(a)     Describe the structure of benzene, C₆H₆.

(b)     State two pieces of evidence that support this description.

(a)     hexagonal / ring of six carbon atoms (each with one hydrogen);

planar;

all carbon-carbon bond lengths equivalent / all carbon-carbon bond lengths intermediate between single and double bonds / carbon-carbon bond order of 1.5;

all C–C–C bond angles 120°;

Allow sp² (hybridization for C’s).

delocalization / resonance;

Allow [2 max] for regular hexagon for M1 and M3.

Award [1 max] for drawing a correct representation of benzene indicating delocalization, but do not award mark for drawing simply a Kekulé structure alone.

If any of these points are stated in (b) award marks in (a).

(b)     enthalpy change of hydrogenation not equal to three times enthalpy change of hydrogenation of cyclohexene;

electron density map (of benzene) showing equal electron density/all carbon-carbon bond lengths equivalent / OWTTE;

Allow diffraction pattern or contour map for electron density map.

only one isomer exists for 1,2-disubstituted benzene compounds / only three disubstituted benzene compounds (rather than four);

undergoes (electrophilic) substitution reactions / does not undergo addition reactions / does not decolorize bromine water;

In (a) describing the structure of benzene was well known by many candidates, but the evidence required in (b) to support this description was not well answered by about half of the candidates and chemical language was used imprecisely.