Br2/CCl4 stereospecificity vs. Cl2/CCl4?!????!!

[krvstak]

Hey guys....

It was to my knowledge that trans addition is just that. TRANS. No matter what. But then I came across some old organic chemistry notes (specifically a practice problem with the correct answer circled) and saw that in an alkene reaction with Br2/CCl4, stereospecificity comes into play... I just needed some clarification in comparison to Cl2/CCl4 which is said to not have stereospecificity??

From my understanding now based on this...
trans alkene & Br2/CCl4 ---> trans dibromo alkane; a meso compound
cis alkene & Br2/CCl4 ---> cis dibromo alkane; either R,R or S,S

trans alkene & Cl2/CCl4 ---> trans dichloro alkane
cis alkene & Cl2/CCl4 ---> trans dichloro alkane????????

Also... would this "rule" have any effect on alkynes at all?

Are there any other reactions that do this? Diels Alder??

PLEASE clarify on any & all of the information above as I am incredibly stumped on this and everyone I've asked so far thinks I'm a bit insane!!


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[Syxx]

Trans and cis refer to alkenes or cyclic alkanes. In this case, your products are straight chain alkanes so you would not refer to them as cis or trans. The halogens are added anti across the double bond for either a trans or cis alkene starting reagent. It's anti because they are added to opposite faces of the double bond due to the formation of the three member ring bromonium or chloronium ion intermediate.

Other than calling the products trans or cis, you steriochemistry was correct for both brominations.
Bromine and chlorine will show the same sterioselectivity in this reaction.

If you react an alkyne with X2 you will get the trans product after one addition. A second addition to the alkene is then possible. Halogenating a cyclopentene will give the trans cyclopentane.

Oxymercation and epoxidation of alkenes have similar mechanisms and show the same sterioselectivity. These are also anti additions and go through three member ring intermediates similar to the halogenation addition. Epoxidation will give products like you suggested with trans-alkenes giving trans-epoxides and cis-alkenes giving cis-epoxides.