# Why should epoxides be more reactive at a position where positive charge is stabilised?

In the above reaction, I am told that the nucleophilic chloride anion attacks the trisubstituted carbon because it has the most partial positive character due to its ability to stabilize positive charge.

How does that statement make any sense?

I'm not following the train of "logic," which is as follows:

1. The nucleophile will want to attack the location with the greatest partial positive charge. Okay, Coulomb's law. I'm good with this statement.

2. The carbon with the greatest partial positive charge is the one that can best stabilize positive charge. (?)

3. Alkyl groups are slightly electron-donating; the tertiary carbon will be able to better stabilize partial positive charge.

4. Thus, the chloride anion attacks the tertiary rather than the secondary carbon. I don't see how this follows from the previous assertions. Doesn't stability correspond negatively with reactivity? If so, what is the real reason the chloride anion attacks the tertiary carbon rather than the secondary carbon?

• Essentially the same question: Regioselectivity of acid-catalyzed ring-opening of epoxides – Martin - マーチン Aug 4 '14 at 4:36
• It is important to state, that in asymmetric epoxides, the $\ce{C-O}$ bond lengths are not equal. Upon protonation, this effect is enhanced, giving rise to a weak and a stronger bond that can be broken. This means the carbon that can better stabilise the charge is more weakly bonded to the oxygen. Thus this will be the carbon suffering the nucleophilic attack. – Martin - マーチン Aug 4 '14 at 4:46