AAMC 9r BS 156 Need help

[russianguy13]

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I have a question with AAMC 9r question 156:
156. The students were careful to keep the temperature of the reaction flask under 90°C to prevent:
A) the unreacted cyclohexanol from distilling.
The students did not want unreacted cyclohexanol to be distilled with the product. Thus, A is the best
answer.

B) the product cyclohexene from distilling.
C) the product cyclohexene from decomposing.
D) rearrangement of the carbocation intermediate.

But in the passage it said that the boiling point of cyclohexanol was 156.

 

[AppleRind]

I have a question with AAMC 9r question 156:
156. The students were careful to keep the temperature of the reaction flask under 90°C to prevent:
A) the unreacted cyclohexanol from distilling.
The students did not want unreacted cyclohexanol to be distilled with the product. Thus, A is the best
answer.

B) the product cyclohexene from distilling.
C) the product cyclohexene from decomposing.
D) rearrangement of the carbocation intermediate.

But in the passage it said that the boiling point of cyclohexanol was 156.

Hi, this is my first time posting in the Q&A forum, but I'll take a stab at your question. Feel free to just jump to the section that is in bold at the bottom.

Let us consider the answer choices:

B. If the lowered temperature prevented this, you would not be able to collect the product. I'm also assuming that the boiling point is such that 90 degrees would not prevent it from evaporating.
C. I don't believe there is any data as to when anything will decompose. If it is not given, chances are its not the answer unless you can eliminate the other choices.
D. Rearrangement is of no consequence here if its cyclohexanol. If rearrangement happened, the product would remain the same. Even if rearrangement did happen, the passage would have likely given some information about thermodynamic vs kinetic control.

A. This response makes the most sense because it is the only one whereby it would affect your results vastly. You may think that the bp of the unreacted cyclohexanol is much higher, but dont forget that as temperature increases, the proportion of molecules with enough energy to escape increases, regardless of what that energy might be.

I hope I made sense and if you have further inquiries, don't hesitate to ask.

-AppleRind

 

[minakosmas]

For me, it does not make sense and it is not reasonable enough because the BP of the Cyclohexanol is 160 degrees; so students could let the Boiling Point up to 100 or 110 with no worries about the Cyclohexanol begin vaporization. On the other hand, as the BP of Cyclohexene is 83 C, it could be possible that elevating the temperature over 90 might decompose / degrade the remaining Cyclohexene in the flask. 🙁 🙁 🙁

However, only because we have to go what the AMCAS considered correct !! I can assume that above 90 includes 160 at which Cyclohexanol vaporizes; also I ma put in consideration that above 90 Cycolhene would be already vaporized and there is no worries of its decomposition. 😉

 

[drechie]

Im really bad at Orgo... So in this example we start with an cyclohexonone. Add H3PO4 to it.... and we get cyclohexene. Does the formation of cyclohexene happen in the reaction flask, and then it is heated so it becomes vapor and travels to the "collecting flask?" Also, what is H3PO4 acting as in this reaction? Is it a reducing agent?

Also --- In the wash to neutralize any lingering acid in the collecting flask, why do we use Sodium Bicarbonate with NaCl? Do the two always go together.