Electrophilic substitution mechanisms

Question 1

the type of reaction benzene undergoes is called

Answer 1

electrophilic substitution

Question 2

benzene has points in common with other species which undergo electrophilic substitution. the three main common points are:

Answer 2

• the benzene ring is electron rich, so although it is electrically neutral, the delocalised electrons in the pi bond above and below the atoms means the molecule attracts electrophiles
• using Y+ to represent the attacking electrophile, when Y+ approaches the delocalised electrons, it attracts two of these e- and forms a covalent bond with is, giving an intermediate species with a positive charge (shown in benzene by an ‘unfinished’ circle). There are now only 4 delocalised electrons
• this intermediate species lacks the stability of an aromatic compound so in the next step the H leaves as H+ and the two electrons in the C-H bond join the delocalised pi bond

Question 3

when drawing the positive intermediate, the positive charge is shown by

Answer 3

a two thirds full circle with a ‘plus’ (+) sign in the middle, with the gap in the circle angled to towards the C atom with the H and Y (electrophile) bonded to it