General Organic Chemistry 1033

Transcription

1 Generl rgni hemistry 0 or Exmple, is, Dihloropentene (igh dipolemoment) Similr is the se with hexene-. isexene (more polr) trns,dihloropentene ( Less dipolemoment ) trns exene (Less polr) (iv) By studying other physil properties: () The is-isomer of ompound hs higher oiling point due to higher polrity, higher density nd higher refrtive index thn the orresponding trns-isomer (Auwersskit rule). is Butene.p m.p. 9 is, Dihloroethene 0 80 trns Butene 0 trns, Dihloroethene 8 0 () The trns-isomer hs higher melting point thn the is-isomer due to symmetril nture nd more lose pking of the trns-isomer. (v) Stility : Trns-isomer is more stle thn is-isomer due to symmetril struture. Terminl lkenes suh s propene, -utene nd -methyl propene do not show geometril isomerism. is-trns isomers re onfigurtionl isomers ut not mirror imges, hene is nd trns isomers re lwys distereomers. on-terminl lkenes with the sme toms or groups either on one or oth the ron toms of the doule ond suh s -methyl-- utene,,-dimethyl utene et. do not show geometril isomerism. () E nd Z system of nomenlture : is nd Trns designtions nnot e used if four different toms or groups re tthed to the ron toms of doule ond. In suh ses, E nd Z system of nomenlture is used. This system is sed on priority system developed y hn, Ingold nd Prelog. In this system, the two toms or groups tthed to eh of the douly onded ron re put in order of preferene on the sis of sequene rules. The symol E is ssigned to n isomer in whih the toms or groups of higher preferene re on the opposite side (E from Germn word Entgegen = ross or opposite). The symol Z is ssigned to n isomer in whih the toms or groups of higher preferene re on the sme side (Z from Germn word, Zusmmen = together). E isomer d e Zisomer signifies higher preferene nd signifies lower preferene. Preferene in most of the ses Z orresponds to is-form nd E to trnsform. owever, there re mny exeptions. The following rules re followed for deiding the preedene order of the toms or groups; (i) igher priority is ssigned to the toms of higher tomi numer. or exmple, the order of preferene in the following toms,,, I, Br is : I (t. no. )> Br (t. no. )> (t. no. 7)> (t. no. ). (ii) If isotopes of the sme element re tthed, the isotope with higher mss numer is given higher order of preferene. or exmple, deuterium D is ssigned higher priority in omprison to hydrogen. (iii) In the groups, the order of preferene is lso deided on the sis of tomi numer of first tom of the group. or exmple, in the following set,,,,, S. The order of the preedene is : S ( t.no7) (t.no.) (t.no.8) (t.no.7) (t.no. ) When the order of preferene of the groups nnot e settled on the first tom, the seond tom or the susequent toms in the groups re onsidered. or exmple, in the set,,, the order nnot e deided on the sis of first tom s it is sme in ll the groups. owever, in, the seond tom is ron, in, the seond tom is hydrogen while in, the seond tom is oxygen. ene, the order of preferene is : (t.no.8) (t.no.) (t.no.) (iv) A douly or triply onded tom is onsidered equivlent to two or three suh toms. or exmple, The group is equl to. is equl to nd the group () umer of geometril isomers in polyenes (i) When ompound hs n doule onds nd ends of polyene re different, the numer of geometril isomers n The given ompound hs four doule onds nd the two ends re different (ne is nd other is ). Therefore, numer of geometril isomers n. (ii) When the ends of polyene re sme. se I : When numer of doule onds (=n) is even then the n [( n / ) ] numer of geometril isomers n,even umer of geometril isomers 8 0. se II : When numer of doule onds (=n) is odd. n n ( n / ) n umer of geometril isomers ( n, odd ) umer of geometril isomers. () Geometril Isomerism in nitrogen ompounds

