The dehydration of 2.3-dimethyl-2-butanol with phosphoric acid yields two alkene

Question

The dehydration of 2.3-dimethyl-2-butanol with phosphoric acid yields two alkenes: A GC analysis of the product mixture using a six-foot, SE-30 packed column at 110 Degree C afforded the following chromatogram: The compound responsible for the major peak (RT = 4.88) was isolated and an IR spectroscopic analysis done: Based on the experimental data (G.C. and IR), which alkene is the major product? Defend your assignment. Calculate the percent composition of the compound responsible for the peak at 4.88. Assume the response factors for the two compounds are the same. Give three distinct changes that could be made to increase the resolution of the two peaks in the gas chromatogram.

Explanation / Answer

a) From Ir analysis we know that

tetra substituted alkenes do not show peaks near 800- 950 cm-1 (below 1000) which are also missing in the given IR.

so the alkene which is the major product is 2,3-dimethyl 2-butene

b) we know that the area under a gas chromatograph peak is proportional to the amount (moles) of the compound eluted.

Total area = 2209 + 53655 = 55864

% of composition of compound responsible for peak at 4.88 = 53655 X 100 / 55864 = 96.05 %

c) we can increase the resolution by

by changing the Temperature

using longer column

Decreasing the flow rate might help

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