Question

Order the following compounds in descending order of frequency for the carbonyl band.

1) 2-propanone, methanal, ethanal

2) 2- pentonone, ethyl butanoate, pentanamide 3) cyclohexanone, 2-cyclohexenone, 3-cyclohexenone.

c) Explain why primary amines and primary amides have two absorption bands

d) Explain why the carbonyl band of a carboxylate salt are displaced at frequencies lower than 1710 cm

Answer 1

1. Methanal > Ethannal > 2-propanone

Ethanal and 2-propanone absorb at lower frequency than methanal because of their aditional alkyl group is electron donating (compared to H) and supplies electrons to C=O bond. This electron releasing effect weakens the C=O bond in the ketone and lowers the force constant and the absorption frequency..

2.) Ethyl butanoate > 2-pentanone > Pentamide

amide have least frequency than other carbonyl group containing compounds.

3.) 3-cyclohexenone > Cyclohexanone > 2-cyclohexenone

Saturated carbonyl group have greater frequency than alpha-beta unsaturated carbonyl compound but less than isolated unsaturated carbonyl comoound.

C) Primary amines and amides have two bands in between 3400-3100 cm^-1which are weaker and sharp than alcohol, due to N-H asymmetrical stretching and N-H symmetrical streching.

D) In carboxylate salt the carbonyl C=O are in conjugation with negative charge on another oxygen (O^-) results a partial double bond character in C=O which is weaker than C=O bond, hence lower the frequency.