[tezha]

Why in the bromination reaction of unsaturated hydrocarbons, when the bromonium ion is formed, the Br- anion, which acts as a nucleophile, adds to the more saturated carbon of the bromonium ion ring? my textbook says it's because the carbon is more positive, which sounds reasonable enough, however, as far as I know, -R groups are electron donating, so the carbon should be actually less positive, right (would be more stable if it was a carbocation)?

[Schrödinger]

Take into consideration the transition state that is getting stabilized thanks to the +I effect of the R groups... The p-orbitals of the R groups also stabilize the transition state is some cases, I think. And since this is not a carbocation mechanism, we cannot use the carbocation theory.

Please correct me if I am wrong.

[Dan]

I think it's perfectly reasonable to view this as what Clayden & Co call "loose S_N2". This is somewhere between S_N1 and S_N2, see acid catalysed hydrolysis of epoxides. There will be significant weakening of one C-Br bond, resulting in partial +ve charge buildup on one of the carbon atoms, but not completely to the point of a carbocation. There is significant carbocation character in the transition state, such that carbocation arguments like:

Take into consideration the transition state that is getting stabilized thanks to the +I effect of the R groups...

can satisfactorily rationalise and predict the result in many cases.