SN2,SN1,E1,E2 MECHANISMS
Summary of Chapter 11 Material
SN2 Reaction
Substrate: NO SN2 ON A TERTIARY HALOALKANE!! order of reactivity is as follows: Methyl > 1 > 2
Sterically less hindered substrates have faster rates in SN2
Nucleophile: If the reacting atom is the same in a series, nucleophilicity parallels basicity (i.e. -OH > -OCH3 > -OCH2CH3 > H2O)
For the halogens in GAS PHASE: F- > Cl- > Br- > IFor the halogens in SOLUTION: I- > Br- > Cl- > F- (due to solvation of nucleophile, rendering it inactive)
Negatively charged nucleophiles give neutral products
Neutral nucleophiles give cationic products
In SOLUTION nucleophilicity increases down a column in the periodic table (i.e. -OCH3 > -SCH3 > -SeCH3)
Anionic nucleophiles are usually stronger than neutral ones
Leaving Group: Best leaving groups are those that best accomodate a negative charge (large anions such as I-)
The best leaving groups are WEAK BASES
According to this criteria, the worst leaving groups are:
F-, OH-, OCH3-, CN-, N3-, NH2
The best leaving groups are: H2O, CH3OH, OTs, ISolvent: Polar protic solvents are the worst for SN2; they solvate the nucleophile so it cannot attack
Polar aprotic solvents stabilize the transition state and speed SN2 reactions up
Polar aprotic solvents are DMSO, DMF, CH3CN, HMPA
Notes: The SN2 reaction is bimolecular - it depends on amounts of nucleophile and leaving group
In synthesis, always convert an OH leaving group into an
OTs leaving group
Converting an OH into an OTs will NOT change the stereochemistry!! SN2 reaction goes with inversion about the stereogenic carbon SN1 Reaction
Substrate: SN1 reactions work best on a TERTIARY!
NO SN1 ON A PRIMARY!!
Order of reactivity: 3 > 2
Benzylic and Allylic carbons have increased reactivity because of resonance stabilization
Nucleophile: Has NO effect upon the rate of reaction!! Rate limiting step does not include the nucleophile!!
To convert OH to a good leaving group use HX
Leaving Group: Same as for SN2
SN2 Reaction
Substrate: NO SN2 ON A TERTIARY HALOALKANE!! order of reactivity is as follows: Methyl > 1 > 2
Sterically less hindered substrates have faster rates in SN2
Nucleophile: If the reacting atom is the same in a series, nucleophilicity parallels basicity (i.e. -OH > -OCH3 > -OCH2CH3 > H2O)
For the halogens in GAS PHASE: F- > Cl- > Br- > IFor the halogens in SOLUTION: I- > Br- > Cl- > F- (due to solvation of nucleophile, rendering it inactive)
Negatively charged nucleophiles give neutral products
Neutral nucleophiles give cationic products
In SOLUTION nucleophilicity increases down a column in the periodic table (i.e. -OCH3 > -SCH3 > -SeCH3)
Anionic nucleophiles are usually stronger than neutral ones
Leaving Group: Best leaving groups are those that best accomodate a negative charge (large anions such as I-)
The best leaving groups are WEAK BASES
According to this criteria, the worst leaving groups are:
F-, OH-, OCH3-, CN-, N3-, NH2
The best leaving groups are: H2O, CH3OH, OTs, ISolvent: Polar protic solvents are the worst for SN2; they solvate the nucleophile so it cannot attack
Polar aprotic solvents stabilize the transition state and speed SN2 reactions up
Polar aprotic solvents are DMSO, DMF, CH3CN, HMPA
Notes: The SN2 reaction is bimolecular - it depends on amounts of nucleophile and leaving group
In synthesis, always convert an OH leaving group into an
OTs leaving group
Converting an OH into an OTs will NOT change the stereochemistry!! SN2 reaction goes with inversion about the stereogenic carbon SN1 Reaction
Substrate: SN1 reactions work best on a TERTIARY!
NO SN1 ON A PRIMARY!!
Order of reactivity: 3 > 2
Benzylic and Allylic carbons have increased reactivity because of resonance stabilization
Nucleophile: Has NO effect upon the rate of reaction!! Rate limiting step does not include the nucleophile!!
To convert OH to a good leaving group use HX
Leaving Group: Same as for SN2