25) Aromatic chemistry

Question 1

What is the molecular formula of benzene?

Answer 1

C6H6

Question 2

Give 3 reasons for the rejection of Kekulé’s model of benzene.

Answer 2

doesn’t undergo electrophilic addition reactions or decolourise Br2 under normal conditions (contains no C=C)
x-ray diffraction has shown benzene to contain bonds of equal length
enthalpy for hydrogenation of benzene was less exothermic than predicted

Question 3

Describe the delocalised model of benzene.

Answer 3

the sideways overlap of p-orbitals above and below the plane form a delocalised ring of electron density above and below the plane of the benzene ring (system of pi bonds)

Question 4

When does benzene become a substituent and the prefix ‘phenyl’ used?

Answer 4

when a benzene ring is attached to an alkyl chain with a functional group / with 7 or more carbon atoms

Question 5

In the electrophilic substitution mechanism for benzene, where are the 2 curly arrows?

Answer 5

from the benzene ring to the electrophile

from the H bond to the positive centre

Question 6

State the reagents and required temperature for the nitration of benzene

Answer 6

conc. HNO3 and conc. H2SO4

50 degrees C

Question 7

What acts as the nucleophile in the nitration of benzene?

Answer 7

NO2 +

Question 8

Give 2 examples of the halogen carriers that can be used for the halogenation of benzene

Answer 8

AlCl3 and FeBr3

Question 9

Describe the alkylation of benzene

Answer 9

substitution of a hydrogen atom in the benzene ring by an alkyl group (carried out with a haloalkane in the presence of AlCl3) which increases the number of carbon atoms by forming C-C bonds

Question 10

Describe the acylation of benzene

Answer 10

when benzene reacts with an acyl chloride in the presence of AlCl3 forming an aromatic ketone e.g. phenylethanone

Question 11

What are phenols?

Answer 11

a type of aromatic compound that contains a hydroxyl -OH functional group directly bonded to an aromatic ring e.g. C6H5OH

Question 12

Phenol is a weak acid. Does phenol react with Na2CO3, a weak base?

Answer 12

No - unlike carboxylic acids, it will only react with strong bases such as NaOH

Question 13

What organic product is formed when phenol undergoes a neutralisation reaction by NaOH?

Answer 13

sodium phenoxide, C6H5O-Na+

Question 14

Explain why phenol is more reactive / can react under milder conditions than benzene.

Answer 14

One of the lone pairs of electrons from the oxygen p-orbital of the -OH group (which overlaps with the delocalised pi system) donates electron density to the ring, making it more susceptible to attack from electrophiles

Question 15

What mixture of organic products will be formed when phenol undergoes nitration with dilute HNO3?

Answer 15

2-nitrophenol and 4-nitrophenol

Question 16

Why is no halogen catalyst required for the bromination of phenol?

Answer 16

the electron density in the phenol ring structure is sufficient to polarise Br2

Question 17

State the organic product and observations of the bromination of phenol

Answer 17

2,4,6-tribromophenol, white ppt

orange to colourless solution

Question 18

Define disubstitution

Answer 18

a second substitution

Question 19

What can different group on the benzene ring have on any second subtituent?

Answer 19

a directing effect

Question 20

What are activating groups?

Answer 20

electron donating groups which activate the benzene ring so the ring reacts more readily with electrophiles; 2- and 4- directing groups
e.g. -NH2, -NHR, -OH, -OR, -R, -F, -Cl, -Br, -I

Question 21

What are deactivating groups?

Answer 21

electron withdrawing groups which deactivate the benzene ring, so the ring reacts less readily with electrophiles; 3- directing groups
e.g. RCOR, -COOR, -SO3H, -CHO, -COOH, -CN, -NO2, -NR3 +

Question 22

Which type of group (activating or deactivating) will cause a mixture of products to be formed?

Answer 22

activating groups

Question 23

If 2 groups are on carbons 1 and 3, would the first substitution have been to put on an activating or deactivating group?

Answer 23

a deactivating group

Question 24

Why could you expect twice as many products with groups on the second carbon as the fourth carbon?

Answer 24

because positions 2 and 6 are equivalent

Question 25

Name 4 key stages to help with planning a two-step organic synthesis

Answer 25

1) identify the type of distribution
2) identify the directing effect of the two substituents
3) write out the steps with the reagents needed for the synthesis
4) note if there is a mixture of organic products to separate

Question 26

What is the empirical formula of benzene?

Answer 26

CH

Question 27

What are arenes?

Answer 27

aromatic hydrocarbons that contain one or more benzene rings