The Effect Of R-group On Rates Of Reactivity

Question 1

How does the number of R-groups bonded to the reacting carbon effect SN1 mechanisms.

Answer 1

• The greater the number of alkyl chains (R-groups) the faster the reaction.
• This is because Alkyl groups are electron-releasing (donating) leading to a positive inductive effect stabilising the intermediate cation.
• The more stable the carbocation, the faster it forms by cleavage of the C-X bond.

Question 2

How does the resonance effect of R-groups effect SN1

Answer 2

If the R-group can stabilise the carbocation by resonance then this will increase the rate of reaction via SN1.

Question 3

How does the size of the R-group affect the rate of an SN2 reaction

Answer 3

• The larger the substituents, the slower the reaction. This is a result of steric hindrance (bulky molecules getting in the way of the backside attack).
• It is the reverse of SN1 mechanisms and in this case from fastest -> slowest its halomethane -> tertiary (Doesn’t undergo) .

Question 4

The effect of electron-donating groups on SN2

Answer 4

Electron-donating groups reduce the partial positive charge in the carbon, making it less attractive to nucleophiles.

Question 5

Why do larger R-groups also effect activation energy and rate of reaction for SN2

Answer 5

Large R-groups also raise the energy of the transition state. Larger activation energy is now needed to be overcome as the transition state is crowded.