Therefore it becomes possible to determine the pk 8 of the weaker base (C) from the simplified eqn, (l-7), 1Uth a lmowledge of the pk a of the base (C) and values of' log [CH+]/[C) ratios it is possible to extend the H 0 scale outside the dilute acid regions by applying the data to eqn, (l-4a), To measure the H 0 values of' mre concentrated acid solutions the value of the pk of' an even treakcr base (D), a the ionisation ratios of' which overlaps that of (C), is calculated, Hence by means of' several indicators the acidity -5-

15 tunotion can be measured over a wide range of concentrations of aqueous solutions of strong acids. About ten indicators are required to measure the acidity f'llnction of sulphuric aeid water l!lixtures aoountely trem dilute solutions up to 98;"M/w sulphuric acid. Jo:rgenson and IIartter ( 4) imposed certain :restrictions on the basic indicators used to give a quantitative fonaul.ation of the aeid1ty t'unction for any particular acid 117stern. First the p.t{a of these indicators :'lust be accurately known and 1.m1versally accepted. Secondly the indicators used must be valid Ho indicators so that the r-atios of the activity co.. eft1cient of the free bases to those of the protonated bases are equal for all indicators so that the Ho soale detem1ned. is 1.ndepement of the indicators employed in ma.k1ng the measurements. 'lllese workers also observed that any 1fo values detei'iil1ned by a p!u'ticular set of imicators ineorporates all the errors arising i'ro!ll any imperfect indicators in the series used. In the original Ho data of Harnmett and De;rrup (2) the pk value of 4 n1tro aniline, whieb. 8 was used as the basis for the calculation of the Ho scale, was taken to be 1,40, the average literature value avauable at that time. Hamrnett and Paul in later work (5) studied the zero point of the acidity tunotion scale and obtained a value of l.u for the px 8 of 4 n1tro aniline. 'lll1s value was obtained by the stepwise treatment of the 1 '11sation data of aminoazobenzene and benzeneazod1jilenylarn1ne and 4 nitro aniline. Even so the pk value shows poor agreement with the so called best value 8-6-

16 of 0,99 quoted by Paul and Long (6), Basoombe and Ball (7) have redetermined the 1on1eat1on data of some P.arnrnett indicators which protonate 1n weak and moderate strensth sul);xlur1c aoid 0 solutions at 25 C by & speotojilotometrio method. 'Ihe ionisation data of 4 nitro aniline, 2 nitro an111rut and 4 ohloro 2 nitro an111ns differ slightly from the data of Ilammett but all the reported values are within the experimental error of the method. It is reasonable to as:rum ther&foro that the ionisation data of ti,e basic indicators st\ldied up to about?l%w/w sul!ilurio acid is accurate and assuming the pk of the strong base is co.:'tect,.;111 give an accurate 8 ll:easura of th9 1\J acidity ecale of sulphuric acid. Taf't (8) has shown that the H scale could be dependent 0 on tha stl'u.otul'e of th& 1nd!oatoi'S in purtlculap Jl!'imary seoondary and tertiary amines, Hamrnett 1 s original 1nd1catol'S partioularly the weaker bases used to stud)" the moro concentrated acid solutions, 1noludec4 compounds of widely differing chemical oonstitution. Recent stulles on the aot1v1ty coefficient behaviour of basee in strong acid l!'edia has shown that the ratios of the activity coefficients of the unprotonated to the protonated are not necessarily equal fot> bases of d1f't'crent chemical types, Observing the ral1abil1 ty of prirrary ani lines as indicators in diluw acid solutions Jorgenson and llal-tter ( 4) took these compounds as the basis of their nelf set of indicator bases. 'Ihe nitro aniline indicators fultill the conditions imposed -7-

17 by these \forkers for valid indicator bases and as they are compounds of' similar chemical oonstitu'-ion it is reasonable to ex~it the Ila.JntNttt act1vit7 coeffloient postulate is applicable to them. These workers augmented the nitre an111nes indicators of Halmnett with speo1all7 selected nitro aniline :tndicators whose pk 8 values were such that they would only ionise 1n model'&tely concenti:ated acid solutions. They did not however redetermine the ionisation data of' the relatively strong nitre an1l1nes used by!lett but assumed his data to b3 correct as far as 24 d1n1tro aniline which ionises 1n sulltlur1o acid c.jncentration 1n the region of '.w/w. When Jorgenson and I!a.rtter repeated the ionisation experiments of Hammett and Deyrup with particularly weak bases such as 2 bromo 46 d1n1tro aniline and 246 tr1n1tro aniline they found that the ionisation data obtained was oons1derebl.y different. Both sets of data were displaced to lower acid conaentrations. These workers did not use such Hammett bas~s as benzalacetoji1enone and anthraquinone both of which have pronounced median effects and req,uire much caretul. stu:!y before e.:ny reliable ionisation data can be obtained. The set or 1nd1cators constructed by Jorgenson and Hartte:r constitutes, at present, the onl7 valid set of llanlnett :tndicators for the calculation or Ho aoid1t7!unctions. '!he na1n disadvantage 1n using their data is that there was no attempt to standardise the temperature at which measurements were made and that the assumx;tion that the ionisation data of -8-

