Effect Of Solvent

Question 1

What is solvation?

Answer 1

This is the process of attraction and the association of molecules of a solvent with molecules or ions of a solute
- surrounding a solute with solvent

Question 2

What is a protic solvent?

Answer 2

This is solvent that contains OH, NH

They are polar solvents

Question 3

What is an aprotic solvent?

Answer 3

This is a solvent that does not contain OH or NH

They can be polar or non polar

Question 4

Which solvents are favoured for SN1 and SN2 reactions?

Answer 4

Aprotic solvents do not solvate nucleophiles and so are good for SN2 reactions
Protic polar solvents are good for SN1 as they help to stabilise the intermediate transition state

Question 5

Describe the effect of increasing polarity on the rate of SN1 reaction?

Answer 5

Increasing polarity increases the rate
They stabilise the carbocation by solvating it
They also provide energy needed for cleavage for CX bond
A protic solvent will solvate the carbocation and the halide in the transition state

Question 6

Describe the effect of using an aprotic solvent on the rate of SN1 reaction?

Answer 6

Aprotic solvents will decrease the rate as they only solvate the carbocation and not the halide in the transition state

Question 7

Why are polar protic solvents favoured for SN1 even though they reduce the reactivity of the nucleophile?

Answer 7

Although polar protic solvent solvate the Nu and decrease its reactivity, the nucleophile is not involved in the rate determining step and so the rate is unaffected

Question 8

Which solvents are favoured for SN2 reactions?

Answer 8

Polar aprotic solvents are better As they cannot solvate the nucleophile so it is still reactive
Nucleophile is involved in the rate determining step and so affects the rate
- favour nonpolar or weakly polar aprotic solvents that do not solvate the attacking nucleophile

Question 9

If the Nu is charged, what is the affect of increasing polarity of solvent in SN2

Answer 9

Increasing polarity decreases the rate as it solvates the nucleophile better than the transition state and so the nucleophile is less reactive

Question 10

If the Nu is neutral, what is the affect of increasing polarity of solvent in SN2

Answer 10

Increasing polarity can increase the rate as it can solvate the transition state better than the nucleophile

Question 11

Why are more basic nucleophiles better in aprotic solvents but worse in polar protic?

Answer 11

This is because in polar protic they are heavily solvated which reduces their reactivity- hydrogen bonds