Shapes of aromatic organic molecules; σ and π bonds — common mistakes

When asked to describe the bonding in benzene, always discuss the formation of the σ framework first, then explain the formation of the delocalised π system from the remaining p-orbitals. Marks are often allocated for both parts separately.

When comparing benzene's stability, always be specific. Don't just say 'benzene is stable'. Instead, say 'benzene is more stable than expected for a structure with three C=C bonds due to delocalisation energy'. This comparative and explanatory detail is what examiners look for.

No. Cyclohexa-1,3,5-triene is a hypothetical molecule with alternating, distinct single and double bonds of different lengths. Benzene is a real molecule where all C-C bonds are identical in length and strength due to a delocalised π system, which makes it significantly more stable.

Each carbon atom is sp² hybridised, forming three σ bonds. According to VSEPR theory, three regions of electron density around a central atom will arrange themselves to be as far apart as possible, resulting in a trigonal planar geometry with 120° bond angles.

While the aromatic ring is very stable, the delocalised π system is a region of high electron density. This makes it susceptible to attack by strong electrophiles (electron-seeking species). The reactions proceed via a substitution mechanism that allows the stable aromatic ring to be regenerated in the final product.

The circle represents the six delocalised π electrons that are shared equally among all six carbon atoms in the ring. It is a more accurate representation of benzene's bonding than the Kekulé structure with alternating double bonds.