54E - Organic Chemistry (Mcmurry)

Question

What spectroscopic method could you use to distinguish among the following three isomeric acids? Tell what characteristic features you would expect for each acid

a)  ${\mathbf{CH}}_{\mathbf{3}}{\left({\mathrm{CH}}_2\right)}_{\mathbf{3}}{\mathbf{CO}}_{\mathbf{2}}\mathbf{H}$ - Pentanoic acid

b)   ${\left({\mathrm{CH}}_3\right)}_{\mathbf{2}}{\mathbf{CHCH}}_{\mathbf{2}}{\mathbf{CO}}_{\mathbf{2}}\mathbf{H}$- 3-Methylbutanoic acid

c)  ${\left({\mathrm{CH}}_3\right)}_{\mathbf{3}}{\mathbf{CCO}}_{\mathbf{2}}\mathbf{H}$ -2,2-Dimethylpropanoic acid

Step-by-Step Solution

Answer

The three isomeric acids, Pentanoic acid, 3-methylbutanoic acid and 2,2-dimethylpopanoic acid, can be distinguished using ¹H NMR spectrum.

The signal for methyl protons in pentanoic acid will be a triplet integrating to three protons

The signal for methyl protons in 3-methylbutanoic acid will be a doublet integrating to six protons

The signal for methyl protons in 2, 2-dimethylpropanoic acid will be a singlet integrating to nine protons

Step 1: Definition of compounds

Any substance composed of identical molecules consisting of atoms of two or more chemical elements is defined as chemical compound

Step 2: Explanations

Pentanoic acid has only one methyl group and the carbon adjacent to it has two protons. Thus the methyl protons signal will integrate to three protons and the signal will be observed as a triplet around $1.0\mathrm{\delta }$ 

3-methylbutanoic acid has two methyl groups and the carbon adjacent to it has only one proton. Thus the methyl signal will integrate to six protons and the signal will be observed as a doublet around $1.0\mathrm{\delta }$ 

2, 2-dimethylpropanoic acid has three methyl groups and the carbon adjacent to it has no protons. Thus the methyl protons signal will integrate to nine protons and the signal will be observed as a singlet around $1.0\mathrm{\delta }$