Aldehydes Ketones and Carboxylic Acids



1   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

Solution :

(i) Cyanohydrin:$\\$ Cyanohydrins are organic compounds having the formula $RR'C(OH)CN,$ where $R $ and $R'$ can be alkyl or aryl groups.$\\$ Aldehydes and ketones react with hydrogen cyanide $(HCN)$ in the presence of excess sodium cyanide $( NaCN )$ as a catalyst to field cyanohydrin. These reactions are known as cyanohydrin reactions.$\\$ $RR'C \underset{Ketone}{=}O+HCN \overset{NaCN}{\to} \underset{Cyanodydrin}{RR'C(OH)CN}$$\\$ Cyanohydrins are useful synthetic intermediates.$\\$ (ii) Acetal:$\\$ Acetals are gem-dialkoxy alkanes in which two alkoxy groups are present on the terminal carbon atom. One bond is connected to an alkyl group while the other is connected to a hydrogen atom.$\\$ General Structure of an acetal When aldehydes are treated with two equivalents of a monohydric alcohol in the presence of dry HCl gas, hemiacetals are produced that further react with one more molecule of alcohol to yield acetal.$\\$ (iii)Semicarbarbazone:$\\$ Semicarbazones are derivatives of aldehydes and ketones produced by the condensation reaction between a ketone or aldehyde and semicarbazide.$\\$ Semicarbazones are useful for identification and characterization of aldehydes and ketones.$\\$ (iv) Aldol:$\\$ A $\beta$-hydroxy aldehyde or ketone is known as an aldol. It is produced by the condensation reaction of two molecules of the same or one molecule each of two different aldehydes or ketones in the presence of a base.$\\$

(v) Hemiacetal:$\\$ Hemiacetals are $\alpha-$alkoxyalcohols$\\$ Aldehyde reacts with one molecule of a monohydric alcohol in the presence of dry $HCl$ gas.$\\$ (vi) Oxime:$\\$ Oximes are a class of organic compounds having the general formula $RR'CNOH,$ where $R$ is an organic side chain and $R'$ is either hydrogen or an organic side chain. If $R'$ is $H$, then it is known as aldoxime and if $R'$ is an organic side chain, it is known as ketoxime. On treatment with hydroxylamine in a weakly acidic medium, aldehydes or ketones formoximes.$\\$ (vii) Ketal:$\\$ Ketals are gem-dialkoxyalkanes in which two alkoxy groups are present on the same carbon atom within the chain. The other two bonds of the carbon atom are connected to two alkyl groups.$\\$ Ketones react with ethylene glycol in the presence of dry $HCl$ gas to give a cyclic product known as ethylene glycol ketals.$\\$ (viii) Imine:$\\$ Imines are chemical compounds containing a carbon nitrogen double bond. Imines are produced when aldehydes and ketones react with ammonia and its derivatives.$\\$

(ix) $2, 4-DNP-$derivative:$\\$ $2, 4-$dinitrophenylhydrazones are $2, 4-DNP-$derivatives, which are produced when aldehydes or ketones react with $2, 4-$dinitrophenylhydrazine in a weakly acidic medium.$\\$ To identify and characterize aldehydes and ketones, $2, 4-DNP$ derivatives are used.$\\$ (x) Schiff's base:$\\$ Schiff’s base (or azomethine) is a chemical compound containing a carbon-nitrogen double bond with the nitrogen atom connected to an aryl or alkyl group-but not hydrogen. They have the general formula $R _1 R _2 C =NR _3$ . Hence, it is an imine. It is named after a scientist, Hugo Schiff. Aldehydes and ketones on treatment with primary aliphatic or aromatic amines in the presence of trace of an acid yields a Schiff’s base.

2   Name the following compounds according to $IUPAC$ system of nomenclature:$\\$ (i)$ CH_3 CH( CH _3 ) CH _2 CH _2 CHO$$\\$ (ii) $CH _3 CH _2 COCH ( C _2 H _5 ) CH _2 CH _2 Cl$$\\$ (iii) $CH _3 CH = CHCHO$$\\$ (iv) $CH _3 COCH _2 COCH _3$$\\$ (v) $CH _3 CH ( CH _3 ) CH _2 C ( CH_ 3 ) _2 COCH _3$$\\$ (vi) $(CH _3 ) _3 CCH _2 COOH$$\\$ (vii) $OHCC_ 6 H _4 CHO - p$

Solution :

(i) $4-$methylpentanal$\\$ (ii)$ 6-$Chloro$-4-$ethylhexan$-3-$one$\\$ (iii) But$-2-$en$-1-$al$\\$ (iv) Pentane$-2,4-$dione$\\$ (v) $3,3,5-$Trimethylhexan$-2-$one$\\$ (vi) $3,3-$Dimethylbutanoic acid$\\$ (vii) Benzene$-1,4-$dicarbaldehyde

3   Draw the structures of the following compounds.$\\$ (i) $3-$Methylbutanal$\\$ (ii)$ p-$Nitropropiophenone$\\$ (iii) $p-$Methylbenzaldehyde$\\$ (iv) $4-$Methylpent$-3-$en$-2-$one$ 4-$chloropentan$-2-$one$\\$ (vi) $3-$Bromo$-4-$phenylpentanoic acid $\\$(vii) $p , p ' -$Dihydroxybenzophenone$\\$ (viii) Hex$-2-$en$-4-$ynoic acid

Solution :

