Dr. Anand Gupta Mr Mahesh Kapil

Transcription

1 Dr. Anand Gupta Mr Mahesh Kapil

2 Preparation of Diazonium salts

3 N N+l - - NaI N N I iodobenzene N N +l - Br, ubr Br - N N bromobenzene

4 ALGENATIN l 2, All 3 l Professor harles Friedel and Professor James rafts Br 2, FeBr 3 Br The alogen is polarised Br Br Br FeBr3 + FeBr 4

5 Aromatic ompounds are resonance stabilized This gives them added stability They undergo Electrophilic Substitution eactions Upon substitution, the fast step is the loss of a proton to regenerate aromaticity Br Br Br FeBr 4 Br + Br FeBr 3 egenerate the catalyst so only a small amount is required

6 eactions eactions of arylhalides Alkyl halides react by S N 1 and S N 2 mechanisms Simple aryl halides (and vinyl halides) are unreactive towards nucleophilic substitution e.g. l hot aq. Na N SUBSTITUTIN 2 l hot aq. Na N SUBSTITUTIN But 3 2 l hot aq. Na 3 2

7 2 l hot aq. Na N SUBSTITUTIN alogen bonds of aryl (and vinyl) halides are shorter and stronger than those of alkyl, allylic and benzylic halides X X (and other resonance forms) X X Note the double-bond character of the X bond of aryl and vinyl halides.

8 Aryl halides and vinylic halides are relatively unreactive toward nucleophilic substitution under conditions that give facile nucleophilic substitution with alkyl halides. eason: (1) Phenyl cations are very unstable. (2) alogen bonds of aryl (and vinylic) halides are shorter and stronger than those of alkyl, allylic, and benzylic halides because of the hybridized state and the resonance. But aryl halides can be remarkably reactive toward nucleophiles if they bear certain substituents or when we allow them to react under the proper conditions.

9 So NMALLY, aryl halides will not undergo nucleophilic aromatic substitution. Some factors/conditions which allow aryl halides to undergo nucleophilic substitution A. When strong electron-withdrawing groups, (e.g. N 2 ) are ortho- or para- to the halogen atom. B. When the aryl halides are allowed to react under forcing conditions e.g. aqueous Na, high P and T ~

10 Nucleophilic Aromatic Substitution l l N 2 N o Na/pressure vs 1 EW gp Na 3 aq 130 o l 2 EW gps vs N 2 Na 3 aq 100 o N 2 2 N vs l 3 EW gps N 2 vs Na 3 aq warm 2 N N 2 N 2 N 2 N 2 N 2 NB as EW gps are increased, milder rxn conditions suffice. EW gps therefore activate the ring towards NAS

11 Nucleophilic aromatic substitution There are two possible mechanism for NAS: A) Bimolecular Displacement mechanism (BDM) or the addition-elimination or S N Ar mechanism or the occurs when powerful EW substituents are on the ring. BDM results in ipso substitution, i.e., substitution at an atom that already had a substituent. B) Elimination-addition mechanism A) For the BDM l l δ+ + Step 1 addition rds Step 2 elimination fast + l N 2 N 2 Meissenheimer complex (delocalized carbanion) N 2

12 Nucleophilic Aromatic Substitution The Meissenheimer complex is stablized by EW gps o or p to the halogen. l l l l N N N N Especially stable. -ve charges are on the -atoms In the last step the -l bond is cleaved because it is more polar, making l a better leaving gp.

13 Benzyne Mechanism Br N 2 NaN 2 liq. N 3, Benzyne - unstable. 3rd bond is due to sp2 sp2 overlap of orbitals which are perpendicular to the π- orbitals of the aromatic ring. Br N 2 i) Br - Br ii) N 2 iii) N2 BENZYNE N2 N 3 + N2 iv)

14 Mechanism: F 3 F 3 l NaN 2 N 3 (-l) 1 2 N 3 X F 3 F N N 2 More stable cabanion Less stable carbanion N 3 F 3 + N 2 N 2 arbanion 3 is more stable than 4 because the carbon atom bearing the negative charge is closer to the highly electronegative trifluoromethyl group.

