Aldehydes & Ketones

Aldehydes & Ketones:

Aldehydes and ketones contains the same functional group, the carbonyl group (> C = O).

Aldehydes behave as reducing agents due to presence of reducing hydrogen atom where as ketones have no such property so

aldehydes easily reacts with oxidising agent. Even HCOOH shows some properties of carbonyl compounds due to presence of –

CHO group. HCOOH behaves as reducing agent while in other acids no reducing hydrogen atom, so no reaction with oxidising

agent.

Preparation Methodology:

(i.) Oxidation of Alcohols:

(a.) From PCC or pyredeniumchlorochromate. Which is pyridine, CrO3 and HCl, in equal ratio

By Jones reagent (CrO3 and aq. CH3COCH3) reaction goes till acids as it is a strong oxidising agent where as by PCC and Collins

reagent reaction stops at aldehyde.

(ii.) Rosenmund’s Reduction:

Reduction of acid halide into aldehydes by Pd and BaSO4 is known as rosenmund reduction, only aldehydes can be prepared by

this reaction.

HCOCl break down into CO + HCl so the first aldehyde from rosenmund reduction is CH3CHO. Here Pd act as catalyst and BaSO4

as catalytic poison to prevent conversion of aldehyde into alcohol by further reduction.

(iii.) Stephan’s Reduction:

When Cyanides are partially reduced by means of stannous chloride and hydrochloric acid followed by hydrolysis yield aldehyde.

Ketones can not be prepared by this process.

Here [H2SnCl4] is formed which works as powerful reducing agent.

(iv) Oxidation of Alkyl halide by dimethyl sulphoxide:

(v) From Grignard Reagent

(vi) Dry distillation of calcium salts of acids

(vii) Oxidation of vicinal glycols by periodic acid

This reaction can also be done by lead tetrs acetate. We have already studied the mechanism

of this reaction in last chapter.

(viii) Action of MnO on acids at 300oC temprature

(ix) By Hydrolysis of Acetoacetic ester

(x) Catalytic Dehydrogenetion by Cu at 300oC

(xi) Oppenauer oxidation

This is specific method for oxidation of secondary alcohol to ketone. Secondary alcohol is

heated with aluminium iso-propoxide in a ketone (usually acetone)- secondary alcohol is

oxidised to ketone whereas ketone is reduced to secondary alcohol.

Actually this is a reversible reaction and equilibrium can be shifted in either direction by

appropriate manipulation.

For example, by taking an excess of secondary alcohol, ketone can be reduced; this reduction

is called Meerwein Ponndorf - Verley reduction which has been discussed later.

The uniqueness of Oppenauer oxidation lies in the fact that it selectively oxidises hydroxy

group which means that if the compound besides hydroxy group has some other oxidisable

functionality, the latter will remain unaffected under these conditions.

For example :

(xii) Hydrolysis of gemdihalide

Properties

(i) Nucleophilic addition reaction

Six electron system of carbocation is formed if primary attack of electrophile takes place,

otherwise it is a eight electron system which is more stable when primary attack of

nucleophile takes place.

So Primary attack here is of the nucleophile due to more stable intermediate oxygen anion of

eight electron system. The reactivity of different carbonyl compounds towards formation of

nucleophilic addition decreases in the following order :

This order arises due to the following two factors :

(i) Electronic factor : We have seen above that carbon of the carbonyl group is susceptible to

the attack of nucleophile due to the presence of positive charge on it (i.e. on carbon). As the

intensity of positive charge on carbon decreases, the nucleophilic attack would occur less

readily. On the other hand, if intensity of positive charge increases nucleophilic attack would

occur more readily. Alkyl group exerts + I effect, due to which it reduces the intensity of

positive charge on carbonyl carbon, which decreases in the following order :

Intensity of positive charge on carbonyl carbon decreases. Hence the reactivity of these

compounds towards nucleophilic addition also decreases in this order.

(ii) Steric factor : The change in C - C - O angle as a result of nucleophilic addition may be

noted.

It is obvious that nucleophilic addition causes decrease in C – C – O bond angle and hence

groups are pushed closer. The bigger groups oppose coming closer, hence the reactivity of

carbonyl group decreases. Size of methyl group is much larger than hydrogen, hence

acetaldehyde is less reactive than formaldehyde. Similarly, acetone will be less reactive than

acetaldehyde.

(a) Reaction with HCN

In the presence of alkali the dissociation of HCN increases because HO- of alkali trap H

+ of

HCN so that ionisation of HCN increases.

