3: Aromatic Compounds

Question 1

What is an aromatic compound

Answer 1

A compound containing a benzene ring

Question 2

What can an aromatic hydrocarbon also be called

Answer 2

arene

Question 3

Does benzene decolourise bromine in the dark

Answer 3

No

Question 4

What reasons are there to reject benzene having 3 localised pi bonds

Answer 4

• Benzene does not easily undergo the typical addition reactions of alkenes (Does not decolourise bromine in the dark)
• All the bonds between C atoms around the ring are the same length
• Hydrogenation of benzene is less exothermic than expected by comparison with alkenes

Question 5

Description of benzene’s structure

Answer 5

• Benzene is a planar molecule
• All six carbon-carbon bonds lengths are the same
• Each carbon atom forms 3 sigma bonds, two to neighbouring carbon atoms and one to a hydrogen atom
• Each carbon atom also has 1 electron in a p orbital, which extends above and below the plane of the molecule, these orbitals overlap to form a delocalised π-electron system

Question 6

What is a delocalised π-electron system

Answer 6

A continuous π cloud of electrons above and below the plane of the molecules

Question 7

How many electrons are in a delocalised π-electron system

Answer 7

6

Question 8

Why does benzene have low reactivity

Answer 8

Delocalisation spreads out the π-electrons more than they would be in a localised π-bond. Hence the electron density is lower in aromatic compounds and they are consequently less attractive to electrophiles

Question 9

Why are the carbon-carbon bond lengths in benzene all the same

Answer 9

π-electrons are evenly distributed around the ring

Question 10

Why is enthalpy change for hydrogenation of benzene lower than expected

Answer 10

Delocalisation lowers the energy of the π-electrons so they are in a more stable state than electrons in localised π bonds.

Question 11

What is an electrophile

Answer 11

an electron pair acceptor

Question 12

Why are aromatic compounds less reactive than alkenes

Answer 12

They have lower π-electron density so electrophiles are less strongly attracted. Consequently, reactions of aromatic compounds tend to occur much more slowly and often require catalysts

Question 13

Do aromatics tend to react by substitution or addition

Answer 13

substitution

Question 14

Why do aromatics tend to react by substitution

Answer 14

Because it is favourable to keep the delocalised π-electron system

Question 15

What is the formula of a nitro group

Answer 15

-NO₂

Question 16

What type of reaction is nitration of an aromatic

Answer 16

Electrophilic substitution

Question 17

What does the aromatic react with in nitration

Answer 17

conc. nitric acid and conc. sulphuric acid

Question 18

Equation for formation of the electrophile in nitration of an aromatic

Answer 18

HNO₃ + H₂SO₄ ⇌ NO₂† + H₂O + HSO₄¯

Question 19

Describe the mechanism of an aromatic reacting with the NO₂† electrophile in nitration reaction

Answer 19

2 electrons move from the delocalised ring to the nitrate ion (curly arrow).
Now the NO₂ is bonded to the aromatic but the ring is now “horse-shoe” shaped and is positive (this is the carbocation).
The hydrogen bonded to the same carbon as NO₂ is eliminated and its bonding pair is put into the horse-shoe to regain the ring
So NO₂ is bonded to the aromatic and there is a separate H† ion

Question 20

Equation for regen of the catalyst in nitration of aromatics reaction

Answer 20

HSO₄¯ + H† → H₂SO₄

Question 21

What is needed for aromatic compounds to react with halogens

Answer 21

halogen carriers

Question 22

Examples of halogen carriers

Answer 22

AlCl₃ or FeCl₃

AlBr₃ or FeBr₃

Question 23

Observations of the halogenation of aromatic compounds

Answer 23

Bromine is decolourised (orange → colourless)

Steamy fumes of HBr

Question 24

Mechanism of reaction between halogen and halogen carrier

in halogenation of an aromatic compound

Answer 24

Br₂ + AlBr₃ ⇌ Br† + AlBr₄¯

Br† is a powerful electrophile

Question 25

Describe the mechanism of an aromatic compound reacting with Br+ in halogenation reaction

Answer 25

2 electrons move from the delocalised ring to the Br† ion (curly arrow).

Now the Br is bonded to the aromatic but the ring is now “horse-shoe” shaped and is positive (this is the carbocation).

The hydrogen bonded to the same carbon as Br is eliminated and its bonding pair is put into the horse-shoe to regain the ring

So Br is bonded to the aromatic and there is a separate H† ion

Question 26

Equation for the regen of the catalyst (halogen carrier) in halogenation of an aromatic compound

Answer 26

H† + AlBr₄¯ → AlBr₃ + HBr

Question 27

Equation for generation of electrophile for aromatic reacting with haloalkane (Friedel - Crafts Alkylation)

Answer 27

RCl + AlCl₃ ⇌ R† + AlCl₄¯

Question 28

Describe the mechanism of an aromatic compound reacting with a positive alkane ion in Friedel-Crafts alkylation

Answer 28

2 electrons move from the delocalised ring to the positive alkane ion (curly arrow).

Now the alkane is bonded to the aromatic but the ring is now “horse-shoe” shaped and is positive (this is the carbocation).

The hydrogen bonded to the same carbon as the alkane is eliminated and its bonding pair is put into the horse-shoe to regain the ring

So the alkane is bonded to the aromatic and there is a separate H† ion

Question 29

Equation for regen of catalyst in Friedel-Crafts alkylation

Answer 29

H† + AlCl₄¯ ⇌ HCl + AlCl₃

Question 30

Why are Friedel-Crafts reactions very important in organic synthesis

Answer 30

They form C-C bonds and therefore allow the carbon framework of an organic molecule to be extended.

Question 31

Equation for generation of the electrophile for aromatic reacting with an acyl chloride

Answer 31

CH₃COCl + AlCl₃ ⇌ CH₃CO† + AlCl₄¯

Question 32

Describe the mechanism of an aromatic compound reacting with CH₃CO† in Friedel-Crafts Acylation

Answer 32

2 electrons move from the delocalised ring to the C† in the CH₃CO† (curly arrow).

Now the CH₃CO† is bonded to the aromatic but the ring is now “horse-shoe” shaped and is positive (this is the carbocation).

The hydrogen bonded to the same carbon as the CH₃CO† is eliminated and its bonding pair is put into the horse-shoe to regain the ring

So the CH₃CO† is bonded to the aromatic and there is a separate H† ion

Question 33

Equation for regen of catalyst in Friedel-Crafts Acylation

Answer 33

H† + AlCl₄¯ ⇌ HCl + AlCl₃

Question 34

What is a benzene ring called when it is in a molecule

Answer 34

It’s a phenyl group

eg. phenylethene is a benzene ring bonded to ethene

Question 35

What is an aliphatic

Answer 35

Any compound that does not contain a benzene ring

Question 36

What is an alicyclic

Answer 36

Any aliphatic compound that contains a non-aromatic ring