Question

Use Table 24-2 to predict the elution order of the following compounds from columns containing (a) poly (dimethyl siloxane), (b) (diphenyl)0.35(dimethyl)0.65polysiloxane, and (c) poly (ethylene glycol): hexane, heptane, octane, benzene, butanol, 2-pentanone.

Short answer

a)hexane, benzene=butanol, 2-pentanone, heptane, octane

b)hexane, heptane, butanol =2-pentanone, benzene, octane

c) hexane, heptane, octane, benzene, 2-pentanone, butanol

Step-by-step solution

 a) poly (dimethyl siloxane)

Retention indexes for straight chain alkanes are fixed and do not depend on the stationary phase. From Table 24-2, the values of the retention index of the given compounds are:

- hexane =600

- heptane =700

- octane =800

The retention index determines the elution order of the compounds; the order is identified based from the lowest to highest value.

Determine the retention indexes of the other given compounds for this stationary phase.

- benzene =651

- butanol = 651

- 2-pentanone =667

Thus, the elution order is:

hexane, benzene=butanol, 2-pentanone, heptane, octane

Step 2:b) ( diphenyl)0.35(dimethyl)0.65 polysiloxane

Determine the retention indexes of the other given compounds for this stationary phase.

- benzene =746

- butanol =733

- 2-pentanone =733

Thus, the elution order is:

hexane, heptane, butanol =2-pentanone, benzene, octane

  Step 3: c) poly (ethylene glycol)

Determine the retention indexes of the other given compounds for this stationary phase.

- benzene =963

- butanol = 1158

- 2 -pentanone =998

Thus, the elution order is:

hexane, heptane, octane, benzene, 2-pentanone, butanol

final answer

a)hexane, benzene=butanol, 2-pentanone, heptane, octane

b)hexane, heptane, butanol =2-pentanone, benzene, octane

c) hexane, heptane, octane, benzene, 2-pentanone, butanol