18 Hammett am Deyrup of the nitro anillnes indicators up to 7(Y;'M/w is correct. Boyd (9) has studied some primary amines of the H=ott set and has noted tu.t'ferenoes in the ionisation data of 24 dichloro 6 n1tro aniline at 25 0 from the recorded data of Hanlnett. 'nle present work which mainly uses the Jorgenson and lrartter eet of pr!mar;y- amines l!'.akes no assumptions about the ionisation data of indicators published by previous workers. The ~K values of the initial 8 4 nitro aniline has been determined am the pk values of subsequent 8 weaker bases have been reo!llcull.ted from the stepdse procedure us:tng eqn. (1 7) Accurate temperature control was maintained thro~out to enable meaningful thermodyna!-4c studies to be performed. In order to studt the thermodynamic: aspects of the equilibrium reaction of the protonation of weak bases it is essential to know the equilibrium rate of reaction or the dissociation constant of the conjusate acid of the weak base, at different temperatures. This will require a knowledge of the acidity :f\motion of the protonsting medium ejj various temperatures. 'l'he only studt of this type available is that of _ Gelbshtein and eo-workers (10) who studied a series of strons acids including sul.jhurio acid at various temperaturea. 'nlere are certain conditions that must be observed 1n the thermodynsin!o studt of acid base equ1llbr1a. King (ll) has shown that the errors 1n the calculation of thermodynsmic quantities can be increased by the injudicious selection of

19 temperature levels. 'n1e temperatures at uhich measurements are made must be evenly spaced and the ternpl"rature intervals must be appt'opriate. If the error in the determination of the pka + ot the base is.o.02 units and the temperature intervals are 2 C the sta.mard deviation in the calculation of 1::. H 1s 970 ~ 0 cals.moles If the temperature interval 1s 5 or 10 C the standard deviations fall to a.orcxirnato!;- 190 and 200 cals. mole'" 1 respectively. Similar effects are observed in the calculation of D. S 0 It is also preferable to avoid extre::lf!s of temperaturo because elevated temperatures can cause changes in the solutions which u1l:i. affect the measurel!'.ents obtained and introduce unnecess~j crror3!he data of Gelbshtein and co-worlrers!suffers :f'roo several disadvantages. First only a ver:r lim1ted number of indicators are used in the measurement of the acid! ty functions ~thich means that the ionisation data had to be ex:trapolated to ver:r high degrees of protcnation Md in ll:l111e cases overlap between consecutive indicatora did not occur. Secondly those workers used 20 0 temperature intervals which can increase the errors in the calculation of!l11 andll's 0 and because a mi.limum of' three temperatures is required. can mean that r.leasu:rements have to be mad.> at elevated temperatures. ndrdly the protonation of the basio indicators were studied by a colorimetric. technique which has been sholm to bo inadequate for the accurate measurement of pka values and Ha data. In this 0 study 10 C temperature intervals were used which give lo11 standard deviations in the measurement of 11 It and allow

20 a sufficient number or temperature levels to be obtained in order to study the thermody!'..amic aspects of protonation dthout coin:; to vary elevated temperature!!. Arnet'l> (l) has shown that the application or acidity function data to the ca111ulation of the pk of the conjugate a acids of a 11eak base requires careful study to obtain accurate and mean11l$ful results. Fer bases tludt adhere to the Hammett activity coefficient postulate the dissociation constants calculated tram H data repre"ents a thermodynamic equ111br1u:n 0 cons'l>ants referred to the standard state free enerf!;:/ of ionisation. All other weak bases t. S amides must be considered as tollowilll! different acidity functions and their pk values merely represent the Ho value of the solutions in which they are half converted to their conjugate acids. ~ failure of lll!uly bases 'l>o obey the Hmmnett activity coefficient postulate may be the result of inadequate treatm:mt of experimental data or it may be that the base in question actually has different activity coefficient behaviour from a true Hammett base. Several r::ethods are available for the mc!u!urement of the pka of a weak base usin:; aoidity function dsta, the principal method involves tho direct sppllcation of eqn.(l-lla). 'lhe values of the log [m+)/[b] ratios for the base IIM obtained over ita principal protonation region in acid solutions ot known H values, this data is then substituted into the equation 0 and the pk of the base calculated directly. 'l'he concentration 8 ratios are caloulated from spectrophotometric data ~ins the relationship -u-

21 f. - E D obs, (1-8) Eobs t.:at ~/here ~ is tho 110lar extinction coefficient of a solution containin~ the unprotc:1ated base at the analytical 11avelength, Rlis the molar extinction coefficient of' the tully' protonated base at the :::ame ''-avolength and C....,... is the molar extinction. ou~ coefficient, measured at the same wavelength, of a solution in tlhieh th~ b..'l::o is partially protonated, 'l"na direct use of eqn, (l-4a) will of necessity requi:oe the accurate measurement of EB and ce!t \fuen the base suffers from medium effeot:j these may be difficult to obtain, l!ost 1!1"11\l!'! effects are lateral effects oaus~ the position of' the absorption maxima of the base to change as the acid media ol~s. Obviously no stat~o analytical 11avelength can be used in these c1roumstance3 because it would gi\'6 erroneous results, Harmoott (12) was the first person to sugsest methods b7 which medium effects could be overcotre and he used two separate methods,!!'1rot the isobastio point rrethod, this involves obtaining the complete spectra of' the base in s.,,utions of varying acid concentrations, These curves are then displaced until they all par:"l through nn isobest1o point, at this point the absorption maxirr.a of ths various speci~s can all be obtained at the :::arr.e l~avelength ~11th some accuracy. S!leondly a oothod that involves treating oi ther eb or CRi+, in eqn, (1 9), as nn unlmo\m quantity pk 8 " u 0 + log E B - Eobs E'ob'l E :st (1-9) in addition to pka' ~n 1men tl-ro sets of data are available the -12-

