Electrophilic addition of bromine

[Clari]

Hi! I am stuck in revising the electrophilic addition of bromine across the alkene molecule... There are two sentences which really confuse me, I hope you can help me.

When a bromine molecule approaches the pi electron cloud of the C=C bond...the bromide ion then attacks the pi complex nucleophilically from a side opposite to the first bromine. This becomes known as a trans attack and the result is a dibromo product.

My question is why the bromide ion have to attach to the pi complex from a different side and not the same side so that leads to the cis attack?

Thanks in advance!

 

[GCT]

Due to steric effects, you'll want to observe the electron density of this reaction.

 

[ravenprp]

Hi there! The reason why the bromide ion attacks the pi complex from a different side is due to the principle of stereoselectivity. In electrophilic addition reactions, the electrophile (in this case, the bromine molecule) typically adds to the alkene in a way that maximizes stability and avoids formation of high-energy intermediates. In the case of bromine addition, a trans attack allows for better overlap of the pi bond with the p-orbital of the bromine, leading to a more stable intermediate. A cis attack, on the other hand, would result in steric hindrance and a less stable intermediate. Therefore, the bromide ion prefers to attack from the opposite side, resulting in a trans product. I hope this helps clarify your confusion! Let me know if you have any other questions.