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Home , Aldehyde

Aldehydes and are simple organic compounds containing a . Carbonyl group contains - double bond. These organic compounds are simple because the carbon atom presents in the carbonyl group lack reactive groups such as OH or Cl.

By Dr. Sayed Hasan Mehdi Assistant Professor Department of Chemistry Shia P.G. College, Lucknow

Dr. S.Hasan Mehdi 6/13/2020 This is to bring to kind notice that the matter of this e- content is for the B.Sc. IV semester students. It has been taken from the following sources. The students are advised to follow these books as well.

•A TEXTBOOK OF ORGANIC CHEMISTRY by Arun Bahl & B.S. Bahl, S. Chand & Company Ltd. Publication. •Graduate Organic Chemistry by M. K. Jain and S.C. Sharma, Vishal Publishing Co. •Pradeep’s Organic Chemistry Vol II by R. N. Dhawan, Pradeep Publication, Jalandhar.

Dr. S.Hasan Mehdi 6/13/2020  An aldehyde is one of the classes of carbonyl group- containing group on one end and on the other end. The R and Ar denote alkyl or aryl member respectively. In the condensed form, the aldehyde is written as –CHO.

Dr. S.Hasan Mehdi 6/13/2020 Dr. S.Hasan Mehdi 6/13/2020 1. From : a. By oxidation of Alcohols: Aldehydes and ketones can be prepared by the controlled oxidation of primary and secondary alcohols using an acidified solution of or . Primary produces aldehydesRef. Last slide. O K Cr O RCH2OH + [O] 2 2 7 + R C H H 10 Alcohol Aldehyde O

CH3CH2OH+ [O] K2Cr2O7 + CH3 C H H Ethyl Alcohol

The aldehydes formed in the above reaction are very easily oxidised to carboxylc if allowed to remain in the reaction mixture.

O O

+ [O] K2Cr2O7 C R C H + R OH H Carboxylic Dr. S.Hasan Mehdi 6/13/2020 KetonesKetones areare prepared by by the the oxidation oxidation of ofsecondary secondary alcohols. alcohols.

OHOH OO

R CHCH K2Cr2O7 R ++ [O][O] K2Cr2O7 ++ RR CC R'R' RR'' HH KetoneKetone

OHOH OO CHCH K2Cr2O7 CHCH33 ++ [O][O] K2Cr2O7 + CHCH CC CHCH CHCH H+ 3 3 22 33 CHCH22CHCH33 H Ethyl MethylKetone 2-propanol Ethyl MethylKetone 2-propanol

Dr. S.Hasan Mehdi 6/13/2020 Ketones are not easily oxidised further and can be obtained in high yield by this methods. Ketones can also be obtained from secondary alcohols by . In this process, the appropriate secondary alcohol is refluxed with excess of acetone in the presence of aluminium tertiary-

butoxide, AlOC(CH3)3. Note: The mechanism of following reaction has already been discussed in Alcohols. 

R R CH CH3 3 CHOH + C O AlOC(CH3)3 C O + CHOH Heat R' R' CH3 CH3 2-Propanol 20 Alcohol Acetone

Dr. S.Hasan Mehdi 6/13/2020 Other reagents used for the oxidation of alcohols to aldehydes and ketones are: - A. Alkaline : KMnO4/OH

B. : H2Cr2O7 (CrO3 or Na2Cr2O7 With dil. H2SO4.

C. chlorochromate (PCC) C5H6NCrO3Cl, in dichloromethane

Pyridine chlorochromate (PCC) C5H6NCrO3Cl, in dichloromethane is a special . It is used to convert primary alcohols to aldehydes. This reagent does not oxidize aldehydes to carboxylic acids.

O PCC CH3CH2CH2CH2OH CH3CH2CH2 C H CH2Cl2 10 Alcohol Aldehyde

Dr. S.Hasan Mehdi 6/13/2020 b. Catalytic of Alcohols: Aldehydes May be Prepared by passing the vapours of primary alcohols over a copper catalyst heated of 0 about 300 C. O Cu RCH2OH R C H 3000C Aldehyde 10 Alcohol

O Cu CH3CH2OH CH3 C H 3000C Ethyl Alcohol Acetaldehyde Similarly Ketones are produced from secondary alcohols.