22 pka of th~ conjugate acid of the base can be obtained by sir.r.lltaneous equations or if more sete -:-~ data are available by a curve fitting ena.j.;ysis. Eoth roothods have been used to reduce or elill'.inate medium effects in this study and are described in more detail later. The protonation of the carboeyl group in organic compounds has alwaya been popular to illustrate the application of acidity function data to study the protonstion of weak bases. Particular attention has been given to aromatic ketones because in the main they are well behaved Ha1nmett indicatora and they provide ooo of the beat uses of u.v. spectroscopy. The particular problems presented by sterio hindrance and increased delooalisation has always been an intriguil!b ons e,g, sterio hindrance in 26 d!~rethyl aoetoibenone loi1grs the basici't'j compared with acetoz;ilenone(lj), Similarly increasing the conjugation of the ll!olecules effects the pka of the compound e,g, increased conjugation in benzal acetophenone increases the basicity compared with acetophenonq, In this stuu.t the basicities of a series of stericall.y hindered benzal acetolbenones are reported end both Ule effects of sterio hindrance and conjugation are discussed, From a knotfledge of Ha data or sulitluric acid at various temperatures obtained from the study of the nitro anilines. pkas of the benzal acetophenones were calculated at different temperatures and thermodynamic calculations of both aeries of compounds were performed. The thermodynamic quantities

23 /:,. H, /:,. G 0 &.."1<1!:; 3 for the protcnation of a.'n1no and carbonyl co'!lpotmds l:'>.ave also b<:!en compared t~!th the indices of molecular orbital theory. In particular /:,. 1f delocallsation energy change:~ G 0 been coml)!lred with the in t.'le compounds caused by protonation.

24 CHAP!'ER 2-15-

25 2 1 NI'ffiO AHILlllES a) Purification of the oommerciall:x:_available nitre ll..:_rl.l1nes 'llic followin$ Hammett indicators of analytical grado 1-rere obtained from th3 Aldr1ch Chemical Compa.ny Ltd,, 4 ni trc aniline, 2 nitro aniline, 4 ohlcro 2 nitro aniline, 2l d1chloro 6 nitro aniline, 24 dinitro an!llne, 2 bromo 4Gdi nitre a.:ullne a."ld 246 tri-ni tre anilina. 'l'besa compounds ~rora f'.ll't.'ler purified hy recrystallisaticn from water. Tha n;alt1ns point:: of there compounds WlM determined t.<d all show~d excellent ngroe ment with the quoted literature values (14), The ne~rer Hammett ind1 ea tort of Jorgenson ar.d IIartter (4), 25 dichloro 4 nitre aniline and 26 di nitro aniline are not available in a form specially purified for use as Hamoett indicators but they are supplied as cotmreroial grade products by the Aldrich Chemical Company, 'Ihe 25 dichloro 4 nitre aniline trs.s purified by colu.m chro!l'.atogmiby using neutral as the ctat1onary Jilase and anhydro\is diethyl ether as the eluent, l!lo compound was then recrystall'\red several tin:es from \ later and ethanol, final P.ll'ification being ac~~eved by vacuum suhlimation. l, l5fc (Ut l57-8 CJ l57 CJ (1!) (15) ) 'lhe 26 d1 nitre aniline was purified by a caries of recrystallisat!ons from water and finally by vacuum sublimation., lj7 C (lit l39-l40 CJ CJ (4) (16) (17) ) b) Preoaration of 3}1e.246 trinitro anili~ 3 ~!athyl 2h6 tri:u tro anillr.o h-all not available ccmmrcially -16-

26 and was prepared, via the N-(3-methylphenyl)-ethyl carbamate, by nitration followed by eydrolysis, aocording to the fol101dni; prooedure. Ethyl chlorooarbamate (:;6 gms.) in dry benzene (50 mls.) was added dropi'tise over a period of' half' an houl" to a stirred mixture of m-toluidine (35 gms.) and anhydrous eod1uiii carbonate (15 gms.) in dry benzene (200 mla.). 'lh" mixture was rofiuxed for one houl", the insoluble salts were removed by filtration and the benzene was removed by distillation under reduced pressure (10 m.m.hg). 'lhe dark brown residual oil was further distilled to give the 11 (3-mathyl f.henyl)-ethyl carbamate, (21 gms. ), as a pale yellow oil. (B.Pt. uc:'c 0,5 m.m.hg). 'lha latter sample was added to ice-cooled t'u nitric acid (500 mla.) and ths m1xtllre was stirred for halt an houl" after tmioh time conoentrated sulphul"io acid (60 mls.) was added, The stirring was continu..nd for a further haul" and then the reaction mixture t~a poured onto ice. 'lhe yall011 precipitate was removed by filtration and was recrtatalliaed ( C) from ethanol. Small portions of the carbamate were hydrolysed by dibsolving them 1n a vert large exoesa of' ~oncentrated sul];ilul"ic acid and by the dro! addition of water until the free nitro aniline was precipitated. [It appeared critical that the water was added slowly to the mixture of sulphul"io acid and carbamate. Rapid addition of water reprecip1tated large amounts of' the undecomposed carbamate]. The precipitated n1tro aniline tractions were 17

