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

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")#