OH O Cu R CH R' R C R' + H 3000C 2 Ketone 20 Alcohol

OH O Cu CH3 CH CH3 CH3 C CH3 3000C + H2 2 Propanol Acetone Dr. S.Hasan Mehdi 6/13/2020 2. From Acyl halides: A. B. By Grignard reagents C. By organocadmium compounds D. By Lithium oragano cuprates

Dr. S.Hasan Mehdi 6/13/2020 2. From Acyl halides: Rosenmund Reduction: Acid Chlorides can be reduced to Aldehydes when they are hydrogenated in the presence of a catalyst such as palladium deposited on BaSO4. A catalyst poison like or Pyridine and sulphur is frequently added to moderate the activity of the catalyst and thus inhibit the further reduction of aldehyde into alcohol. This conversion of COCl into –CHO is known as Rosenmund O reduction. Pd, BaSO4 CH CH CH C Cl + 3 2 2 H2 CH3CH2CH2CH O + HCl Quinoline/Sulphur Butanal Butanoyl chloride

O O d, BaSO4 + P CH CH + HCl CH3 C Cl H2 3 Xylene

4-methyl benzoyl chloride 4-methyl

Dr. S.Hasan Mehdi 6/13/2020  Mechanism of Rosenmund reduction:

O OH O H Pd, BaSO4 -Cl- R C Cl + H2 R C Cl R C H H

O + H + R C H

Dr. S.Hasan Mehdi 6/13/2020 b. By Grignard reagents: reacts with acyl halides under controlled conditions to form ketones. The resulting ketone can otherwise react further with excess grignard reagent to form Mechanism: tertiary alcohol.

O CH OM gBr + CH 3 R C 3 M gBr C CH3 Cl Cl Heat

CH3 C O Br + Mg CH3 Cl Acetone

Dr. S.Hasan Mehdi 6/13/2020 c. By organocadmium compounds. Ketones can be prepared from acid chlorides by the reaction of suitable organocadmium compounds.

R2Cd + 2R'COCl 2R'COR+ CdCl2

(R is preferablly a primary alkyl group bur R' cab be alkyl or Aryl group)

O O + CdCl2 2NO C (C H )Cd 2NO C 2 + 2 5 2 Cl CH2CH3

p-nitrobenzoyl chloride Diethyl cadmium Ethyl-p-nitrophenylketone

Dr. S.Hasan Mehdi 6/13/2020 D. By Lithium organocuprates (for ketones only). Lithium dialkylcuprates react readily with acid chlorides to form ketones in good yields.

O O

C Cl C R' + R + R'2CuLi R R'Cu + LiCl (R and R' can be alkyl or aryl groups)

Lithium dialkylcuprates needed for this purpose are themselves prepared as below:

2R'X 4Li CuX 2R'Li R'2CuLi + LiX -2LiX The advantage of this method is that Lithium dialkylcuprates do not ract with many of the functional

group like NO2, CN, COOR etc with which grignard reagents and organolithiums do react.

O O Dry 2NO2 C (CH3) CuLi 2NO2 C + CH3Cu + LiCl + 2 196K Cl CH3

p-nitrobenzoyl chloride Lithium dimethyl cuprate p-nitroacetophenone

Dr. S.Hasan Mehdi 6/13/2020  3. From 1,3 Dithiane:  1, 3-Dithiane is a weak protonic acid and can be converted into its anion by treatment with strong like n-butyl lithium. The 1,3-Dithane anion being a strong reactions with alkyl halide by SN2 mechanism to form alkyl derivatives of 1,3 dithiane which upon subsequent with HgCl2 in or aq. CH3CN gives aldehydes.

+ + CH3CH2CH2CH3 S S CH3CH2CH2CH2 S S _ Li+ n- H H Li H 1,3-Dithiane RX -LiX

O HgCl2, CH3OH.H2O C + S S HS SH Hydrolysis H R H R

Aldehyde Thioacetal

Dr. S.Hasan Mehdi 6/13/2020  Similary ketones can be eaisly prepared from 1,3-dithiane by first converting it into its disubstitued derivatives, thioketal in two stages followed by hyrdolysis under the similar above conditions.

