Q 7P - Organic Chemistry (Mcmurry)

Question

Question: What functional groups might the following molecules contain?

(a) A compound with a strong absorption at 1710 ${\mathbf{cm}}^{\mathbf{-}\mathbf{1}}$

(b) A compound with a strong absorption at 1540 ${\mathbf{cm}}^{\mathbf{-}\mathbf{1}}$

(c) A compound with strong absorptions at 1720 ${\mathbf{cm}}^{\mathbf{-}\mathbf{1}}$ and 2500 to 3100  ${\mathbf{cm}}^{\mathbf{-}\mathbf{1}}$

Step-by-Step Solution

Answer

(a) Aldehyde or ketone

(b) Nitro group

(c) Carboxylic acid

Step 1: Absorption spectrum

Absorption means the presence of dark lines in the spectrum of electromagnetic radiation, which is transmitted through a substance indicating absorption at specific wavelengths.

Step 2: Chart for absorption by specific functional group

The given chart for absorption of a particular wavelength by a specific functional group tells which functional group absorbs at which wavelength.

Step 3: Part (a) absorption at 1710 cm - 1

Here compound has a strong absorption at 1710 ${\mathrm{cm}}^{-1}$ which is only present in the case of the carbonyl compound. The carbonyl compound can be aldehyde or ketone. The weak intensity would be observed if it was an aromatic functional group.

Step 4: Part (b) absorption at 1540 cm - 1

Here it can be seen a compound has a strong absorption at 1540 ${\mathrm{cm}}^{-1}$which is seen in the case of the nitro functional group so the functional group is nitro. 

Step 5: Part (c) absorption at 1720 cm - 1 and 2500 to 3100 cm - 1

Here compound is with strong absorptions at 1720 ${\mathrm{cm}}^{-1}$and 2500 to 3100${\mathrm{cm}}^{-1}$ which is seen in the case of carboxylic acid as the carbonyl group of carboxylic acid has absorption at around 1710 ${\mathrm{cm}}^{-1}$ and the hydroxyl group of carboxylic acid has absorption between 2500 to 3100 ${\mathrm{cm}}^{-1}$ .