substitution

substitution

alkyl halide is subbed in with nucleophile

What is the rate law for S_N2?

rate=k[alkyl halide][nucleophile]

What is the rate law for S_N1?

rate=k[alkyl halide]

What type of solvent does S_N2 prefer? and why?

Aprotic bc the salts of the nucleophile can dissolve without H-bonding. the anion is exposed and highly reactive

What type of solvent does S_N1 prefer? and why?

Protic bc they have one hydrogen atom bonded to an electronegative atom, which enables them to stabilize the rate-determining formation of the carbocation intermediate and the anionic leaving group through solvation and hydrogen bonding.

What strength of nucleophile and base does S_N2 prefer?

strong nucleophile & weak base

What strength of nucleophile and base does S_N1 prefer?

weak nucleophile & weak base

characteristics of strong nucleophile

• negative charge

• high polarizability (big), but not bulky

• low electronegativity

common strong nucleophiles

OH- (oxides), CN-, N3-, RS-, HS-,

common weak nucleophiles

H2O, ROH, CH3OH (methanol), CH3CH2 (ethanol)

characteristics of weak base

• is conjugate to a strong acid

what happens in S_N2 reactions?

In one quick step, the halide leaves and the nucleophile attacks from the back resulting in inversion

what happens in S_N1 reactions?

in multiple steps, a leaving group departs first, forming a carbocation intermediate, followed by attack from a weak nucleophile

What substitution is preferred by S_N1? and why?

Tertiary bc it forms the most stable carbocation intermediate

What substitution is preferred by S_N2? and why?

primary bc it has the least steric hinderance

products of S_N1 and why?

racemic mixture bc nucleophile attacks front and back

products of S_N2 and why?

inversion due to backside attack

in which case would a carbocation shift be necessary?

if there is a higher substituted C in S_N1

examples of carbocation shifts?`

ring expansion, methyl shift, hydride shift

why is substitution not concerned with regioselectivity?

bc it happens at a specific spot so there is no question about the isomer formed

solvolysis is common in which reaction?

S_N1 bc it happens in polar protic solvents