Unit 7: Benzene & Aromatic Compounds
Discovering the unique structure, stability, and reactivity of benzene and its derivatives.
7.23 Structure of Benzene (Kekulé, Delocalisation, Resonance)
Benzene ($C_6H_6$) is the parent compound of a large family of substances called aromatic compounds. Its structure puzzled chemists for a long time.
The Kekulé Structure
The first plausible structure, proposed by August Kekulé, was a flat, six-membered carbon ring with alternating single and double bonds. However, this model could not explain all of benzene's properties. For instance, it predicted two different isomers for 1,2-dibromobenzene, but only one is ever found. It also suggested that the C-C bonds should have different lengths, but experiments show they are all identical.
The Modern Delocalised Structure
The modern, accepted model of benzene describes a planar hexagonal ring of six carbon atoms. Each carbon atom is bonded to one hydrogen atom and two other carbon atoms via sigma ($\sigma$) bonds. The remaining p-orbital on each carbon atom overlaps sideways with the p-orbitals of its neighbours on both sides.
This continuous overlap creates a ring of electron density above and below the plane of the carbon atoms. The six pi ($\pi$) electrons are not located between specific atoms but are delocalised over the entire ring. This delocalisation, also known as resonance, is what gives benzene its unique properties. The structure is often represented as a hexagon with a circle inside.
Solved Examples:
- What is the molecular formula of
benzene?
Solution: $C_6H_6$. - What was the main problem with Kekulé's model of
benzene?
Solution: It predicted alternating single and double bond lengths, but experimental evidence shows all C-C bonds in benzene are identical in length. - What does the circle inside the hexagon in the symbol for benzene
represent?
Solution: The delocalised ring of six pi electrons. - How many pi electrons are in the delocalised system of
benzene?
Solution: Six. - What is the shape of the benzene
molecule?
Solution: It is a planar hexagon. - What is "delocalisation" in the context of
benzene?
Solution: It means the pi electrons are not confined to specific double bonds but are spread out over the entire six-carbon ring. - What is the bond angle between the carbon atoms in the benzene
ring?
Solution: 120°. - Are the C-C bonds in benzene longer or shorter than a typical C=C double
bond?
Solution: Longer. Their length (0.140 nm) is intermediate between a C-C single bond (0.154 nm) and a C=C double bond (0.134 nm). - What is another term for the delocalisation of electrons in
benzene?
Solution: Resonance. - What is the general term for compounds containing a benzene
ring?
Solution: Aromatic compounds.
7.24 Stability & Reactions (Substitution vs. Addition)
The delocalisation of the pi electrons makes the benzene molecule unusually stable. A significant amount of energy, known as the delocalisation energy or resonance energy, is required to break up this stable aromatic system.
This stability dictates benzene's chemical reactivity. Although it is unsaturated, it does not readily undergo the addition reactions that are characteristic of alkenes. An addition reaction would destroy the delocalised ring and the stability that comes with it.
Substitution Reactions:
Instead, benzene's typical reaction is electrophilic substitution. In this reaction, a hydrogen atom on the ring is replaced by another atom or group, but the stable delocalised pi system is preserved.
Example: Bromination of benzene.
This reaction requires a catalyst (a halogen carrier like $FeBr_3$) to make the bromine
a stronger electrophile.
$C_6H_6 + Br_2 \xrightarrow{FeBr_3} C_6H_5Br + HBr$
Under harsh conditions (e.g., high temperature and pressure, UV light), benzene can be forced to undergo addition reactions, such as hydrogenation to form cyclohexane ($C_6H_{12}$).
Solved Examples:
- Why is benzene more stable than the hypothetical
cyclohexatriene?
Solution: Due to the delocalisation of its six pi electrons, which spreads the electron density over the entire ring and lowers the molecule's overall energy. - What is the characteristic reaction type of
benzene?
Solution: Electrophilic substitution. - Why does benzene not readily undergo addition reactions like
ethene?
Solution: Because an addition reaction would destroy the very stable delocalised aromatic ring system. - What is needed for benzene to react with
bromine?
Solution: A halogen carrier catalyst, such as iron(III) bromide ($FeBr_3$). - Does benzene decolourise bromine water under normal lab
conditions?
Solution: No. This is a key test to distinguish it from an alkene. - What is the product when benzene undergoes a substitution reaction with
chlorine in the presence of a catalyst?
Solution: Chlorobenzene ($C_6H_5Cl$) and hydrogen chloride ($HCl$). - What is the product when benzene is hydrogenated under high pressure and
temperature with a nickel catalyst?
Solution: Cyclohexane ($C_6H_{12}$). - Is the delocalised ring preserved in a substitution
reaction?
Solution: Yes. - What is an electrophile?
Solution: An electron-pair acceptor; a species that is attracted to regions of high electron density. - What is the role of the $FeBr_3$ catalyst in the bromination of
benzene?
Solution: It polarises the $Br_2$ molecule, making it a stronger electrophile that is able to attack the stable benzene ring.
7.25 Uses of Benzene
Benzene and its derivatives are important industrial chemicals, although the use of pure benzene is now restricted due to its toxicity and carcinogenic nature.
Major Uses:
- Starting material for synthesis: Benzene is a crucial starting material (feedstock) for the production of many other organic chemicals.
- Production of Polymers: It is used to make ethylbenzene, which is then converted to phenylethene (styrene), the monomer for making polystyrene. It is also used in the production of nylon.
- Pharmaceuticals and Dyes: The benzene ring is a common structural feature in many drugs, dyes, and pesticides.
- Former Uses: In the past, benzene was widely used as an industrial solvent and as an additive in petrol to increase the octane number, but these uses have been largely phased out in favour of safer alternatives like methylbenzene (toluene).
Solved Examples:
- Name a major polymer that is produced from a benzene
derivative.
Solution: Polystyrene. - What is the monomer used to make
polystyrene?
Solution: Phenylethene (styrene). - Why is the use of benzene as a solvent now
restricted?
Solution: Because it is toxic and a known carcinogen (cancer-causing agent). - Name a safer alternative to benzene that is used as a solvent and petrol
additive.
Solution: Methylbenzene (toluene). - What is a "feedstock" chemical?
Solution: A basic chemical that is used as a starting material to synthesise other, more complex chemicals. - Besides polystyrene, what other type of polymer is benzene used to
produce?
Solution: Nylon. - What was a historical use of benzene related to
fuels?
Solution: It was added to petrol to increase the octane number. - What is the relationship between ethylbenzene and phenylethene
(styrene)?
Solution: Ethylbenzene is dehydrogenated (hydrogen is removed) to produce phenylethene. - Are aromatic compounds important in the pharmaceutical
industry?
Solution: Yes, the benzene ring is a common feature in the structure of many medicinal drugs. - What is the IUPAC name for styrene?
Solution: Phenylethene.