2 0 Generl rgni hemistry (i) Geometril isomerism due to ond. The importnt lss of ompounds exhiiting geometril isomerism due to ond re oximes, nitrones, hydrzones nd semirzones. But the most ommon ompound is oxime. ximes : In ldoxime, when hydrogen nd hydroxyl groups re on the sme side, the isomer is known s syn. (nlogous to is) nd when these groups re on the opposite side, the isomer is known s nti (nlogous to trns) Syn -enzldoxime Anti enzldoxime In ketoximes the prefixes syn nd nti indite whih group of ketoxime is syn or nti to hydroxyl group. or exmple: this ompound will e nmed s; () Syn-ethyl methyl ketoxime nd re syn or () Anti-methyl ethyl ketoxime nd re nti. Similrly onsider the following struture Syn methylethylketoxime or Antiethylmethylketoxime (ii) Geometril isomerism due to = ond. Syn -zoenzene Anti zoenzene () Geometril isomerism show y umultrienes : umultrienes (Trienes with three djent doule onds) show only geometri isomerism. This is euse their moleule is plnr, s suh the terminl groups nd - toms lie in the sme plne. Therefore, in this se their plnr struture n exist in two distereoisomeri forms, is- nd trnsut no enntiomeri forms re possile. isex,, triene trns ex,, triene (7) Geometril isomerism in ylolknes : Disustituted ylolknes show geometril isomerism. is-,-ylohexnediol is-,-dimethyly Trns-,-ylopent lopropne nediol ertin ompounds show geometril s well optil isomerism. Suh type of isomerism is known s geometril enntiomerism. ptil isomerism () ompounds hving similr physil nd hemil properties ut they hve the ility to rotte the plne of polrised light either to the right (okwise) or to the left (Antilokwise) re termed s optilly tive or optil isomers nd the property is lled optil tivity or optil isomerism. The optil tivity ws first oserved in orgni sustnes like qurtz, rok-rystls nd rystls of potssium hlorte K ), potssium romte ( KBr ) nd sodium periodte ( I ). ( () Mesurement of optil tivity : The mesurement of optil tivity is done in terms of speifi rottion whih is defined s the rottion produed y solution of length of 0 entimetres (ne deimetre) nd unit onentrtion ( g/ml) for the given wvelength of the light t the given temperture. t Speifi rottion, wvelength os l Where os is the rottion oserved, l is the length of the solution in deimeters nd is the numer of grms in ml of solution. The speifi rottion of the surose t 0 using sodium light (D-line, =89Å) is +. nd is denoted s: 0 D /.( 0.0g ml wter) + sign indites the rottion in lokwise diretion. () n the sis of the study of optil tivity, the vrious orgni ompounds were divided into four types : (i) The optil isomer whih rottes the plne of the polrised light to the right (okwise) is known s dextrorottory isomer (Ltin: dextro = right) or d-form or indited y +ve sign. (ii) The optil isomer whih rottes the plne of the polrised light to the left (Antilokwise) is known s levorottory isomer (Ltin; levo = left) or l-form or indited y ve sign. (iii) The optil powers of the ove two isomers re equl in mgnitude ut opposite in sign. An equimolr mixture of the two forms, therefore, will e optilly intive due to externl ompenstion. This mixture is termed s remi mixture or dl-form or () mixture. (iv) ptil isomer with plne of symmetry is lled meso form. It is optilly intive due to internl ompenstion, i.e., the rottion used y upper hlf prt of moleule is neutrlised y lower hlf prt of moleule. () hirlity, (i) Definition : A moleule (or n ojet) is sid to e hirl or dissymmetri, if it is does not possess ny element of symmetry nd not superimposle on its mirror imge nd this property of the moleule to show non-superimposility is lled hirlity. n the other hnd, moleule (or n ojet) whih is superimposle on its mirror imge is lled hirl (non-dissymmetri or symmetri). To understnd the term hirl nd hirl let us onsider the lphet letters P nd A wheres P is hirl, A is hirl s shown in fig. Mirror Mirror on-superimposle Superimposle (hirl or dissymmetri) (Ahirl or non-dissymmetri) (ii) Elements of symmetry : There re three elements of symmetry, () Plne of symmetry : It my e defined s plne whih divides moleule in two equl prts tht re relted to eh other s n ojet nd mirror imge. e.g.,

