Factors affecting Subsitution Mechanisms, SN1&SN2

Question 1

What is an SN1 reaction?

Answer 1

• Polar Protic and Polar Aprotic Solvents
• Tertiary substrates favour SN1
• weak Nu favours SN1
• Unimolecular
• carbocation intermediate
• 2 steps
• racemisation

Question 2

What is an SN2 reaction?

Answer 2

• Polar Aprotic solvents only
• Primary substrates favour SN2
• strong nucleophile favours SN2 (e.g NC-)

Question 3

Tertiary substrates are never…

Answer 3

SN2 due to steric hindrance

Question 4

Inductive effects of R groups in an SN1…

Answer 4

• 3XR groups stabilise intermediate via inductive effects therefore rate of SN1 reaction is increases

Question 5

Alkyl groups are…

Answer 5

electron donating inductive effect

Question 6

Hammond Postulate principle

Answer 6

• anything that stabilises intermediate also stabilises the transition state

Question 7

SN1 reactions are X steps/stages…

Answer 7

Two steps/stages.

• slow RDS and fast step
• carbocation intermediate formed

Question 8

SN1 forms a…

Answer 8

Racemic mixture as nucleophile can come from LHS or RHS

Question 9

Factors affecting the substitution mechanism - Leaving group

Answer 9

• DOESN’T AFFECT
• Equilibrium to right hand side if A-/X- is a weak base
• As pKa of (HX) decreases leaving group increases from F- to I- and basicity increases from I- to F-
• Efficacious (successful) leaving groups have conjugate acids (HX) with low pKa’s. (i.e. derived from a strong acid)

Question 10

Factors affecting the substitution mechanism - Substrate

Answer 10

• DOES AFFECT
• From primary to tertiary in SN1 rate increases
• From primary to tertiary in SN2 rate decreases

Question 11

Factors affecting the substitution mechanism - Nucleophile

Answer 11

• DOES AFFECT
• hard Nu ( e.g F-) interacts with hard and soft (e.g I-) with soft
• Hard: proton, small, non polarisable
• Soft: Large, polarisable, e- spread out

Question 12

Factors affecting the substitution mechanism - Solvent

Answer 12

• HUGE AFFECT
  1) Non Polar solvent - CCl4, cannot solvate, poor solvent for SN1
• cannot solvate either M+ or Nu- therefore M+/Nu- remain as ion pair therefore Nu- not free to react therefore poor solvent for SN2
  2) Polar Protic - H2O, alcohols, SN1.
• carbocation stabilised so SN1 favoured
• M+ stabilised but so is Nu- so SN2 disfavoured
  3) Polar Aprotic - THF, DMSC,
• Carbocation stabilised so SN1 favoured
• M+ stabilised and Nu- not stabilised so SN2 also favoured

Question 13

A nucleophile is…

Answer 13

• species that attacks centres of positive charge, can be neutral or charged

Question 14

An electrophile is…

Answer 14

• species that is attacked by centres of negative charge, e.g by S+ or + centres

Question 15

In an SN2 reaction…

Answer 15

• Bimolecular
• Nu enters as LG leaves
• Inversion of chirality

Question 16

For Secondary substrates which SN1/SN2 is favoured?

Answer 16

• could be either
• look at solvent choice
• look at nucleophile - if strong then favours SN2

Question 17

What makes a good leaving group?

Answer 17

• A leaving group is a nucleophile acting in reverse; it accepts a lone pair as the bond between it and its neighbour is broken.
• More stable that lone pair is, the better a leaving group it will be.
• Good leaving groups are weak bases. This is due to the dissociation of an acid H–A to give H+ and A– . The species A– is the conjugate base of HA. It accepts a pair of electrons from the H-A bond. It’s a base acting in reverse.
  The more stable A- is, the greater the equilibrium constant will be that favors dissociation to give A- .