Aromatics Directing Effects 3+4

Question 1

Why is the nitro group a meta director?

Answer 1

• It deactivates the ortho and para positions to electrophilic substitution.
• So only the meta position can undertake electrophilic substitution.

Question 2

What are ortho para directing compounds in relation to inductive and resonance effects?

Answer 2

-I

+R

Question 3

What are meta directing compounds in relation to inductive and resonance effects?

Answer 3

• I

- R

Question 4

Substituents that are -I and +R and are ortho/para directing?

Answer 4

OH, NH2, SH, halogens etc (see table on slide 11)

Question 5

Halogens deactivate the benzene ring, so how come it is still ortha/para directing and not meta directing?

Answer 5

there are 2 opposing effects:

1. conjugative electron donation to the ring (+R)
2. inductive electron withdrawal from the ring (-I)
   - the inductive electron withdrawal deactivates the ortha position more than the para position as it is closer to the halogen. However, the resonance effect takes precedence and the resonance effect makes the ortha and para positions more reactive than the meta position.

Question 6

Which halogens are more para directing?

Answer 6

more electronegative halogens i.e. F and Cl

Question 7

How does a meta directing substituent affect the binding of the reactant to the benzene ring at the ortho and para positions?

Answer 7

it makes the ortho and para positions more positively charged by withdrawing electrons from there (-I). This makes it difficult/impossible for an electrophile to bind at the ortho and para positions.

Question 8

Which has more effect, O/P directors or M directing deactivators?

Answer 8

O/P directors

Question 9

Which has dominance, electron donating groups or withdrawing groups?

Answer 9

electron donating groups

Question 10

what type of nucleophiles are used to form a benzyne?

Answer 10

strong bases.

Question 11

What is a benzyne?

Answer 11

an unstable intermediate, which has a structure similar to benzene with an additional pi bond. The third pi bond is formed by the overlap of sp2 hybridized orbitals outside the ring.
(gets a lone electron pair from the removal of hydrogen from one of the carbons hence forming a carbocation)