BENZENEWHY ELECTROPHILIC SUBSTITUTION?

ELECTROPHILIC SUBSTITUTION…

Delocalised electrons are what makes benzene so stable and allow it to resist addition reactions.

Electron rich benzene rings are prone to attack from electrophiles

One or more of the 6 hydrogen atoms is replaced by a reacting functional group.

REACTIONS OF ARENES

THE MECHANISM E+ = electrophile. The overall equation is:

What do you think happened?

+ E+ + H+

E

GENERIC MECHANISM

STAGE 1

Positive electrophile attacks the delocalised electron ring.

A temporary intermediate is formed where the delocalisation is partially lost and the ring will join to both the original hydrogen and the new electrophile.

The overall charge of the molecule is positive so make sure you add this to your diagram.

STAGE 2

Y- comes from the system (remember that ions dissociate in solution).

It will be attracted to the H on the benzene ring and cause H to donate its electrons back to the ring.

1. NITRATION

2. FRIEDEL-CRAFTS ACYLATION

NITRONIUM ION The nitronium ion/nitryl cation is NO2

+

Firstly need to generate the nitronium ion

CONCENTRATED!!!

H2SO4 + HNO3 H2NO3+ +

HSO4-

H2NO3+ NO2

+ + H2O

MECHANISM

LOOKING AT THE BIG PICTURE…

NITROBENZENE TO PHENYL AMINE

NO2 group is reduced to NH2 group. In industry, iron is used In lab, tin is used with conc HCl as

reductant. Phenylamine is widely used in dye

industry.

WHAT IS ACYLATION?

Adding an acyl C=O group to a benzene ring….

BACKGROUND… Benzene reacts with an acyl chloride

Ethanoyl chloride is the most commonly used acyl chloride

AlCl3 catalyst

Forms a phenyl ketone and HCl.

AlCl3

GENERATING THE ELECTROPHILE…

RCOCl + AlCl3 RCO+ + AlCl4-

ELECTROPHILIC SUBSTITUTION…

Where E+ = CH3CO

EXAM QUESTIONS

Do questions 1, 4 and 5 in your books

MOREEXAM

QUESTIONS