remember from chapter 6 (alkyne chapter): The SN2 mechanism Two possible mechanistic pictures for SN displacement:

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Presentation transcript:

remember from chapter 6 (alkyne chapter):

The SN2 mechanism Two possible mechanistic pictures for SN displacement:

The SN1 mechanismThe SN1 mechanism

Which mechanism? Depends on: structure of electrophile (alkyl halide) structure, concentration of nucleophile solvent

example of an SN2

SN2 has second order kinetics – rds is collision between two molecules

Structure of the alkyl halide: less hindered = faster

(demonstrate with models)

SN2 results in inversion of configuration

Rate of SN2 reaction influenced by leaving group: weak base = good leaving group

...but not so fast!

polarizability also is a factor!

which factor predominates? depends on the solvent

protic solvent shields nucleophile, stronger solvent interactions with stronger bases

in protic sovent, polarizability determines nucleophilicity

polar, aprotic solvent solvates nucleophiles, but with less shielding. base strength determines nucleophilicity These solvents used for SN2

less hindered nucleophiles are better nucleophiles quiz: why is tert-butoxide stronger base? (pKa of alcohols 18 vs. 15.9)

The SN1 reaction first order rate expression: rds is unimolecular rate = k[bromide] [H2O] does not influence rate

think about stability of carbocation intermediate!

SN1 results in racemization of asymmetric center

Again, weaker bases are better LG Notice: reactivity of Nu does NOT influence rate of SN1 rxn Solvent effect: more polar solvents better at stabilizing carbocation, so polar solvents used in SN1 reactions (solvolysis – solvent is nucleophile)

SN1 reactions can undergo carbocation rearrangements!

not always 50:50 mix – why?

intimate ion pair influences stereochemical outcome

Benzylic and allylic alkyl halides: primary can undergo SN1, because stable intermediate

can get mixed products (compare to 1,2 vs 1,4 addition)

vinylic, aryl halides do not undergo SN reactions

SN1 or SN2? methyl, primary: SN2 only secondary: both primary and secondary allylic/benzylic: both tertiary: SN1 only vinylic/benzylic: neither When both are possible: high concentration of good Nu, polar aprotic solvent favors SN2 poor Nu, polar solvent favors SN1 (typically solvolysis)

SN2!

SN1

Which is likely to occur? Depends on concentration of reactant, ring size

5,6-membered rings: intra favored tethering: entropy!