topic 18A

Question 1

aliphatic chemicals

Answer 1

straight or branched chain organic substances

Question 2

aromatic/arene chemicals

Answer 2

includes one or more rings of 6 carbon atoms with delocalised bonding

Question 3

structure of benzene

Answer 3

• C6H6
• six C atoms in a hexagonal ring with one H atom bonded to each C
• bonded by single covalent sigma bonds
• one unused electron on each C atom in a p orbital

Question 4

Kekule’s representation

Answer 4

• he suggested that the structure of benzene consisted of alternate single and double covalent bonds between the carbon atom
• this structure is incorrect

Question 5

bonding in benzene

Answer 5

• 6 electrons in the pi bonds are delocalised
• they spread out over the whole ring
• benzene is planar molecule
• all the C-C bonds are the same
• bond energy between a C-C single and C=C double bond

Question 6

bond angle in benzene

Answer 6

120 degrees

Question 7

how can the stability of benzene be measured

Answer 7

• by comparing the enthalpy changes of hydrogenation

Question 8

evidence for the delocalised model

Answer 8

• hydrogenation of cyclohexene = -120 kJ/mol
• you would assume because there are 3 double bonds in benzene, the ΔH = -360 kJ/mol
• the actual ΔH is less; -208 kJ/mol
• more energy is actually required to break the bonds in benzene
• this suggests benzene is more stable than cyclohexene
• this stability is due to the delocalised electrons

Question 9

why is benzene resistant to bromination

Answer 9

• benzene is stable so doesn’t undergo electrophilic addition reactions
• benzene has high electron density hence undergoes electrophilic substitution
• delocalised ring of electrons is attracted to electrophiles

Question 10

why does benzene have high electron density

Answer 10

• due to the delocalised ring of electrons

Question 11

why does benzene prefer electrophilic substitution rather than addition

Answer 11

• the addition of an electrophile would disrupt the stability of the ring of electrons

Question 12

reaction of benzene with oxygen (combustion)

Answer 12

• C6H6 + 7.5 O2 -> 6 CO2 + 3 H2O
• produces a smoky flame (soot)

Question 13

halogenation of benzene
- change in functional group
- reagents
- conditions
- mechanism

Answer 13

• benzene to bromobenzene
• bromine
• iron bromide catalyst
• electrophilic substitution

Question 14

equation for the formation of electrophiles

Answer 14

• AlCl3 + Cl2 -> AlCl4- + Cl+
• FeBr3 + Br2 -> FeBr4- + Br+

Question 15

nitration of benzene
- change in functional group
- reagents
- mechanism
- electrophile

Answer 15

• benzene to nitrobenzene
• conc nitric acid in the presence of conc sulfuric acid
• electrophilic substitution
• NO2+

Question 16

equation for the formation of electrophile

Answer 16

• HNO3 + 2H2SO4 -> NO2+ + 2HSO4- + H3O+

Question 17

importance of nitration

Answer 17

• synthesise useful compounds
• done at 60 degrees

Question 18

hydrogenation of benzene
- change in functional group
- reagents
- conditions
- mechanism

Answer 18

• benzene to cyclohexane
• hydrogen
• nickel catalyst at 200 C and 30 atm
• addition and reduction

Question 19

friedel crafts alkylation
- change in functional group
- reagents
- conditions
- mechanism

Answer 19

• benzene to alkylbenzene
• chloroalkane in the presence of anhydrous aluminium chloride catalyst
• heat under reflux
• electrophilic substitution

Question 20

formation of the electrophile

Answer 20

• AlCl3 + CH3CH2Cl -> CH3CH2+ + AlCl4-

Question 21

Friedel crafts acylation
- change in functional group
- reagents
- conditions
- mechanism

Answer 21

• benzene to phenyl ketone
• acyl chloride in the presence of anydrous aluminium chloride catalyst
• heat under reflux (50C)
• electrophilic substitution

Question 22

equation for the formation of electrophile

Answer 22

AlCl3 + CH3COCl -> CH3CO+ + AlCl4-`

Question 23

phenols

Answer 23

OH group attached directly to benzene ring

Question 24

are phenols weakly or strongly acidic

Answer 24

• very weakly acidic

Question 25

phenol's and carboxylic acids

Answer 25

- weaker acids than carboxylic acids - both react with sodium metal and sodium hydroxide - only carboxylic acids react with sodium carbonates

Question 26

why can a phenol not react with sodium carbonate

Answer 26

it is not strong enough an acid to react

Question 27

reaction of phenol with bromine - reagent - conditions

Answer 27

- bromine water - room temperature

Question 28

what is more reactive - phenol or benzene?

Answer 28

phenol is more reactive

Question 29

why is phenol more reactive

Answer 29

- due to the electron density in the ring being higher - electrons in the p-orbital of the oxygen overlap with the delocalised ring structure - electron density increases within the ring structure hence it is more susceptible to attack from electrophiles - Br2 is more polarised

Question 30

bromination of phenol compared to benzene

Answer 30

- phenol does not need an FeBr3 catalyst like benzene - undergoes multiple substitution whereas benzene will only add one Br

Question 31

what are phenols used in the production of?

Answer 31

- plastics - antiseptics - disinfectants - resins for paints

Question 32

bromination of phenol reaction

Answer 32

phenol + 3Br2 --> 2,4,6-tribromophenol + 3HBr

Question 33

phenol reaction with alkalis

Answer 33

phenol + NaOH --> sodium phenoxide + water phenol + alkali -> salt + water

Question 34

observation of bromination of phenol

Answer 34

brown to colourless