+ + CH3CH2CH2CH3 S S CH3CH2CH2CH2 S S _ Li+ n-Butane H H Li H 1,3-Dithiane R-X -LiX

O n-butyl lithium HgCl , CH OH.H O 2 3 2 S S -C4H10 S S + C Hydrolysis R'X HS SH R' R R' R H R Thiokettal Thioacetal Ketone

Dr. S.Hasan Mehdi 6/13/2020  4. From Carboxylic acids:  Reaction of lithium with carboxylic acids provides a useful route for the synthesis of ketones. For e.g.

O O R C OH + + 196 K R' Li R C O Li+ + R'H Dry ether

R' = 10, 20, 30 alkyl or aryl group

OH O OLi H+/H O - 196 K 2 R C O Li+ + R' Li R C O Li+ R C OH Dry ether Dry ether R' R'

The overall reaction may be represented as Unstable

O OLi O + 2R'Li H /H2O R C OH R C Li R C R' -R'H Dry ether O R' Ketone R C R'

Ketone

Dr. S.Hasan Mehdi 6/13/2020 5. From : By Reaction with Grignard Reagents or organolithium compounds: Grignard reagents or organolithium compounds add to alkyl or aryl nitriles to form addition products which upon subseqent hydrolysis gives ketones. O Dry ether + R C N + R'MgX R C NMgX+ H /H2O R C R' -NH -Mg(OX)X R' 3, Alkyl or aryl Ketone Addition product

O R C N Dry ether + + + R'Li R C NLi H /H2O R C R' Alkyl or aryl nitrile -NH -LiOH Alkyl or R' 3, aryl Lithium Addition product

CH C N + Dry ether H+/H O 3 CH3CH2MgBr CH3 C NMgBr+ 2 -NH3, -Mg(OX)X Acetonitrile ethyl Mag. Bromide CH2CH3

Addition product O

CH3 C CH2CH3 Butanone Dr. S.Hasan Mehdi 6/13/2020 6. Oxidation of (): Aldehydes and ketones can be obtained by ozonolysis of alkenes. This involves the treatment of the alkenes with to give . The ozonides are not isolated because they are explosive often in dry state. They are decomposed with Zn + H2O to form aldehydes and ketones. H H H H 1. O3 CH3CH2C CCH3 + CH3CH2C O O CCH3 2. Zn + H2O 2-Pentene Propanal Acetaldehyde This is not a good method because a mixture of carbonyl compounds is often produced. However, if the starting is symmetrical, only one carbonyl compound will be obtained. For e.g.

H H H H 1. O3 CH3 C CCH3 CH3 C O + O CCH3 2. Zn + H2O 2-Butene Acetaldehyde Acetaldehyde

Dr. S.Hasan Mehdi 6/13/2020 7. Hydration of : Hydration of Acetylene yields acetaldehyde. Wate r adds to acetylene in the presence of mercuric sulpahte and sulphuric acid to form the unstable -intermediate which tautomerise to give acetaldehyde. H H H H CH CH + H OH HgSO4 H C C O H2SO4 H C C OH Acetylene H Unstable Hydration of alkynes other than acetylenes gives ketones.Acetaldehyde Water adds according to Markonikov rule to give an unstable enol-intermediate which rearranges to form ketones.

O OH HgSO CH CH3C CH + H OH 4 3 C CH3 H2SO4 CH3 C CH2 Acetone Propylene Unstable

Dr. S.Hasan Mehdi 6/13/2020 8. Hydrolysis of gem-Dihalides: Aldehydes are prepared by alkaline hydrolysis of those gem-dihalides in which two halogen atoms are attached to the terminal carbon atom. For e.g.

Cl O H2O OH -H2O CH3 CH Cl CH3 C H NaOH CH3 CH OH

1,1 Dichloroethane Unstable Acetaldehyde Ketones are produced by alkaline hydrolysis of those gem-dihalides in which the two halogen atoms are attached to a non-terminal carbon atom. For e.g.

Cl O OH H2O -H2O CH3 C CH3 CH3 C CH3 NaOH CH3 C CH3 Cl OH 2,2 Dichloroethane Unstable Acetone

Dr. S.Hasan Mehdi 6/13/2020 9. Pyrolysis of Calcium Salts of Acids: Symmetrical ketones may be prepared by heating calcium Salts of acids at 4000C.