But in presence of acid the dissociation is suppresed due to common ion effect. Thus the

reaction shifts to backward direction.

(b) Reaction with NaHSO3 (sodiumbisulphite)

This reaction is most versatile test for separating carbonyl compounds from noncarbonyl

compounds, it first form white crystalline solid which when passed with dry HCl gas re-form

carbonyl compound.

Here the carbonyl compounds are reobtained by reaction with dry HCl, so these reaction are

used for separation test.

But always remember that for separation of aldehydes & ketones is done by Tollen’s reagent

(ammonical AgNO3 ) i.e. [Ag(NH3)2+], where aldehydes reacts to form white metallic silver.

(c) Reaction with sodio - derivatives of 1 - alkynes

(d) Reaction with C2H5SH (Ethanethiol)

Aldol Condensation

(ii) Aldol Condensation

Aldehydes or ketones with when treated with dilute alkali like NaOH,K2CO3 etc. undergo nucleophilic addition

by active intermediate carbanion to form known as aldols.

Various basic reagents such as potassium hydroxide, aqueous alkali carbonate, alkali metal alkoxides, etc., may be used. The

reaction is not favourable for all type of ketones but applicable in some special cases like CH3COCH3.

Aldol condensation are of different type and it can occur between

(i) two identical or different aldehydes,

(ii) two identical or different ketones and

(iii) an aldehyde and a ketone.

When the condensation is between two different carbonyl compounds, it is called crossed aldol condensation.

(a) Simple Aldol

Above reaction is also called as Claisen Schimdt Condensation where one is aromatic carbonyl compound and other is aliphatic

carbonyl compound.

In cross aldol condensation possibility of more than one product is always there so these reactions are not very much useful for

synthatic chemistry. For example :

Acetone can also undergo aldol type reaction in the presence of dilute alkali (base catalyst) or dry HCl gas.

Intraaldol condensation: Dicabonyl compounds, having two carbonyl groups within the same molecule, undergo intramolecular

aldol condensation reactions. Even bases as weak as sodium carbonate are adequate in these reactions.

Intramolecular aldol condensations proceed best when five- or six- membered rings result because these are of minimum strain

ring.

(iii) Cannizaro’s Reaction

Aldehydes which do not have any , when treated with concentrated solution of NaOH or KOH, undergo

simultaneous oxidation and reduction (disproportionation) forming a salt of carboxylic acid and alcohol.

Here 'R' should be –H or phenyl but not alkyl group with presence of .

The reaction follows third-order law (second order in aldehyde and first order in base),/ i.e.,

rate

. This suggests the reaction between the first-formed anion (from base and aldehyde) and another molecule of aldehyde in the rate-

determining step.

In the presence of a high concentration of base, the reaction follows fourth-order law (second order in both two molecules of base),

i.e., rate

The hydride ion is directly transferred from one molecule of the aldehyde to the other, and does not become free in solution has

been proved by the observation that the recovered alcohol does not contain deuterium when the reaction is performed in the

presence of D

2

O.

Cannizaro Reaction of HCHO

Transfer of hydride ion due to back donation of oxygen charged electron give out hydride ion which is used for reduction of other

carbonyl molecule so oxidation of one molecule gives acid salt while other by reduction forms alcohol.

(a) Simple cannizaro's reaction

Mechanism of Cannizaro's of benzaldehydeMechanism of Cannizaro's of benzaldehyde Mechanism of Cannizaro's of benzaldehyde

(b) Cross Cannizaro's Reaction

Reaction of carbonyl compounds (does not having

) with alkali forms each molecules of acid and alcolol.

Whenever HCHO is present, there are only two main products otherwise there will be four products in all other cases. Because

HCHO contains maximum reducing hydrogen atom so easily undergo oxidation process.

One of the most important applications is the

crossed Cannizzaro

reaction between formaldehyde and other aldehydes containing

(iv) Bimolecular Reduction or Pinacol Reaction

Two molecules of acetone undergo reduction in the presence of Mg/Hg to form Pinacol. Upon treatment with mineral acids, 2,3 -

dimethyl 2,3- butane diol (pinacol) is converted into methyl ter-butyl ketone (pinacolone). The 1,2-diol undergo dehydration in

such a way that rearrangement of the carbon skeleton occurs. Other 1,2 diols undergo analogous reactions, which are known as

pinacol pinacolone

type rearrangement.

Mechanism

Pinacol–Pinacolone type Rearrangement

As the migrating group migrates with its electron pair, the more nucleophilic group might be expected to migrate. Thus, the order

of migratory attitude amongst the aryl groups is

p

-anisyl >

p

-tolyl > phenyl >

p

-chlorophenyl, etc.

