Textbooks quote typical relative SN2 rates of 30 for a methyl bromide, 1 for a primary bromide and…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Textbooks quote typical relative rates of for a methyl bromide, for a primary bromide and for a secondary bromide, with the same nucleophile and solvent. They are illustrative values and depend on the case. Under the conditions of an experiment the primary bromide reaches a given conversion in . Use the relative rates to estimate the time that the methyl bromide needs for the same conversion, and the time that the secondary bromide needs, in minutes and in hours.
Need a reminder? The key ideaShow
is one step with the rate law , and it inverts the shape of the carbon. The speed falls from methyl to primary, secondary and tertiary. Rates of reactions of different halides are compared through their rate constants. Time for the same conversion is inversely proportional to the rate constant. Yield calculations use the ratio of the balanced equation.
The worked solution
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.
Plan. For the same conversion, .
Step 1. Methyl. .
Step 2. Secondary. .
Step 3. In hours. .
Answer: for the methyl bromide and , or , for the secondary bromide.
Check. The secondary bromide takes more than a day, as expected for a hindered carbon. A tertiary bromide is slower still, and in practice it reacts by another route.
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