Chapter 9, Problem 1Q

Interpretation Introduction

Interpretation: The order of elution of a mixture of triphenylmethanol, biphenyl, benzoic acid, and methyl benzoate from an alumina column should be predicted.

Concept introduction: A separation technique used for the segregation of components of chemical mixture based on the difference between the adsorption of different components in stationary and mobile phase is called column chromatography.

It is used for purification or segregation of compounds. It consists of two phases namely:

1. Stationary phase (solid)

2. Mobile phase (generally liquid)

Silica gel is commonly used as the stationary phase.

Elution is the process of extraction of one component from another with the help of the solvent. Eluent is the carrier part of mobile phase. Eluate is the material that comes up through the chromatogram.

Answer to Problem 1Q

The order of elution of the compounds in the given mixture is,

  $\text{Biphenyl}<\text{Methyl benzoate}<\text{Triphenylmethanol}<\text{Benzoic acid}$ .

Explanation of Solution

The structures of the given compounds are as follows:

  

Triphenylmethanol has $-\text{OH}$ group, benzoic acid has $-\text{COOH}$ group, methyl benzoate has $-\text{COOR}$ group and biphenyl is an aromatic hydrocarbon. Alumina $\left({\text{Al}}_2{\text{O}}_3\right)$ is a polar adsorbent. The mobility of the less polar compound is more than that of the more polar compound because such compound will be less adsorbed on the polar stationary phase. Polar compound will spend most of the time in stationary phase and will be eluted last. So the elution of less polar compound occurs first, followed by more polar compound.

The order of polarity of the given compounds is as follows:

  ${\text{C}}_{12}{\text{H}}_{10}<\text{COOR}<\text{OH}<\text{COOH}$

Biphenyl is the least polar and therefore it is eluted first, followed by methyl benzoate, triphenylmethanol, and benzoic acid.

So the order of elution of the mixture of the given compounds is as follows:

  $\text{Biphenyl}<\text{Methyl benzoate}<\text{Triphenylmethanol}<\text{Benzoic acid}$