O

CH3 C O O Heat CH3 C CH3 Ca2+ O

CH3 C O

Calcium acetate Acetone

Dr. S.Hasan Mehdi 6/13/2020 Nucleophilic Addition Reactions: The carbonyl group of aldehydes and ketones is highly polar group.

C O C O C O

The positive charged carbon is readily attacked by electron-rich . The negatively charged oxygen is attacked by electron-deficient electrophiles.

Dr. S.Hasan Mehdi 6/13/2020 Mechanism of Nucleophilic Addition Reactions:

O OH O + C H+ C C Nu Nu Nu Addition Produc The nucleophilic addition reactions of carbonyl compounds may be catalyzed by acids or bases.

Dr. S.Hasan Mehdi 6/13/2020 Base catalysed addition: Base convert a weak neutral electrophile to a strong one by removing a proton. The strong nuclophile then adds to the carbonyl group as shown below.

Nu H B Nu + BH neutral nucleophile Nucleophile (weak) (strong)

Dr. S.Hasan Mehdi 6/13/2020 Acid catalysed addition: Step 1: The hydrogen ion from the acid attacks the negatively charged carbonyl oxygen to give protonated carbonyl group.

O H O H O + C C C H+

Protonated carbonyl group

Step 2: The nucleophile attacks the protonated carbonyl group to form the addition product.

O H O H C + Nu C Nu

Addition produc

Dr. S.Hasan Mehdi 6/13/2020 Acidity of α-: The hydrogen on carbon alpha to carbonyl group is acidic because the carbanion resulting after removal is stablized as shown below:

O O O O C -H C C C C C C C

H carbanion:resonance stabilized Resonance hybrid

These carbanion acts as a nucleophile and undergoes nucleophilic addition with its own counterpart or with other aldehyde or ketone to form condensation products.

Dr. S.Hasan Mehdi 02/04/2020 Aldehydes containing α-hydrogens undergo self-addition in the presence of a base to form products called Aldols. The reaction is called . For e.g.

O O OH O

CH3 C H + CH2 C H OH CH3 C CH2 C H H H Acetaldehyde Acetaldehyde 3-hydroxy butanol (aldol)

Aldols are easily dehydrated either by heating or by treatment with dilute acid to form unsaturated α, β-aldehydes. For e.g.

H OH O O OH CH3 C CH C H CH 3 C CH C H + H2O H H 3-hydroxy butanal 2-butenal or crotonaldehye (aldol)

Dr. S.Hasan Mehdi 02/04/2020 H O O OH O CH OH + C H + 2 C H C CH2 C H H H or heat

Benzaldehyde Acetaldehyde 3-hydroxy butanol (aldol)

O CH CH C H

Cinnamaldehyde

Dr. S.Hasan Mehdi 02/04/2020 O O CH C H OH H O Step 1 2 CH2 C H + 2 H ion Acetaldehyde (Nucleophile)

Step 2 O O O O

CH3 C H CH2 C H CH3 C CH2 C H

Enolate ion H Acetaldehyde (Nucleophile) Step 3

HO H

O O OH O CH3 C CH2 C H CH3 C CH C H H H H 3-hydroxy butanol (aldol)

Dr. S.Hasan Mehdi 6/13/2020 O H KCN + C C H H C H2O + C2H5OH C O OH O

Benzaldehyde Benzoin Benzaldehyde

Dr. S.Hasan Mehdi 6/13/2020 Mechanism of Benzoin Condenstion

O O OH C H CN C H C CN CN

Benzaldehyde A

OH O OH O C H C C C CN CN H

Proton Shift

H - O OH -CN C C C C OH O CN H

Benzoin

Dr. S.Hasan Mehdi 6/13/2020 O O CH3COONa + (CH3CO)2O 0 CH CH C OH C H 180 C -CH3COOH Cinnamic acid

Dr. S.Hasan Mehdi 6/13/2020 H O O O C CH C 1 CH2 2 C OH + CH3COO O CH3 O CH C CH3 C O 3 O A O O O O CH C CH CH C C H 2 2 2 O O C C CH3 O + CH3 O 3 H A B O OH H O CH CH C -H2O CH CH O C 4 O CH3 C O CH3 C O 5 H2O D C O O

