Organic Question

Question

Organic Question

abb0t · Answer

"Comparison of several series of solvolysis reactions which proceed via carbocation intermediates reveals that an $\sf \alpha$-cyano group retards the reaction by a factor of about 10$^3$. A $\sf \beta$-cyano group is even more strongly rate-retarding, with the factor being as high as 10$^7$." 

I don't understand why! Why is the $\sf \beta$-cyano is more rate-retarding than the $\sf \alpha$-cyano group??

abb0t · Answer

@aaronq

abb0t · Answer

My bad for all the Q's lately, but my research and other course is taking up more time thnan I have to put in for this course and Im trying to cram the past 5 weeks this week. Lol

aaronq · Answer

no worries man, i honestly don't mind looking at q's, specially when it's stuff i'm not familiar with, like this. I can look into it tmrw because i got 2 midterms today, so hopefully it can wait.

aaronq · Answer

Sorry, i took so long to reply, i found it has to do with the dipole moment of the cyano group and it's "$field~effect$".

"..substituent effects are a combination of resonance and polar effects, individual substituents may have both electron-donating and electron-withdrawing components. For example, the methoxy group is a $\pi$-donor but a $\sigma$-acceptor. As resonance effects are generally dominant in aromatic systems, the overall effect of a methoxy group is electron release (in the ortho and para positions). For other groups, such as NO2 and CN, the resonance and polar effects are reinforcing. The main polar effect seems to be electrostatic (through space) and is sometimes referred to as a field effect, to distinguish it from an inductive effect (through bonds)."

pg 338 Carey, Advanced OC (i found that your q is from this book which I've been reading).

i'm attaching the paper cited in the book

abb0t · Answer

Sorry for the late response. Haha. Yeah i figured it had something to do with resonance and its ability to be a good pi donor, but i think i was just thrown off by the whole alpha/beta  thing, i wasnt clear about that. But thanks!! that article is interesting.