3 Generl rgni hemistry 0 () entre of symmetry : It my e defined s point in the moleule through whih if line is drwn in one diretion nd extended to equl distne in opposite diretion, it meets nother similr group or tom, eg. isdimethyldiketo piperzine nd Trns Dimethyldiketo piperzine Sine trns form ontins entre of symmetry, it is optilly intive. () Alternting xis of symmetry : A moleule is sid to possess n lternting xis of symmetry if n oriention indistinguishle from the originl is otined when moleule is rotted Q degree round n xis pssing through the moleule nd the rotted moleule is refleted in mirror tht is perpendiulr to the xis of rottion in step (I). (iii) Symmetri, Asymmetri nd Dissymmetri moleules () Symmetri moleules : If ny symmetry is present in the moleule then moleule will e symmetri moleule. () Dissymmetri moleules : Moleule will e dissymmetri moleule if it hs no plne of symmetry, no entre of symmetry nd no lternting xis of symmetry. () Asymmetri moleules : Dissymmetri moleule hving t lest one symmetri ron is known s symmetri moleule. All symmetri moleules re lso dissymmetri moleules ut the reverse is not neessrily true. 80o rottion round xis o plne of symmetry Dissymmetrimoleule Asymmetri moleule Plne of symmetry o plne of symmetry Dissymmetrimoleule Asymmetri moleule (iv) hirl or symmetri ron tom : A ron onded to four different groups is lled hirl ron or hirlity entre. The hirlity entre is indited y sterisk. e.g., Refletion perpendiulr to xis rottion entre of symmetry * d * Lti id rons tht n e hirlity entres re sp -hyridised rons; sp nd sp -hyridised rons nnot e hirl rons euse they nnot hve four group tthed to them. Isotopes of n tom ehve s different group in stereoisomerism. D * Br T * Lti id ron of the following groups will not e hirl ron,, X,, Z Mlei id ( ) show geometril isomerism while mli id ( ) show optil isomerism. () lultion of numer of optil isomers (i) If moleule is not divisile into two identil hlves nd moleule hs n symmetri ron toms then umer of optilly tive forms n umer of enntiomeri pir / umer of remi mixture / umer of meso form 0 (ii) If moleule is divisile into two identil hlves, then the numer of onfigurtionl isomers depends on the numer of symmetri ron toms. se I : When ompound hs even numer of ron toms, i.e., n,,8,0,,... : (i) umer of optilly y tive forms (ii) umer of enntiomeri pirs / (iii) umer of remi mixture / (iv) umer of meso forms m n ( n / ) (v) Totl numer of onfigurtionl isomers m se II : When ompound hs odd numer of ron toms, i.e., n,,7,9,,... : ron (i) umer of optilly tive forms (ii) umer of enntiomeri pirs / (iii) umer of remi mixutre / (iv) umer of meso forms ( n ) / m (v) Totl numer of onfigurtionl isomers n m 7 ( n)/ () ptil tivity of ompounds ontining one symmetri Exmples :

4 0 Generl rgni hemistry ; Lti id ; Glyerldehyde -hloro--phenylethne Any moleule hving one symmetri ron tom exists in two onfigurtionl isomers whih re nonsuperimposile mirror imges. (I) (II) (I) nd (II) hve the sme moleulr formul, the sme struture ut different onfigurtions, hene (I) nd (II) re known s onfigurtionl isomers. (I) nd (II) re nonsuperimposle mirror imges, hene (I) nd (II) re optil isomers. onfigurtionl isomers whih re nonsuperimposle mirror imges re known s enntiomers. Thus (I) nd (II) re enntiomers. Pir of (I) nd (II) is known s enntiomeri pir. (i) Properties of Enntiomers : All hemil nd physil properties of enntiomers re sme exept two physil properties. Mode of rottion : ne enntiomer rottes light to the right nd the other y n equl mgnitude to the left diretion. (ii) Remi Mixture : An equimolr mixture of two enntiomers is lled remi mixture (or remte, form, (dl) form or remi modifition). Suh mixture is optilly intive euse the two enntiomers rotte the plne polrised light eqully in opposite diretions nd nel eh other s rottion. This phenomenon is lled