chapter 25

Question 1

benzene- appearance

Answer 1

-colourless, sweet smelling, flammable liquid
-found naturally in crude oil, petrol & cigarette smoke
- a carcinogen
C6H6
aromatic hydrocarbon/ arene

Question 2

The kekule model & evidence to disprove

Answer 2

-6C ring w/ alternate single + double bonds
evidence against:
1) lack of reactivity; should decolourise bromine. BUT- doesn’t & doesn’t undergo electrophilic addition
2) lengths of c-c bonds; x-ray diffraction measure. all bonds in benzene= 0.139 nm between single (0.153nm) & double (0.134nm)
3) hydrogenation enthalpies; expected 3x cyclohexene (-120 KJmol-1 x 3= -360), BUT benzene only -208 KJmol-1

Question 3

the delocalised model

Answer 3

• benzene is a planar, cyclic, hexagonal hydrocarbon C6H6
• each c uses 3 of its 4 electrons bonding to 2 cs and one H
• each c has 1 electron in a p orbital at right angles to bonded plane
• adjacent p orbitals overlap sideways above & below plane of c atoms–> ring of electron density
• overlapping creates a system of pi bonds, 6 electrons in pi system are delocalised

Question 4

substituent groups

Answer 4

-when a benzene ring is attached to an alkyl chain w/ a functional group of 7 OR more c atoms, benzene= substituent group— phenyl
eg phenyl ethananone or 2-phenyloctane
-some exceptions; Benzoic acid, phenylamine & benzaldehyde

Question 5

more than one substituent group

Answer 5

-groups listed in alphabetical order using smallest numbers possible

Question 6

nitration of benzene

Answer 6

-reacts w/ nitric acid
-catalysed by sulphuric acid at 50 degrees- waterbath maintain temp (if above temp further substitution– dinitrobenzene) - nitrobenzene- dyes, pharmaceuticals & pesticides- preparation of paracetamol
-electrophile= nitronitium ion NO2+- produced by reaction of nitric acid w/ conc sulphuric
-NO2+ accepts pair if e from ring to form a dative covalent bond- intermediate= unstable & breaks down to form nitrobenzene & H+
step 1 - HNO3 + H2SO4–> NO2+ +HSO4- + H2O
step 2- intermediate- nitrobenzene + H+
step 3- H+ + HSO4- –> H2SO4

Question 7

bromination of benzene

Answer 7

• RTP w/ halogen carrier, electrophilic substitution
• one H atom replaced by Br

benzene + Br2 –FeBr3 or AlBr3–> bromobenzene + HBr

-electrophile= bromonium ion Br+, generated when halogen carrier reacts with bromine
-Br+ accepts a pair of electrons to form a dative covalent, unstable intermediate breaks down forming bromobenzene and H+
step 1- Br2 + FeBr3 –> FeBr4- + Br+
step 2- intermediate–> bromobenzene + H+
step 3- H+ + FeBr4- –> FeBr3 + HBr

Question 8

chlorination of benzene

Answer 8

halogen carrier- FeCl3 or AlCl3

benzene + Cl2 –FeCl3 or AlCl3–> chlorobenzene + HCl

Question 9

Alkylation reactions

Answer 9

substitution of h atom by an alkyl group (C2H5)
benzene w/ haloalkane in presence of AlCl3- generates electrophile

benzene + C2H5Cl–AlCl3–> ethylbenzene + HCl

Question 10

Acylation reactions

Answer 10

• benzene reacts w/ acyl chloride in presence of AlCl3- aromatic ketone is formed
• electrophilic addition

benzene + ethanoyl chloride –AlCl3–> phenylethanone + HCl
(CH3COCl)

Question 11

comparing reactivity of alkenes with arenas

Answer 11

-benzene doesn’t react w/o a halogen carrier- because delocalised electrons, electron density is less than in C==C in alkene, insufficient pi electron density to polarise the molecule

Question 12

phenols

Answer 12

• OH functional group bonded C6H6OH= phenol

- compounds that have an OH group bonded to carbon side chain rather than the ring classified as alcohols not phenols

Question 13

uses of phenol

Answer 13

-disinfectants, detergents, plastics, paints & aspirin

Question 14

phenol as a weak acid

Answer 14

• less soluble in water than alcohols due to non-polar benzene ring
• partially dissociates in water–> phenoxide ion (C6H6O-) and a proton
  -more acidic than alcohols but less than carboxylic acids
  -ethanol doesn’t react w/ NaOH (strong base) or sodium carbonate (weak base)
  -phenols + carboxylic acids react w/ strong bases
  -only carboxylic acids are strong enough to react w/ weak bases
  distinguish between phenol & carboxylic acid- carboxylic acid reacts with sodium carbonate to produce CO2

Question 15

reaction of phenol with NaOH

Answer 15

C6H6OH + NaOH –> C6H6O-Na+ + H2O

Question 16

Bromination of phenol

Answer 16

reacts with aq bromine to form a white precipitate of 2,4,6- tribromophenol. Decolourises bromine water
halogen carrier not required. RTP
phenol + 3Br2 –> 2,4,6- tribromophenol + 3HBr

Question 17

Nitration of phenol

Answer 17

nitric acid at RTP

mix of 2-nitrophenol & 4-nitrophenol formed

Question 18

comparing the reactivity of phenol & benzene

Answer 18

Bromine and nitric acid react more readily w/ phenol than benzene
- increased reactivity from lone pair electrons from oxygen p-orbital of OH being donated to pi system- electron density is increased- more susceptible to attack from electrophiles– no halogen carrier required

Question 19

activation and deactivation

Answer 19

activation- bromine requires halogen carrier to react w/ benzene. NH2 group activates the ring to react w/ electrophiles
or NO2 group deactivates to react less readily
-NH2 group directs the second substituent to positions 2 or 4
-NO2 group directs second substituent to position 3

Question 20

directing effects

Answer 20

different groups have a directing effect on any substituent on the ring

• 2 and 4 directing groups (Ortho & para) are activating groups except halogens
• all 3 directing groups (meta) are deactivating groups

Question 21

using directing effects

Answer 21

used when planning organic synthesis
-when carrying out more than one electrophilic substitution, you have to consider the order to create the correct substitution pattern for product.