There are two major classes of organic chemicals aliphatic : straight or branched chain organic substances aromatic or arene: includes one or more ring of six carbon atoms with delocalised bonding. All of the organic substances we have looked at so far have been aliphatic. Benzene belongs to the aromatic class. Benzene’s Structure The simplest arene is benzene. It has the molecular formula C6H6 Its basic structure is six C atoms in a hexagonal ring, with one H atom bonded to each C atom. Each C atom is bonded to two other C atoms and one H atom by single covalent σ-bonds. This leaves one unused electron on each C atom in a p orbital, perpendicular to the plane of the ring. The Six p electrons are delocalised in a ring structure above and below the plane of carbon atoms. Benzene is a planar molecule. The evidence suggests all the C-C bonds are the same and have a length and bond energy between a C-C single and C=C double bond. H H H H H H H H H H H H The six electrons in the pi bonds are delocalised and spread out over the whole ring. Delocalised means not attached to a particular atom. In formulae we draw a circle to show this delocalised system. Abbreviated formula Displayed formula The H-C-C bond angle is 120o in Benzene N Goalby chemrevise.org + H2 + 3H2 + 3H2 H = -120 kJ/mol H = -360 kJ/mol H = -208kJ/mol Using Enthalpies of Hydrogenation to show Thermodynamic Stability cyclohexene cyclohexane Non delocalised structure delocalised structure Theoretically because there are 3 double bonds in the theoretical cyclohexa-1,3,5-triene one might expect the amount of energy to be 3 times as much as cyclohexene. x3 However, in actual benzene the amount of energy is less. The 6 pi electrons are delocalised and not arranged in 3 double bonds -360 kJ/mol Theoretical value H = -208kJ/mol actual value enthalpy H = -152kJ/mol delocalisation energy This when represented on an energy level diagram shows that the delocalised benzene is more thermodynamically stable than the theoretical structure . The increase in stability connected to delocalisation is called the delocalisation energy. In cyclohexa-1,4-diene, there would not be delocalisation as the pi electrons are too far apart and so don’t overlap. The hydrogenation value would be -240 kJ/mol In cyclohexa-1,3-diene, there would be some delocalisation and extra stability as the pi electrons are close together and so overlap. The hydrogenation value would be less negative than -240 kJ/mol (showing more stable) N Goalby chemrevise.org 2 Naming aromatic molecules Naming aromatic compounds can be complicated. The simplest molecules are derivatives of benzene and have benzene at the root of the name CH3 C2H5 Cl Br NO2 CO2H CHO Methylbenzene ethylbenzene chlorobenzene bromobenzene nitrobenzene benzenecarboxylic acid benzaldehyde If two or more substituents are present on the benzene ring, their positions must be indicated by the use of numbers. This should be done to give the lowest possible numbers to the substituents. When two or more different substituents are present, they are listed in alphabetical order and di, tri prefixes should be used. CH3 CH3 CH3 NO2 NO2 O2N COOH OH CH3 Cl 1,3-dimethylbenzene 1-chloro- 4-methylbenzene 4-hydroxybenzenecarboxylic acid 2,4,6-trinitromethylbenzene In other molecules the benzene ring can be regarded as a substituent side group on another molecule, like alkyl groups are. The C6H5 – group is known as the phenyl group. NH2 CH CH2 H3C CH CH2 CH3 C CH3 O H3C C O O phenylamine phenylethene 2-phenylbutane phenylethanone phenylethanoate Reactions of Benzene Benzene does not generally undergo addition reactions because these would involve breaking up the delocalised system. Most of Benzene’s reactions involve substituting one H for another atom or group of atoms. Benzene has a high electron density and so attracts electrophiles. It reactions are usually electrophilic substitutions. Toxicity of Benzene Benzene is a carcinogen (cancers causing molecule) and is banned for use in schools. Methylbenzene is less toxic and also reacts more readily than benzene as the methyl side group releases electrons into the delocalised system making it more attractive to electrophiles.
126.96.36.199 Bonding (A-level only)
The nature of the bonding in a benzene ring, limited to planar structure and bond length intermediate between single and double.
Delocalisation of p electrons makes benzene more stable than the theoretical molecule cyclohexa-1,3,5-triene.
Students should be able to:
• use thermochemical evidence from enthalpies of hydrogenation to account for this extra stability
• explain why substitution reactions occur in preference to addition reactions.