chapter 4 (1).pdfhhhhhhhhhhhhhhhhhhhhhh6yyyyyuj

Goo

1. Oxidation and Reduction
Reactions in Organic Chemistry
1
UNIT-4

2. ❑Oxidation Reactions
2
▪Introduction
➢The word oxidation and reduction are used somewhat differently in
organic chemistry from what you might have learned previously.
➢In general chemistry,
✓Oxidation is defined as the loss of one or more electrons by an atom.
➢However, in organic chemistry
✓Oxidation is a reaction that results in a loss of electron density by carbon,
caused by either
•Bond formation between carbon and a more electronegative atom (usually oxygen,
nitrogen or halogen), or
•Bond breaking between carbon and a less electronegative atom (usually hydrogen).
i.e. Oxidation : Decreases electron density on carbon by
•forming one of these: C-O, C-N, C-X
•breaking this: C-H
Note:
An oxidation often adds oxygen, while a reduction often adds hydrogen.

3. ❑Oxidation of Alcohols
3
➢The most valuable reaction of alcohols is their oxidation to
yield carbonyl compounds.
Primary alcohols yield aldehydesor carboxylic acids.
Secondary alcohols yield ketones.
Tertiary alcohols don’tnormally react with most oxidizing
agents.

4. Cont’’’
4
➢The oxidation of a 1º or 2º alcohol can be accomplished by
any of a large number of reagents.
➢Some of the reagents
✓KMnO4
✓CrO3
✓Na2Cr2O7
➢1º alcohols are oxidized to either aldehydesor carboxylic
acids, depending on
✓the reagentschosen
✓the conditions used

5. Cont’’'
5
➢One of the best methods for preparing aldehydefrom a 1º
alcohol is
✓Using pyridiniumchlorochromate(PCC) in dichloromethane
solvent.

6. Cont’’’
6
➢Most other oxidizing agents, such as chromium trioxide
(CrO3) in aqueous acid, oxidize 1º alcohols directly to
carboxylic acids.
➢In this reaction, an aldehydeis involved as an intermediate
but can’t usually be isolated because
✓it is further oxidized too rapidly.
➢2º alcohols are oxidized easily and in high yield to give
ketones

7. Cont’’’
7
➢All these oxidations occur by a pathway that is closely
related to the E2 reaction.
➢The first step involves reaction between the alcohol and
Cr (VI) reagent to form a chromate intermediate. Followed
by expulsion of chromium as the leaving group to yield the
carbonyl product.

8. ❑Oxidation of Aldehydes
8
➢Aldehydesare easily oxidized to yield carboxylic acids, but
ketonesare generally inert towards oxidation.
➢The difference is a consequence of structure:
✓Aldehydeshave a –CHO protonthat can be abstracted during
oxidation, but ketonesdo not.

9. Cont’’’
9
➢Manyoxidizingagents,includingKMnO4andhotHNO3,
convertaldehydesintocarboxylicacids,but
CrO3inaqueousacidisamorecommonchoice.
✓Theoxidationoccursrapidlyatroomtemperatureand
resultsingoodyields

10. Cont’’’
10
➢One drawback to this CrO
3oxidation is that
✓it takes place under acidic conditions, and sensitive molecules
sometimes undergo side reactions.
➢In such cases, the oxidation of aldehydecan be carried out using
a solution of silver oxide (Ag
2O) in aqueous ammonia, the so-
called Tollens’ reagent.

11. Cont’’’
11
➢Aldehydes are oxidized by Tollens’ reagent in high yield
without harming
✓Carbon-carbon double bonds, or
✓Acid-sensitive functional groups in a molecule.

12. Cont’’’
12
➢Aldehyde oxidations occur through intermediate 1,1-diols or
hydrates,
✓which are formed by a reversible nucleophilic addition of water
to the carbonyl group.
➢Even though formed to only a small extent at equilibrium, the
hydrate reacts like any typical 1º or 2º alcohol and is oxidized
to a carbonyl compound.

13. ❑Reduction Reaction
13
▪Introduction
➢In general chemistry, a reduction is defined as the gain of one or
more electrons by an atom.
➢In organic chemistry, however, a reduction
✓is a reaction that results in a gain of electron density by
carbon, caused either
•by bond formation between carbonand a less
electronegative atom, or
•by bond breaking between carbonand amore
electronegative atom.
i.e. Reduction: Increases electron density on carbon by
✓forming this: C-H, or
✓breaking one of these: C-O, C-N, C-X

14. ❑Catalytic Hydrogenation
14
▪Catalytic Hydrogenation of Alkenes
➢Alkenesreact with H2in the presence of a metal catalyst
to yield the corresponding saturated alkaneaddition
products.
➢Thus, the double bond has been hydrogenated, or reduced.

15. Cont’’’
15
➢The most common catalysts for alkenehydrogenations are
•Platinum
•Palladium
➢Palladium is normally used as a very fine powder supported on an
inert material such as charcoal (Pd/C) to maximize the area.
➢Platinum is normally used as PtO
2, a reagent called Adams’ catalyst.
➢Hydrogenation usually occurs with synstereochemistry, in which
both hydrogensare added to the double bond from the same face
(from the same catalyst surface).

16. Cont’’’
16
➢Alkenes are much more reactive than other unsaturated functional groups
towards catalytic hydrogenation, and the reaction is therefore quite selective.
➢Other functional groups such as aldehydes, ketones, esters, and nitrilessurvive
normal alkenehydrogenation conditions unchanged.
For example,

17. ▪Catalytic Hydrogenation of Aromatic Rings
17
➢Aromaticringsareinerttocatalytichydrogenationunder
conditionsthatreducetypicalalkenedoublebonds.However,itis
possibletoreduceanalkenedoublebondselectivelyinthe
presenceofanaromaticring.
For example,
4-Phenyl-3-buten-2-one is reduced to 4-phenyl-2-butanone at room
temperature and atmospheric pressure using a palladium catalyst.
i.e. neither the benzene ring nor the ketonecarbonyl group is affected.

18. ❑Hydride Reduction
18
➢Additionofhydrideion(:H¯)toanaldehydeorketoneyieldsan
alcohol.
➢Althoughthedetailsofcarbonyl-groupreductionarecomplex,
LiAlH4andNaBH4actsasiftheyweredonorsofhydrideionina
nucleophilicadditionreaction.
➢Additionofwateroraqueousacidafterthehydrideadditionstep
protonatesthetetrahedralalkoxideintermediateandgivesthe
alcoholproduct.

19. ❑Dissolved Metal Reduction
19
➢TreatmentofanaldehydeorketonewithaGrignardreagent
(RMgX)yieldsanalcoholbynucleophilicadditionofacarbon
anion(carbanion).
➢Acarbon-magnesiumbondisstronglypolarizedsoaGrignard
reagentreactsasR:¯
+
MgX.
➢TheGrignardreactionbeginswithanacid-basecomplexationof
Mg²+tothecarbonyloxygenatomofthealdehydeorketone,
therebymakingthecarbonylgroupabetterelectrophile.
➢NucleophilicadditionofR:¯thenproducesatetrahedral
magnisiumalkoxideintermediate,andprotonationbyadditionof
waterordiluteaqueousacidinaseparatestepyieldstheneutral
alcohol.

20. 20