Introduction & Definitions Catalytic Hydrogenations Dissolving Metal Reduction Reduction by Addition of H- and H+ Oxidation of Alcohols Oxidation of

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1 CEM 241- UNIT 4 xidation/reduction Reactions Redox chemistry 1

2 utline Introduction & Definitions Catalytic ydrogenations Dissolving Metal Reduction Reduction by Addition of - and + xidation of Alcohols xidation of Aldehydes & Ketones ydroxylation of Alkenes xidative Cleavage of 1,2-diols xidative Cleavage of Alkenes ozonolysis xidation with Permanganate 2

3 Introduction to Redox Chemistry Redox chemistry is concerned with net electron flow to and from a defined centre during a chemical reaction. A defined centre can be: 3

4 Introduction Cont d A defined centre is said to be oxidized if the electron density decreases, and reduced if electron density increases, during a reaction. The rule is: Loss of electrons equates with xidation Gain of electrons equates with Reduction 4

5 Introduction Cont d The oxidation of a defined centre can be changed in two ways. Firstly by Single Electron Transfer (SET) to the defined centre (reduction) or from the defined centre (oxidation). For example, the iron(iii) ion, Fe 3+, can be reduced to iron(ii), Fe 2+. The reaction can also occur in the oxidation direction. 5

6 Introduction Cont d 6

7 Introduction Cont d The reduction electron can either be provided by a chemical reducing agent (often a metal) or electrochemically. Electron flow by way of single electron transfer oxidation and reduction can be predicted using standard reduction potential data 7

8 Introduction Cont d The second method of changing the oxidation number is by reversal of bond polarization at the defined centre. ydrogen is electropositive and it renders the carbon of methane, C 4, electron rich and it is defined as having an oxidation number of -4. owever, the carbon of carbon dioxide has an oxidation number of +4 because oxygen is more electronegative than carbon. (Each bond contributes once.) 8

9 9

10 Introduction Cont d The combustion of methane to carbon dioxide is an oxidation of carbon because the oxidation number of carbon increases from -4 to +4 10

11 Introduction Cont d It follows that Redox Chemistry can proceed by three types of redox reaction: 11

12 Introduction Cont d 12

13 In general, an organic compound will be reduced if the no. of C- bonds increases. A compound will be oxidised if the no. of C- bonds decreases or if the no. of C- bonds increases. RC CR 2 Pt RC 2 C 2 R reduction RC 2 Cr 4 RC oxidation 13

14 Catalytic ydrogenations Alkenes and alkynes are both reduced to alkanes C 3 C 2 C C Pt, Pd, or Ni 1-butene C 3 C 2 C 2 C 3 butane Pt, Pd, or Ni C 3 C 2 C 2 C C C 3C 2C 2C 2C 3 1-pentyne pentane The catalytic hydrogenation of an alkyne can be stopped at a cis alkene if a partially deactivated catalyst is used. C 3 Lindlar's catalyst C 3 C CC C C 2-butyne cis-2-butene C 3 14

15 nly the alkene is reduced in this reaction. The very stable benzene ring can be reduced only under special conditions. C C 2 2, Pd/C C 2 C 3 Catalytic hydrogenations can also be used to reduce C=N bonds and carbon nitrogen triple bonds. These reactions produce amines. C 3 C 2 C NC Pd/C C 3 C 2 C 2 C N + 2 Pd/C C 3 C 2 C 2 NC 3 methylpropylamine C 3 C 2 C 2 C 2 N 2 butylamine 15

16 Reduction of ketones and aldehydes by catalytic hydrogenation C 3 C 2 CC 3 a ketone C 3 C 2 C 2 C 2 C 3 C 2 CC 3 Raney Ni a secondary alcohol 2 Pd/C C 3 C 2 C 2 C 2 a primary alcohol 16

17 An acyl chloride is reduced by catalytic hydrogenation to an aldehyde which is further reduced to a primary alcohol. C 3 C 2 CCl an acyl chloride 2 Pd/C C 3 C 2 C an aldehyde C 3 C 2 C 2 a primary alcohol The reaction can be stopped at the aldehyde using a partially deactivated palladium catalyst. C 3 C 2 CCl an acyl chloride 2 partially deactivated Pd C 3 C 2 C an aldehyde 17

18 The carbon-oxygen double bonds of carboxylic acids, esters, and amides are less reactive, so they are harder to reduce than the carbon-oxygen double bonds of aldehydes and ketones. They cannot be reduced by catalytic hydrogenation (except under extreme conditions). C 3 C 2 C 2 Raney Ni N REACTIN C 3 C 2 CC 3 C 3 C 2 CNC 3 2 Raney Ni 2 Raney Ni N REACTIN N REACTIN 18

