Question

Which of the following aldehydes does not under go Cannizzaro's reaction? (A) propanal (B) trichloro acetaldehyde (C) trimethyl acetaldehyde (D) benzaldehyde. Please explain.

Answer 1

All but (A) propanal will undergo the Cannizzaro reaction.

Explanation:

The Cannizzaro reaction is the disproportionation in concentrated base of an aldehyde lacking an α-hydrogen atom to form a carboxylic acid and a primary alcohol.

The overall equation for the reaction is

`underbrace("2RCHO")_color(red)("aldehyde") stackrelcolor(blue)("1. OH"^"-"; "2. Acid work-up"color(white)(mm))(→) underbrace("RCH"_2"OH")_color(red)("1° alcohol") + underbrace("RCOOH")_color(red)("carboxylic acid")`
`stackrelcolor(blue)("R" = "alkyl, aryl (no α-H)")(color(white)(mmm→)`

(A) Propanal

`"CH"_3"C"color(red)("H")_2"CHO"`

Propanal has two α-hydrogens, so it will not undergo the Cannizzaro reaction.

(B) Trichloroacetaldehyde

`"Cl"_3"CCHO"`

Trichloroacetaldehyde has no α-hydrogens, so it will undergo the Cannizzaro reaction.

`underbrace("2Cl"_3"CCHO")_color(red)("trichloroacetaldehyde") → underbrace("Cl"_3"CCH"_2"OH")_color(red)("1,1,1-trichloroethanol") + underbrace("Cl"_3"CCOOH")_color(red)("trichloroacetic acid")`

(C) 2,2-Dimethylpropanal

`("CH"_3)_3"CCHO"`

2,2-Dimethylpropanal has no α-hydrogens, so it will undergo the Cannizzaro reaction.

`underbrace(2("CH"_3)_3"CCHO")_color(red)("2,2-dimethylpropanal") → underbrace(("CH"_3)_3"CCH"_2"OH")_color(red)("2,2-dimethylpropan-1-ol") + underbrace(("CH"_3)_3"CCOOH")_color(red)("2,2-dimethylpropanoic acid")`

(D) Benzaldehyde

`"C"_6"H"_5"CHO"`

Benzaldehyde has no α-hydrogens, so it will undergo the Cannizzaro reaction.

`underbrace("2C"_6"H"_5"CHO")_color(red)("benzaldehyde") → underbrace("C"_6"H"_5"CH"_2"OH")_color(red)("benzyl alcohol") + underbrace("C"_6"H"_5"COOH")_color(red)("benzoic acid")`