3.3.10 Aromatic chemistry

Question 1

Describe the structure of bezene

Answer 1

1. Benzene has the formula C₆H₆
2. It has a planar cyclic structure
3. Each carbon atom forms single colvalent bonds to the carbons on either side of it and to one hydrogen atom. The final unpaired electron on each carbon atom is located in a p-orbital that sticks out above and below the carbon ring. The p-orbitals on each atom combine to form a ring of delocalised electrons
4. All the carbon-carbon bonds in the ring are the same, so they are the same length, 140pm. This lie inbetween the length of a single C-C bond, 154 pm, and a double C=C bond, 135 pm

Question 2

How is benzene drawn in skeletal formula?

Answer 2

A hexagon with a ring in the middle symbolising delocalised electrons

Question 3

How is the Kekule structure drawn

Answer 3

A hexagon with alternating double bonds

There are not really alternating double and single bonds in benzene

Question 4

How does the delocalised ring of electrons in benzene make it more stable than the Kekulé structure?

Answer 4

Benzene is far more stable than the theoretical compound cyclohexa-1,2,3-triene would be. You can see this by comparing the enthalpy change of hydrogenation for benzene with cyclohexene:
1. Cyclohexene has one double bond. When it’s hydrogenated, the enthalpy change is -120kJ mol^-1. If benzene had three double bonds you’d expect it to have an enthalpy of -360kJ mol^-1
2. But the experimental enthalpy of hydrogenation for benzene is -208kJ mol^-1 - far less exothermic than expected
3. Energy is put in to break bonds and released when bonds are made. So more energy must have been put in to break the bonds in benzene than would have been needed to break the bonds in cyclohexa-1,2,3-triene

This difference indicates that benzene is more stable than cyclohexa-1,2,3-triene would be, this is thought to be due to the delocalised ring of electrons

Question 5

What are molecules containing a benzene ring called

Answer 5

Arenes or aromatic compounds

Question 6

Give the two ways arenes can be named

Answer 6

1 - The functional group is written before benzene, e.g:
* Chlorobenzene
* Nitrobenzene
* 1,3-dimethylbenzene

2 - Others are named as compounds with a phenyl group (C₆H₅) attached, e.g:
* Phenol
* Phenylamine

Question 7

Explain why arenes undergo electophillic substitution reactions

Answer 7

The benzene ring is a region of high electron density, so it attracts electrophiles. As the bezene ring’s so stable, it doesn’t undergo electrophillic addition reactions, which would destroy the delocalised ring of electrons. Instead, it undergoes electrophillic substitution reactions where one of the hydrogen atoms (or another functional group on the ring) is substituted for the electrophile

Question 8

Explain how Friedal-Crafts Acylation reactions produce phenylketones

Answer 8

• Many useful chemicals such as dyes and pharmaceuticals contain benzene rings. But because benzene is so stable, it’s fairly unreactive, therefore it’s more difficult to make chemicals that contain benzene.
• Friedal-Crafts acylation reactions are used to add an acyl group (RCO-) to the benzene ring, once an acyl group has been added, the side chains can be modified using further reactions to make useful products
• An electrophile has to have a strong positive charge to be able to attack the benzene ring - most aren’t polarised enough. But some can be made into stronger electrophiles using a catalyst called a halogen carrier
• Friedal-Crafts uses an acyl chloride as an electrophile and AlCl₃ as a halogen carrier

Question 9

Explain how AlCl₃ makes the acyl chloride electrophile stronger

Answer 9

AlCl₃ accepts a lone pair of electrons from the acyl chloride. As the lone pair of electrons is pulled away, the polarisation in the acyl chloride increases and it forms a carbocation. This makes it a much stronger electrophile, and gives it a strong enough charge to react with the benzene ring

RC^δ+OCl^δ-: + AlCl₃ -> RC⁺O + AlCl₄^-

Question 10

Explain how the electrophile is substituted into the bezene ring in Friedal-Crafts Acylation

The mechanism for electrophillic substitution

Answer 10

1. Electrons in the benzene ring are attracted to the positively charged carbocation. The two electrons from the benzene bond with the carbocation. This partially breaks the delocalised ring and gives it a positive charge.
2. The negatively charged AlCl₄^- ion is attracted to the positively charged ring. One chloride ion breaks away from the aluminium chloride ion and bonds with the hydrogen ion. This removes the hydrogen from the ring forming HCl. It also allows the catalyst to reform.
3. The reactant needs to be heated under reflux in a non-aqueous solvent (like dry ether) for the reaction to occur.

Question 11

Give the equations for the production of the nitronium ion, where sulfuric acid acts as a catalyst

Answer 11

HNO₃ + H₂SO₄ -> H₂NO₃⁺ + HSO₄^-

H₂NO₃⁺ -> NO₂⁺ + H₂O

The sulfuric acid gains its hydrogen again later

Question 12

Give the mechanism and conditions required to nitrate benzene

Answer 12

Electrophillic addition:
1. The nitronium ion attacks the bezene ring - electrons from the ring of delocalised electrons are attracted to the positive charge on the nitronium ion
2. An unstable intermediate is formed - the ring of delocalised electrons is broken and has a positve charge
3. A pair of electons bonding a hydrogen to the benzene ring (on the same carbon as the nitronium ion) is attracted to the positive charge - this causes the bond to break and the H⁺ goes to react with the sulfuric acid catalyst

Bezene is warmed with concetracted nitric and concentrated sulfuric acid to produce nitrobenzene.
Sulfuric acid acts as a catalyst - it helps to make the nitronium ion, NO₂⁺, which is the electrophile.

If you only want one NO₂ group added (mononitration), the temperature must be kept bellow 55^oC. Above this temperature you’ll get more subtitutions

Question 13

What can nitration reactions be used to produce?

Answer 13

• Nitrocompounds can be reduced to form aromatic amines, which can be used to manufacture dyes and pharmaceuticals
• Some nitro compounds can be used as explosives, such as 2,4,6-trinitromethylbenzene (trinitrotoluene - TNT)