15 Electrophilic Subtitution eactions alogenation Nitration Sulphonation Friedel raft Acylation Friedel raft Alkylation

16 eaction With Metals Wurtz eaction Wurtz Fittig eaction Fittig eaction Frankland eaction Ullman eaction eaction with Magnesium

17 Questions Q 1: ow will you distinguish between hlorobenzene & benzyl chloride and hloroethane & bromoethane Q 2 Why -I becomes violet or brown on long standing in presence of light. Q 3: onvert i) Bromomethane into propanone. ii) 2-bromobutane to 1-bromobutane iii) Benzene into biphenyl Q4: Which reagent can be converted in ethane and methane in single step reactions. Give reactions.

18 USES F ALGENALKANES Synthetic The reactivity of the -X bond means that halogenoalkanes play an important part in synthetic organic chemistry. The halogen can be replaced by a variety of groups via nucleophilic substitution. Polymers Many useful polymers are formed from halogeno hydrocarbons Monomer Polymer epeating unit chloroethene poly(chloroethene) PV - ( 2 - l) n - tetrafluoroethene poly(tetrafluoroethene) PTFE - (F 2 - F 2 ) n - hlorofluorocarbons - F s dichlorofluoromethane Fl 2 refrigerant, aerosol propellant, blowing agent trichlorofluoromethane F 3 l refrigerant, aerosol propellant, blowing agent bromochlorodifluoromethane BrlF 2 fire extinguishers l 2 FlF 2 dry cleaning solvent, degreasing agent

19 PBLEMS WIT F s s AND TE ZNE LAYE F s have been blamed for damage to the environment by thinning the ozone layer zone absorbs a lot of harmful UV radiation owever it breaks down more easily in the presence of F's F s break up in the atmosphere to form radicals F 2 l 2 > F 2 l + l Free radicals catalyse the breaking up of ozone 2 3 > 3 2 F s were designed by chemists to help people hemists are now having to synthesise alternatives to F s to protect the environment This will allow the reversal of the ozone layer problem

20 PBLEMS WIT F s s AND TE ZNE LAYE There is a series of complex reactions but the basic process ozone in the atmosphere breaks down naturally 3 > + 2 F's break down in UV light to form radicals l 2 F 2 > l + lf 2 is :- chlorine radicals then react with ozone 3 + l > l + 2 chlorine radicals are regenerated l + > 2 + l verall, chlorine radicals are not used up so a small amount of F's can destroy thousands of ozone molecules before they take part in a termination stage.

21 Dr. Anand Gupta Mr Mahesh Kapil

22 Alcohols Nomenclature Isomerism Preparation Properties

23 Preparation eactions eduction of carbonyl compounds eduction of arboxylic acids/esters ydration of Alkenes Grignard reactions

24 eduction of Aldehydes/Ketones ydrogenation 2 Pt 2 Primary ' 2 Pt ' Secondary

25 eduction of Aldehydes/Ketones ydride eductions LiAl 4 or NaB 4 2 Primary ' LiAl 4 or NaB 4 ' Secondary

26 eduction of arboxylic Acids and Esters Lithium Aluminum ydride eduction LiAl ' LiAl '

27 ydration of Alkenes Acid catalyzed ydration xymercuration-demercuration ydroboration-xidation

28 Acid-atalyzed ydration of Alkenes Markovnikov addition Formation of most stable carbocation Shifts/rearrangements possible + 2 ' '' + 2 ' ''

29 ydration of Alkenes via xymercuration/demercuration Markovnikov addition Typically no shifts/rearrangements Mercurinium ion involvement g(ac) 2 NaB 4 2 ' ' g(ac) 2 NaB 4 '' 2 ''