27 :ror:~oved by filtration as a :f'1ne orance-brorm powder. A sample of pure aniline (bri~t yell011 crystals) ttas obtained :from the impure product by vac= subllmation. U.Pt )8 C. (lit ;;8.s 0 c, 1~c. 138 C, (4) (18) (19) ) '.IJm lm!zal ACETOP!IEll'OllE:S a~. Introduction. Ilenzal acetolilenones aro normally prepared by the Claisen..Seh!:lidt condensation reaction between the appropriate aeetophenone and benzaldehyde. 'Jhl.s method, as described by Hill, Spear and Lachowitz (20), has been used in this - study' and is referred to as l~thod 1. A secor.d method is available for the production of' benza.lacetolimnones, provided they aro stericall;r hindered, by the use of organo-mgtallio compounds. Y..ohler and matz;r (21) have found that stericall;r hindered ketones and Gr1gnard reagents do not cive the normal reaction the carbonyl group. SUbsequently l'uson, F\lgate and F!scbor (22) showed that the reaction of 2,4,6,tr1 methyl acetojilcnone and Jilen;rl magnesium produced an intermediate product that behaved as a true organo ootallio compound; where R is the mesityl group. (2 1) 'nlese workers stated that when benzaldehyde was reacted with the above intermediate a carb1nol,2(2,4,6, tr1 methyl benzoyl)- 1-:lilen;rl ethanol, was produced. The method described here uses phenyl Uthil.l.'ll instead of ):ilenyl magnesium bromide to produce

28 reactivity ot orgeno lithium compounds compared with orgeno magnesium compounds an organo-metallic 1ntamed1ate was obtained and subsequent reaction of this intermediate gave high :;ields or the carbinol. Dehydration of the carb1nols gave the required product. ~e structures of' the compounds prepared by this method (l~ethod 2) were confirmed by inf:ra red and nuclear mae;oet1o resonance spectrosco:w. l>/henever possible the ident1 ties of the compounds were verified further by comparing their spectra and melting points with the corresponding compounds produced by the more conventional Method 1, ~1ethod 2 has particular importance in the production of the 13 substituted etericall7 hindered benzal acetoxnenonea (e.g. 11 Me. benzal 2 1,4',6',tr1me~l aceto:>henone) which cannot be mad3 by the normal condensation reaction. Methods 1 and 2 are described in detail below for typical examples prepared by these reactions and thereafter onl7 details o:r any special conditions used are recorded. tjnless otherwise stated all the substituted aceto:>henones used 1n the preparations were purified coi:i!:lercial samples. b. l Nomenclature. All the compounds were numbered as 1n FIQ, 2-1 according to the regulations of the Chemical Society and Chemical Abstracts. 0

29 c.) Hethod 1. Benzal acetoffienone. Sodium hydroxide ( 4.4 GDS.) was dissolved in lfater ( 40 mls.) and ethanol (25 rnls.). acetor.ucnone (10.2 gms.) and freshly distilled benzaldehyde (9.2 sms ) were added. The solution was agitated for 24 houre after which time an equal volurne of water was added. The product was extracted with chloroform and drl.ed over anhydrous sodium sulphate. 'll1e chloroform was distilled off under reduced pressure and the productt a yellow oil was allowed to stand in a re:f'rigerstor for 24 houre. Pale yellow cr:rstals were formed which were recr:rstalllsod :t'rolll eth.anol to constant melting point. Yield 15 gms. M.Pt c. (lit. 58 c.. (23)) Benzal 2 1 MG. Aceto!ilenone. Benzaldehyde (9.2 g.) and 2 M3 acetoliwnone (ll.4g.) were condensed as above!lt>.e final product t:as obtained as a yellow on. Distilled at Smn Hg. B.Pt.1TI 9 C. (lit. 197 C. 7rmt.Hs.(24 )). Yield 14.7 gms. Finally purified by preparative gas chromtograiil:f. Benzsl J'l-le. Benzaldehyde (9.2g.) and ~.acetophenone (U.4g.) were condensed as above. The product was a yelloy r.'lld. reor:rstall1sed from ethanol. M.Pt. 6o 0 c. (lit. 61 c. (24)) Yield 16 sus.

30 Benzal 4 1 Ne.acetoroenone. Benzaldehyde (9 25 ) and lfme,acetophenone (ll.4g,) were condensed as above. 'lhe product was a yellow l!olid recry~~tal.lil!cd from ethanol. Yield 17,3gms, Benzal diiija,aoetojilenone, Benzaldehyde (9,25,) and d!me,acetophenone (l2.5g.) were condensed as above, Yellow cr;ystals were produced, re cry~~tal.used from ethanol. M,Pt, 70-1 C. (Ut, 7l 2 C, Yield l8.ssms. (26)) Benzal dime,acetojilenone, Benzaldehyde (9.2g,) and dime,acetojilen(l!le (l2.5g,) were condensed as above. A yellow oil was produced, '..0 0 Distilled at, 207 /j c. (Ut, c. lmm.hg, (27)) Yield ljgm:s, Finally purified by PNI8l'ative gas chromatogralil;r. d.) M~thod 2. Ilenzal d1me, acetojilenone, A solution of 2,6 dime, aceto}ilonone (Sz,), preparation outlined below, in dry ether (20 ml.s,) was added slowly over a per.l.od of 30 minutes to an ethereal solution containing a alight e.jtcesl! of }ilen;vl lithium, 'lhe rel!ulting m1xture was refluxed for a further 30 minutes, Freshly distilled benzaldehyde ( in lll1hydrous ether (25 mls.) was added over a period of one hour, Refluxing was continued for 18 hours, during which -21-

