organic standard answers

Question 1

Explain the term carbonyl compound

Answer 1

Compound containing an aldehyde or ketone functional group

Question 2

Describe a test to show that a compound contains either aldehyde or ketone functional group

Answer 2

· Add 2,4-DNPH

· Red/orange/yellow solid formed

Question 3

Explain how the crystals from the test above can be used to confirm the identity of a specific carbonyl compound

Answer 3

filter precipitate

· purify by recrystallisation and allow crystals to dry

· measure m.p. of crystals and compare with database of known values

Question 4

Describe the evidence for the delocalized model of benzene

Answer 4

Benzene does not react with bromine

· The enthalpy change of hydrogenation of benzene is less exothermic than anticipated

· All C-C bonds in benzene have the same length

Question 5

Describe the bonding in the delocalized model of benzene

Answer 5

One delocalised π bond

· containing 6 delocalised electrons between 6 carbon atoms

· formed by the sideways overlap of six p-orbitals

· above and below the plane of the ring

Question 6

Describe the bonding in the Kekule model of benzene

Answer 6

· Three localised π bonds

· each containing 2 electrons between 2 carbon atoms

· formed by the sideways overlap of two neighbouring p-orbitals

· above and below the plane of the ring

Question 7

Describe the similarities between the Kekule and delocalised model of benzene

Answer 7

· π bond formed by the sideways overlap of p-orbitals

· above and below the plane of the ring

Question 8

Describe the differences between the Kekule and delocalised model of benzene

Kekule

Answer 8

· p-orbitals overlap in one direction

· 3 alternating π bonds

· each localised π bond contains two electrons

· higher electron density than delocalised model

Delocalised model

· p-orbitals overlap in both directions

· one π ring system

· the π bond contains six delocalised electrons

· lower electron density than delocalised model

Question 9

Describe and explain the relative resistance to bromination of benzene compared to cyclohexene.

Answer 9

· Benzene does not react with bromine

· benzene has a delocalised π bond

· cyclohexene reacts with bromine

· cyclohexene has a localized π bond with greater electron density

· cyclohexene is better able to attract an electrophile

Question 10

Explain why phenol is more reactive than benzene

Answer 10

· In phenol the lone pair of electrons on p orbital of oxygen is delocalised into the π ring system

· increased electron density in the π ring system

· more able to attract to attract an electrophile

Question 11

Explain why the reaction of phenylamine with dilute aqueous nitric acid produces a mixture of 2-nitrophenylamine and 4-nitrophenylamine

Answer 11

· -NH2 is electron releasing

· Directs substitution to positions 2 and 4 (or 6

Question 12

Explain why the reaction of nitrobenzene with Cl2 / AlCl3 produces mainly 1-chloro-3-nitrobenzene

Answer 12

· -NO2 is electron withdrawing

· Directs substitution to position 3 (or 5)