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ALDEHYDES AND KETONES
In ketones, the carbonyl group is linked to two carbon containing groups which may be same or different alkyl, aryl group. If two R and R’ groups are same, the ketone is called simple or symmetrical ketone and if R and R’ are different, then ketone is known as mixed or an unsymmetrical ketone.
STRUCTURE
Carbonyl carbon of both aldehyde and ketones is sp^2 – hybridised, One of the three sp^2 hybridised orbital get involved in σ- bond formation with half – filled p-orbital of oxygen atom whereas rest of the two are consumed in σ-bond formation with hydrogen and carbon depending on the structure of aldehyde or ketone. Unhybridised p-orbital of carbonyl carbon form π-bond with another half-filled p-orbital of oxygen atom by sideways overlapping.
ISOMERISM IN ALDEHYDES AND KETONES
(a) Chain isomerism: Aldehydes ( with 4 or more carbon atoms) and ketone ( with 5 or more carbon atoms) show chain isomerism. Example i) C 4 H 8 O CH 3 - CH 2 - CH 2 - CHO ( butanal)
In aldehydes, the carbonyl group is linked to either two hydrogen atom or one hydrogen atom and one carbon containing group such as alkyl, aryl or aralkyl group Examples
ii) C 5 H 10 O
(b) Position isomerism: aliphatic aldehydes do not show position isomerism, because – CHO group is always present at the end of carbon chain. Aromatic aldehyde show position isomerism. Example
(c) Metamerism: Higher ketones show metamerism due to presence of different alkyl groups attached to the same functional group C 5 H 10 O
(d) Functional isomerism : Aldehydes and ketones show functional isomerism in them. In addition, they are also related to alcohols, ethers and other cyclic compounds. Example C 3 H 6 O
- From alkenes (i) Reductive ozonolysis of alkenes.
(ii) Wacker process.
(iii) OXO process [Carbonylation / Hydroformylation]
- From alkynes
- From Grignard reagent (1) By addition to ester
(iii) By addition to nitriles
- From carboxylic acids (i) Catalytic decomposition of carboxylic acid.
- From gem-dihalides by hydrolysis
- From nitriles by reduction (i) Stephen’s reduction.
(ii) Reduction with LiAlH 4
- Preparation of aromatic carbonyl compounds. (i)
This is known as Etard reaction
(ii) By side chain chlorination followed by hydrolysis
(iii) Gatterman – Koch reaction
(iv) Friedel Craft Acylation
(v) Reimer – Tiemann reaction
group and hence, lower is its reactivity towards nucleophilic addition reactions. Thus, the following decreasing order of reactivity is observed
(ii) Steric effect In formaldehyde there is no alkyl group while in all other aldehyde there is one alkyl group so here the nucleophile attack is relatively more easy but in ketones there are two alkyl groups attached to carbonyl group and these causes hinderance, to the attacking group. This factor is called steric hinderance (crowding). In other words the hindrance increases, the reactivity decreases accordingly. Thus order of reactivity is
(b) Aromatic aldehydes and ketones In general, aromatic aldehydes and ketones are less reactive than the corresponding aliphatic aldehydes and ketones. It is due electron releasing resonance effect of bezene ring
Due to electron withdrawing resonance effect (-R effect) of benzene ring, the magnitude of positive charge on carbonyl group decreases and consequently it becomes less susceptible to nucleophilic attack.
The order of reactivity of aromatic aldehydes and ketones is
CHEMICAL PROPERTIES OF ALDEHYDES AND KETONES
Nucleophilic addition reaction
In this reaction carbon atom of carbonyl group changes from sp^2 to sp^3 hybridised (i) Addition of hydrogen cyanide (HCN)
Mechanism Step I : The hydrogen cyanide interacts with the base to form nucleophile
Formaldehyde form a primary alcohol
Higher aldehydes give secondary alcohol
Ketone give tertiary alcohols
(iv) Addition of alcohols
Dry HCl protonates the oxygen atom of the carbonyl compounds and therefore, increases the electrophilicity of the carbonyl carbon and hence facilitating the nucleophilic attack by the alcohol
molecule. Dry HCl gas also absorbs the water produced in these reactions and thereby shifting equilibrium in forward direction.
Ketals can be prepared by treating the ketone with ethyl ortho formate
(v) Addition of ammonia derivative
Z = OH, NH 2 , NHC 6 H 5 , NHCOCH 2 etc.
The reaction of ammonia derivatives to aldehydes and ketones is called by acids
Mechanism
Step I: In acidic medium, the carbonyl oxygen gets protonated.
Step II : In ammonia derivatives, the nitrogen atom has a lone pair of electrons, which attack the positively charged carbonyl carbon and results in positive charge on nitrogen atom
Step III : The unstable intermediate loses a proton, H+^ and water molecule to form stable product (imines)
IV. Reduction with HI + P (red)
V. Reduction to pinacols
- Oxidation reactions
i. Oxidation with mild oxidizing agents
Ketones are not oxidized by mild oxidizing agents (a) Aldehydes reduces Tollen’s reagent to metallic silver which appears as a silver mirror on wall of test tube. Thus the reaction is also known as silver mirror test.
(b) Reduction of Fehling’s solution Fehling’s solution is an alkaline solution of CuSO 4 mixed with Rochelle slat i.e. sodium potassium tartarate. Aldehydes reduces cupric ion (Cu2+) of Fehling’s solution to cuprous ions (Cu+) to form red precipitate of cuprous oxide
Fehling’s solution is reduced by aliphatic aldehydes only. Aromatic aldehydes and ketones so not give this reaction. ii. Oxidation with strong oxidizing agent
iii. Haloform reaction
- Condensation reactions (1) Aldol condensation
Mechanism
A-B Condensation
B-A Condensation
(3) Claisen – Schmidt condensation
- Cannizzaro reaction
Mechanism Step I : The OH-^ ion attacks the carbonyl carbon to form hydroxyl alkoxide
Step II : Anion (I) acts as hybride ion donor to the second molecule of aldehyde. In the final step of the reaction, the acid and the alkoxide ion transfer H+^ to acquire stability.
- Reaction with chloroform
Chloretone is used as hypnotic.
- Reaction with primary amine
