Aromatic chemistry

Question 1

What is the formula of benzene and what does it look like?

Answer 1

• C6H6
• It has a hexagonal six-sided ring structure

Question 2

What are arenes?

Answer 2

• They are hydrocarbons like benzene which conatin rings stabilised by elctron delocalisation.
• Benzene is the simplest arene

Question 3

Why was the bonding and structure of benzene such a puzzle?

Answer 3

• Despite it being unsaturated it didn’t readily undergo addition reactions
• All carbon atoms were equivalent meaning all carbon-carbon bonds were same

Question 4

What is the actual structure of benzene?

Answer 4

• It is a regular flat hexagon of carbon atoms each of which is bonded to 1 hydrogen atom each .
• The bond angles are 120 degrees each
• The C-C bonds are intermediate between those of a carbon double and single bond meanING its equivalent to 1.5 bonds.

Question 5

What is the actual bonding in benzene?

Answer 5

• Each C atom is bonded to 2 other C atoms and 1 H atom via single covalent bonds leaving 1 electron on each C atom in P orbital perpendicualr to the plane of the ring.
• Each P orbital overlaps with neighbouring P orbitals to form a π - bond leaving an overall result of a ring of negative charge/electron cloud above and below the plane of the ring.

Question 6

Explain the structure of benzene occuring today:

Answer 6

• Some of the electrons are delocalised therefore spread over more than 2 atoms so the 6 C atoms that form the ring.
• Each C has 3 covalent bonds to H and 2 C. 4th electron of each C is in P-orbital ands there’s 6 of these one on each C atom.The P-orbitals overlap to from π-bond and electrons in them are delocalised which form a region of electron density above and below the ring.
• The delocalisation in the π system means electrons more spread out and will repel each other less making molecule more stable

Question 7

What was Kekule’s proposed structure for Benzene?

Answer 7

• He believed in a ring structure of carbon atoms joined to 1 H atom with alternating 3 double and single bonds.It would be called cyclohexa-1,3,5-triene.

Question 8

What was the thermochemical evidence disproving Kekule’s structure?

Answer 8

• The enthalpy change for hydrogenation of cyclohexene = -120 kJ mol-1
• So the proposed benzene structure with alternating double bonds would be expected to be 3x -120 so -360 kJ mol-1
• However, enthalpy change for benzene actually is -208 kJ mol-1 so benzene is 152 kJ mol-1 more stable than proposed structure

Question 9

What other 2 pieces of evidence disprove Kekule’s structure?

Answer 9

1. C-C bond length - all C-C bonds are of same length and in between carbon single and double bonds.If benzene was triene then would expect 3 longer C single bonds and 3 C shorter double bonds.
2. Addition reactions - benzene doesn’t readily undergo addition reactions. If triene would expect it to undergo addition reactions but it doesn’t.

Question 10

What are the physical properties of Arenes?

Answer 10

• Benzene is a colourless non-polar liquid at room temperature and is immiscible with water.

Question 11

How do you name aromatic compounds?

Answer 11

• Substituted arenes are named with -benzene at the end so C6H5CH3 is called methylbenzene. If more than 1 substituent the ring is numbered.
• Compounds derived from arenes - so benzene ring with different functional groups so benzoic acid,benzaldehyde and benzyl alcohol.
• Sometimes can also be named after phenyl group/C6H6 e.g. phenol and phenylamine.

Question 12

Describe the reactivity of aromatic compounds:

Answer 12

1. The delocalised electron ring structure gives an area of high electron density which is attractive to electrophiles.
2. The aromatic ring is very stable and tends to remain preserved during reactions of arenes.

• This means benzene ring structures undergo electrophilic substitution reactions.

Question 13

Why are substitution reactions preferred over addition reactions?

Answer 13

• Substitiution reactions are preferred to addition reactions as stability of benzene ring is maintained.However. they are slower becasue in 1st step delocalised electron system disrupted.
• The full delocalised ring is not preserved by addition reactions as it requires delocalisation energy to destroy aromatic system.

Question 14

Describe the Nitration of Benzene:

Answer 14

• It involves substituting a nitro group/NO2 into benzene
• Benzene reacts with a mixture of conc.HNO3 and H2SO4 and the temp has to be less than 50-55 degrees to make nitrobenzene
• A nitronium ion is produced when H2SO4 protonates HNO3 and then that breaks down to form a nitronium ion:

HNO3 + 2H2SO4 → (NO2+) + (2HSO4-) + (H30+)

Question 15

What is nitration important for?

Answer 15

• It is an important step in the production of explosives like TNT.
• It is also the first step needed to make aromatic amines whch are used in industrial dyes.

Question 16

What is the reduction of nitro compounds to give amines?

Answer 16

Question 17

Describe Friedel-Crafts acylation reactions:

Answer 17

• They are useful reactions as provide a way off adding C atoms to benzene ring.
• The electrophile in the reaction is called an acylium ion / RCO+ and AlCl3 is used as a catalyst.
• An acyl chloride reacts with aluminium chloride to form the electrophile and AlCl4-.

RCOCl + AlCl3 → RCO+ + AlCl4-