31 time the solution turned a vivid yellow. 'lhe organo-metallic compound was decomposed blf ice water, the ethereal solution was removed and the ether distilled off to leave a brown oil. 'lbis was distilled ('TJmn. Hg, Yield 6sns. l80-2 C) to produce a yellow oil. 'lbis oil was further distilled (5mm, Hg, l80-2 C) to give 5g, of a yellow oil, 'lbis oil did not crystallise on standing, on cooling in a refrigerator or in liquid nitrogen, Analysts:. Found: c-86,55% Expected: H 6.g2% B:mzal tril<ie. acetornenone, Ha6,8% 'lhe procedure was followed as above using the lithium derivative of 2,4,6,tri."le. acetor:henone (0.02 moles) which was condensed with freshly distilled benzaldeeyd.e (5.5g,). 'lhe product was obtained as yellow oil. B,Pt. 208-l0 C, (l4rnrn, Hg.) On standing in a refrigerator, yellow crystals were produced, which were recrystallised from ethanol, Yield 6.2 SllS M.Pt. 62 C, (Ut, 63 C. (28)) 'lhe compound was also prepared ey condensing 2,4,6,tr1Me, acetor:henone (6,Jg,) and benzaldehyde (4.5g.) according to I<Iethod l, Yellow crystals were produced, recrystallised from ethanol. Yield 8,lgms, C. Eenzal tetra11~,acetophenone, 2,3,5,6,tetrar<Ie. acetornenone (7g, ), preparation outlined below, -22-

32 was reacted with phenyl lithium and benzaldehyde (7 5 g.) as above, 'lbe product, a yellow oil, was distilled, 200 C Yield loe;ms, After coollng for 12 hours yellow green c:rystala were f'o:rmed, these were recrystallised f'rcm ethanol, 0 ( 0 M,Pt, 93 C. 95 c. (29.)) 'lbe compound was also prepared b7 condensing 2,3,5,6-tetra Me, acetojilenone (3,5 gm.s,) and benzaldehyde (3gm.s,) to Method 1, A yellow solid was produced, Recrystalllsed f'rcm ethanol, Yield 5gm.s. 0 93 c. (3-Meth;yl Beru!al aeetophenone. AcetoJilenone (6.8g,) in xylene (20mls.) was added to aluminium tertiary buto:dde (6,8 g,), 'lbe mixture was stirred and heated to c, af'ter 30 mins. tertiary butyl alcohol distilled over at 80 C, Heating was continued f'or a further 2 hours hy which.. time the reaction mixture had 0 turned a :yellowish green. 'lbe mixture was cooled to 110 C and water was added dropdse until a :yellow solution containing a white precipitate was obtained, Further coollng was ca!'l'ied out 1n iced water and the prec1p1 tated alundnium hydroxide was centrifuged off'. 'lbe xylene was removed f'rcm the yellow solution by distillation (48 C ;.onrn.hg. )J an orange liquid remained, Yield 3,5gms

33 ~ oil was distilled at 200-2l0 C at l4mm.hg. F1naJ. Im'it7 was obtain"...d b:r a further distillation at 208 2l0 C at l4mm.hg. (li t.225 C/22mm.Hg. (30)) only the middle fraction was retained. a-~tlvl Eenzal tr1.'le. aceto).ilenone. Acetophenone (8.0g.) 1n ether (20mls.) was added over a period of one hour to a suspension of 0.05 moles of the lithium derivative of 2,4,6, acetophenone in ether (5()mls. ). Ret'lux!ng was continued for 18 hours, (the mixture became vivid yellow), the ethereal solution was removed and the ether allowed to evaporate ot't' spontaneousl71 this caused the organometallic compourxl to decompose and a yellow oil was obtained. Eventual~ after cooling in liquid nitrogen and the addition of a little petroleua ether a ;vellow solid cr;vstall1sed. Yield 8.6~. ~ compourxl was recr:rstall1sed trom ethanol. C (lit. 84 C (31))!!-Phenyl Eenzal acetop!lenone. rus compourxl wao prepared b:r the dell;;drobromination ot the a bromo flfld!phenyl propiophenone as outlined by' Kohler and Johnstin (32). a Bromo flfld!phenyl propiothenone (38 ) was renuxed in an ethanolic solution of potassium h;vdroxide (O.Sz/l.OOmls.) for 24 hours. 'lbe solution became bright :rellow and the potassium bromide was precipitated, the ethanol was removed by' vacuum distillation and the residue poured into water, the product was extracted with ether and dried

34 over anhydrous sodium sul!flate, The ethereal solutio:~. was decanted l'm was allowed to evaporste sponta11.oousl.y, A yellow oil was producd. which solid1t1 'ld on standing to giye yellow crystals. Yield 1,9gms, Reoeystallised f'rom ethanol and. n hexane to oonetant melting point. M.Pt. 85 C (11t.87.s-aa.s 0 c, (32)), 'nle compound was further p~:r 1 tled by col= chromtocraphy, with neutral aluminium oxide as the stationary ]i'lase and anhydrous ether as t!luent. A nerrow ora~ ba.nd was retained on the column and the Main bulk of' the sa:nple passed through, 'nlis solution was retained and the ether removed by evaporation to give a yellow-green oil which soud1t1ed to g1 ve yellow CI'Y'Stals. Reei'Y'stallised f'rom ethanol. ~c. Phenyl l'len:o:al tr!me. eoetop:tenone. To a suspension of 0,05 moles of the lithium derivative of 2,4,6, tr1me. acetophenone 1n ether (loomls.), benz!ljilenor.e (log,) in ether was added over 2 hours, 'nlo mixture was refluxed for 24 hours, during which timt'! t.":e solution turned bright yellow. 'nle reaction product was doc:o'llposod by iced water and the ether layer remo\'ed a."ld dried over anhydrous sodium sulph11te, The ether was removed by distillation and the yellow oil that was produced cooled, :yellow crystals were deposited, and were reci'y'stallised f'rom ethanol. Yield losms. M.Pt. 103 C. (lit, 104 c (:~1.)) Anal:rsisa Found1 c-88.5~ H-6.80%

