25.1 introducing benzene

what is benzene like

• a colourless, sweet smelling, highly flammable liquid

• found naturally in crude oil, a component of petrol, found in cigarette smoke

• classed as a carcinogen

the kekule model

based on a six membered ring of carbon atoms joined by alternated single and double bonds

evidence to disprove kekules model

the lack of reactivity of benzene

• benzene does not undergo electrophilic addition reactions

• benzene does not decolourise bromine under normal conditions

the lengths of the carbon-carbon bond

• all of the bonds in benzene are the same length which is somewhere in between a single and a double bond

hydrogenation enthalpies

• if it did have the structure that kekule proposed, it would have an enthalpy of hydration which is 3 times greater than that of cyclohexene (-360kj/mol)

• the actuall enthalpy change of hydration is only -208 therefore benzene is much more stable than the kekule model

features of the delocalised model of benzene

• benzene is a planar, cyclic, hexagonal hydrocarbon containing 6 carbon atoms and 6 hydrogen atoms

• each carbon uses 3 of its available 4 electrons in bonding to two other carbon atoms and one hydrogen atom

• each carbon atom has one electron in a p-orbital at right angles to the plane of the bonded carbon and hydrogen atoms

• adjacent p-orbital electrons overlap sideways, in both directions above and below the plane of the carbon atoms to form a ring of electron density

• this overlapping of the p-orbitals creates a system of pi bonds which spread over all 6 of the carbon atoms in the ring structure

• the 6 electron occupying this system of pi bonds are said to be delocalised

naming an aromatic compound with one substituent group

• the benzene ring is considered to be the parent chain

• alkyl groups, halogens, and nitro groups are considered the prefixes to benzene

• eg: ethylbenzene, chlorobenzene

• when an alkyl chain has a functional group attached, benzene is considered to be a substituent so the prefix phenyl is used in the name

• eg. phenylethanone

naming aromatic compounds with more than one substituent group

• the ring is now numbered like a carbon chain starting with one of the substituent groups

• the substituent groups are listed in alphabetical order using the smallest numbers possible