externl ompenstion. Remi mixture n e seprted into (+) nd ( ) forms. The seprtion is known s resolution. The onversion of (+) or ( ) form of the ompound into remi mixture is lled remistion. It n e used y het, light or y hemil regents. ron Remi mixture is designted s eing ( ) or (dl). (7) ptil tivity of ompounds ontining two symmetri se I : When moleule is not divisile into two identil hlves. The numer of optil isomers possile in this se is four ( ). urther there will e two pirs of enntiomers nd two remi modifitions. In prtie lso it is found to e so. onfigurtionl isomers whih re not mirror imges re known s distereomers. Properties of Distereomers : Distereomers hve different physil properties, e.g., melting nd oiling points, refrtive indies, soluilities in different solvents, rystlline strutures nd speifi rottions. Beuse of differenes in soluility they often n e seprted from eh other y frtionl rystllistion; euse of slight differenes in moleulr shpe nd polrity, they often n e seprted y hromtogrphy. Distereomers hve different hemil properties towrds oth hirl nd hirl regents. either ny two distereomers nor their trnsition sttes re mirror imges of eh other nd so will not neessrily hve the sme energies. owever, sine the distereomers hve the sme funtionl groups, their hemil properties re not very dissimilr. se II : When moleule is divisile into two identil hlves. umer of optil isomers umer of meso forms m 0 Totl numer of onfigurtionl isomers (8) ptil tivity in ompounds ontining no ssymmetri ron : Although the lrgest numer of known optilly tive ompounds re optilly tive due to the presene of hirl ron tom, some ompounds re lso known whih do not possess ny hirl ron tom, ut on the whole their moleules re hirl (suh moleules were erlierly lled dissymmetri); hene they re optilly tive. Vrious types of ompounds elonging to this group re llenes, lkylidene ylolknes, spiro ompounds (spirns) nd properly sustituted iphenyls. (i) Allenes : Allenes re the orgni ompounds of the following generl formule. Allenes exhiit optil isomerism provided the two groups tthed to eh terminl ron tom re different, i.e., or (ii) Alkylidene ylolknes nd spiro ompounds : When one or oth of the doule onds in llenes re repled y one nd two rings, the resulting systems re respetively known s lkylidene ylolknes nd spirns. -Methylylohexylidene-- eti id (Alkylidene ylolkne) (iii) Biphenyls : Suitly sustituted diphenyl ompounds re lso devoid of individul hirl ron tom, ut the moleules re hirl due to restrited rottion round the single ond etween the two enzene nulei nd hene they must exist in two non-superimposle mirror imges of eh other. Suh types of stereoisomerism whih is due to restrited rottion out single ond, is known s tropisomerism nd the stereoisomers re known nd tropisomers. Exmples Spirns The ove disussion leds to the onlusion tht the essentil ondition for optil isomerism is the moleulr disymmetry or moleulr hirlity nd not the mere presene of hirl entre. owever, it my e noted tht the moleules hving only one hirl entre re lwys hirl nd exhiit optil isomerism. (9) isher projetion formule : The rrngement of the toms or groups in spe tht hrterises stereoisomer is lled its onfigurtion. Emil isher (89) provided n esy method to represent the three dimensionl formule of vrious orgni moleules on pper. isher projetion is, thus, plnr representtion of the three dimensionl struture. x y

