6.1.1 aromatic compounds

Question 1

aliphatic

Answer 1

straight or branched chain organic substance

Question 2

aromatic/arene

Answer 2

includes one or more ring of six carbon atoms with delocalised bonding

Question 3

benzene structure

Answer 3

simplest arene

C6H6

6 delocalised electrons over 6 bonds above & below plane of carbon atoms

has a lower electron density

so it cannot induce a dipole/polarise

Question 4

kekule model

Answer 4

3 double bond alternating benzene

Question 5

problems with kekules model

Answer 5

lacks reactivity
- if benzene contained the C=C bond, it should decolourise Br2 in electrophilic addition
- but, doesn’t undergo electrophilic addiction & doesn’t decolourise Br2 so no C=C

all bonds were found to be equal length, inbetween the value of the single and double bond length

enthalpy change of hydrogenation expected to be 3x of cyclohexenes (double bonds), -360 but only -208 so less exothermic
- more stable than thought

Question 6

phenyl group

Answer 6

instead of benzene

C6H5-

e.g. phenylamine, phenylethene

Question 7

alkenes reaction with bromine

Answer 7

react easily at room temperature

localised pi electrons above and below plane of 2 carbon atoms so has a higher electron density

can polarise/induce dipole in Br2

Question 8

toxicity of benzene

Answer 8

carcinogen

methylbenzene is less toxic and reacts more readily than benzene as methyl side group releases electrons into delocalised system making it more attractive to electrophiles

Question 9

nitration of benzene

Answer 9

benzene > nitrobenzene

conc nitric acid in presence of conc H2SO4

electrophilic substitution

electrophile: NO2+

50/60C as doing at higher temperatures results in further substitution

Question 10

equation for formation of nitrification electrophile

Answer 10

HNO3 + 2H2SO4 > (NO2+)+ (2HSO4-) + H3O+

Question 11

halogenation of benzene

Answer 11

benzene > bromobenzene or chlorobenzene

reagents: bromine or chlorine

conditions: FeBr3 or AlCl3 catalyst

electrophilic substitution

Question 12

formation of electrophiles for halogenation

formation of catalyst

Answer 12

AlCl3 + Cl2 > AlCl4- + Cl+

FeBr3 + Br2 > FeBr4- + Br+

to reform AlCl3 catalyst

H+ + AlCl4- > AlCl3 + HCl

Question 13

Friedel crafts alkylation

Answer 13

benzene > alkylbenzene

reagents: chloroalkane in presence of anhydrous aluminium chloride catalyst

conditions: heat under reflux

mechanism: electrophilic substitution

Question 14

formation of electrophile in alkylation

Answer 14

AlCl3 + Ch3CH2Cl > CH3CH2+ AlCl4-

reform AlCl3 catalyst:

H+ + AlCl4- > AlCl3 + HCl

Question 15

friedel crafts acylation

Answer 15

benzene > phenyl ketone

reagents: acyl chloride in presence of anhydrous aluminium chloride catalyst

conditions: heat under reflux (50C)

mechanism: electrophilic substitution

Question 16

equation for formation of electrophile in acylation

Answer 16

AlCl3 + CH3COCl > CH3CO+ AlCl4-

reform catalst: H+ + AlCl4- > AlCl3 + HCl

Question 17

phenol

Answer 17

OH group directly attached to benzene ring

lone pair of electrons on oxygen p-orbital is donated into pi system
- higher electron density so can polarise

weakly acidic - weaker than carboxylic acid

Question 18

phenol + sodium

Answer 18

H from alcohol replaced by Na+ (sodium phenoxide) + 0.5H2

Question 19

phenol + sodium hydroxide

Answer 19

H from alcohol replaced by Na+ (sodium phenoxide) + H2O

sodium phenoxide more soluble than phenol so solid phenol dissolves on addition of NaOH

Question 20

phenol + bromine

Answer 20

Br2 reagent

room temperature

doesn’t need a FeBr3 catalyst like benzene and undergoes multiple substitutions whereas benzene will only add one Br

white solid formed (2,4,6-tribromophenol) and HBr

Question 21

phenol use

Answer 21

production of plastics

antiseptics

disinfectants

Question 22

phenol + nitric acid

Answer 22

4M HNO3 reagent - single substitution

room temp

forms 2-nitrophenol or 4-nitrophenol

Question 23

effects of side groups on substitution (nitric and phenol )

Answer 23

position on ring of substitution reactions

electron-donating groups (OH, NH2) will force further substitutions to occur on 2nd and 4th positions of ring

electron-withdrawing (NO2) will have a 3-directing effect of in electrophilic substitution of aromatic compounds

Question 24

effect of delocalisation of side groups with lone pairs

Answer 24

if a -OH group, Cl atom or NH2 group is directly attached to benzene ring, delocalisation in benzene ring will extend to include lone pairs on N, O and Cl > changes properties and reactions of side group

Question 25

delocalisation on chlorobenzene

Answer 25

C-Cl bond is made stronger

halogenoalkane substitution and elimination reactions do not occur

electron rich benzene ring will repel nucelophiles

Question 26

delocalisation on phenol

Answer 26

makes the C-O bond stronger and the O-H bond weaker

doesn’t act like an alcohol , it is more acidic and doesn’t oxidise

Question 27

delocalisation on phenylamine

Answer 27

less basic than aliphatic amines as lone pair is delocalised and less available for accepting a proton