Lecture 19 - Aromaticity part 2/2

Question 1

Give equation for the Friedel crafts acylation reaction of Benzene.

Answer 1

Question 2

Give mechanism for Friedel crafts alkylation reaction.

Answer 2

Question 3

Give sulfonation mechanism of Benzene

Answer 3

Question 4

Give the nitration reaction of Benzene.

Answer 4

Question 5

Give halogenation mechanism of benzene - show resonance forms

Answer 5

Question 6

Benzene can undergo mono-substitution reactions to form three isomers- name them and explain where groups are located around ring.

Answer 6

• ortho = 1,2
• meta = 1,3
• para = 1,4

Question 7

Draw monosubstituted benzene at the ortho position

Answer 7

Question 8

Draw monosubstituted benzene at the meta position

Answer 8

Question 9

Draw monosubstituted benzene at the para position

Answer 9

Question 10

What do directing groups allow us to predict?

Answer 10

Where the electrophile will be substituted in the benzene ring.

Question 11

Give the general mechanism for ortho substitution of benzene and show the three resonance forms. Circle the most stable intermediate (if R group is electron donating) and explain why.

Answer 11

R group is bonded to the positively charged carbon- electron donating

Question 12

Give the general mechanism for meta substitution of benzene and show the three resonance forms.

Answer 12

Question 13

Give the general mechanism for para substitution of benzene and show the three resonance forms. Circle the most stable intermediate (if R group is electron donating) and explain why.

Answer 13

R group is bonded to the positively charged carbon- electron donating. This stabilises the positively charged intermediate.

Question 14

If the R group is strongly electron donating (activating), where does it direct the electrophile to substitute?

Answer 14

• the most stable positions for the electrophile to substitute are the ortho and para positions - they have resonance forms where the carbocation is bonded to the R group
• the electron donating R group stabilises the positive charge.

Question 15

Give examples of strong electron donating R groups. How do they donate electron density?

Answer 15

NH2
NHR
NR2
OH
OR

donate via the mesomeric effect

Question 16

If the R group is weakly electron donating (activating), where does it direct the electrophile to substitute?

Answer 16

• the most stable positions for the electrophile to substitute are the ortho and para positions - they have resonance forms where the carbocation is bonded to the R group
• the electron donating R group stabilises the positive charge.

Question 17

Give examples of weakly electron donating groups. How do they donate electrons?

Answer 17

-Me
-Et
-Ph

Donate electron density via the inductive effect

Question 18

If R groups are strongly electron withdrawing (deactivating), where do they direct the electrophile to substitute and why?

Answer 18

The R group is strongly electron withdrawing so would not stabilise the carbocation. Therefore, directs to the meta position where the R group is never located next to the carbocation in any of the resonance forms.

Question 19

Give examples of strongly electron withdrawing groups

Answer 19

aldehehyde, ketone, amide, NO2, CN

Question 20

If R groups are weakly electron withdrawing (deactivating), where do they direct the electrophile to substitute and why?

Answer 20

If R is weakly electron withdrawing (deactivating), it would stabilise ortho and para so would direct electrophile to ortho and para positions

Question 21

Give examples of weakly electron withdrawing groups. How do they donate electron density?

Answer 21

-F, -Cl, -Br
they are weakly electron withdrawing but can donate through the mesomeric effect.

Question 22

What does SnAR stand for?

Answer 22

nucleophilic aromatic substitution

Question 23

Give the general equation for an SnR reaction

Answer 23

Question 24

Draw the mechanism for production of Sanger’s reagent. Show how the position and type of R group is worked out.

Answer 24