Give plausible explanation for each of the following:
(i) Cyclohexanone forms cyanohydrin in good yield but 2,2,6-trimethylcyclohexanone does not.
(ii)There are two –NH2 groups in semicarbazide.However,only one is involved in the formation of semicarbazones.
(iii)During the preparation of esters from a carboxylic acid and an alcohol in the presence of an acid catalyst,the water or the ester should be removed as soon as it is formed.
Give plausible explanation for each of the following:
(i) Cyclohexanone forms cyanohydrin in good yield but 2,2,6-trimethylcyclohexanone does not.
(ii)There are two –NH2 groups in semicarbazide.However,only one is involved in the formation of semicarbazones.
(iii)During the preparation of esters from a carboxylic acid and an alcohol in the presence of an acid catalyst,the water or the ester should be removed as soon as it is formed.
Solution and Explanation
(i)Cyclohexanones form cyanohydrins according to the following equation.
In this case,the nucleophile \(CN^-\)can easily attack without any steric hindrance.However,in the case of 2,2,6 trimethylcydohexanone,methyl groups at α-positions offer steric hindrances and as a result,CN-cannot attack effectively.
For this reason,it does not form a cyanohydrin.
(ii)Semicarbazide undergoes resonance involving only one of the two\(-NH2 \) groups,which is attached directly to the carbonyl-carbon atom.
Therefore, the electron density on-NH2 group involved in the resonance also decreases.As a result,it cannot act as a nucleophile.Since the other -\(NH_2\) group is not involved in resonance; it can act as a nucleophile and can attack carbonyl-carbon atoms of aldehydes and ketones to produce semi carbazones.
(iii)Ester along with water is formed reversibly from a carboxylic acid and an alcohol in presence of an acid.
If either water or ester is not removed as soon as it is formed, then it reacts to give back the reactants as the reaction is reversible.Therefore,to shift the equilibrium in the forward direction i.e., to produce more ester, either of the two should be removed.
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Write IUPAC names of the following compounds and classify them into primary, secondary and tertiary amines.
(i) (CH3 )2CHNH2 (ii) CH3 (CH2 )2NH2 (iii) CH3NHCH(CH3 )2
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(ii) Secondary and tertiary amines
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(iv) Aniline and benzylamine
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(i) pKb of aniline is more than that of methylamine.
(ii) Ethylamine is soluble in water whereas aniline is not.
(iii) Methylamine in water reacts with ferric chloride to precipitate hydrated ferric oxide.
(iv) Although amino group is o– and p– directing in aromatic electrophilic substitution reactions, aniline on nitration gives a substantial amount of m-nitroaniline.
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Top CBSE CLASS XII Aldehydes, Ketones and Carboxylic Acids Questions
What is meant by the following terms? Give an example of the reaction in each case.
\((i)\)\(Cyanohydrin\)
\((ii)Acetal \)
\((iii)Semicarbazone \)
\((iv)Aldol \)
\((v)Hemiacetal \)
\((vi)Oxime \)
\((vii)Ketal \)
\((vii)Imine \)
\((ix)\)\(2,4-DNP-derivative \)
\((x)Schiff's base\)Name the following compounds according to IUPAC system of nomenclature:
\((i) CH_3CH(CH_3)CH_2CH_2CHO \)
\((ii) CH_3CH_2COCH(C_2H_5)CH_2CH_2Cl \)
\((iii) CH_3CH=CHCHO \)
\((iv) CH_3COCH_2COCH_3 \)
\((v) CH_3CH(CH_3)CH_2C(CH_3)_2COCH_3 \)
\((vi) (CH_3)_3CCH_2COOH \)
\((vii) OHCC_6H_4CHO-p\)Write the IUPAC names of the following ketones and aldehydes. Wherever possible, give also common names.
\((i) CH_3CO(CH_2)_4CH_3 \)
\((ii) CH_3CH_2CHBrCH_2CH(CH_3)CHO \)
\((iii) CH_3(CH_2)_5CHO \)
\((iv) Ph-CH=CH-CHO\)
\((v)\)
\((vi) PhCOPh\)Draw structures of the following derivatives.
(i)The 2,4-dinitrophenylhydrazone of benzaldehyde
(ii)Cyclopropanone oxime
(iii)Acetaldehydedimethylacetal
(iv)The semicarbazone of cyclobutanone
(v)The ethylene ketal of hexan-3-one
(vi)The methyl hemiacetal of formaldehydePredict the products formed when cyclohexanecarbaldehyde reacts with following reagents.
(i)\(PhMgBr\) and then \(H3O + \)
(ii)Tollens' reagent
(iii) Semicarbazide and weak acid
(iv)Excess ethanol and acid
(v) Zinc amalgam and dilute hydrochloric acid
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Concepts Used:
Aldehydes, Ketones and Carboxylic Acids - Chemical Reactions
Chemical Reactions of Aldehydes and Ketones:
- Cross aldol condensation
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