SN1 and SN2??

[predental89]

Can someone explain why this is SN2 and not E2? this is driving me crazy!!!


CH3CH2CH2BR + (CH3)3P in (CH3OH) ------>


please help!!!

---

Members don't see this ad.

 

[UnimaasMED]

Can someone explain why this is SN2 and not E2? this is driving me crazy!!!


CH3CH2CH2BR + (CH3)3P in (CH3OH) ------>


please help!!!

its because you have a primary halide. For E2 to happen you need a tertiary or secondary halide. As a rule of thumb. Whenever you see a primary halide it is most probably a SN2 reaction.
Remember, Phosphor and Nitrogen can act as strong nucleophiles. Since you have a primary halide the phosphor can still attack the primary carbon even though it seems a little bulky.

 

[predental89]

Thank You!

 

[DoCt0rPeTe]

Can someone explain why this is SN2 and not E2? this is driving me crazy!!!


CH3CH2CH2BR + (CH3)3P in (CH3OH) ------>


please help!!!

Eh the above response is partially correct.

It is primary, so IF YOU DON'T HAVE A BULKY BASE, its SN2. If you are reacting with something like tert-butoxide however, it is E2.

Also, E2 reactions usually have strong bases. (CH3)3P is a strong Nucleophile, not a strong base.
You should memorize those (NaH, NaOH, NaNH2...basically anything with a - sign. Note that the ones I just named ARE negatively charged, but they are ionically complexed with a metal. So ignore the metal when you see it, and recognize the base with a charge. ie. NaOH is really -OH. But that only works if its a metal in front. ie. CH3OH will not split apart/dissociate in solution, thus it is not -OH, but CH3OH, a weaker base than -OH (or NaOH as it is commonly written).

Also, know the most common nucleophiles- anythign with a P (ie. (CH3)3P in the above rxn) or an S are the most common. Nuceophiles react in SN reactions, and bases react in E reactions.

Weak nucleophiles generally are in SN1 reactions.
Strong nucleophiles are usually in SN2 rxns

Weak Bases E1
Strong Bases E2

Don't just memorize the trend. Know which bases/nucleophiles are strong/weak.

Also know which leaving groups are best. For instance, if the above substrate had an I, would it react faster or slower?

You want to make sure you have a great handle on this for the DAT, as it will DEFINITELY show up.

 

[predental89]

Thank you so much that was very helpful! 🙂I have my second ochem midterm on monday and I'm freaking out about it :/

 

[losinghair]

Eh the above response is partially correct.

It is primary, so IF YOU DON'T HAVE A BULKY BASE, its SN2. If you are reacting with something like tert-butoxide however, it is E2.

Also, E2 reactions usually have strong bases. (CH3)3P is a strong Nucleophile, not a strong base.
You should memorize those (NaH, NaOH, NaNH2...basically anything with a - sign. Note that the ones I just named ARE negatively charged, but they are ionically complexed with a metal. So ignore the metal when you see it, and recognize the base with a charge. ie. NaOH is really -OH. But that only works if its a metal in front. ie. CH3OH will not split apart/dissociate in solution, thus it is not -OH, but CH3OH, a weaker base than -OH (or NaOH as it is commonly written).

Also, know the most common nucleophiles- anythign with a P (ie. (CH3)3P in the above rxn) or an S are the most common. Nuceophiles react in SN reactions, and bases react in E reactions.

Weak nucleophiles generally are in SN1 reactions.
Strong nucleophiles are usually in SN2 rxns

Weak Bases E1
Strong Bases E2

Don't just memorize the trend. Know which bases/nucleophiles are strong/weak.

Also know which leaving groups are best. For instance, if the above substrate had an I, would it react faster or slower?

You want to make sure you have a great handle on this for the DAT, as it will DEFINITELY show up.

wow, that was an awesome explanation. can you recommend some material that would help me learn this stuff better?