CH CH C OH + CH3 C OH

Cinnamic acid Dr. S.Hasan Mehdi 6/13/2020 O COOC2H5 COOC2H5 CH2 Piperidine, heat CH3 C H + CH3 CH C (-H O) COOC2H5 2 COOC2H5

Condensation Product Acetaldehyde Malonic OH) O 5 2 H H 2 / + H Hydrolysis (-C

COOH -CO2 CH3 CH CH COOH CH3 CH C COOH Crotonic acid

Dr. S.Hasan Mehdi 6/13/2020 O O O C OC H C OC H C OC H B H 2 5 H 2 5 H 2 5

C C C + H + BH C OC2H5 C OC2H5 C OC2H5 O O O

A Resonace stabilised carbanion O O O C OC2H5 O C OC2H5 CH3 C H C CH3 C CH H H C OC2H5 C OC2H5 O O A B + BH -B O

C OC2H5 O OH C OC H CH3 C C 2 5 -H2O CH C H C 3 C H B OC2H5 H O C OC2H5 D O OH) 5 O 2 H 2

H C / + H Hydrolysis (-2C

COOH -CO 2 CH CH CH COOH CH3 CH C 3 COOH Crotonic acidDr. S.Hasan Mehdi 6/13/2020 E O OH RCH NH + H O R C H + NH Nucleophilic R C NH 2 3 addition 2 Aldimine

O N + 4NH 6H C H 3 (CH2)6N4 + 6H2O CH2 CH2 CH2 Utropine N

CH2 CH2 N N

CH2

CH3 C O + HCH2COCH3 Acetone CH3 CH3 Acetone C CH2COCH3 + HNH 2 CH3 NH2 Diacetone (4-Amino-4-methyl-2-pentanone)

Dr. S.Hasan Mehdi 6/13/2020 + C O + NH2 Z C N Z H2O

Step 1 H H Proton transfer C O H N C O N C OH Z Z Unstable NH2 Z

Step 2 H N C OH C N Z + H2O Z Unstable Dr. S.Hasan Mehdi 6/13/2020 Product Reactant Z Reagent

Aldehyde OH Hydroxyl amine Aldehyde

Acetone OH Hydroxyl amine Ketone oxime

Aldehyde NH2 Aldehyde Hydrazine Ketone hydrazone Ketone NH2

Aldehyde C6H5 NH Aldehyde phenylhydrazone

Ketone C6H5 NH Phenylhydrazine Ketone phenylhydrazone

NO 2 Aldehyde 2,4 dinitroPhenylhydrazine Aldehyde 2,4 dinitrophenylhydrazo NHNH2 NO 2

NO 2 Ketone 2,4 dinitroPhenylhydrazine Ketone 2,4 dinitrophenylhydrazon NHNH2 NO 2

Dr. S.Hasan Mehdi 6/13/2020 C O R C P(C6H5)3 2 C CR2 + O P(C6H5)3

Carbonyl compound Alkene from the ylide

Phosphorous are prepared from primary alkyl halides and triphenylphosphone.

H (C H ) P R CH Br C2H5ONa 6 5 3 2 (C6H5)3P CR2 Triphenylphosphine primary alkyl halide

(C6H5)3P CR2 + C2H5OH An ylide

Dr. S.Hasan Mehdi 6/13/2020 Step 1

C O C O R2 C P(C6H5)3

R2C P(C6H5)3 Carbonyl compound Ylide Betaine Step 2

C O C O C CR2 + O P(C6H5)3 R2C P(C6H5)3 R2C P(C6H5)3 Triphenylphosphine oxid Betaine Alkene

Dr. S.Hasan Mehdi 6/13/2020 O O C H C CH HCl - 6 5 3 + HCHO + (CH3)2NH C6H5 C CH2 CH2 NH(CH3)2Cl Acetophenone Dimethyl amine (20 Amine) O NaOH C6H5 C CH2 CH2 N(CH3)2 -NaCl, -H O 2 ß-dimethylaminopropio- phenone

