dehydration..

[SaintJude]

Kaplan Subject Test 5-Orgo

Which of the phrases below best describes the kinetics of the dehydration of 3-methyl-3-hexanol in concentrated (85%) phosphoric acid?


independent of concentration
first order in alcohol, first order overall
first order in acid, first order overall
first order in alcohol, second order overall


Answer: B. Watch out for the acid-catalyzed dehydration reaction!

I've learned a lot from doing orgo subjects tests in the past few days. I got too stuck on looking for the "typical" on this one.

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[chiddler]

Here's a question that helped me earlier today:

propene + HBr. First order or second order?

Answer is second order because the slow step involves attack of a proton by the pi bond. Therefore, the slow step involves both the acid proton and the propene.

With your question, the acid protonates the OH which is not the slow step of reaction. Rather, it is the actual dehydration which makes it first order.

always look for slow step.

 

[SaintJude]

hm, it looks like you may have a better grasp of orgo b/c I don't understand what you're saying.

I knew propene + HBr = second order only b/c I could not identify a leaving group .

But I admit my focus on leaving group got me in trouble the first time I saw the OP question. Now I see, that in the presence of a strong acid that dissociates all of its H + protons, the OH will get protonated and become a good leaving group. The loss of the water is thus the "slow" step = first order.