35 2 3 PREPARATION OF IN'lERMEDIATE COMPOUNDS, 2,6, dir-te. acetownone 'lhe reaction scheme followed WIIB that proposed b.1 Schwartman and Corson (33), PN>paration of 2,6, dime. ffienzl meth;rl carbinol. \ solution ot 2,6, dimee iodo benzene (20~;, ), preiji'red b.tlitera't'.tre method, (34), in ether (loomls,) was added to lith1urn ribbon (l.4s.) in dl7 ether (40mls.) and the l'lixture refluxed. for 6 hours. 'lhe mixture was cooled in iced-water and acetaldehyde (SS ) in ether (loomls, ) was added drcpldse over a period of 2 hours. Heat was generated and the mixture was gently refluxed for a further 14 hours, th'9n it ~ms deco"!posed with iced-water, The ether layer was separated ahd WIIS washed successively with water, a dilute solution or sodiurn thiosulphate and ~rater. Af'ter drying over anhydrous sodium sulpiulte the ether was removed by distillation to give 2,6, dir1e, phenyl rnthyl carbinol as white crystals. Yield Bgms, 0 B,Pt, 108-UO C, at 'jrrn,hg, PrePI!I.ration of 2,6, dime, acetophenone, 2,6, d1j1e, phenyl meth;rl carbinol (Bs.) was added with st1rrir13 over 90 minutes to a cold solution of potassiurn dichromate (JOg,) and 9~ su1pn!r1o acid (24 mls.) in water (150mls,). 'Ihe mixture was saturated with sodiurn chloride and the product was extracted with ether, af'ter removal of the ether the product was distilled at 2}nm.H!::, the main fraction dh"illins over at l08-ll3 C, Yield ssms.

36 2,,,5,6,tetrat.!a. acetopllenone. Durene (40g,) was dissolved 1n carbon disulphide (400mlB.) a'to 0 C, anhydrous aluminium chloride was added slo11ly to the stirred solution followed by acetyl chloride (24g,) added droprlse over 30 minutes. 'lhe mixture was stirred at 0 C for 8 hours o.nd allcmed to stnnd overnizh,t at room temperature. 'l'he mixture uas decomposed by pouring into an ice hydrochloric acid trater mixture and the product was extracted wit: ether. 'lhe ethereal solution was dried over anhydrous sodium sulifulte, after which it was decanted ar..d the etheto tras allowed to evaporate spontaneously, A pale bro1m solid was deposited much \'IS.s recrystalll!oied :f'rom 40/GO petroleum ether to si ve llhi te crystals. Yield 25gms. n,pt, 72 C. (Ut. 75 C {;55)) a-bromo!3!3 diphenylprop!ophenone. «-Bromobenzal acetophenone (8g,), prepared by Uteraturo methods (,36), was added in ether (25 rnls.) to a solution obtained by dissolving magnesium (l.~e;.) in bromobenzcne (9.5g,) and anhydrous ether (loo:nls, ), 'n'.e reaction tilts carried out in boillng ether. Immediately all the a-brorco benzal acetophenone had been added the solution tumed orange brown. 'lhe mixture was poured into iced-water, the organic productg were extracted with ether, and dried over!ulhydrous sotium sulphate. 'nle ether was removed by evaporation and a conta':lill<l.ted product 1\'aS produced. 'lhe contall'inate, an oranee oil, could not be -27

37 removed by recrystallisation using common,"solvents or by column chromatogratily. Kohler and Jol:mstin (32) reported this reaction to be satisfactory with a m yield of' pure product, but although the reaction was attempted several times a pure sample could not be obtained. 'lhe preparation was repeated with the same quantity as above 1 the ethereal solution ot a bromo benzal aoeto];henone was added quickly to a cooled ethereal solution of' Iilen:vl magnesium bromide with immediate decomposition of' the products with iced dilute hydrochloric acid. A small yield of a white product was obtained, this was very slightly contaminated by a yellow compound,but was readily purified by washing with ethanol and recrystallisation from acetone to give creamy white crystals. Yield 4.2 &lll3 15'fc. (ut. 160 c (32)).


