Aromatic Compounds

Question 1

What are aromatic compounds

Answer 1

Benzene and structurally related compounds

Question 2

What are common aromatic compounds

Answer 2

Phenol (hydroxybenze, OH), Aniline (aminobenzene, NH2), Toulene (methylbenzene, CH3), Benzaldehyde (HC=O)

Question 3

Describe bonding in benzene

Answer 3

Benzene is planar and all the carbon bonds are the same length. Bonds are between a single and double bond (1.39A)

Question 4

What are the canonical forms of benzene

Answer 4

Due to resonance electrons arranged in 2 ways,Real structures is a mixture of the two canonical forms.

Question 5

Why does benzene not react like an alkene

Answer 5

It does not do electrophilic additions

Question 6

What is benzene more stable to than alkenes

Answer 6

Hydrogenation

Question 7

Why is benzene more stable than expected

Answer 7

P orbitals on carbon atoms interact with each other to form pi bonds. Benzene has a doughnut shaped electron cloud above and below the ring- electrons are delocalised

Question 8

What is the Huckel rule for aromatic compounds

Answer 8

There are 4n+2 pi electrons, where n= an integer. 4n + 2 = 2, 6, 10, 14

Question 9

How can you tell if cyclic conjugated alkenes are aromatic

Answer 9

1. They are planar

2. There are 4n+2 pi electrons

Question 10

How many pi electrons are 1 double bond or 1 lone pair of electrons

Answer 10

2 pi electrons

Question 11

Is cyclobutane (2 double bonds) aromatic

Answer 11

No. Has 4 pi electrons which does not follow the Huckel rule. Very unstable

Question 12

Is benzene (3 double bonds) aromatic

Answer 12

Yes. Has 6 pi electrons (3 double bonds x 2 = 6 pi electrons), follows the Huckel rule. Very stable

Question 13

Is cyclooctatetraene (4 double bonds) aromatic

Answer 13

No. Has 8 pi electrons so does not follow the Huckel rule. Reacts like an alkene

Question 14

Is pentalene (4 double bonds) aromatic

Answer 14

No. Has 8 pi electrons so does not follow the Huckel rule. Never been isolated

Question 15

Is napthalene (5 double bonds) aromatic

Answer 15

Yes. Has 10 pi electrons so follows the Huckel rule. Reacts like benzene

Question 16

What happens to lone pairs of electrons substituents (e.g. NH2) of aromatic rings

Answer 16

Lone pair becomes partially delocalised (decreases reactivity of lone pair, but increases reactivity of aromatic ring towards electrophiles). E.g. nitrogen lone pair is partially delocalised (makine aniline less basic and less nucleophilic than alkylamine)

Question 17

Why is phenol more acidic than methanol

Answer 17

The negative charge is stabilised by delocalisation in the phenolate anion

Question 18

What reactions do aromatic compounds undergo with electrophiles

Answer 18

Electrophilic substitution

Question 19

Describe bromination

Answer 19

Benzene (+Br2 and FeBr3) -> bromobenzene

Question 20

Why is the FeBr3 catalyst required in bromination of benzene

Answer 20

It makes Br2 a better electrophile as it polarises it.

Question 21

What is nitration

Answer 21

Reaction with concentrated nitric acid and sulfuric acid

Question 22

What is the electrophile in nitration of benzene

Answer 22

NO2+

Question 23

How is NO2+ formed

Answer 23

HO-NO2 + H2SO4 H2O(+)-NO2 + HSO4- H20 + NO2+ + H2SO4-

Question 24

Why is nitration a useful reaction for making substituted benzenes

Answer 24

Nitro (NO2) groups can be converted into a variety of other functional groups

Question 25

What does activated mean

Answer 25

More reactive than benzene

Question 26

What does deactivated mean

Answer 26

Less reactive than benzene

Question 27

What are electron donating groups

Answer 27

Activating

Question 28

What are electron withdrawing groups

Answer 28

Deactivating

Question 29

Why are alkybenzenes activiating

Answer 29

The alkyl group is electron donating (inductive effect) and is a activator

Question 30

Why are phenol (-OH substituent) activating

Answer 30

Oxygen donates a lone pair to the benzene ring that stabilizes the intermediate (so activating)

Question 31

Why are carbonyl and nitro groups deactiviating

Answer 31

These substituents withdraw electrons and hence slow the rate of reaction.