6.1.1 Benzene

Question 1

what is the general formula of benzene

Answer 1

C₆H₆

Question 2

describe Kekulé’s model

Answer 2

Kekule’s model suggests that benzene consists of alternating double and single bonds

Question 3

Why is kekule’s model no longer accepted

Answer 3

• lack of reactivity of benzene
• lengths of the C-C bonds
• hydrogenation enthalpies

Question 4

explain how kekule’s model doesnt account for the lack of reactivity of benzene

Answer 4

• If benzene contained C=C, as kekule suggests, then it should decolourise bromine water which it does not do.
• This suggests benzene doesnt have any C=C bonds

Question 5

explain how length of bonds in benzene disproves kekule’s model

Answer 5

• using X-ray diffraction its possible to measure bond lengths in a molecule.
• Kekule’s model suggests that the length would alternate - longer-shorter-longer-shorter
• however all the bonds have been found to be the same length

Question 6

Explain why the hydrogenation enthalpy of benzene disproves kekule’s model

Answer 6

• The enthalpy change of hydrogenation of benzene is less exothermic than the one predicted by kekule
• this suggests the actual structure of benzene is more stable than kekules structure.

Question 7

describe the delocalised model of benzene

If you draw the picture

Answer 7

• Sideways overlap of p-orbitals gives a ring of delocalised π-electrons above and below the plane of carbon atoms.
• There are sigma bonds between all the C-C and C-H
• All the bond angles are 120°
• benzene is planar
• all the C-C are the same length

Question 8

Describe the model of Benzene without the aid of a diagram

Answer 8

• Benzene is a planar, cyclic, hexagonal hydrocarbon containing six carbon atoms and 6 hydrogen atoms
• each carbon atom uses three of its available four electrons in bonding to two other carbon atoms and to one hydrogen.
• Each carbon atom has one electron in a p-orbital at a right angle to the plane of the bonded carbon and hydrogen atoms
• Adjacent p-orbital electrons overlaps sideways in both directions, above and below the the plane of the carbon atoms to form a ring of electron density.
• This overlapping of the p-orbitals creates a system of π-bonds which spread over all 6 carbon atoms in the ring structure.
• The six electrons occupying this systrm of π-bonds are delocalised.

Question 9

when is benzene the parent and when is it the substituent in naming aromatic compounds with one substituent group

Answer 9

• in monosubstituted aromatic compounds the benzene is often considered the parent chain to alkyl groups, halogens, and nitrogroups.
• However when a benzene ring is connected to an alkyl chain with a functional group or that is made up of 7 or more carbon atoms it is considered the substituent. In this case the prefix ‘phenyl’ is used.

Question 10

name: benzene connected to a nitro group

Answer 10

nitrobenzene

Question 11

name: benzene connected to a halogen (e.g. Cl) group

Answer 11

chlorobenzene

Question 12

name: benzene connected to an alkyl group (e.g. C₂H₅)

Answer 12

ethylbenzene

Question 13

name: a benzene connected to COCH₃

Answer 13

phenylethanone

Question 14

name: a benzene connected to CH(CH₃)(CH₂)₅CH₃

Answer 14

2-phenyloctane

Question 15

name: a benzene connected to NH₂

Answer 15

phenylamine

Question 15

name: a benzene connected to COOH

Answer 15

benzoic acid

Question 16

name a benzene connected to CHO

Answer 16

benzaldehyde

Question 17

how do you name arenes with more than one substituent groups

Answer 17

The ring is now numbered starting with one of the substituent groups.
The substituent groups are listed in alphabetical order using the smallest numbers possible

Question 18

what reaction does benzene undergo

Answer 18

electrophilic substitution

Question 19

what is formed in the nitration of benzene when the temperature ≤ 50°

Answer 19

one of the hydrogen atoms on the benzene will be replaced by a nitro -NO₂ group.

Question 20

what is formed in the nitration of benzene when the temperature is > 50°

Answer 20

at a temperature greater than 50° further substitution may occure leading to the production of dinitrobenzene

Question 21

what is the electrophile in the nitration of benzene

Answer 21

• NO₂⁺
• this ion is formed from:
  HNO₃ + H₂SO₄ → NO₂⁺ + H₂SO₄⁻ + H₂O

The H₂SO₄ catalyst is reformed at the end from the H⁺ off of benzene

Question 22

What is required for the halogenation of benzene

Answer 22

halogens do not react with benzene unless a halogen carrier is present

a halogen carrier is a type of catalyst

Question 23

list some halogen carriers

Answer 23

• FeCl₃
• AlCl₃
• AlBr₃
• FeBr₃

Question 24

Why does a benzene require a halogen carrier to react with halogens

Answer 24

benzene is too stable to react with a non-polar bromine/chlorine molecule.

Question 25

What is the first step of a halogen reacting with benzene

e.g. Bromine

Answer 25

formation of the electrophile
Br₂ + FeBr₃ → FeBr₄⁻ + Br⁺

Question 26

compare the reactivity of alkanes with arenes

Answer 26

• alkenes react in addition reactions, benzene reacts in substitution reactions - though both are elecrophilic
• π-electrons in alkenes are localised to the C=C bond. So there is enough electron density to polarise the Br₂ and induce a dipole. HOWEVER the π-electrons in benzene are delocalised around the ring resulting in a lower electron density so a halogen carrier is requires to form Br⁺