Q12-12-10 - Organic Chemistry (Mcmurry)

Question

Where might the following compounds have IR absorptions?

Step-by-Step Solution

Answer

a. Absorption bands at 1715 \(c{m^{ - 1}}\)and 1640-1680 \(c{m^{ - 1}}\) 

b. Absorption bands at 1730 \(c{m^{ - 1}}\), 2100 - 2260 \(c{m^{ - 1}}\) and 3300 \(c{m^{ - 1}}\). 

c. Absorption bands at 1710-1760 \(c{m^{ - 1}}\), 2500-3100 \(c{m^{ - 1}}\), 3400-3650 \(c{m^{ - 1}}\), 3030 \(c{m^{ - 1}}\) and 1450 - 1600 \(c{m^{ - 1}}\). 

  

Step 1: Absorption spectrum

The absorption spectrum shows the presence of dark lines in the spectrum when transmitted through a substance indicating absorption at specific wavelengths of electromagnetic radiation.

Step 2: Chart for absorption by specific functional group

The given chart shows absorption of particular wavelength radiation by specific functional group.

                              

                                         Functional group absorption band 

Step 3: Part (a) absorption spectrum

Here two absorption bands will be seen. One for ester group at 1715 \(c{m^{ - 1}}\)and one for alkene group \( - C = C - \) at to 1640-1680 \(c{m^{ - 1}}\). The value for ester group absorption is less due to presence next to double bond.

 

Step 4: Part (b) absorption spectrum

Here three absorption bands will be seen. One for aldehyde carbonyl group at 1730 \(c{m^{ - 1}}\), one for alkyne \( - C \equiv C - \)at 2100 - 2260 \(c{m^{ - 1}}\) and one for alkyne \( \equiv C - H\) at 3300 \(c{m^{ - 1}}\). 

 

Step 5: Part (c) absorption spectrum

Here five absorption bands will be seen. One for carbonyl group of carboxylic acid at 1710-1760 \(c{m^{ - 1}}\), one for hydroxyl group in carboxylic acid at 2500-3100 \(c{m^{ - 1}}\), one for hydroxyl group at 3400-3650 \(c{m^{ - 1}}\), two for aromatic ring at 3030 \(c{m^{ - 1}}\) and 1450 - 1600 \(c{m^{ - 1}}\).