Confused about SN2 vs E2

[pandalove89]

So from Chad's video, he says that: SN2 is favored when there is a strong nucleophile and weak base over E2.... However, doesn't the strong nucleophile make the base strong...? So how can there be a weak base? So what I'm asking is, I thought strong nucleophiles meant strong bases...?

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[Impulse155]

So from Chad's video, he says that: SN2 is favored when there is a strong nucleophile and weak base over E2.... However, doesn't the strong nucleophile make the base strong...? So how can there be a weak base? So what I'm asking is, I thought strong nucleophiles meant strong bases...?

Its a general rule of thumb organic chemistry teachers give you as a short cut. However the grimey ass professor I had would tell us that, and then try and trick us on exams. There are some strong nucleophiles that are not strong bases.

SN2 rules:
Strong Nucelophile Rules (in rank of priority)
Negatives always better (strong base)
Row: C>N>O>F
Size (halogens): Protic = large Aprotic = small
Size: Larger the better

E2 rules:
Mainly anything with a negative, or with a Na, K etc since these yield negative charges.

Other ways to tell if its a E2/Sn2 reaction:

SN2 never occurs with 3 degree carbons its vinyl >1st >2nd never 3rd
e2 favors allyl, 3rd then 2nd but will never happen with 1st

Also remember e2 needs a higher temp.

 

[needzmoar]

So from Chad's video, he says that: SN2 is favored when there is a strong nucleophile and weak base over E2.... However, doesn't the strong nucleophile make the base strong...? So how can there be a weak base? So what I'm asking is, I thought strong nucleophiles meant strong bases...?

Let me clarify something for ya:

Nucleophilicity is defined as the rate of attack. Faster the rate, stronger the Nucleophile.

Basicity is defined as the equilibrium constant for abstracting a proton.

Generally, most strong bases are in fact strong nucleophiles. Not all strong nucleophiles are strong bases. Example:
Bromide ion. It's a strong nucleophile but not a strong base.
Hydroxide ion. It's a strong base and can also serve as a nucleophile for an SN2 attack IF AND ONLY IF THE SUBSTRATE IS A PRIMARY HALIDE.

Here's a mnemonic device to help you with the common strong nucleophiles and strong bases

ROCCN BRaIn SHRiNC (Rocking brain shrink)

Stong bases
RO alkoxides *
CC Acetylide anions *
N Amines *

Strong Nucleophiles
Br Bromide
a nothing
I Iodide
n nothing

Shr Thiol anion (example: CH3SH- )
I nothing
NC nitrile anion

* will undergo SN2 if substrate is a primary halide. In the case of amines, be weary of polyakylation.

ROCCN BRaIn SHRiNC

 

[pandalove89]

noice! thanks guys!