5 Generl rgni hemistry 07 By onvention, the following points re followed in writing the isher formul. (i) The ron hin of the ompound is rrnged vertilly, with the most oxidised ron t the top. (ii) The symmetri ron tom is in the pper plne nd is represented t the intertion of rossed lines. (iii) Vertil lines re used to represent onds going wy from the oserver, i.e., groups tthed to the vertil lines re understood to e present ehind the plne of the pper. (iv) orizontl lines represent onds oming towrds the oserver, i.e., groups tthed to the horizontl lines re understood to e present ove the plne of the pper. (0) me of optil isomers : ollowing three nomenltures re used for optilly tive ompounds, (i) D,L. System of nomenlture : This nomenlture is minly used in sugr hemistry or optilly tive polyhydroxy ronyl ompounds. This nomenlture ws given y Emil isher to designte the onfigurtions of vrious sugrs reltive to the enntiomeri (+) nd ( ) gluose s referene. All sugrs whose isher projetion formul shows the group on the hirl ron tom djent to the terminl group on the right hnd side elong to the D -series. Similrly if is on the left hnd side, then the sugrs elong to the L -series. Dseries Exmples : D( d)glyerldehyde or D( )glyerldehyde Lseries L( l)glyerldehyde or L( )glyerldehyde It must e noted tht there is no reltion etween the sign of rottion (+, or d, l) nd the onfigurtion (D nd L) of n enntiomer. Any ompound tht n e prepred from, or onverted into D(+) glyerldehyde will elong to D-series nd similrly ny ompound tht n e prepred from, or onverted into L( ) glyerldehyde will elong to the L-series. This nomenlture is lso used in -mino ids. (ii) Erythro nd Threo system of nomenlture : This nomenlture is used only in those ompounds whih hve () nly two hirl rons nd () The following struture, Asymmetri ron tom R R i.e., out of six sustituents on two symmetri rons, t lest two should e sme. When two like groups in isher projetion formul re drwn on the sme side of the vertil line, the isomer is lled erythro form; if these re pled on the opposite sides, the isomer is sid to e threo form. R R Erythro form Br Erythro form () R,S omenlture (Asolute onfigurtion) Br Threo form The order of rrngement of four groups round hirl ron (stereoentre) tom is lled the solute onfigurtion round tht tom. System whih indites the solute onfigurtion ws given y three hemists R.S. hn,.k. Ingold nd V. Prelog. This system is known s (R) nd (S) system or the hn-ingold Prelog system. The letter (R) omes from the ltin retus (mens right) while (S) omes from the ltin sinister (mens left). Any hirl ron tom hs either (R) onfigurtion or (S) onfigurtion. Therefore, one enntiomer is (R) nd other is (S). A remi mixture my e designted (R) (S), mening mixture of the two. (R) (S) nomenlture is ssigned s follows : Step I : By set of sequene rules given elow the toms or groups onneted to the hirl ron re ssigned priority sequene. Sequene Rules for rder of Priority Rule : If ll four toms diretly tthed to the hirl ron re different, priority depends on their tomi numer. The tom hving highest tomi numer gets the highest priority, i.e., (). The tom with the lowest tomi numer is given the lowest priority, i.e., (), the group with next higher tomi numer is given the next higher priority () nd so on. Thus, I Br Br I Inresing tomi numer Inresing priority Br Br Inresing prioritywith tomi numer Rule : If two or more thn two isotopes of the sme element is present, the isotope of higher tomi mss reeives the higher priority. Inresing priority

6 08 Generl rgni hemistry Rule : If two or more of the toms diretly onded to the hirl ron re identil, the tomi numer of the next toms re used for priority ssignment. If these toms lso hve identil toms tthed to them, priority is determined t the first point of differene long the hin. The tom tht hs tthed to it n tom of higher priority hs the higher priority. I nd I nd Br nd Br In this exmple the toms onneted diretly to the hirl ron re iodine nd three rons. Iodine hs the highest priority. onneted, to the three rons re nd Br, nd nd nd. Bromine hs the highest tomi numer mong, nd Br nd thus Br hs highest priority mong these three groups (i.e., priority no. ).The remining two rons re still identil ( nd ) onneted to the seond rons of these groups re nd I nd nd. Iodine hs highest priority mong these toms, so tht I is next in the priority list nd hs the lst priority. I I Br Rule : If doule or triple ond is linked to hirl entre the involved toms re duplited or