19 Dissolving metal reduction C 3 C CC 3 2-butyne Na or Li N 3 (liq) 3 C C C C 3 trans-2-butene Na or Li donates an electron and N 3 donates a proton. Na/Li and N 3 cannot reduce a C=C, which makes it a useful reagent for reducing a triple bond in a compound that also has a double bond. C 3 C 3 C CC 2 C CC 3 Na or Li N 3 (liq) C 3 C 3 C C C 3 C CC 2 19

20 Reduction by addition of - and + The reducing agents are NaB 4 or LiAl 4. ydride ion adds to the carbonyl carbon and the alkoxide ion that is formed is protonated by water. - C - C 2 C + - Aldehydes and ketones are reduced by NaB 4 C 3 C 2 C 2 C an aldehyde C 3 C 2 C 2 CC 3 a ketone 1. NaB , 2 1. NaB , 2 C 3 C 2 C 2 C 2 a primary alcohol C 3 C 2 C 2 CC 3 a secondary alcohol 20

21 The metal-hydrogen bonds in LiAl 4 are more polar than those in NaB 4 and as a result, LiAl 4 is a stronger reducing agent. Both will reduce aldehydes and ketones, but only LiAl 4 will reduce carboxylic acids, esters and amides. C 3 C 2 C 2 C C 3 C 2 CC 3 1. LiAl , 2 1. LiAl , 2 C 3 C 2 C 2 C 2 a primary alcohol C 3 C 2 C 2 a primary alcohol

22 C 3 C 2 C 2 CN 2 C 3 C 2 C 2 CNC 3 1. LiAl LiAl C 3 C 2 C 2 C 2 N 2 a primary amine C 3 C 2 C 2 C 2 NC 3 a secondary amine C 3 C 3 1. LiAl 4 C 3 C 2 C 2 C NC 3 C 3 C 2 C 2 C 2 NC a tertiary amine In order to obtain a neutral (non-protonated) amine, acid is not used in the second step of the reaction. 22

23 Because NaB 4 cannot reduce an ester, an amide, or a carboxylic acid, it can be used to selectively reduce an aldehyde or a ketone group. C 3 CC 2 C 2 CC 3 1. NaB C 3 CC 2 C 2 CC 3 Sterically bulky donors can be used to deliver only one equivalent of hydride ion. E.g esters and and acyl chlorides are reduced to aldehydes. C 3 C 2 C 2 CC 3 an ester C 3 C 2 C 2 C 2 CCl an acyl chloride 1. [(C 3 ) 2 CC 2 ] 2 Al, -78 o C LiAl[C(C 3 ) 3 ] 3, -78 o C 2. 2 C 3 C 2 C 2 C an aldehyde C 3 C 2 C 2 C 2 C an aldehyde 23

24 Multiply bonded carbon atoms of alkenes and alkynes do not react with NaB 4 and LiAl 4. It can therefore be used to selectively reduce a carbonyl group in a compound that has multiply bonded carbon atoms. C 3 C 2 C C 2 NaB 4 No reaction C 3 C 2 C 3 2 C C 3 C CC 2 CC 3 NaB 4 1. NaB No reaction C 3 C CC 2 CC 3 24

25 xidation of Alcohols Most common reagent used is 2 Cr 4 (chromic acid) formed by dissolving Cr 3 or Na 2 Cr 2 7 in aqueous acid. C 3 C 2 CC 3 Cr 3 2 S C 3 C 2 CC 3 4 Na 2 Cr S 4 CC 2 C 3 2 Cr 4 CC 2 C 3 2 alcohols are oxidised to ketones 25

26 1 o alcohols are first oxidised to aldehydes and then further oxidised to carboxylic acids. C 3 C 2 C 2 C 2 A primary alcohol Cr 2 Cr 4 C 3 C 2 C 2 C an aldehyde further oxidation Mechanism of oxidation of alcohols + Cr - + RC 2 RC 2 Cr + C 3C 2 C 2 C - + a carboxylic acid 2 Cr RC RC Cr 2 A chromate ester 26

27 xidation using pyridinium chlorochromate (PCC) as the oxidising agent and an anhydrous solvent e.g. dichloromethane stops the reaction at the aldehyde. C 3 C 2 C 2 C 2 a primary alcohol PCC C 2 Cl 2 C 3 C 2 C 2 C an aldehyde Chromium is very toxic which led to the development of other methods e.g. the widely used Swern oxidation. C 3 C 2 C 2 a primary alcohol C 3 C 2 CC 3 a secondary alcohol 1. C 3 SC 3, Cl C C Cl, -60 C 2. triethylamine 1. C 3 SC 3, Cl C C Cl, -60 C 2. triethylamine C 3 C 2 C an aldehyde C 3 C 2 CC 3 a ketone 27

28 Mechanism for the Swern oxidation R R() C 3 C 3 C S R() Cl R C S C 3 C R() + R C S C 3 C 3 C 3 SC 3 + R C R() (C 3 C 2 ) 3 N The Swern oxidation uses an E2 reaction to form the aldehyde or ketone. 28

