Aromatic chemistry

Question 1

what group of compounds does aromatic refer to

Answer 1

compounds containing a benzene group

Question 2

what is the bond angle in a benzene ring?

Answer 2

120 degrees. It is a planar molecule

Question 3

what is the result when bromine is added to benzene

Answer 3

there is no colour change. rejects the usual observation with compounds containing a double bond

Question 4

explain the bonding and the shape of a benzene molecule

Answer 4

• each carbon has 3 bonds
• spare electrons overlap, to form a pi bond
• forms a ring of delocalisation
• the shape of benzene is planar
• it has a bond angle of 120 degrees
• the C-C bonds within the molecule have a length that is between the bond length for a C-C single bond and the bond length for a C=C double bond.

Question 5

compare the bond hydration of cyclohexa-1,3,5-triene with that of benzene

Answer 5

• benzene is more stable than cyclohexa-1,3,5-triene
• the expected hydrogenation of C6H6 = 3(-120) = -360 kJmol-1
• the actual hydrogenation for benzene is different from expected. it is less exothermic.
• Because of delocalisation. electrons spread out.

Question 6

suggest why the enthalpy of hydrogenation of cyclohexa-1,3-diene is not exactly double that of cyclohexene

Answer 6

• benzene is more stale because of the delocalised electrons
• there may be some delocalisation in cyclohexa-1,3-diene, making it more alike benzene
• each double bond is separated by one single bond

Question 7

what reactions are likely to occur with benzene

Answer 7

elecrophilic substitution reactions. this is because the delocalised electrons in benzene make it more stable, preventing it from reacting as a alkene. the energy required to break the ring in electrophilic addition is too high and so is unlikely to occur.

Question 8

why must the electrophile be extremely powerful

Answer 8

so it is able to react with such a stable molecule

Question 9

what reagents and conditions are used to generate the electrophile used in a nitration reaction with benzene

Answer 9

reagents :conc H2SO4 and conc HNO3

conditions: warm to 50 degrees C

Question 10

write two reactions for the formation of the electrophile

Answer 10

H2SO4 + HNO3 –> HSO4- + H2NO3+

H2NO3+ H2O + NO2+

Question 11

why is nitration a useful process

Answer 11

it produces compounds that can be converted into other molecules e.g. to produce phenylamine, that is used to synthesise dyes
nitro compounds are also used for the manufacture of explosives

Question 12

what is nitrobeneze treated with to form phenylamine

Answer 12

Sn and conc HCl

Question 13

write an overall equation for this reaction

Answer 13

C6H5(NO2) + 6[H] –> C6H5(NH2) + 2H2O

Question 14

how is the acylium ion generated for a friedel-crafts acylation reaction

Answer 14

AlCl3 is reacted with an alcyl chloride. The AlCl3 is a catalyst that is regenerated at the end.
R-COCl + AlCl3 –> R-CO+ + [AlCl4]-

Question 15

give the overall equation for the reaction of the acyl carbocation that is formed, with benzene.
what is the name of this mechanism

Answer 15

C6H6 + RCOCl –> C6H5RCO + HCl

electrophilic substitution