Try it on paper first. A hint, the key idea and the worked solution are here when you need them.
For a secondary bromide in ethanol, elimination has a higher activation enthalpy than substitution by and a more favourable activation entropy by . These are values chosen for this problem. The ratio of the two rate constants is , with and . Find the temperature at which the two routes are equally fast, then the percentage of elimination at and at .
Elimination takes a beta hydrogen and the halogen away and leaves a double bond. The more substituted alkene is the main product (Saytzeff). Each double bond with different groups on both ends counts as cis and trans. Alcoholic KOH and high temperature favour elimination. The ratio of two rates is .
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Crossover. gives .
Step 2. At 298 K. . Then . The share is .
Step 3. At 350 K. . Then . The share is .
Answer: , at and at .
Check. Below the share is under and above it is over . Both results agree.
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