Nucleophilic Substitution Reactions (Sn1 and Sn2) Flashcards
what factors affect the rate of Sn2 reactions?
reactivity of the nucleophile. (as basicity increases, rate increases).
leaving group ability. (rate increases as leaving group ability increases).
substituents attached to the electrophilic carbon atom. (rate decreases as steric bulk at electrophilic carbon atom increases).
nature of the solvent. (dipolar aprotic solvents favour Sn2).
what does lower pKa value mean?
more dissociated = more stronger acid.
what is the effect of a nucleophile on the rate of Sn2?
decreasing nucleophilicity, how good a nucleophile something is, which causes the rate to decrease too.
what is the effect of the leaving group on the rate of Sn2?
increasing LG ability leads to an increase in rate.
it correlates with the increase in acidity of the LG pKa(h)
what are the effects of substituents on rate of Sn2?
the more complex a structure is, the amount of LUMO accessible to the nucleophile decreases.
rate decreases as steric bulk increases.
what factors affect the rate of Sn1 reactions?
LG ability. (rate increases as LG ability increases).
substituents attached to the electrophilic carbon atom. (those that stabilise the intermediate, increase rate).
nature of the solvent. (dipolar protic solvents favour Sn1).
what effects does an aprotic and protic solvent have on the Sn2 mechanism?
How is this achieved?
aprotic = stabilises nucleophile, making it more reactive.
protic = solvates the nucleophile, making it less reactive.
aprotic reduces electrostatic attraction between cation and anion of Nu
reduces energy required for the Nu to approach the substrate.
protic has H that bond to Nu, reducing ability to interact with the substrate.
why does a better LG ability increase the rate?
for both Sn mechanisms.
more stable after it leaves substrate, transition state more stable, lowers activation energy.
in Sn1:
more stable carbocation intermediate, more likely to form, increased rate.
Sn2:
more easily displaced by Nu, increased rate.
how do the substituents attached to the electrophilic carbon atom impact the Sn1 and Sn2 mechanisms?
Sn1:
stabilise the carbocation intermediate by donating electrons through inductive and resonance effects.
formation of intermediate is more favourable.
Sn2:
block the approach of the Nu to the substrate.
electron-withdrawing substituents can make the carbon atom more electrophilic, more difficult for Nu to attack.
what are the relative rates of Sn1 mechanism in primary, secondary and tertiary molecules?
rate increases as you go down.
more hyperconjugation in tertiary molecules, which stabilises the cation.
smaller Ea, faster reaction.
what is Hammonds Postulate?
factors that stabilise a high energy intermediate will also stabilise a TS that is close in energy.
