chapter 25 - benzene

activating group

makes the aromatic ring react more readily with electrophiles

generally 2-4 directing

deactivating group

makes the aromatic ring react less readily with electrophiles

generally 3 directing

is -NH2/ -NHR 2 and 4 directing or 3 directing

2 and 4 directing

is -OH 2 and 4 directing or 3 directing

2 and 4 directing

is -OR 2 and 4 directing or 3 directing

2 and 4 directing

is -R/ -C6H5 2 and 4 directing or 3 directing

2 and 4 directing

are the halides 2 and 4 directing or 3 directing

2 and 4 directing

is RCOR 2 and 4 directing or 3 directing

3 directing

is -COOR 2 and 4 directing or 3 directing

3 directing

is SO3H 2 and 4 directing or 3 directing

3 directing

is CHO 2 and 4 directing or 3 directing

3 directing

is COOH 2 and 4 directing or 3 directing

3 directing

is -CN 2 and 4 directing or 3 directing

3 directing

is -NO2 2 and 4 directing or 3 directing

3 directing

is NR3+ 2 and 4 directing or 3 directing

3 directing

phenol as a week acid

• phenol is less soluble in water than alcohols due to the presence of the non-polar benzene ring

• when dissolved in water, phenol partially dissociates forming the phenoxide ion and a proton

• because of this ability to partially dissociate to produce protons, phenol is classed as a week acid

• it is more acidic than alcohols but less acidic than C.A so can only react with strong bases eg. sodium hydroxide

reaction of phenol with sodium hydroxide

phenol reactions with sodium hydroxide to form the salt, sodium phenoxide and water in a neutralisation reaction

bromination of phenol

• phenol reacts with bromine water to form a white precipitate of 2,4,6-tribromophenol 

• the reaction colourises the bromine water

• with phenol, a halogen carrier catalyst is not required and the reaction is carried out a room temperature   

nitration of phenol

• phenol reacts readily with dilute nitric acid at room temperature

• a mixture of 2-nitrophenol and 4-nitrophenol is formed

comparing the reactivity of phenol with benzene

• bromine and nitric acid react more readily with phenol than they do with benzene

• the increased reactivity is caused by a lone pair of electrons from the oxygen p-orbital of the -OH group being donated into the Pi system of phenol

• the electron density of the benzene ring in phenol is increased. This increased electron density attracts electrophiles more strongly than with benzene

• the aromatic ring in phenol is therefore more susceptible to attack from electrophiles than in benzene. For bromine, the electron density in the phenol ring structure is sufficient to polarise bromine molecules and so no halogen carrier is needed

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

conditions of nitrogenation of benzene and why they are important

• catalysed by sulphuric acid

• heating to 50 degrees

• if heated above 50 degrees further substitution reactions may occur leading to the production of dinitrobenzene

reaction mechanism for nitrogenation of benzene

how is a halogen carrier used in the bromination of benzene

• benzene is too stable to react with a non-polar bromine molecules

• the electrophile is the Br+ which is generated when the halogen carrier reacts with bromine

• the bromium ion accepts a pair of electrons from the benzene ring to form a dative covalent bond

• the organic intermediate is unstable and breaks down to form the organic product bromobenzene and a H+ ion

• The H+ ion reacts with the FeBr4-  ion to regenerate the FeBr3 catalyst

bromination of benzene mechanism6

alkylation reactions

• the substitution of a hydrogen atom in a benzene ring by an alkyl group

• reacting benzene with a haloalkane in the presence of AlCl3, which acts as a halogen carrier catalyst forming the electrophile

comparing the reactivity of alkenes with arenes (using bromination reaction)

in alkenes

• the pi bind in the alkene contains localised electrons above and below the plane of the two carbon atoms in the double bond. This produces an area of high electron density

• the localised electrons in the pi bond induce a dipole in the non-polar bromine molecules making one bromine atom of the Br2 molecjules slightly more positive and the other slightly negative

• the slightly positive bromine atom enables the bromine molecule to act like an electrophile

in arenes

• benzene will not react with bromine unless a carbon carrier is present

• this is because benzene has delocalised pi electrons spread above and below the plane of carbon atoms in the ring structure.

• the electron density of carbons in benzene rings is less than in a c=c 

• when bromine approaches the benzene, there in insufficient pi electron density around two carbon atoms to polarise the bromine molecule. This prevents any reaction from taking place