Chapter 8: Aldehydes, Ketones, and Carboxylic Acids

(i) Cyanohydrin:

Cyanohydrin is an organic compound with the formula RR’C(OH)CN, where alkyl or aryl groups may be R and R ‘.

In the presence of excess sodium cyanide (NaCN) as a precursor to using cyanohydrin, aldehydes and ketones react with hydrogen cyanide (HCN). Such reactions are classified as reactions to cyanohydrins.

The useful synthetic intermediates are Cyanohydrins.

(ii) Acetal:

The gem-dialkoxy alkanes at the terminal carbon atom consisting of two groups of alkoxy are called Acetal. One bond is connected to an alkyl group, and the other to a hydrogen atom.

Hemiacetals are formed in the presence of dry HCl gas when aldehydes are treated with two monohydric alcohol equivalents. In addition, hemiacetals react with another alcohol molecule to yield acetal.

(iii) Semicarbazone:

Semicarbazone comes from aldehydes and ketones and is formed by the condensation reaction between a ketone or aldehyde and semicarbazide.

Semicarbarzones serve to identify and characterize aldehydes and ketones.

(iv) Aldol:

An aldol is a β-hydroxy aldehyde or ketone. It is produced in the presence of a base by the condensation reaction of two molecules of the same or a single molecule, each of two different aldehydes or ketones.

(v) Hemiacetal:

Hemiacetals are α−alkoxyalcohols

The general structure of a hemiacetal.

In the presence of dry HCl gas, aldehyde reacts with one single molecule of monohydrated alcohol.

(vi) Oxime:

General formula – RR′CNOH

Where R is an organic side chain and R′ is either an organic side or hydrogen side chain. If R′ is an organic side chain, then ketoxime is known, and if R′ is H, it is called aldoxime.

Aldehydes or ketones form oximes when treated with hydroxylamine in a weakly acidic medium.

(vii) Ketal:

Ketals are gem-dialkoxyalkanes in which one carbon atom is comprised of two groups of alkoxy within the chain. The other two carbon atom bonds are related to two alkyl groups.

A cyclic product is produced in the presence of dry HCl gas, called ethylene glycol ketals when ketones react with ethylene glycol.

(viii) Imine:

Imines are chemical compounds which have a double bond between carbon-nitrogen.

The reaction of aldehydes and ketones with ammonia or 1º-amines forms imine derivatives

(ix) 2,4−DNP−derivative:

2, 4-dinitrophenylhydrazone are 2,4−DNP−derivatives which are produced in a weakly acidic medium when aldehydes or ketones react with 2,4−dinitrophenylhydrazine.

The 2,4−DNP derivatives are used to identify and characterise aldehydes and ketones.

(x) Schiff’s base:

Schiff’s base is a chemical compound that contains a double bond between carbon-nitrogen and the nitrogen atom connected to an aryl or alkyl group-but, not hydrogen. They have the general R₁R₂C formula= NR₃. Thus, it’s an imine. It’s named for Hugo Schiff, a scientist.

Aldehydes and ketones in the presence of a residue of acid on treatment with primary aliphatic or aromatic amines yield a base of a Schiff.

(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

(i) CH₃CO(CH₂)₄CH₃

IUPAC name: Heptan-2-one

Common name: Methyl pentyl ketone

(ii) CH₃CH₂CHBrCH₂CH(CH₃)CHO

IUPAC name: 4-Bromo-2-methylhaxanal

Common name: (γ-Bromo-α-methyl-caproaldehyde)

(iii) CH₃(CH₂)₅CHO

IUPAC name: Heptanal

(iv) Ph-CH=CH-CHO

IUPAC name: 3-phenylprop-2-enal

Common name: β-Pheynolacrolein

(v)

IUPAC name: Cyclopentanecarbaldehyde

(vi) PhCOPh

IUPAC name: Diphenylmethanone

Common name: Benzophenone

Solution coming soon.

Aldehydes and ketones having at least one α-hydrogen undergo aldol condensation. The compounds (ii) 2−methylpentanal, (v) cyclohexanone, (vi) 1-phenylpropanone, and (vii) phenylacetaldehyde contain one or more α-hydrogen atoms. Therefore, these undergo aldol condensation.

Aldehydes having no α-hydrogen atoms undergo Cannizzaro reactions. The compounds (i) Methanal, (iii) Benzaldehyde, and (ix) 2, 2-dimethylbutanal do not have any αhydrogen.

Therefore, these undergo Cannizzaro reactions.

Compound (iv) Benzophenone is a ketone having no α-hydrogen atom, and compound (viii) Butan-1-ol is alcohol. Hence, these compounds do not undergo either aldol condensation or Cannizzaro reactions.

(i) Ethanal produces 3-hydroxybutanal giving butane-1, 3-diol on reduction when it is treated with dilute alkali.

(ii) Ethanal gives 3-hydroxybutanal which on heating produces but-2-enal when it is treated with dilute alkali.

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

(i) One molecule of propanal acts as a nucleophile, and the other molecule acts 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 act 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.

The compound having molecular formula C₉H₁₀O forms a 2,4-DNP derivative and reduces Tollen’s reagent. Therefore, the given compound must be an aldehyde. Again, the compound gives 1, 2 benzene dicarboxylic acids and undergoes a Cannizzaro reaction followed by oxidation. Therefore, the −CHO group is directly attached to a benzene ring, and this benzaldehyde is ortho-substituted. Hence, the compound is found to be 2-ethylbenzaldehyde.

The given reactions can be explained by the following equations.