triplited respetively. ; ; By this rule, we otined the following priority sequene : Inresing priority Step : The moleule is then visulised so tht the group of lowest priority () is direted wy from the oserves (At this position the lowest priority is t the ottom of the plne). The remining three groups re in plne fing the oserver. If the eye trvels lokwise s we look from the group of highest priority to the groups of seond nd third priority (i.e., with respet to ) the onfigurtion is designted s R. If rrngement of groups is in ntilowise diretion, the onfigurtion is designted s S. or exmple: R,,,,, okwise rrngement of Antilokwise rrngement of, nd R, nd S Let us pply the whole sequene to romohlorofluoro methne. Br In this isher projetion the lest priority numer is not t the ottom of the plne. In suh ses the isher projetion formul of the ompound is onverted into nother equivlent projetion formul in suh mnner tht tom or group hving the lowest priority is pled vertilly downwrd. This my e done y two interhnges etween four priority numers. The first interhnge involves the two priority numers, one is the lest priority numer nd other is the priority numer whih is present t the ottom of the plne. In the ove se first interhnge will tkes ple etween nd. (A) (B) irst interhnge of two groups t the hirl entre inverts the onfigurtion nd this gives enntiomer of the originl ompound. Thus (A) nd (B) re enntiomer. The seond interhnge involves the remining two groups. Seond interhnge etween remining groups, i.e., nd (B) (A) Exmple : Arrngement of, nd re lokwise, hene onfigurtion is R L-Glyerldehyde Glyerldehyde (or exmple) hs one symmetri ron, hene it hs two onfigurtionl isomers (I) nd (II). ( R)glyerl dehyde (I) irst interhnge Between nd D-glyerldehyde irst interhnge Between nd Seond interhnge Between nd okwise R-onfigurtion (i) irst interhnge etween nd (ii) Se. interhnge etween nd ( S )glyerl dehyde (II) Antilokwise S-onfigurtion ne n drw numer other onfigurtions for glyerldehyde ut eh of them will e repetition of either (I) or (II). In this onnetion it is

7 Generl rgni hemistry 09 importnt to note tht if two projetion formule differ y n odd numer of interhnges (,,, 7,..) of positions of groups on the hirl ron, they re different. But if the two differ y n even numer of interhnges (,,,..) they re identil. or exmple : ( I), irst interhnge ( IV) ( II) Seond interhnge ( III) etween, Third intrerhnge ourth interhnge,, ( V) Thus (I), (III) nd (V) re identil. Similrly (II) nd (IV) re identil. () Resolution of remi modifitions : The seprtion of remi mixture into its enntiomers is known s resolution. Group X rets with group to give new group W. () Asymmetri synthesis nd Wlden inversion (i) Asymmetri synthesis : The synthesis of n optilly tive ompound (symmetri) from symmetril moleule (hving no symmetri ron) without resolution to form (+) or ( ) isomer diretly is termed symmetri synthesis. or exmple the redution of pyruvi id ( ) in presene of nikel tlyst gives () lti id (remi mixture). n the other hnd, pyruvi id is redued to ( ) lti id only y yest. (Pyruvi id) (-Keto propnoi id) Redution ' i' Yest ( ) Lti id mixture ( ) Lti id ( ) Lti id (ii) Wlden inversion: The onversion of (+) form into ( ) form nd vie-vers is lled Wlden inversion. When n tom or group diretly linked to n symmetri ron tom is repled; the onfigurtion of the new ompound my e opposite to (inverse) tht of the originl, i.e., ( ) Mli id P P ( ) hloro suini id ( ) hloro suini id Ag Ag ( )Mli id ( ) Mli id onformtionl isomerism () Definition : The different rrngement of toms in moleule whih n e otined due to rottion out ron-ron single ond re lled onformtionl isomers (onformers) or rottionl isomers (rotmers). This type of isomerism is found in lknes nd ylolknes nd their sustituted derivtives. It my e noted tht rottion round sigm ond is not ompletely free. It is in ft hindered y n energy rrier of to 0 kj mol in different onds. There is possiility