SN1 or SN2 - classifying rxns

[am24]

Can you tell me if I've categorized these reactions correctly? It is quite possible that I didn't, so I'd love to hear from you!

 

SN1:

NaI + n-BuBr

NaI + t-BuBr

NaI + n-BuCl

AgNO3 + n-BuCl

AgNO3 + t-BuCl

AgNO3 + n-BuBr

AgNO3 + n-BuI

 

SN2:

NaI + i-BuBr

NaI + s-BuBr

AgNO3 + s-BuCl

 

I was unsure of how to approach classifying the "normal" n substrates, so any help would be appreciated! Thank you so much for helping

[big314mp]

"n" means a straight chain alkane (it stands for "normal").

 

Think about why a tertiary carbon goes through SN1, then go back and consider what is different between a tertiary carbon group and a "normal" carbon group.

[lavoisier]

Hi am24,

sometimes the boundary between SN1 and SN2 is a bit blurred. These exercises are OK for learning the concept, but in practice my advice is don't be too dogmatic about it.

 

The basic message I think you need to grasp is that all the conditions that make the carbocation more stable favour the SN1 mechanism, and all those that make it more unstable favour SN2.

 

It is known that carbocations can be stabilized by field effect (e.g. alkyl substituents - that's why tert-butyl cation is more stable than isopropyl cation) and by resonance (e.g. benzyl cation is more stable than cyclopropylmethyl cation).

The better the leaving group, the easier the formation of the carbocation. In general, I->Br->Cl- for alkyl halides.

Adding Ag+ favours the formation of the carbocation by making the halogen a better leaving group (because it can form a strong bond with the metal).

In addition to that, the solvent can form interactions that stabilize the carbocations. And temperature plays a role too.

 

Therefore, just writing down any two reagents and asking for a definite prediction of the mechanism is a bit pretentious. But there you go, I'm afraid you'll have to live with it as long as you're a student.