29 xidation of aldehydes and ketones C 3 C 2 C aldehydes CC 3 Na 2 Cr S 4 2 Cr 4 C 3 C 2 C C carboxylic acids Ag 2 in N 3 (aq) (tollens reagent) will oxidise and aldehyde but is too weak to oxidise an alcohol. 1. Ag2, N C 3 C 2 C , 2 C 3 C 2 C + Ag metallic silver 29

30 C 3 C 2 C 2 C Baeyer-Villiger oxidation + RC C 3 C 2 C 2 C an aldehyde a peroxyacid a carboxylic acid C 3 C 2 CC 2 C 3 a ketone + RC a peroxyacid C 3 C 2 CC 2 C 3 an ester + RC + RC If two alkyl substituents are attached to the carbonyl group, on which side will the oxygen be inserted? CC 3 RC CC 3 or CC 3 cyclohexyl methyl ketone methyl cyclohexanecarboxylate cyclohexyl acetate To answer this, we need to look at the mechanism. 30

31 Mechanism of the Baeyer-Villiger oxidation R C R' + C 3 C - R C R' CC R C R' CC 3 C 3 C + R CR' Several studies have established the order of group migration tendencies: > tert-alkyl > sec-alkyl = phenyl > primary alkyl > methyl 31

32 An alkene can be oxidised to an epoxide by a peroxyacid RC C 2 RC + RC C 2 + RC an alkene a peroxyacid an epoxide a carboxylic acid The epoxidation of an alkene is a concerted reaction C C C C + R C C R 32

33 Alkyl substituents increase the electron density of the double bond and therefore it will react in preference to another double bond with fewer alkyl substituents RC + + RC The addition of oxygen to an alkene is a stereospecific reaction RC C C 3 C C 3 C C 3 C C 3 cis-2-butene cis-2,3-dimethyloxime 3 C C C C 3 RC 3 C C C C 3 trans-2-butene trans-2,3-dimethyloxime 33

34 ydroxylation of alkenes An alkene can be oxidised to a 1,2-diol ether by KMn 4 in cold basic solution or by s 4. C 3 C CC 3 KMn 4, -, 2 cold C 3 C CC 3 a vicinal diol 1. s C 3 C 2 C C NaS 3, 2 C 3C 2 CC 2 a vicinal diol 34

35 Both KMn4 and s4 form a cyclic intermediate when they react with an alkene. A cycloalkene forms only a cis-diol. cyclopentene Mn - Mn - a cyclic manganate intermediate 2 + Mn 2 cis-1,2-cyclopentanediol cyclohexene s s a cyclic osmate intermediate 2 NaS 3 + s 3 cis-1,2-cyclohexanediol 35

36 xidative cleavage of 1,2-diols 1,2-diols are oxidised to ketones and/or aldehydes by periodic acid (I 4 ). C 3 C 3 C CC 3 C 3 I 4 C 3 C CC 3 I C C 3 3 C C + C a ketone an aldehyde + I 3 C 3 I 4 CC 2 C 2 C 2 C 2 CC 3 + I 3 36

37 xidative cleavage of alkenes: zonolysis Alkenes can be directly oxidised to aldehydes and ketones by 3. C C 1. 3, -78 C 2. work up C + C R C C R R R R R R R C C rearrangement C C R molozonide ozonide Zn, 2 or (C 3 ) 2 S 2 2 R R R R C ketone C ketone + + C C R aldehyde R carboxylic acid 37

38 Examples of oxidative cleavage of alkenes by ozonolysis. C 3 C 2 C CC 2 C 3 C C 3 C 2 C C 3 C 2 C CC 2 C (C 3 ) 2 S 2 C 3C 2 C + C 3 CC 2 C 3 C Zn, 2 C 3CC 2 C 2 C 2 C 2 C 1. C 3 C 2 C 2 C C 3 2 C 3C 2 C 2 C 2. Zn, 2 C 3 C 2 C 2 C C C 3 C 2 C 2 C + C + C 2 C CC 2 C C + C 3 C 2 C 38

39 xidation with permanganate C 3 KMn 4, - C 3 C 2 C CC 3 heat C 3 C 2 CC 3 + C 3 C - KMn C 4 3 C 2 C C 2 C 3 C 2 C + C 2 + C 2 KMn 4, - heat + C 2 39

40 xidative cleavage of alkynes Alkynes are oxidised to diketones by a basic solution of KMn4 and are cleaved by ozonolysis to carboxylic acids. C 3 C CC 2 C 3 KMn 4 2-pentyne - C 3 C CC 2 C 3 a diketone C 3 C CC 2 C 3 2-pentyne C 3 C + C 3 C 2 C C 3 C 2 C 2 C 1-pentyne C C 3 C 2 C 2 C + C 2 40