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Alkenes, for the most part (unless there is a phenyl group or highly ionising solvent) undergo almost exclusively anti addition of bromine. However, under similar conditions alkynes, although undergo preferential anti-addition, it is not as exclusive as alkenes. Even with electron withdrawing substituents like two $\ce{COOH}$, only 70% is anti. (Peter Sykes, page 182) Why?

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    $\begingroup$ I've checked two of my books but I didn't find anything. If I could make an assumption I would say that maybe the mechanism followed by alkynes differentiates from the one followed by alkenes? In alkenes' mechanism a three-membered ring is formed among one Br atom and the two double bonded carbons thus leading the other Br to bond with the carbons from the other side of the bond to create an anti-bromination $\endgroup$ – Αντώνιος Κελεσίδης Aug 13 '17 at 22:15
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From what I know of the mechanism, Addition of Br2 takes place in the following mechanismenter image description here

For an alkyne, the intermediate would not be stable due to the ring strain on 3 member ring with a double bond in it, thus reducing the rate of reaction.

Mechanism referred from: https://en.wikipedia.org/wiki/Halogen_addition_reaction

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One factor that can be responsible for lesser extent of anti brominationot of alkynes is may be to do with intermediate formed during the reaction - cyclic bromonium ion.

In case of alkyne, the intermediate consists of 3 membered ring including the halogen. The ring has a double bond thereby increasing the angle strain in the ring. This makes alkynes less reactive.

enter image description here

In alkenes, the cyclic intermediate does not have a double bond in the ring and so there is far less angle strain as compared to the one in alkynes. Hence they are more likely to undergo bromination i.e. to a far greater extent as compared to alkynes.

enter image description here

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In alkynes, the bonding arrangement is linear (180 degrees) and as an advantage of this fact, they do not suffer the effect of steric hindrance as do alkene or more specifically bonding arrangements that contain only 1 set of Pi electrons. Also, electrophilic addition reaction mechanisms tend to initiate by forming an intermediate-complex with the Pi electrons being shared in the substrate, and as a result, the nucleophile has 2 sides of electrophilic attack in alkenes compared to alkenes in which there is only one. That being said, anti-arrangement will dominate in the reaction regardless due to the steric repulsion between the two Bromine atoms, however there are mechanisms and methods of achieving a greater percentage of syn products such as using a inorganic catalyst like Palladium.

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