Question

Predict the characteristic infrared absorptions of the functional groups in the following molecules.

(a) cyclopentene

(b) hexan-2-ol

(c) hexan-2-one

(d) hex-1-yne

(e) dimethylamine

(f) hexanoic acid

(g) hexanenitrile

(h) methyl acetate

(i) hexanamide.

Short answer

Answer

a. Cyclopentene has only C-H stretch due to double bond which shows IR absorption at around 3000-2800 cm^-1.

b. In Hexan-2-ol, alcohol is present. Hence, it shows IR absorption at around 3700-3500 cm^-1.

c. In Hexan-2-one, aliphatic ketone is present. Hence, it shows IR absorption at around 1725-1705 cm^-1.

d. Hex-1-yne shows presence of triple bond and therefore, it shows a weak band at 2100cm^-1.

e. Dimethylamine has amine group and it shows IR absorption at around 3500-3300 cm^-1 due to N-H stretching.

f. Hexanoiccid has carboxylic acid group and hence, it shows IR absorption band at 1720cm^-1. Due to the presence of –OH and CH, streching peaks at 2900cm^-1 are observed.

g. Hexanenitrile has –CN group and hence, it shows absorption at 2200-2300cm^-1.

h. Methylacetate has ester group and it shows absorption at 1750-1735cm^-1.

i. Hexanamide has amide group and it shows absorption at 1690-1730cm^-1.

Step-by-step solution

 IR spectrum characteristic peaks

The IR spectrum is highly helpful in predicting the functional group present in the given organic molecule. Every functional group has its own characheristic IR frequency.

 Explanation

a) Cyclopentene only has C-H stretch due to double bond which shows IR absorption at around 3000-2800 cm^-1.

b) In Hexan-2-ol, alcohol is present. Hence, it shows IR absorption at around 3700-3500 cm^-1.

c) In Hexan-2-one, aliphatic ketone is present and hence, it shows IR absorption at around 1725-1705 cm^-1.

d) Hex-1-yne shows presence of triple bonds and therefore, it shows weak band at 2100 cm^-1.

e) Dimethylamine has amine group and shows IR absorption at around 3500-3300 cm^-1 due to N-H stretching.

f) Hexanoiccid has carboxylic acid group ahd hence, it shows IR absorption band at 1720cm^-1. Due to the presence of –OH and CH streching peaks at 2900 cm^-1 is observed.

g) Hexanenitrile has –CN group and hence, it shows absorption at 2200-2300cm^-1.

h) Methylacetate has ester group and it shows absorption at 1750-1735cm^-1.

i)Hexanamide has amide group and it shows absorption at 1690-1730cm^-1.