Aromatic Chemistry

Question 1

Describe the nature of the bonding in a benzene ring.

Answer 1

• Benzene (C₆H₆) consists of a flat (planar), regular hexagon of carbon atoms, each of which is bonded to a single hydrogen atom.
• 120^o bond angles.
• The C-C lenghts in benzene are intermediate between those expected for a carbon-carbon single bond and a carbon-carbon double bond; each bond is intermediate between a single and a double bond.
• This is represented by a hexagon with a circle inside.

Question 2

What does delocalisation refer to?

Answer 2

• Delocalisation of electrons confers stability to the molecule; the electrons are spread over more than two atoms.
• Delocalised electrons form a region of electron density above and below the plane of the ring, from an electron from each carbon’s p-orbital; these electrons overlap and are delocalised.
• Overall, each carbon-carbon bond has a total of 3 electrons, thus making it between a single and a double bond, making benzene unusually stable;
• *aromatic stability**.

Question 3

What is the thermochemical evidence for benzene’s stability?

Answer 3

• The enthalpy change for the hydrogenation (adding H₂) of cyclohexene (one C=C bond) to cylcohexane is -120 kJ mol-1.
• One would expect the hydrogenation of a ring with alterante double bonds (Kekulé Model) to be three times this; -360 kJ mol^-1
• However, the actual enthalpy change for benzene is in fact -208 kJ mol-1; more energy must have been put in to break the bonds in benzene than would have been needed to break the bonds in the Kekulé structure.
• Thus Benzene is more stable than the Kekulé stucture, due to the ring of delocalised electrons; benzene is 152 kJ mol-1 more stable than an unsaturated ring structure (plain hexagon).

Question 4

What reactions do arenes/aromatic systems undergo?

Answer 4

• Electrophilic substiution reacitons; electrophiles (has a positve charge; postive ion or positive end of a dipole) attack the ring; due to the high electron density caused by the delocalised electrons.
• Due to the sheer stability of the aromatic (benzene) ring, it tends to undergo electrophilic substituion reactions which preserve the delocalised ring; the aromatic ring takes energy to be put in to break the ring before the system is destroyed; the delocalisation energy.
• Ring almost always remains intact in the reactions of arenes.

Question 5

What occurs when ethanoyl chloride is added to benzene?

(Name of reaciton, mechanism, equations)

Answer 5

• A Friedal-Crafts acylation reaction occurs, producing a phenylketone product, HCl by-product, and AlCl₃ (catalyst reformation).
• AlCl₃ is a halogen carrier that makes the acyl chloride (e.g. ethanoyl chloride) electrophile stronger; most electrophiles are not polarised enough to attack the stable benzene ring, not postively charged enough, hence the action of AlCl_3.
• AlCl₃ accepts a lone pair of electrons from the acyl chloride; as the lone pair is pulled away, the polarisation in the acyl chloride increases and it forms a carbocation.
  (makes it a much stronger electrophile; strong enough to react with the benzene ring).
• The alumunium atom in AlCl₃ only has 6 electrons in its outer main level thus readily accepts a lone pair from the Cl atom of RCOCl.

CH3COCl + AlCl₃ → CH₃CO⁺ + AlCl₄^-

RCOCl + AlCl₃ → RCO+ + AlCl₄^-

AlCl₄^- + H⁺ → AlCl₃ + HCl

1. Electrons in the benzene ring are attracted to the positively charged carbocation. Two electrons from 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 aluminum chloride ion and bonds with the hydrogen ion (the C loses the H+ ion to regain the stability of the aromatic system which is destroyed when the electrons bond with the carbocation). This removes the hydrogen from the ring, forming HCl.
   It allows the catalyst to reform.

Conditions:

• Heated under reflux
• Anhydrous (non-aqueous environment)

Question 6

Why are Friedal-Crafts acylation reactions important?

Answer 6

• Friedal-Crafts acylation reactions are used to add an acyl group (-C(=O)-R) to the benzene ring of dyes and pharmaceuticals (benzene’s stability made the compound fairly reactive).
• The acyl group allows the side chains to be modified by further reactions to produce useful products, hence its important in synthesis.

Question 7

What reaction occurs when you warm benzene with concentrated nitric and sulfuric acids?

(Mechanism, reaction equations to form catalyst)

Answer 7

• Nitration; a nitrobenzene product is achieved.
• During nitration, the substitution of NO2 for one of the hydrogen atoms on the benzene rings, occurs.
• The electrophile NO₂⁺ is generated in the reaction mixture of conc. nitric and sulfuric acid; H₂SO₄ is a stronger acid than HNO₃, and donates a proton (H⁺) to HNO₃ (which is acting as a Bronsted-Lowry base).
• H₂NO₃⁺ then loses a molecule of water to give the electrophile NO₂⁺; the nitronium ion/nitryl cation.

H₂SO₄ + HNO₃ → H₂NO₃+ + HSO₄^-

H₂NO₃⁺ → NO₂⁺ + H₂O

• The H+ ion from the benzene ring reacts with HSO₄^- to reform the catalyst, H₂SO₄.

Question 8

Why is the nitration of arenes important?

Answer 8

• Nitration is an important step in synthesis; nitro compounds can be reduced to form aromatic amines, which are used to manufacture dyes and pharamceuticals.
• Also used in the manufacture of explosives; nitro compounds decompose violently upon heating, e.g. 2,4,6-trinitrotoluene, TNT.