sn1 + non-basic nucleophile

[lovemcat]

why does sn1 favour a non-basic nucleophile?

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[MD Odyssey]

why does sn1 favour a non-basic nucleophile?

By this, I assume that you're asking why a non-basic nucleophile favors Sn1 over an E1 reaction.

The reason is pretty simple. After the leaving groups is gone, the reaction becomes a question of which is more likely - nucleophilic attack on the carbocation left behind or deprotonation of a proton from an adjacent carbon. If you have a good nucleophile present, then obviously nucleophilic attack is going to predominate. However, if you have a basic compound around, then it is most likely going to do what bases do: deprotonate the adjacent carbon and form a double bond instead.

In reality, for many substrates, both elimination and substitution reactions are able to occur to some degree, so you definitely see some inadvertent side products.

Does this help?

 

[whiteshadodw]

😕 SN1 rxns are often accompanied by E1 rxns

If you want to preferentially do SN1 rxns (over E1), then you would want to use a good a nucleophile that is not a good base. If you wanted to favor E1 rxns over SN1, you would want to use a strong base that is a bad nucleophile.

Does that help? (Maybe I don't understand what you are asking)

that's not exactly right since good nucleophiles tend to be good bases. but you can avoid an elimination by using a smaller nucleophile and messing with the temperature. big bulky bases aren't good nucleophiles (great for E1 and E2), small bases are good nucleophiles (great for SN1/SN2).

essentially:

1) can't be good nucleophile without being a good base
2) can't be a good nucleophile if you're fat as hell

 

[lovemcat]

hey everyone,

thank you so much for your replies! i guess i should have been a bit more clearer :S I was confused with why non-basic nucleophiles favor unimolecular substitutions (i.e. non-basic nucleophiles like sn1 more than sn2)?

 

[lovemcat]

that's not exactly right since good nucleophiles tend to be good bases. but you can avoid an elimination by using a smaller nucleophile and messing with the temperature. big bulky bases aren't good nucleophiles (great for E1 and E2), small bases are good nucleophiles (great for SN1/SN2).

essentially:

1) can't be good nucleophile without being a good base
2) can't be a good nucleophile if you're fat as hell

btw lol @ whiteshadodw - love this mnemonic!