Dr. S.Hasan Mehdi 6/13/2020 Step 1

H + H OH H (CH3)2NH -H O C O C OH C 2 N(CH ) H H 3 2 H Imminium salt H H

C N(CH3)2 C N(CH3)2 H H Step 2 Electrophile

O OH OH + C6H5 C CH3 + H C6H5 C CH2 H C6H5 C CH2 Acetophenone (Enol Form) (keto form)

Step 3 OH OH H C N(CH ) C H C CH CH + 3 2 6 5 2 2 N(CH3)2 C6H5 C CH2 H

O O - Proton OH C6H5 C CH2CH2 NH(CH3)2 C6H5 C CH2CH2 N(CH3)2 transfer -H2O ß-dimethylaminopropiophenone ()

Dr. S.Hasan Mehdi 6/13/2020 O O R C H + 2Ag(NH ) OH R 2Ag 3 2 C ONH4 + + H2O+ 3NH3 Fehling’s solution: It is an alkaline solution of cupric Sionilver mcomplexedirror with sodium potassium tartrate ions. On treating with an aldehyde, the complex cupric ion (deep blue) is reduced to cuprous oxide (red). The presence of red precipitate of cuprous oxide indicates the presence of aldehyde group in a molecule. O O R C H + 2Cu(OH)2 + NaOH R C ONa + Cu2O+ 3H2O (red)

Bendict’s solution: It is an alkaline solution of cupric ion complexed with citrate ions. It reacts in the same way as Fehling’s solution.

Dr. S.Hasan Mehdi 6/13/2020 Cannizaro reaction: Aldehydes which lack an α-hydrogen, when heated with conc. NaOH, undergo reaction. One half of the aldehyde molecules are oxidised to a and on half are reduced to alcohol. This reaction is known as Cannizaro reaction. Aldehyde with α-hydrogen undergo an aldol condensation under these conditions. Step 1: O O H C H + OH H C H OH Step 2: Hydride Transfer

O O O H H C H C C + CH OH + C O H + CH3O H 3 OH H OH O

Dr. S.Hasan Mehdi 6/13/2020 Baeyer-Villiger Oxidation: Oxiation of ketones (acyclic/cyclic) into / by means of peracids such as perbenzoic acid (PAA), trifluroperacetic acid (TFPAA) etc is known as Baeyer-Villiger Oxidation. for e.g. O O R C R' TFPAA R C OR' Ketone Ester

O O

TFPAA O

Cyclopentanone Aliphatic aldehydes are oxidised to acids by the migration of hydrogen.

O O O R'COOOH R C H R C OH + R' C OH

Dr. S.Hasan Mehdi 6/13/2020 Mechanism: It involves 1. protonation of carbonyl oxygen, 2. nucleophilic attack of per benzoic acid on carbonylic carbon, 3. migration of R to electron deficient oxygen with release of carboxylate anion and 4. deprotonation of protonated ester to ester. H O O O H OH O O O C C6H5 + R C R + H R C R R C O O C C6H5 Nucleophilic addition R H

OH O OH + - -H -C6H5COO 1,2 -Migration of R from C to O R C O O C C6H5 R C O R R

O H O H O + R C OR R C OR R C OR + H Ester

Most probably the two steps involving the 1,2-migration of group R and loss of carboxylate ion are concerted i.e. occurs simultaneously.

Dr. S.Hasan Mehdi 6/13/2020 Mechanism continued……..

OH O OH - R C O O C C6H5 R C OR + -C6H5COO R This shows that the carbonyl oxygen of the ester group is one that is on the original ketone.

18 18 O O C H COOOH C6H5 C C6H5 6 5 C6H5 C OC6H5 -C6H5COOH (O18 = isotopic oxygen) Phenyl benzoate In unsymmetrical ketones, that group migrates which is more electron-releasing. Among 0 0 0 alkyl group, the order of migration is 3 >2 > 1 > CH3. Among aryl group the migratory order is:

CH3O >CH3 > > Cl

Dr. S.Hasan Mehdi 6/13/2020 Meerwin-Pondorrf-Verley reduction: This reaction converts carbonyl group into alcoholic group on reduction. It consists in treating aldehyde or ketone with in isopropyl alcohol. for e.g. R R (Me CHO) Al 2 3 CHOH (CH ) C O C O + (CH3)2CHOH 3 2 R R Ketone Isopropyl alcohol Sec. Alcohol Acetone

(Me CHO) Al 2 3 (CH ) C O O + (CH3)2CHOH OH + 3 2

CyclohexanoneIsopropyl alcohol Cyclohexanol Acetone

This method is also useful for the selective reduction of carbonyl group without affecting the alkene double bond, when present in conjugation with aldehyde group.