4   Write the $IUPAC$ names of the following ketones and aldehydes. Wherever possible, give also common names. (i)$ CH_3 CO ( CH _2 )_4 CH _3 $$\\$ ( ii )$ CH _3 CH _2 CHBrCH _2 CH ( CH _3 ) CHO$$\\$ ( iii )$ CH _3 ( CH _2 ) _5 CHO$$\\$ ( iv )$ Ph - CH = CH - CHO$$\\$ (v)$\\$ (vi)$ PhCOPh$

Solution :

(i)$ CH _3 CO ( CH _2 ) _4 CH _3$$\\$ $IUPAC$ name: Heptan-2-one$\\$ Common name: Methyl n-propyl ketone$\\$ (ii) $CH _3 CH _2 CHBrCH _2 CH ( CH _3 ) CHO$$\\$ $IUPAC$ name: 4-Bromo-2-methylhaxanal$\\$ Common name: ($\gamma$-Bromo-$\alpha$-methyl-caproaldehyde)$\\$ (iii) $CH _3 ( CH _2 )_ 5 CHO$$\\$ $IUPAC$ name: Heptanal$\\$ (iv)$ Ph - CH = CH - CHO$$\\$ $IUPAC$ name: 3-phenylprop-2-enal$\\$ Common name: $\beta$-Pheynolacrolein $\\$ (v)$\\$ $IUPAC$ name: Cyclopentanecarbaldehyde$\\$ (vi)$ PhCOPh$$\\$ $IUPAC$ name: Diphenylmethanone$\\$ Common name: Benzophenone

5   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 formaldehyde

Solution :

6   Predict the products formed when cyclohexanecarbaldehyde reacts with following reagents.$\\$ (i) $PhMgBr$ and then $H _3 O ^+$ $\\$ (ii)Tollens’ reagent $\\$ (iii) Semicarbazide and weak acid$\\$ (iv)Excess ethanol and acid $\\$ (v) Zinc amalgam and dilute hydrochloric acid

Solution :

7   Which of the following compounds would undergo aldol condensation, which the Cannizzaro reaction and which neither? Write the structures of the expected products of aldol condensation and Cannizzaro reaction. $\\$ (i) Methanal $\\$ (ii)$ 2-$Methylpentanal $\\$ (iii) Benzaldehyde $\\$ (iv) Benzophenone $\\$ (v) Cyclohexanone $\\$ (vi)$ 1-$Phenylpropanone $\\$ (vii) Phenylacetaldehyde $\\$ (viii) Butan$-1-$ol $\\$ (ix)$ 2, 2-$Dimethylbutanal $\\$

Solution :

Aldehydes and ketones having at least one $\alpha-$hydrogen undergo aldol condensation. The compounds$\\$ (ii) $2-$methylpentanal,$\\$ (v) cyclohexanone, $\\$ (vi) $1-$phenylpropanone, and $\\$ (vii)phenylacetaldehyde $\\$ contain one or more $\alpha -$hydrogen atoms. Therefore, these undergo aldol condensation.$\\$ Aldehydes having no $\alpha -$hydrogen atoms undergo Cannizzaro reactions. The compounds$\\$ (i)Methanal,$\\$ (iii) Benzaldehyde, and$\\$ (ix)$ 2, 2-$dimethylbutanal$\\$ do not have any $\alpha -$ hydrogen.$\\$ Therefore, these undergo cannizzaro reactions. Compound$\\$ (iv) Benzophenone$\\$ is a ketone having no $\alpha -$hydrogen atom and compound $\\$(viii) Butan$-1-$ol $\\$ is an alcohol.$\\$ Hence, these compounds do not undergo either aldol condensation or cannizzaro reactions.$\\$ Aldol condensation (ii)

8   How will you convert ethanal into the following compounds?$\\$ (i) Butane$-1, 3-$diol (ii) But$-2-$enal (iii) But$-2-4$enoic acid

Solution :

(i) On treatment with dilute alkali, ethanal produces 3-hydroxybutanal gives butane-1, 3-diol on reduction.

(ii) On treatment with dilute alkali, ethanal gives 3-hydroxybutanal which on heating produces but-2-enal.

(iii) When treated with Tollen’s reagent, But-2-enal produced in the above reaction produces but-2-enoic acid.

9   Write structural formulas and names of four possible aldol condensation products from propanal and butanal. In each case, indicate which aldehyde acts as nucleophile and which as electrophile.

Solution :

(i) Taking two molecules of propanal, one which acts as a nucleophile and the other as an electrophile.

(ii) Taking two molecules of butanal, one which acts as a nucleophile and the other as an electrophile.

(iii) Taking one molecule each of propanal and butanal in which propanal acts as a nucleophile and butanal acts as an electrophile.

(iv) Taking one molecule each of propanal and butanal in which propanal acts as an electrophile and butanal acts as a nucleophile.

10   An organic compound with the molecular formula $ C_ 9 H_{ 10} O$ forms $2, 4-DNP$ derivative, reduces Tollens’ reagent and undergoes Cannizzaro reaction. On vigorous oxidation, it gives $1, 2-$benzenedicarboxylic acid. Identify the compound.

Solution :

It is given that the compound (with molecular formula $C_ 9 H _{10 }O $) forms $2, 4-DNP$ derivative and reduces Tollen’s reagent. Therefore, the given compound must be an aldehyde. Again, the compound undergoes cannizzaro reaction and on oxidation gives $1,2-$benzenedicarboxylic acid. Therefore, the $-CHO$ group is directly attached to a benzene ring and this benzaldehyde is ortho-substituted. Hence, the compound is $2-$ethylbenzaldehyde.

The given reactions can be explained by the following equations.