30 ydroboration-xidation of Alkenes Anti-Markovnikov addition No shifts/rearrangements Syn addition (B 3 ) ' ' (B 3 ) 2 - '' 2 2 ''

31 Grignard Addition eactions Addition to Aldehydes/Ketones Addition to Esters Addition to Epoxides

32 Grignard Additions to Aldehydes/Ketones ' Formation of primary, secondary, and tertiary alcohols MgX MgX " ' MgX " ' 2 Primary ' Secondary Tertiary

33 Grignard Additions to Esters Formation of secondary and tertiary alcohols + 2'MgX ' 2 + Secondary ' " + 2'MgX " + ' Teriary

34 Grignard Addition to Epoxides + MgX 2 2 Primary + MgX ' ' ' ' Secondary ' ' + MgX ' ' ' ' ' ' Tertiary

35 Typical Alcohol eactions Salt formation Dehydration xidation Alkyl halide formation Ester formation Ether synthesis Periodic acid cleavage of glycols aloform reaction of methyl carbinols TP acetal formation

36 onversion of Alcohols to Salts eaction with Active Metals Na - Na + + 2

37 Dehydration of Alcohols E-1 E rds Pl 3 Pl 2 a dichlorophosphate intermediate Pl 2 N 1,2-shifts/rearrangements possible Anti periplanar (coplanar) elimination No 1,2-shifts/rearrangements possible

38 xidation of Alcohols Primary 2 P KMn 4 or K 2 r 2 7 ² Secondary 2 P or KMn 4 or K 2 r 2 7 ² Tertiary 3 P or KMn 4 ² no reaction

39 Alcohol onversion to Alkyl alides eaction with ydrogen halides eaction with Thionyl chloride eaction with Phosphorus trihalides or pentahalides

40 ydrogen alide onversion of Alcohols to Alkyl alides 2 X 2 X S N 2 predominantly 2 X 2 X S N 1 or S N 2 3 X 3 X S N 1 predominantly where X = I, Br, or l

41 onversion of Alcohols to Alkyl hlorides via Thionyl hloride Sl2 S primary or secondary alcohol l alkyl chlorosulfite l - S N 2 l +S 2 +l

42 onversion of Alcohols to Alkyl alides via Phosphorus alides PX 3 + X - PX 2 S N 2 X + PX 2 primary or secondary alcohol protonated alkyl dihalophosphite

43 Ester Formation from Alcohols l ' ' + l ' ' + ' + ' + 2

44 Periodic Acid leavage of Glycols ' 2 + I I I I 3 4 ' 2 ' + I I 3 4

45 aloform eaction Methyl carbinol cleavage to give arboxylic acids and aloform 3 X 2 + X X X 3 +

46 Disguising an Alcohol reating a tetrahydropyranyl acetal verall Transformation + + Dihydropyran (DP) (an acetal) Mechanism (a heteroatom-stabilized carbocation)

47 Ethers Nomenclature Properties Preparation eactions

48 Preparation of Ethers Dehydration of Alcohols Williamson synthesis Alkoxymercuration- Demercuration Peroxyacid Epoxidation of Alkenes

49 Ether Formation via Acid atalyzed Dehydration of Alcohols 2 + S 1 N S N

50 Williamson Synthesis of Ethers Bimolecular Substitution by Alkoxide on a suitable substrate - Na + + ' S N 2 X ' + NaX Primary Alkyl alide

51 Alkoxymercuration-Demercuration of Alkenes Markovnikov Addition Typically no rearrangements/shifts Mercurinium ion involvement MM ' NaB 4 ' ' ' MM NaB 4 '' ' ' '' Where MM = modified mercury reagent = Mercury trifluoroacetate

52 Epoxidation of Alkenes Prilezhaev reaction

53 Ether eactions X leavage Epoxide ing pening

54 X leavage of Ethers Unimolecular or Bimolecular leavage Pathways X + X - Protonation S N X - X S N 2 + X - X +

55 Epoxide ing pening Unimolecular or Bimolecular S N S N