izaakha
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Hi.
The question is:
The molecule shown below is reacted with hydrogen bromide:

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Which of the following are the products of this reaction?

A-4-bromo-3,4-dimethylpentane and 3-bromo-3,4-dimethylpentane
B-3-bromo-2,3-dimethylpentane and 2-bromo-2,3-dimethylpentane
C-3-bromo-2-dimethylpentane and 2-bromo-2-dimethylpentane
D-2-bromo-3,4-dimethylpentane and 3-bromo-2,3-dimethylpentane


The correct answer is B.

I don’t understand which one of the carbon atoms within the double bond will receive the H or the Br. I know that the Markovnikov's Rule says that the Hydrogen is attached to the carbon atom with the most amount of hydrogens on it; but don’t they both have the same amount of hydrogens on the carbon atoms here??

Please could someone tell me which carbon receives what????
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Kian Stevens
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There's no image to go with the question, and the attachment doesn't work
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izaakha
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(Original post by Kian Stevens)
There's no image to go with the question, and the attachment doesn't work
Hi, sorry about that, I was trying to upload the image from college - I have uploaded it again, I think you will be able to see it now.
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Kian Stevens
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From a general view:
This question is mostly whether you can properly name molecules or not; A, C and D are all instantly recognisable as being incorrect combinations, and so this leaves B as the only correct pair

From a mechanistic view:
Markovnikov's rule essentially states that the electrophile (H+ in this case) will attack the alkene carbon with the least hydrogen atoms, in an attempt to form the most stable carbocation

However, both alkene carbons have no hydrogen atoms attached... This therefore means that both potential carbocations will be tertiary, and also of roughly equal stability (ignoring inductive effects), meaning that the electrophile will attack both alkene carbons

Because of this, it can be assumed that the two products in answer B will be produced in roughly equal proportion
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izaakha
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(Original post by Kian Stevens)
From a general view:
This question is mostly whether you can properly name molecules or not; A, C and D are all instantly recognisable as being incorrect combinations, and so this leaves B as the only correct pair

From a mechanistic view:
Markovnikov's rule essentially states that the electrophile (H+ in this case) will attack the alkene carbon with the least hydrogen atoms, in an attempt to form the most stable carbocation

However, both alkene carbons have no hydrogen atoms attached... This therefore means that both potential carbocations will be tertiary, and also of roughly equal stability (ignoring inductive effects), meaning that the electrophile will attack both alkene carbons

Because of this, it can be assumed that the two products in answer B will be produced in roughly equal proportion
Thank you, I understand now - basically, I had to rule out the other options.
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