3.10.1 bonding

aromatic compound

a cyclic, planar organic molecule with a continuous ring of delocalised pi electrons

this delocalisation gives the molecule extra stability

benzene

an aromatic cyclic hydrocarbon consisting of a ring of 6 carbon atoms with 6 hydrogen atoms, and a central ring of delocalised pi electrons

C₆H₆

describe the nature of bonding in a benzene ring

• planar structure, all bond angles are 120°

• each bond in the ring is the same length and has an intermediate length between a single and a double C to C bond

• the outer electron from the p-orbital of each C atom is delocalised. these p-orbitals overlap sideways to form a continous delocalised ring above and below the ring of C atoms.

  • the delocalised electrons form pi bonds spread around the ring of C atoms

• delocalisation of p electrons makes benzene more stable

what was Kekulé’s proposed benzene structure?

• he suggested cyclohexa-1,3,5-triene, which has alternating single and double bonds in its ring (3 of each bond type)

• this structure was predicted from empirical measurements

how does the reactivity of benzene disprove Kekulé’s proposed benzene structure?

• if benzene contained double bonds, it would react like an alkene

  • benzene does not carry out electrophilic addition and does not decolourise bromine water

  • so, benzene does not contain double bonds

how do the bond lengths in benzene disprove Kekulé’s proposed benzene structure?

• C-C bonds are longer than C=C bonds. if benzene contained both single and double C to C bonds, it would have bond of different lengths in its ring.

  • benzene is perfect regular hexagon. it has equal C to C bond lengths and this bond length is an intermediate between single and double.

  • so, benzene does not contain a mix of C-C bonds and C=C bonds.

how does isomerism disprove Kekulé’s proposed benzene structure?

• if benzene had the cyclohexa-1,3,5-triene structure, it would have 2 isomers of 1,2-dibromobenzene. isomer 1 would have a Br atom on either side of the C=C bond, and isomer 2 would have a Br atom on either side of the C-C bond.

  • these isomers cannot be made.

  • so, Kekulé’s proposed structure is incorrect.

use thermochemical evidence from enthalpies of hydrogenation to:

• disprove Kekulé’s proposed benzene structure, and

• compare the stability of benzene and the theoretical molecule cyclohexa-1,3,5-triene

• cyclohexene evolves 120 kJ mol^-1, so expect cyclohexa-1,3,5-triene to evolve 3 x 120 = 360 kJ mol^-1

• 360 - 208 = 152 kJ mol^-1, the enthalpy of hydrogenation of benzene is 152 kJ mol^-1 less exothermic

• due to delocalisation of p electrons in benzene

• so benzene is more stable than cyclohexa-1,3,5-triene

what was correct about Kekulé’s proposed benzene structure?

both models are planar/hexaganol

arenes

aromatic organic compounds that contain a benzene ring as part of their structure

• benzene can become a side group: the phenyl group C₆H₅ 

• the prefix ‘phenyl’ can be used when naming the compound

properties of arenes

• high melting points due to high stability of the delocalised ring

• low boiling points as they are non-polar - van der Waals forces between molecules are easily overcome

• often insoluble in water

why doesn’t benzene react with bromine in the same way as alkenes?

• in benzene, pi electrons are delocalised above and below the whole ring of C atoms

• so, the electron density between any two adjacent C atoms is less than in the C=C bond in alkenes

• so, benzene cannot polarise Br₂ and induce a dipole

• so, electrophilic addition does not occur

• for benzene and bromine to react, we need halogen carier catalyst

why can benzene be attacked by electrophiles?

how does benzene react with electrophiles?

• the delocalised ring is an area of high electron density which attracts electrophiles

• benzene cannot induce an electrophile, so a catalyst may be used to generate an electrophile with a + charge

• benzene undergoes electrophilic substitution

delocalisation energy

the energy required to break the pi bonding system of delocalised electrons in benzene

explain why electrophilic substitution reactions occur in preference to electrophilic addition reactions

• to maintain the stable delocalised ring in the product

• delocalisation energy is too high

• benzene cannot induce an electrophile