Remember, electron-attracting groups will retard the migration. The migratory aptitude amongst the alkyl groups is Me

3

C > Me

2

CH > Me. However, the stability of the initially formed carbocation may offset the migratory attitude order. Thus, in the compound

1, 1-dimethyl-2, 2-diphenyl glycol, the resonance-stabilized carbocation (I) is formed instead of (II) and so it is the methyl group

and not the phenyl group which migrates, contrary to the above sequence.

Steric hinderance may affect the rate of migration–

p

-anisyl group migrates 1000 times faster than

o

-anisyl group.

Migrating group attacks from the trans side or back side of to the leaving group. This has significant aspect in cyclic systems.

Thus, the two isomers of 1, 2-dimethyl-cyclohexane-1, 2-diol give different products due to different orientations of the methyl and

hydroxyl groups. The one (III) in which the Me and OH groups are

trans

to each other gives 2, 2-dimethylcyclohexanone by methyl shift. The other (IV) in which the Me and OH groups are

cis

to each other undergoes ring methylene group shift instead of Me-shift with consequent ring contraction to give 1-acetyl-1-

methylcyclopentane (V).

Applications

Ketones from cyclic diols

Pinacol rearrangement has been also applied to prepare ketones which are very difficult to prepare by other method.

(v) Reaction with NH3

When carbonyl compounds reacts with ammonia, nucleophilic addition reaction occurs but products are formed according to

praportions between ammonia and carbonyl compound. For example

(vi) Reaction with Ammonia Derivatives

NH

2

– Z is an example of nucleophilic addition elemination reaction where – Z groups are following in nature. First attack from

nucleophilic site of nitrogen atom to electrophilic carbon atom then elimination of water occurs simultaneously.

These reactions are usually carried out in weakly acidic medium because weak acid catalyses the reaction by protonating carbonyl

oxygen but in presence of excess of acid nucleophile is also protonated which reduces its reactivity, so optimum pH is necessary.

When zine compounds reacts with carbonyl compound they form crystalline solid as zone derivatives.

The reaction with 2,4–DNPH is used for sepration of carbonyl compound with noncarbonyl compound.

Always remember, presence of traces of

creates better polarity in the carbonyl compound and more electrophilic nature of carbon atom but on the other hand presence of

excess H +

in the ammonia derivative makes it a bad nucleophile. For example when NH

2

NH

2

(hydrazine) in presence of excess of acid converted into NH

2

N +

H

3

cation (hydrazinium) which is a bad nucleophile. Hence optimum pH is a very important condition for this reaction,only tracer

amount of H +

are required.

Distinction between aldehydes & Ketones

(vii) Distinction between aldehydes & Ketones

Aldehydes having reducing hydrogen atom whereas ketones don’t, thus only aldehydes reacts with oxidising agent and forms

respective product.

(a) Ammonical AgNO3 (Tollen’s reagent)

Aldehydes reacts with silveroxide to form white precipitate of metallic silver while ketones cannot, due to absence of reducing

hydrogen atom.

(b) Reaction with HgCl2 (Mercuric chloride)

Aldehydes reacts with mercuric chloride to form white precipitate of mercurous chloride which changes into black precipitate of

metallic mercury.

(c) Fehling's solution

There are two solutions which contains cupric oxide as oxidising agent, when reacts with aldehydes it forms red precipitate o f

cuprous oxide.

C

6

H

5

CHO and their aromatic derivatives do not give test with Fehling's solution because aromatic aldehydes are not good reducing

agents. It gives reaction with tollen's reagent due to more oxidising nature of Ag

2

O than CuO. (Ag contains higher reduction potential than copper in electrochemical series)

(d) Benedict's Solution

Similar chemical reaction and cupric oxide are present in benedict's solution but presence of citrate gives differant complex. It

reacts with aldehydes to form red precipitate of Cu

2

O. It cannot reacts with benzaldehyde and its aromatic derivatives.

(e) Schiff’s Reagent

When dilute solution of p-rosaniline hydrochloride, pink in colour, passed through sulphurdioxide gas forms colourless solution

known as schiff's reagent. This restore it colour by reducing nature of aldehyde while ketones gives no response.

(viii) Reformatsky Reaction

When an

usually an

reacts with carbonyl compound in the presence of zinc metal to produce a

. This is known as

Reformatsky reaction.