39 3=1. INFRA-RED SmcTRA. a) Expel'!"'lE!ntal, 'nle :1n ra red spectra or all the benzal aoetophenones P''epared were recorded on a Unicam S,P. 200 G Inf'ra-Red Speotromater. All the spectra were obtained 1'or 5 w/ ~ solutions 1n carbon tetrachloride w1 th a carbon tetra chloride blank, 'nle solvent used was or spectrosol grade; the instrument was standardised by a pol,j"styrene standard, b) Discussion, 'nle 1nf'ra red spectra were recorded to prove the authenticity of the samples, particularly those prepared by ~bthod 2,. Benzal 110etoibenone PIG. 3=l should have some highl,j" characteristic bsnds due to the styryl grou;p, phenyl group, the ethylenio group and the carbonyl group :>' PIQ, 3=1. B:lllamy (37) has assigned these absorptions 1n benzal acetoibenones to bsnds at the following wavelengths: llmd A, styryl group l 1570cm Band B, tilenyl group 1610 cm" 1 Band C, et.hylenic group, 1650 cm l. Band D, carbonyl group 1670 cmn 1, Also the undehydrated carbinol which would be prepared as an 30-

40 intermediate product 1n l~ethod 2 should hve a characteristic band corresponding to the o-h stretching frequency at about 3!"00om l. Particular attention was paid to the i.r. spectra of the compounds for the presence or otherwise of these buns. 'lhe band positions of the gt'oups 1n the benzal acetopbenones corresponding to :Bellamy 1 s assignments are tabulated 1n table (3-J.), As can be seen all the benzal aceto]flenones prepared show the presence of this quartet of bands and this can be taken as good evidence that the compounds prepared are authentic. None of the compounds prepared b,v Method 2, e.g. 'bsnzal ]flenone, had a band corresponding to a hydroxyl gt'oup so dehydration of the carbinol was completed to give the required unsatunted ketone. 'nle pos1 t1on of tne bandlil Il, C and D 1n the i.r. spectra appear to be affected b.v steric hindrance of the molecules caused b,v the methyl gt'oups, 'lhe variation of band C would suggest that reduction of conjugation caused b.v ster1c-d1stort1on affects the ethylene bond 1n the l!l)lpcule. Sim1lsrly increasing ster1o hindrance causes the carbonyl gt'oup frequency, band D, to alter l!l)vilig it to a lower frequency, \ le would also expect that 1f the ethylenio gt'oup frequenc:r is affected b.v steric hindrance so the st:n7l gt'oup frequency, band A, would be sim1lsrly affected, but this does not happen. This may be due to a wrong a5signment of bands b,v :Bellamy (:~7) and further work 1e being carried out 1n these laboratories to investigate thie problem (38). Even 1f the band assignment is 31

41 wrong however 1 t does not completel7 1nval.!.t!ate the above argument because the spectral pattern of benzal acetoj:henone is well docume'lted and ever,y bc-nzal acetoitlenone produced here showed a similar spectral pattern, 32

42 INFRA-RED SPEcTRAL DATA FOR Tfm BENZAL ACE'roPHENONES BAND A BAND B BAND c B\ND D (cm. 1 ) (cm- 1 ) (cm- 1 ) (cm- 1 ) mnzal ACE'roPHENONE M J'}le Me d1ma o d1me d1me trllle '5'6'tetraMe JlMe EENZAL ACETOPHENONE tr1!4~ Jll'h.mNZAL ACETOPHENONE tr1me

43 3=2. PRO'ION MAGNETIC RF.SONA!a SPECTRA. a) Exper:l.mental. 'l'he p.m.r. spectra of the benzal aceto J;ilenones were obtained on a Perkin Elmer RlO Uuclear Magnetic Resonance Spectrometer (60 mega cycles). All the spectra were obtained 1n approximately 15 w/ V'% solutions Ull~ Spectrosol grade as solvent with tetmmethyl s1lane as internal standaro. b) Discussion, Benzal aceto];ilenones possess tlto ];ilenyl groups and it was hoped thet the p.m.r. spectra would show different chemical shifts for each phenyl group depending on the conformation of the molecule. In addition the compounds possess two olefinic protons which should appear as a pair ot doublets, the splitting of lihich should indicate the geometr:l.e is01l'.er:l.sat1on present 1n the molecule. Blt the complex conjugate structure of the benzal acetolilenones made assignment of individual peaks veey difficult and no separate resolution of the ];ilenyl protons 1n the simple benzal aceto];ilanones was possible. 'l'he p.m.r. spectra of aceto];ilanone shows that the position of the o:rtho protons are at a lower field than the other aromatic protons, such a ];ilenomenon was observed in benzal aceto];ilenone, 11 methyl benzal acetophenone and 11 ];ilecyl banzal aceto];ilenone. It was possible to assign resonance frequencies to the methyne proton 1n the fj substituted benzal aceto];ilenones, in.ji methyl benzal acetophenone it appeared at,.351:., while the substitution of a 11 phenyl group deshielded this proton ani it appeared at,.10 "'C In the none 11 substituted benzal acetolilenones, hereafter referred to as simple benzal acetophenones, the doublets that should ap~ar for these protons were absorbed into the

44 main mul.tiplet of the aromatic protons. A :f'.zrt..her attempt was lll!lde to 11ssign poaks to the aromatic protons in ring A, FIG. (J-1), cy comparing the spectra of the substituted bet>..zal acetophenones with the parent acetojitenoncs used in the synthesis of the benz!u acetojitenones, but no concluaive assignments could be made, For compounds prepsred cy Method II incomplete dehydration of the carbinol intermediate compound would leave a compound with a saturate a:-!3 carbon-carbon bon1 (FIG.)-1) whose protons would have different resonance frequencies to those of an unsaturated carbon-carbon bond. For this reason the p.m.r. spectl'ul:l of p,f1,d1jitenyl propiojitenone (FIG.3-2) was obtained. FIG.)-2. It was f'ound that the spectrum of' this co:npound showed peaks at appro:x1mately 5,25 '( and 6.Z!1: in addition to a complex mul.tiplet at about 2,7 "t The peaks at the higher l: v!uues integrated in ratio of' lt2 and were attributed to the protons b and a respectively(fig.3-2). The absence of' theee peaks in the p.m,r, spectra of' the benzal aceto!itenones studied, even in ve:ey concentrated solutions, is a good indication that the dehydration of' the carbinol was complete

