Regioselectivity

Question 1

What is meant by the term regioselectivity?

Answer 1

This is the preferential reaction of a molecule at one of a number of positions bearing the same functional group

Question 2

What is regioselecitivity determined by?

Answer 2

Regioselectivity is determined by the substituents already present on the aromatic ring

Question 3

What two types of directors are there?

Answer 3

All substituents fit into two categories: ortho/para (2,4) directors and meta (3) directors

Question 4

Which director is activating groups?

Answer 4

All activating groups are ortho or para directors

Question 5

Why do more of the para isomer form?

Answer 5

There are 2 ortho and 1 para so you would expect more ortho
But para isomer is less sterically hindered so more forms
If the substituents or electrophile are very large, sometimes only the para isomer forms

Question 6

What can stabilise the transition state?

Answer 6

Anything that stabilised wheland will also stabilise transition state leading to it as the transition state has a partial positive charge

Question 7

What does regioselectivity depend on in terms of energy

Answer 7

Regioselectivity is determined by transition state energy not product energy
This is because aromatic substations are irreversible, kinetically controlled reactions

Question 8

How do number of resonance forms effect stability of the carbenium ion?

Answer 8

The more resonance forms there are of a carbenium ion, the more stable it is

Question 9

Why are activating groups ortho/ para directors?

Answer 9

Ortho and para reactions with activating groups have 4 resonance forms whereas meta only has 3.
This means wheland intermediate is more stable for ortho and para
This means the transition state will also be lower in energy for these reactions and so the reaction will be faster than meta

Question 10

In a resonance form, where can a positive charge never be placed?

Answer 10

A positive charge can never be placed next to an oxygen as this is too unstable

Question 11

When methyl benzene (activating) is brominated, there are 3 resonance forms for meta, para and ortho. Why is para and ortho still favoured?

Answer 11

This is because for the ortho- para substitution, there is a resonance form in which the carbenium ion can be adjacent to and hence stabilised by the electron donating methyl group

Question 12

Which director is a deactivating group?

Answer 12

Any group which is -I or -M will deactivate the ring and be a meta director

Question 13

What are deactivating groups meta directors?

Answer 13

This is because although they all give 3 resonance forms, ortho and para give very unstable resonance forms
This is because the positive charge of the intermediate is adjacent the electron withdrawing group

Question 14

What directors are halogens?

Answer 14

Halogens have both -I and +M effect although the -I effect is stronger so they are deactivating groups
However they are para and ortho directors due to the +M effect
The para isomer is favoured for steric reasons

Question 15

Why is para and ortho favoured for halogens?

Answer 15

Halogens have a lone pair to act as a +M effect
This is because ortho and para have 4 resonance forms compared to 3 which means a more stable intermediate and the transition state will be lower in energy
This means reaction occurs more rapidly at these positions
Meta only has 3 as you cannot have a positive charge next to a halogen as it is very unstable

Question 16

How should you explain why a certain director is favoured?

Answer 16

Draw out wheland intermediate to explain directing effects and then relate to the 1st transition state

Question 17

What should you do if there are multiple substituents on the ring?

Answer 17

Consider each in turn and decide which effect is the strongest
Remember you can never get electrophilic aromatic substitution at a position that is already occupied by another group other than hydrogen