When, a mixture of the carbonyl compound,

and zinc in dry solvent benzene is carefully heated under reflux when zinc undergo dissolution. Zinc may be activated by adding

traces of iodine, or copper powder. The mixture is then treated with ice-cold dilute sulphuric acid and benzene layer separated.

Benzene is distilled off when

is obtained.

Aldol condensation, Knovengel’s reaction, Perkin reaction and Reformatsky reaction are

base catalysed

reaction so these reactions are carbanian active process. The advantage of using zinc in place of magnesium is that the organo-zinc

compounds are less reactive than the organo-magnesium derivatives of

so that they do not normally react with their own ester groups.

(ix) Perkin synthesis

In Perkin reaction, synthesis has been effected between aromatic aldehydes and aliphatic acid anhydrides in the presence of s odium

or potassium salt of the acid corresponding to the anhydride, to yield

In this reaction active species also comes in the presence of base as carbanion (C–H2COOCOCH3).

Besides simple aromatic aldehydes, their vinyl derivatives, heterocyclic aldehydes and even phathalic anhydride (as the carbonyl

component) give this reaction.

When carbonyl compounds reacts with acetic anhydride in the presence of base to form active carbanian species, which give

nucleophilic addition with carbonyl compounds. But remember, absence of base gives simple fission of anhydride (no carbanian)

which on reaction with carbonyl forms stable alkyledene acetates.

Toluene oxidation by chromic acid forms benzoic acid while in the presence of acetic anhydride reaction stops at benzaldehyde due

to formation of stable intermediate benzeledene di-acetate which on hydrolysis again form –CHO group. So protection of –CH

3

group oxidation at –CHO group, we use chromic acid and anhydride mixture. Following reactions are given below

Witting reaction

(x) Witting reaction

Witting reaction gives an important and useful method for the preparation of alkenes by the reaction of aldehydes or ketones with

alkylidenetriphenylphosphorane (Ph3P = CR2) or simply called as phosphorane.

The

Witting reagent

, alkylidenetriphenylphosphorane, is prepared by reaction of trialkyl or triarylposphine usually the latter with an alkyl halide in

ether solution. Finally resulting phosphonium salt is reacted with a strong base (such as C

6

H

5

Li, BuLi, NaNH

2

, NaH, C

2

H

5

ONa, etc.) which removes a haloacid to give the reagent, methylenetriphenyl phosphorane (II).

In end, carbonyl compound is directly treated with the ethernal solution of the above reagent to form many compounds.

Mechanism

The reaction go through by the nucleophilic attack of the ylide on the carbonyl carbon. The dipolar complex (betain) so formed

undergo electronic exchange decomposes to olefin and triphenylphosphine oxide through a four -centred transition state.

The mechanism is strongly supported by an example that an optically active phosphonium salt reacts to produce a phosphine oxide

with retention of configuration in the final product.

(xi) Lederer Manasse’s Reaction

When phenol is treated with 40% aqueous solution of formaldehyde (formalin) in the presence of a dilute acid or alkali at low

temperature, a mixture of o-and p-hydroxy benzyl alcohol is formed.

This reaction is called

Lederer-Manasse reaction.

On heating for a short time, these compounds unergo condensation reaction with themselves and unchanged phenol and give linear

polymers by elimination of water.

These reactions are the basis of the preparation of phenol formaldehyde resins. These materials were developed by Backland and

are hence called

bakelite.

They are thermoplastic solids soluble in many organic solvents. When warmed with hexa methylene tetramine. (CH

2

)

6

N

4

, which splits up to formaldehyde and ammonia, further methylene bridges are formed and a three-dimensional polymer results.

(xii) Baeyer–villiger rearrangement

Baeyer--Villiger rearrangement

is an example of the migration of a group from carbon to electron-deficient oxygen.The reaction first involves the oxidation of

ketones to esters by the treatment with peracids such as peracetic acid, performic acid, meta chloroperbenzoic acid (MCPBA),

perbenzoic acid, pertrifluoroacetic acid, permonosulphuric acid, etc. This reaction can also be done by H

2

O

2

and base.

Cyclic ketones are converted to lactones with expansion of ring.

(xiii) Beckmann rearrangement

The acid-catalyzed conversion of ketoximes to

N

-substituted amides is known as

Beckmann rearrangement

. The reaction is catalysed by acidic reagents such as, H

2

SO

4

, SOCl

2

, P

2

O

5

, PCl

5

, Al

2

O

3

C

6

H

5

SO

2

Cl, H

3

PO

4

and many others.

The reaction proceeds by the migration of a group from carbon to electron-deficient nitrogen.

