alkynes - addition of halogens

[aro00oo]

Not really a particular MCAT question or anything, but I'm currently taking the Organic Chemistry sequence offered at my University. I know that during the addition of halogens in alkynes, two equivalent amount of X^2(X; Cl/Br/I) are needed. First addition forms a trans dihalide from the anti-addition. Second addition inevitably forms a tetrahalide, with all four halogens around the original two carbons from the triple bond.

Question is, after the first product is formed, the trans dihalide, wouldn't the reaction not proceed due to the electron withdrawing halogens that are trans on both carbons? The pi bond on the formed alkene would be electron deficient; thus, they wouldn't act as a nucleophile with the second concentration of X^2.


EDIT

well i kinda realized that it may depend on the halogen? particularly only Cl might do that because it's the most electronegative.. not sure. I have a hard time understanding electron density/charge distribution.

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[MD Odyssey]

Not really a particular MCAT question or anything, but I'm currently taking the Organic Chemistry sequence offered at my University. I know that during the addition of halogens in alkynes, two equivalent amount of X^2(X; Cl/Br/I) are needed. First addition forms a trans dihalide from the anti-addition. Second addition inevitably forms a tetrahalide, with all four halogens around the original two carbons from the triple bond.

Question is, after the first product is formed, the trans dihalide, wouldn't the reaction not proceed due to the electron withdrawing halogens that are trans on both carbons? The pi bond on the formed alkene would be electron deficient; thus, they wouldn't act as a nucleophile with the second concentration of X^2.


EDIT

well i kinda realized that it may depend on the halogen? particularly only Cl might do that because it's the most electronegative.. not sure. I have a hard time understanding electron density/charge distribution.

You forget that the electrons in the π-bond are extremely reactive. The electron-withdrawing effect of the chlorine atoms affects the electrophilic carbons, but the π-electrons are still available for bonding.

 

[aro00oo]

You forget that the electrons in the π-bond are extremely reactive. The electron-withdrawing effect of the chlorine atoms affects the electrophilic carbons, but the π-electrons are still available for bonding.

Ooh.. that makes sense. Thanks!