Addition of chlorine to styrenes is often accompanied by the formation of 2-chlorostyrene. In some s — Physical Chemistry — Kinetics Chemistry Question
Chlorination of styrenes
Addition of chlorine to styrenes is often accompanied by the formation of 2-chlorostyrene. In some solvents, the formation of solvent-incorporated products is also observed. For example, chlorination of styrene in acetic acid yields a 1-acetoxy-2-chloro derivative. The overall process can be illustrated by the following scheme:
Formation of each product obeys the same rate law: the reaction order is 1 with respect to both styrene and chlorine.
The product distribution during the chlorination of cis-1-phenylpropene at 25oC is given in the Table.
The rate constant of the overall reaction is 1.45 · 10^4 mol–1 dm3 s–1 at 25 oC. What are the rate constants for the formation of 1,2-dichloro and 1-acetoxy-2-chloro adducts and 2-chlorostyrene?
Model Answer
Since all reaction pathways obey the same rate law, the quantity of the product is proportional to the respective rate constant. The overall constant equals the sum of constants for different pathways. Hence
for 1,2-dichloro:
k = 1.45 · 10^4 · 61% / 100 % = 8.8 · 10^3 mol-1 dm3 s-1
for 1-acetoxy-2-dichloro:
k = 1.45 · 10^4 · 30 % / 100 % = 4.4 · 10^3 mol-1 dm3 s-1
for 2-chlorostyrene:
k = 1.45 · 10^4 · 9 % / 100 % = 1.3 · 10^3 mol-1 dm3 s-1
Products of this reaction can be separated by chromatography. If the achiral sorbent is used, the determined number of products in cis-1-phenylpropene + chlorine reaction is 6. Why? What is the determined number of products if the sorbent is chiral?
Model Answer
This reaction is not stereospecific and leads to the formation of diastereomeric addition products in comparable amounts. The following products are obtained (approximate ratios of product quantities at 25oC are given as an illustration):
1,2-dichloro:
enantiomers ~1:3 enantiomers
1-acetoxy-2-chloro:
enantiomers ~1:1 enantiomers
2-chlorostyrene:
~1:3
The achiral sorbent is unable to separate enantiomers, so only 6 different fractions can be obtained. The chiral sorbent allows full separation, so in this case the determined number of products is 10.