Aromatic Compounds

Question 1

Describe benzene

Answer 1

Colourless sweet smelling Highly flammable
Found naturally in crude oil
Carcinogen

Question 2

How length of carbon carbon bonds disapprove kekules model

Answer 2

Using X-ray diffraction possible to measure bond length. Bond length of benzene was between the length of a single bind and a double bond

Question 3

How do hydrogenation enthalpies disprove kekules model

Answer 3

If Benz did have kekule structure then it would be expected to have an enthalpy change of hydrogenation that is three times that of cyclohexene

Question 4

Delocalised model of benzene

Answer 4

Benz is a planar cyclic hexagonal hydrocarbon
Each C atom uses 3 of 4 electrons in binding to 2 C and one H
Each C has 1 e in p orbital at right angles to the plane of bonded c and h atoms
Adjacent p orbital e create system of pi bonds which spread over whole structure
E are delocalised

Question 5

Delocalised model of Benzene diagram

Answer 5

See Diagram 1

Question 6

When benzene ring attached to an alkyl chain with functional group or alkyl chain with 7 or more C atoms

Answer 6

Benzene is considered substituent, prefix phenyl is used

Exceptions include see diagram 2

Question 7

Benzene and electrophilic substitution diagram

Answer 7

See diagram 3

Question 8

Nitration of benzene

Answer 8

See diagram 4

Question 9

What happens if temp of reaction of nitration of benzene rises above 50

Answer 9

Further substitution reactions can occur leading to production of dinitrobenzene
See diagram 5

Question 10

Uses of nitrobenzene

Answer 10

Starting material in prep of dyes pharmaceuticals and pesticides and paracetamol

Question 11

Reaction mechanism for nitration of benzene electrophilic substitution

Answer 11

Step 1: Electrophile is NO2+ produced by reaction of concentrated nitric acid and concentrated sulphuric acid
Step 2: NO2+ accepts pair of e from benzene ring to form dative covalent bond, intermediate is unstable and breaks down to form product nitrobenzene and the H+ ion. Stable benzene ring is reformed.
Step 3: H+ formed reacts with HSO4- ion from step 1 to regenerate catalyst H2SO4
See diagram 6

Question 12

Benzene reacts with bromine under what conditions

Answer 12

Room temp and pressure
Presence of halogen carrier
Electrophilic substitution
See diagram 7

Question 13

Reaction mechanism of bromination of benzene

Answer 13

Step 1: Benzene too stable to react with non polar Br. The electrophile is the bromonium ion Br+ generated when halogen carrier catalyst reacts with Br
Step2: Br+ accepts a pair of e from the benzene ring to form a dative covalent bond. Intermediate is unstable and breaks down into bromobenzene and H+
Step 3: H+ reacts with FeBr4- to regenerate FeBr3 catalyst
See diagram 8

Question 14

Chlorination of benzene

Answer 14

See diagram 9

Question 15

Alkylation of benzene

Answer 15

Substitution of H Atom in benzene ring by an alkyl group
Benzene + haloalkane + AlCl3 which ascots as halogen carrier generating electrophile
See diagram 10

Question 16

Acylation reaction

Answer 16

Benzene + acyl chloride + AlCl3
Aromatic ketone formed
Ethanoyl chloride CH3COCl is first member of acyl chloride
Ethanoyl chloride + benzene = phenylethanone
See diagram 11

Question 17

Cyclohexene + bromine

Answer 17

See diagram 12

Question 18

Reaction mechanism of cyclohexene with bromine

Answer 18

Br adds across double bond in cyclohexene
pi bond in aliens has localised e above and below the plane of the 2 C atoms in double bond, produces area of high e density
Localised e in pi bond induce dipole in non polar Br making one Br of Br2 slightly positive and other slightly negative
+ Br enables the Br molecule to act like an electrophile
See diagram 13

Question 19

Why benzene does not react with bromine unless halogen carrier present

Answer 19

Benzene has delocalised pi e spread above and below the plane of the C atoms in ring structure, the e dens around any two C atoms in benzene ring is less than that in a c double c double bind in an Allene
When non polar molecule eg Br approaches benzene insufficient pi e dens around 2 C atoms to polarise Br

Question 20

Diff reaction mechanism of alleges and benzene

Answer 20

Alkenes: electrophilic addition
Benzene: electrophilic substitution

Question 21

Phenol

Answer 21

Contains OH directly bonded to aromatic ring
Simplest member: C6H5OH
Some compounds contain OH grp bonded to C side chain rather than ring, these are alcohols not phenols
Reactions between alc and phe can be diff because proximity of delocalised ring influences the OH group
See diagram 14

Question 22

Phenol as weak acid

Answer 22

Less soluble in water than alc because of non polar benzene ring
When dissolved in water partially dissociates.
Weak acid
More acidic than alc but less than carb acid
Ethanol: doesn’t react with strong or weak base
Phenol: react with strong base
Carb acid: only these strong enough to react with weak base
See diagram 15

Question 23

Phenol + NaOH

Answer 23

See diagram 16

Question 24

Phenols undergo

Answer 24

Electrophilic substitution
Mild conditions r
React more readily than reactions of benzene

Question 25

Bromination of phenol

Answer 25

See diagram 17

Question 26

Nitration of phenol

Answer 26

See diagram 18

Question 27

Compare reactivity of phenol to benzene

Answer 27

Br + HNO3 react more readily with phenol than with Benz
Phenol nitrated with dilute HNO3 rather than conc
Increased reactivity because lone pair of e from O p orbital of OH group donated into pi system of phenol
E dens of Benz in phenol is increased which attracts electrophiles more strongly than with Benz
So phenol more susceptible to attack

Question 28

Activation and deactivation

Answer 28

See diagrams 19 and 20

Question 29

Directing effects

Answer 29

Different groups attached to benzene ring can have directing effects on any second substitute on ring
All 2 and 4 (ortho and para) are activating the except halogens
All 3 ( meta) are deactivating
See diagram 21