Arenes

Chemistry: IB Diploma Higher Level, topic 3.7.2

Structure of Benzene

Benzene is an aromatic hydrocarbon with delocalised electrons above and below the plane of the ring.

Benzene is a cyclic hydrocarbon structure containing six carbons and six hydrogens (C₆H₆).
The carbons in benzene are sp² hybridised.
Despite its unsaturation, the reactivity of benzene is different from alkenes.

The structure of benzene is aromatic and contains delocalised p-electrons (𝜋-bonds), due to overlapping p orbitals.
These delocalised electrons give benzene the aromatic structure, which has increased stability compared to the theoretical cyclohexa-1,3,5-triene molecule and forces the structure into a planar geometry.
The overlap of electron density above and below the plane of the ring dictates the structural features and reactivity of benzene and its derivatives (arenes).

To represent the mobility of double bonds in benzene, the skeletal structure shows that double bonds are not localised between two carbon atoms and the electrons can move around the entire structure.
The skeletal formula represents the true structure, which is a resonance hybrid.

The structural parameters of benzene have been characterised by modern analytical techniques.
Benzene shows key differences from both alkanes and alkenes.
The crystal structure of benzene demonstrates that all carbon-carbon bond lengths are the same length (0.139 nm).
The bond lengths are intermediate between C-C single-bond (0.154nm) and C=C double-bond (0.134nm) lengths, implying a bond order = 1.5.

Benzene is planar with a trigonal planar geometry around each carbon (120° bond angles).

Enthalpies for hydrogenation reactions of benzene can be compared to theoretical values for cyclohexa-1,3,5-triene to give evidence for increased stability of aromatic structures.
Hydrogenation of benzene is -208 kJ mol^-1, which is 151 kJ mol^-1 less exothermic than the theoretical value.
This increased stability is attributed to the delocalised electrons (aromaticity) and losing the aromaticity in addition reactions, such as hydrogenation, is unfavourable.

Benzene typically undergoes electrophilic substitution reactions, rather than addition.

Reactions of benzene and its derivatives are typically substitution and not addition.
This is different from what we observed with alkenes, which normally undergo electrophilic addition reactions.

As with alkenes, the reaction pathway starts with high-electron density in 𝜋-bonds attracting and being attacked by an incoming electrophile.
Contrary to alkenes, in benzene, substitution and not addition occurs after the electrophilic attack.
Substitution regenerates the complete delocalisation of electrons throughout the ring and achieves a higher stability aromatic structure.

Which of these is NOT evidence to support the aromatic structure of benzene?
Which of the following would benzene undergo an electrophilic substitution reaction with?