25.1 aromatic compounds

what is benzene?

• naturally occurring aromatic hydrocarbon with a ring structure

• liquid at room temperature

• key ingredient added to gasoline

what were features when Benzene was first discovered?

• colourless, sweet smelling, highly flammable liquid

• naturally in crude oil, components of petrol, cigarette smoke

• carcinogen

what is structure of benzene?

• hexagonal ring of 6 carbons atoms

• each carbon joined to 2 carbons and 1 hydrogen

how did Kekule come up with the kekule model?

• he thought of the ring shape of benzene whilst day dreaming about a snake seizing its own tail

what is the kekule’s model?

• structure of benzene is made up of 6 membered ring of carbon atoms joined by alternate single and double bonds

why has not all chemists accepted kekule model?

• not able to explain all chemical and physical properties

what are the 3 reasons to disprove kekule’s model?

• lack of reactivity of benzene

• lengths of carbon-carbon bonds in benzene

• hydrogenation enthalpies

explain the lack of reactivity as a way to disprove kekule’s model

• if benzene contained C=C it should decolourise bromine in electrophilic addition reaction

• but benzene doesn’t undergo electrophilic addiction reactions and it doesn’t decolourise bromine under normal conditions

• led scientists to believe benzene has no C=C bonds

explain lengths of carbon-carbon bonds in benzene as a way to disprove kekule’s model

• X-ray diffraction can measure bond lengths of a molecule

• Lonsdale, a crystallographer examined benzene found all bonds were 0.139nm which was in-between single bond length of 0.153nm and double bond 0.134nm

explain hydrogenation enthalpies as a way to disprove kekule’s model

• if benzene has kekule strucure expected to have a hydrogenation of x3 of cyclohexene

• but cyclohexane required -120 and x3 would be -360

• but benzene enthalpy change is only -208

what are features of delocalised model of Benzene?

• planar, cyclic, hexagonal

• canon uses ¾ electrons to bond

• each carbon has 1 electron in p-orbital at right angles to plane of carbon and hydrogen atoms

• addict p-orbitals overlap sideway in both directions above and believe the plane of carbon bonds to form a ring of electron density

• 6 electrons occupying pie-bonds are delocalised

when is benzene used as main parent chain?

• when one substituent groups are monosubstitued like short alkyl chains, halogens and nitrogen groups

what is benzene ring considered if attached to alkyl chain with functional group, or alkyl group longer than 7 chains?

• substituent with prefix phenyl

what are exceptions to naming rules of benzene?

• benzoic acid

• phenylamine

• benzaldehyde

what happens in substation reactions with benzene?

• hydrogen atom on benzene ring replaced by another atom/group of atoms usually with electrophile and so most of time electrophilic substituion

what happens during nitration of Benzene?

• benzene reacts slowly with nitric acid to form nitrobenzene

• reaction catalysed by sulphuric acid and heated to 50 degrees

• water bath used to maintain steady temperature

• 1 hydrogen replaced by nitro group

why is temperature used in nitration of Benzene 50 degrees?

• further substitution may occur leading to production of dinitrobenzene

what is nitrobenzene useful for?

• starting material in preparation of dyes, pharmaceuticals and pesticides such s starting material to make paracetamol

what are the steps involved in nitration of Benzene?

what do halogens need to react with benzene?

• a catalyst called a halogen carrier

what are common halogen carriers?

• AlCl₃

• FeCl₃

• AlBr₃

• FeBr₃

what are conditions for bromination of benzene?

• RTP

• halogen carrier

why do you need a electrophile Br⁺ instead of non-polar bromine molecule for bromination?

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

what are the steps for bromination of benzene?

what is alkylation?

• substation of hydrogen atom in benzene ring by an alkyl group

• needs halogen carrier catalyst

what halogen carrier catalyst is in alkylation?

• AlCl₃

what is alkylation reaction of benzene with chloroethane?

what is acylation reaction?

• benzene reacts with acyl chloride in presence of AlCl₃ catalyst, an aromatic ketone formed

what is reaction between benzene and ethanol chloride?

what is an acyl?

• Carbon double bonded to oxygen and single bonded to R group

what is differences between alkenes and benzene that leads to its difference in reactions with bromine?

• alkene has localised n-electrons

• benzene and delocalised n-electrons

• so electric density around any 2 carbons in benzene ring is less than double bond

what are observations of bromination of benzene?

• orange solution to colourless solution

what are observations of bromination of phenol?

• orange solution to a white precipitate in colourless solution

what are the reaction conditions of bromination of benzene?

• RTP

• halogen carrier required

what are reaction conditions of bromination of phenol?

• RTP

• halogen carrier not required

what is overall reaction Bromination of Benzene?

what is overall reaction of Bromination of Phenol?

why is halogen career required for benzene but not phenol bromination?

• phenol more reactive than benzene during electrophilic substitution reactions

why is phenol more reactive than benzene?

what are reaction conditions for nitration of benzene?

• conc sulphuric and nitric acid

• reflect at 50 degrees

what are reaction conditions for nitration of phenol?

• RTP

• dilute nitric acid

• no sulphuric acid

what is overall reaction for nitration of benzene?

what is overall reaction for nitration of phenol?

why does trisubstitution not take place when phenol nitrated?

why is phenol partially soluble in water?

• hydroxyl group in phenol make hydrogen bonds with water molecules

• but aromatic ring is non-polar so can’t form hydrogen bonds with water

how is penal acidity compared to alcohols and carboxylic acids?

• more acidic than alcohols, less acidic than carboxylic acids

what don’t alcohols react with and why?

• strong or weak bases as not acidic

what only do phenols react with?

• strong bases

what do carboxylic acids react with?

• strong and weak bases

what is directing effect?

• how functional group that is attached directly to aromatic ring affects which aromatic hydrogen atoms are more likely to undergo electrophilic substitutions

what is directing group of -NH₂

• 2 and 4 directing

what are some only go to 2,4 positions for phenol?

• lone pair on oxygen atom of the hydroxyl group is delocalised into aromatic ring

• this activates only 2,4 positions only

• so hydroxyl is 2 and 4 directing group

what are deactivating groups called?

• 3 directing groups

what is a 3 directing group we need to know?

• NO₂

why are 3-directing groups activate ring at position 3 only?

• resonance structures show that electrophiles will not attack aromatic ring at 2 or 4 because aromatic ingredients is least electron rich at these positions

• this is due to electron withdrawing effect of the nitro group which withdraws n electron from aromatic ring towards itself which makes ring only electron rich at position 3