of wek repulsive intertions etween the onds or eletron pirs of the onds on djent ron toms. Suh type of repulsive intertion is known s torsionl strin. () Differene etween onformtion nd onfigurtion : The term onformtion should not e onfused with the onfigurtion whih reltes to those sptil rrngements of the toms of moleule tht n e hnged only y the reking nd mking of onds wheres the sptil rrngements in onformtion re hnged simply y rottion out single ond. () Representtion of onformtions : onformers n e represented in two simple wys. These re : (i) Sw horse representtion nd (ii) ewmn projetion (i) Sw horse representtion : In this projetion, the moleule is viewed long the xis of the model from n olique ngle. The entrl ron-ron ond ( ) is drw s stright line slightly tilted to right for the ske of lrity. The front ron is shown s the lower left hnd ron nd the rer ron is shown s the upper right hnd ron. The three onds round eh ron tom ( in ethne or in higher lknes) re shown y three lines. ront ron Sw horse (ii) representtion ewmn projetion ewmn projetion : This is simple method to represent the onformtions. In this method, the moleule is viewed from the front long the ron-ron ond xis. The two ron toms forming the -ond re represented y two irles; one ehind the other so tht only the front ron is seen. The front ron tom is shown y point wheres the ron further from the eye is represented y the irle. Therefore, the onds of the front ron re depited from the entre of the irle while onds of the k ron re drwn from the irumferene of the irle t n ngle of 0 to eh other. () onformtion in lknes (i) onformtions of ethne : When one of the ron tom is kept fixed nd other is rotted out ond n infinite numers of isomers re possile. ut of ll the onformtions for ethne, only two extreme onformtions re importnt nd these re: () Stggered onformtion () Elipsed onformtion Stggered onformtion Bk ron 0 Elipsed onformtion Bk ron ront ron

8 00 Generl rgni hemistry Stggered onformtion of ethne is more stle thn elipsed. (ii) onformtions of propne : The next higher memer in lkne series, propne ( ) lso hs two extreme onformtions, the energy rrier in propne iskjmol, whih is slightly higher thn tht in ethne. Stggered propne Elipsed propne ewmn projetion of propne (iii) onformtions of utne : As the lkne moleule eomes lrger, the onformtion sitution eomes more omplex. In utne ( ), for exmple, the rottion out the single ond etween two inner toms ( nd ) is onsidered. In this se, ll the stggered s well s elipsed onformtions will not hve sme stility nd energy euse of different types of intertion etween (of methyl) nd onds. The lowest energy onformtion will e the one, in whih the two methyl groups re s fr prt s possile i.e., 80 wy form eh other. This onformtion will e mximum stggered, most stle nd is lled nti or trns onformtion (mrked I). ther onformtions n e otined y rotting one of the or ron toms through n ngle of 0 s shown hed. I Anti II Elipsed IV V VI ully elipsed Skew or Guhe Elipsed As is ler from the (sme ove s ewmn III) projetion (sme the s Guhe II) or Skew onformtions (III nd V) re lso stggered. owever, in these onformtions, the methyl groups re so lose tht they repel eh other. This repulsion uses guhe onformtions, to hve out.8 kj mol- more energy thn nti onformtion. This onformtions II nd VI re elipsed onformtions. These re unstle euse of repulsions. These re kj mol- less stle thn nti onformtion. onformtion IV is lso elipsed nd it is lest stle hving energy 9 kj mol- more thn nti onformtion. This is euse of repulsion etween methyl-methyl groups whih re very losed together. It is lled fully elipsed onformtion. The order of stility of these onformtions is, Anti > Skew or Guhe > Elipsed > ully elipsed. () onformtions in ylolknes III Skew or Guhe (i) Stility of ylolknes : ompounds with three nd four memered rings re not s stle s ompounds with five or six memered rings. The Germn hemist Beyer ws the first to suggest tht