Some aldoximes undergo the rearrangement process in the presence of polyphosphoric acid (PPA) but the reaction is not a general

one. The migration of the group not depends on the migrational activity but upon the orientation of the group in relation to the OH

group. It is found that the migrating group is always anti (i.e.,

trans

) to the hydroxyl group. So we can say that, the reaction is stereospecific.

Mechanism of reaction

(xiv) Benzilic acid rearrangement (modified intra molecular cannizaro's reaction)

When we mix a strong base to a carbonyl group first the formation of an anion takes place and the reversal of the anionic charge

may cause removal of the attached group, but in case of 1,2-diketone the attched group may migrate to the adjacent electron-

deficient carbonyl carbon forming

Thus, benzil on reaction with a strong base forms benzilic acid (salt), reaction is known as

benzilic acid rearrangement

. Basically this reaction is intracannizaro reaction where formation of benzillic acid takes place.

Barium hydroxide (barayta water) is more effective than sodium or potassium hydroxides due to strong basic nature. Alkoxide ions

(methoxide, ethoxide,

t

-butoxide, etc.) in place of hydroxide ion give the corresponding esters.

Phenoxide ions are too weak for nucleophile to attack. Besides aromatic 1, 2-diketones, aliphatic and heterocyclic diketones, for

example

o

-quinones can also undergo this type of reaction.

Polymerization Reactions

(xv) Polymerization Reactions

When two or more molecules combine to form bigger molecule accompanied by the loss of simple molecule like water, alcohol or

ammonia, etc. the process is called condensation polymerisation.

But if the number of molecules (of the same substance or of different substances) combine to form larger molecule, the process is

called polymerisation.

Condensation and polymerisation products of some carbonyl compounds are as follows -

Condensation and polymerisation product of formaldehyde

(a) Condensation with ammonia : Formaldehyde condenses with ammonia to form hexa methylene tetramine (urotropine)

The reaction occurs in the following manner :

(b) Condensation with phenol - Lederer - Manasse’s reaction :

Formaldehyde condenses with phenol in alkaline medium to form o-and p-hydroxy benzyl alcohol which further condenses with

phenol to give polymer,

Bakelite

. This is called

Lederer-Manasse reaction.

(c) Formation of penta erythritol :

Formaldehyde and acetaldehyde combine to form trihydroxy aldehyde

(Claisen-Schmidt reaction)

which reacts with alkali to give pentaerythritol (Cannizaro reaction) :

(d) Formation of formose :

On treating formaldehyde with dilute barium hydroxide (baryta water) , a mixture of several sugars (monosaccharides) is formed

which is called

formose.

Above reaction resembles aldol condensation although formaldehyde does not have

Polymers of formaldehyde :

Formaldehyde forms three polymers :

(a) Trioxan or trioxymethylene :

On distilling formaldehyde with a small quantity of dilute sulphuric acid,

trioxan

is formed. On allowing formaldehyde to stand, it slowly changes to trioxan.

(b) Paraformaldehyde :

When a concentrated aqueous solution of formaldehyde is evaporated off to dryness, a long linear polymer is obtained which is

called

paraformaldehyde.

On heating paraformaldehyde, formaldehyde is regenerated. Hence formaldehyde is transported in the form of paraformaldehyde.

when paraformaldehyde is heated in a sealed tube at 115 o

C, trioxan is formed.

(c ) Polyoxymethylene :

When dilute sulphuric acid is added into cold aqueous solution of formaldehyde, a white insoluble solid is formed which is ca lled

polyoxymethylene

. This is also a linear polymer. In this case chain length is longer than in paraformaldehyde.

Condensation and polymerisation products of acetaldehyde

(a) Reaction with ammonia :

Ammonia reacts with acetaldehyde to form simple nucleophilic addition product- acetaldehyde ammonia which subsequently

losses water to give acetaldemine.

(b) Polymerisation:

Acetaldehyde when treated with hydrogen chloride gas at 0°C forms a solid tetramer called

metaldehyde

. However, if temperature is not controlled, a trimer, called

paraldehyde

is formed which is liquid.

Metaldehyde burns with a smokeless non-luminous flame and is used as a killer for snails and slug and is marketed under the name

‘Snarol’.

Paraldehyde is used as a hypnotic (sleep-inducing drug).

Both the polymers of acetaldehyde get depolymerised on heating with dilute sulphuric acid and thus they act as source of

acetaldehyde reactions in acid medium.

The polymeric aldehydes do not show characteristic aldehyde reactions. Higher aldehydes and ketones do not give definite isolable

polymers.