45 and the compounds prepared by the organo-metallic reaction = indeed benzal aceto~nones. Th'.! difference in thet position of the methyl protons 1n the mono methyl substituted benzal acetophenone show:j no significant variation 1n going from ortho to para substitution. :rut, relative to the mono ortho me't;'lyl compounds, the methyl protons 1n the veey sterically hindered compounds, such as benzal ~'6 1 d1i~.acctophenone, are subjected. to some shielding. '.1113 peaks are shifted from 7,6 -c. to 7.81: 'lhis could ba caused by either the carbonyl group or the <X-Il unsaturated carbon-carbon group, (FIG,>l). '!be p.m.r. spectra of benzal tetra Me.acetophenone shows tmt the crtho_metbyl protons resonate at hig!er t'requenoies than the corresponding protons in the 2 1 6'~.compound.. 'l'his would. indicate that the meta meth;y l protons force the ortho protons in the former co:npound by a buttressins effect I!Xlre into the environment of the deshieldins group, The positions and types of bands produced by the!!'ethyl protoruj for tl1e sl1ple benzal aoeto phcnones are tabulated 1n table (3-2), The p.m.r. technique was also used to estimate the purity of the prepared compounds. Ver;r coneentrated solutions of the compounds in carbon tetraehloride were prepared tu'ld the p,m.r. speetra obtsined., The spectra of all the compounds showed steady and o~ooth bme lines with n.;> dbtortion produced ltj cxtra.nco~m prototl.'j ~lhich ~rould indicate i'llp'jri tie:s. '!'IUs toclmique oh01~e1d tlut.t all the prepared compounds were or high. purity,

46 TABLE 3=2 'llle POSITION 01' THR ~IE'mYL PRO'roNS IN 'llle'LE mnzal ACETOP!lENONE AS FOUND BY p.m.r. SPECTROSCOP'l COMPOUND POSITION BENZAL ACETOPHENONE Me. 7,6o 1:. SINGLET 3 1 Me. 7,63 ~ SD!GLE'l' 4 1 Me; 7.65 "'C SINGLET d1me "t' DOum.ET dil-b "C DOum.E'l' du.!e "C SINGlET tr1me "t DOum.E'l' ! ;tetra!-!a. 7.Bo-98 't. DOtll:lLET

47 3:3 'lhe UL'mA VIOLET SPECTRA OF THE BENZAL ACETOPHENONES, a) Experimental. 'lbs u.v. spectra of all the benzal aoeto.. Ihenones were obtained by a H1lger Watts Uvispek Spectrothoto meter using matched lcm. silica cells, all the u.v. spectra were obtained at 25 C. 'lhe strength of the solutions was approximately 5 x lo 5:~ in absolute ethyl alcohol, en absolute ethyl alcohol blank was used for each determination. 'lhe ethyl alcohol was checked before use for aromatic impurities and turther purified by passing it down a l metre column packed w1 th fine ( meah) colourless silica gel and by a final fractional distillation. A single standard flask was used throughout; this flask was blackened to avoid any possibility of cis trans isomerisation occurring. b) Discuss~.on. Benzal acetothenones (FIG. 3-l) possess a complex conjugated system since the carbonyl group is interposed between Iilenyl and styeyl groups. Trans benzal acetoiilenones ~If two main regions of u.v. absorption, one near 230 m r and the other near )CO m JL 'lbsse two maxima were attributed by Ferguson and Barnes (39) to the partial benzyl (l' and cinnamoyl (Ph-cH=CH..CO ) chromothores respectively. Further work by Syzmant and Basso (40)has shown that the long wavelength band is a rr - 1T* band which involves the whole conjugate system and the short wavelength band is due to secondaey absorption A so-called "middle" band near 250 r.1 f has been show by mack and Lutz (41) to be present in all cis benzalacetoiilenones -36-

48 and some trans bem:al acetopwnones, This balld probsbl:r results f'ro!ll the partiall;y benzoyl or benzoyl-vinyl (Hl-co-cH=CH-) chromophores. All the simple benzal acetolilenones show the complete absence of this "middle" balld and are clearly trans isomersa 13 Methyl benzal acetolilenones and!3 phenyl benzal acatophenones which have clearly been shown to be trans (42) do possess a "middle" band in their spectra, The preseno& of this band ~3 caused by the substitution of the olefinic a:-!3 bond in the s:rstem and this increases the chromolilore responsible for this band. mack and IAltz (41) have argued that this band in!3 substituted benzal acetolilenones would be deleted if a mesityl group was substituted for ring A in the benzal acetophenone (FIG.)-1), 'lhis substitution will cause a disruption of tha coplanarity of the 8l'OY1 system and destrey the chromosnora. This effect was indeed observed when this substi tl.uon was made. 'Ihe spectraldata for the benzal acetolilenones are tabulated in Tables (3-J. )-4 and 3-5), In the case of the simple benzal acetolilenones only the spectral data for the long TT "'T * band are recorded. :Benzal acetophenone 1s a planar unsaturated molecule and two hypothetical resonance structures are possible as shown in FIG, )-), 0 0 o~() + 37 \ I