the instility of these smll rings ompounds ws due to ngle strin. This theory is known s Beyer-strin theory. Beyer strin theory ws sed upon the ssumption tht when n open hin orgni ompound hving the norml ond ngle 09. is onvert into yli ompound, definite distortion of this norml ngle tkes ple leding to the development of strin in. the moleule. devition. devition Beyer ssumed tht yli rings re plnr. Assuming tht the rings re plnr, the mount of strin in vrious ylolknes n e expressed in terms of ngle of devition (d). ( n ) d or d [09. ] n Where n = numer of ron-ron onds in ylolkne ring; = inner ond ngle in the ylolkne ring. Anglestrin d ; Stility innerngle( ) inner ngle d ow let us tke the se of three to eight memered yli ompounds. ylopropne = 0 d =. 09. ylohexne = yloutne = 90 d = 9. ylopentne = 08 d = 0. d =. d = 9. d =. The positive nd negtive vlues of (d) indite whether the inner ngle is less thn or more thn the norml tetrhedrl vlue. Beyer thus predited tht five memered ring ompound would e the most stle. e lso predited tht six memered ring ompounds would e less stle nd s the yli ompounds eome lrger thn five memered ring, then they would eome less nd less stle. ontrry to wht Beyer predited, however ylohexne is more stle thn ylopentne. urthermore, yli ompounds do not eome less nd less stle s the numer of sides inrese. Thus Beyer strin theory is pplile only to ylopropne, yloutne nd ylopntne. The mistke tht Beyer mde ws to ssume tht ll yli ompounds re plnr. But only ylopropne is plnr nd other ylolknes re not plnr. yli ompounds twist nd end in order to yloheptne = 8. 0 ylootne =

9 Generl rgni hemistry 0 hieve struture tht minimises the three different kinds of strin nd tht n destilise yli ompound. () Angle strin is the strin tht results when the ond ngle is different from desired tetrhedrl ond ngle of 09.. () Torsionl strin is used y repulsion of the onding eletrons of one sustituent with onding eletrons of nery sustituent. () Steri strin is used y toms or groups of toms pprohing eh other too losely. (ii) onformtion of ylohexne : Despite Beyer s predition tht five-memered yli ompounds would e the most stle, the six memered yli ompound is the most stle. Six memered yli ompound re most stle euse they n exist in onformtion tht is lmost ompletely free of strin. This onformtion is lled the hir onformtion. In hir onformtion of ylohexne ll ond ngles re 09.8 whih is very lose to the 09. nd ll the djent ronhydrogen onds re stggered. hir onformtion of ylohexne Eh ron in hir onformtion hs n xil ond nd n equtoril ond. Axil onds re perpendiulr to the plne of the ring nd equtoril onds re in the plne of the ring. If xil ond on ron- is ove the plne of the ring then xil ond on ron- will e elow the plne of the ring. Thus, nd xil onds re ove the plne ewmnn projetion of the hir onformtion As result of rottion out ron-ron single onds ylohexne rpidly interonverts etween two stle hir onformtions. This interonversion is known s ring flip. When the two hir forms interonvert, xil onds eome equtoril nd equtoril onds eome xil. lipping ylohexne n lso exist in ot onformtion. Like the hir onformtion, the ot onformtion is free of ngle strin. owever, the ot onformtion is less stle thn the hir onformtion y kl/mole. Bot onformtion is less stle euse some of the ronhydrogen onds in ot onformtion re elipsed. The ot onformtions is further destilised y the lose proximity of the flgpole hydrogens. These hydrogens re.8 Å prt ut the vnder Wl s rdii is. Å. The flgpole hydrogens re lso known s trns nuler hydrogens. The reltive stilities of the four onformtions of ylohexne derese in the order: hir > twist ot > ot > hlf hir., nd xil onds re elow the plne Thus xil nd xil re trns to eh other. Similrly nd xils re is to eh other. If xil ond on ron- will e ove the plne then equtoril ond on this ron will e elow the plne. () Aove (e) Below (e) Aove () Below () Thus equtoril nd equtoril onds re trns. () xil nd equtoril will e is.