6.1: Aromatic compounds, carbonyls and acids

Question 1

What were the 3 pieces of evidence that did not support Kekule’s model?

Answer 1

1. Benzene resists to typical ADDITION
2. Enthalpy of hydrogenation is more STABLE than expected
3. All bonds are the same length

Question 2

Explain what was expected of Benzene and what is actually true in terms of “Resistance to addition”

Answer 2

• Benzene expected to undergo electrophilic addition
• Benzene actually undergoes electrophilic substitution

(Kekule thought that benzene experiences a rapid dynamic equilibrium and double bonds alternate)

Question 3

Explain what was expected of Benzene and what is actually true in terms of “Enthalpy of Hydrogenation”

Answer 3

• Benzene expected to have a ▲H of -360 kJ mol^-1
  -What actually happened:
  the ▲H of cyclohexene is -120 kJ mol^-1
  the ▲H of Kekule’s benzene is -360 kJ mol^-1
  but Benzene is the difference between these two: 360-120 = -280 kJ mol^-1 MORE stable than predicted

Question 4

Explain what was expected of Benzene and what is actually true in terms of “Bond lengths”

Answer 4

• Benzene expected to have alternating double bonds (different lengths)
• Benzene’s 6 bonds actually are of the same length, according to X-Ray diffraction (0.140 nm)

Question 5

What is the Revised Model of Benzene?

Answer 5

The DELOCALISED ELECTRON structure

Question 6

Describe the Delocalised Electron structure (2)

Answer 6

• EACH carbon donates an electron from its P-ORBITAL

- Forms a ring of electrons ABOVE and BELOW the plane

Question 7

What does the Delocalised Electron structure show in terms of its electron density and energy required to disrupt the delocalisation?

Answer 7

• LOW electron density

- so MORE ENERGY required to disrupt the delocalisation

Question 8

What are “derivatives” when naming aromatic compounds?

Answer 8

Aromatic compounds where a H is substituted for a different group

Question 9

What are 4 common groups and their prefixes?

Answer 9

1. Chlorine: Chloro-
2. Bromine: Bromo-
3. Nitro group: Nitro-
4. Alkyl chains: Methyl,ethyl, etc.

Question 10

When a Hydrogen is removed from Benzene what is the prefix of that derivative called?

Answer 10

Phenyl (C6H5)

Question 11

When a derivative is attached to a compounds but is NOT the focus, is it named as a prefix or a suffix?

Answer 11

Prefix (e.g. Benzene with a NH2 group is called PHENYLamine)

Question 12

Give two examples of single-substituted aromatic compounds.

Answer 12

Methylbenzene (CH3 attached)

Chloromethylbenzene (CH2Cl attached)

Question 13

How are double-substituted aromatic compounds named (3)?

Answer 13

1. Carbon attached to 1st group, Labelled ‘1’
2. Relative position of 2nd group found
3. Prefix written in ALPHABETICAL ORDER

Question 14

Give an example of a double-substituted aromatic compound

Answer 14

2-chloromethylbenzene (CH3 is on the 1st carbon, Cl attached to 2nd carbon)

Question 15

How are aromatic compounds named when multiples of the same group are substituted?

Answer 15

We use ‘DI (2)’ or ‘TRI (3)’

Question 16

Give an example of a multiple-substituted aromatic compound.

Answer 16

1,3,5-trimethylbenzene

Question 17

Define electrophilic substitution.

Answer 17

A SUBSTITUTION reaction where an ELECTROPHILE is attracted to an ELECTRON-RICH area and accepts a PAIR OF ELECTRONS.

Question 18

Why do electrophilic substitution reactions occur in Benzene?

Answer 18

Because of it’s high e- density.

Question 19

Describe the general mechanism of electrophilic substitution.

Answer 19

1. Electrophile attracted to the delocalised e- ring
2. Accepts e- pair (this is the rate-determining step)
3. Delocalisation DISRUPTED and intermediate formed, the positive charge is spread over the ring.
4. Intermediate releases H+ (joins with the -ve ion)

Question 20

State the reagents and conditions needed for the Nitration of Benzene (an electrophilic substitution reaction).

Answer 20

Reagents: CONCENTRATED SULFURIC ACID, CONCENTRATED NITRIC ACID
Conditions: 50 C, a higher temperature may cause multiple substitutions

Question 21

State the overall reaction for the nitration of benzene.

Answer 21

C6H6 + HNO3 → C6H5NO2 + H2O

Question 22

Explain, step by step, the mechanism of the nitration of benzene (3).

Answer 22

1. Electrophile formed: HNO3 + H2SO4 → NO2+ + 2HSO4- + H3O+
2. NO2+ accepts lone pair, attaches to benzene, forms an intermediate (positive horseshoe)
3. H joins with the HSO4- ion to form nitrobenzene and H2SO4 (h2so4 is catalyst)