Organic III- Topic 18

Question 1

Evidence for Kekule’s structure of Benzene (alternating double bonds) being wrong

Answer 1

Should undergo addition reactions due to double bonds, but it doesn’t (so doesn’t react with bromine water). All Carbon to carbon bonds same length in x-ray but double should be shorter therefore no double bonds.
Enthalpies of hydrogenation expected to be more exothermic but is instead less exothermic therefore more positive and more STABLE

Question 2

Describe the delocalised model.

Answer 2

six C atoms in a hexagonal ring, with one H atom bonded to each C atom. Each C atom is bonded to two other C atoms and one H atom by single covalent σ-bonds. This leaves one unused electron on each C atom in a p orbital, perpendicular to the plane of the ring. The Six p electrons are delocalised in a ring structure above and below the plane of carbon atoms

Question 3

How does benzene react in the air?

Answer 3

very smoky flame as too little oxygen in the air for complete combustion (particles of soot = smoky)

Question 4

What are the bond angles in benzene?

Answer 4

120*

Question 5

Why is Benzene not used in school labs?

Answer 5

Carcinogenic

Question 6

What mechanisms do Arenes react via?

Answer 6

Electrophilic substitution

Question 7

Explain why phenol decolourises bromine water

Answer 7

The lone pair on the oxygen atom delocalises with electrons in pi bond. therefore increases electron density in benzene ring.

Question 8

Suggest why nitration becomes increasingly difficult as each successive nitro group is added to benzene. [2]

Answer 8

As the nitro group contains electronegative oxygen/nitrogen which withdraws electron density from the delocalised pi bond.

Question 9

Nitrobenzene can be prepared using two acids. Name them.

Answer 9

Concentrated nitric acid and concentrated sulfuric acid

Question 10

How will a phenol react with bromine water?

Answer 10

Decolourises it and further substitutes to an insoluble product which forms white ppt and antiseptic smell