(Me2CHO)3Al CH3CH CH CH O + (CH3)2CHOH CH3CH CH CH2OH + (CH3)2C O

Crotonaldehyde Isopropyl alcohol Crotyl Alcohol Acetone

Dr. S.Hasan Mehdi 6/13/2020 Mechanism of MPV reduction:

R R C O + (Me2CHO)3Al C O Al(OHCMe2)2 R R O -Me2C O Aluminium isopropoxide H CMe2 (Cyclic cooordination complex) Aldehyde or ketone

R R C O Me2CHOH CHOH Al(OHCMe2)2 R (Me2CHO)3Al + H R

R = CH3, C2H5 or H Mixed aluminium 10 or 20 alcohol

Dr. S.Hasan Mehdi 6/13/2020 Wolff-Kishner reduction: It involves the treatment of carbonyl compounds with hydrazine followed by strong base like KOH or potassium tert-butoxide in solvents like diethylene glycol or dimethyl

sulphoxide (DMSO). This rection reduces C=O into CH2 Group.

R R C O + KOH CH2 + N + H2O NH2 NH2 2 R' (DEG) R' [R, R' are Alkyl or aryl group

Ketone Hydrazine

Dr. S.Hasan Mehdi 6/13/2020 : It reduces aldehyde or ketone by heating with amalgamated zinc (Hg-Zn) and conc. HCl to form

corresponding hydrocarbon (-C=O into CH2 Group). This reduction works well for ketones but not for aldehydes. The ketones which are sensitive to alaklies are well reduced by this reduction. R R 4H C O CH2 + H2O (Zn-Hg), HCl R' R' [R, R' are Alkyl or aryl group

Ketone Hydrocarbon

R + R R - Mechanism: H + + 2e from Zn C O C OH C OH R' R' R'

R + R + R H H -H2O C OH CH OH CH OH2 R' R' R'

R R - R + +2e H CH CH CH2 from Zinc R' R' R' Hydrocarbon

Dr. S.Hasan Mehdi 6/13/2020 Reduction with Lithium (LiAlH4) and (NaBH4)

Lithium aluminium hydride (LiAlH4) and sodium borohydride (NaBH4) converts aldehydes and ketones to primary and secondary alcohols respectively. In these reduction hydride ion (H- ion) acts as a nucleophile. For e.g.

or NaBH4 RCH O + LiAlH4 RCH2OH Aldehyde

LiAlH4 C6H5COCH3 C6H5CH(OH)CH3 or NaBH4 Acetophenone 1-phenyl (sec. alcohol

Dr. S.Hasan Mehdi 6/13/2020 R R R 3R C O Mechanism: - 2 C O H: shift CH OAlH3]Li CH O Al]Li R R R 4

+ H AlH3]Li Lithium aluminium salt of alcoho

R R H + Al(OH) CHO Al]Li + 4H2O CHOH 3 + LiOH R 4 R Alcohol Sodium borohydride also reacts in the same way. It has an additional advantage that the carbon-carbon double bond, if present in conjugation to –CH=O), remain unaffected. For. e.g.

NaBH4 LiAlH4 C6H5CH=CH-CH OH C6H5CH=CH-CH=O C6H5CH2CH2CH2OH 2 Selective reduction Cinnamyl alcohol Dihydrocinnamyl alcoho

Dr. S.Hasan Mehdi 6/13/2020 References:

•A TEXTBOOK OF ORGANIC CHEMISTRY by Arun Bahl & B.S. Bahl, S. Chand & Company Ltd. Publication. •Graduate Organic Chemistry by M. K. Jain and S.C. Sharma, Vishal Publishing Co. •Pradeep’s Organic Chemistry Vol II by R. N. Dhawan, Pradeep Publication, Jalandhar.

Dr. S.Hasan Mehdi 6/13/2020