Explain: 1.Benzyl carbonium ion is more stable than ethyl carbonium ion. 2.Out of BF3 and BCL3, which is a stronger lewis acid? 3. which is stronger Lewis acid: chcl3 or chf3? What is allyl cation.? why it is more stable than ethyl cation?
Answers
1) Benzyl carbonium ion is more stable than ethyl carbonium ion because of resonance in benzyl carbonium ion.
2) Out of these two, BF3 is more acidic than BCl3. The strength of a Lewis acid is the measure of the ability to attract a lone pair of electron from the molecule that is behaving as lewis base. Fluorine is more electronegative than chlorine so they will withdraw electron density from boron atom leaving it more positive. On this basis we would expect BF3 to be a stronger lewis acid.
3) Though F is more electronegative than Cl, Therefore I effect of F would be more and CHF3 would be more acidic. But this is not the correct explanation as M effect also plays role and we know that M effect is considered before I effect. If we compare the conjugate base of both of them C-Cl3 and C-F3, Cl has vacant 3d orbital whereas F doesn't. Therefore Cl can exert M effect due to availability of vacant d orbital but F cannot. M effect stabilizes the negative charge and increases acidic strength. So CHCl3 is more acidic than CHF3
4) Allyl cation: The allyl cation is resonance stabilized species
CH2==CH—CH2+ <-->CH2+ —CH= CH2
It is more stable than ethyl cation because in allyl cation, resonance happens which makes it more stable.