Benzene

Question 1

Type of reactions of benzene

Answer 1

Electrophilic substitution

Question 2

Nitration of benzene:

Answer 2

Benzene + nitric acid -> nitrobenzene + water
C6H6 + HNO3 -> C6H5NO2 + H2O
{{H2SO4, 50C}}

Question 3

Nitration of benzene:

What happens if temp is over 50C?

Answer 3

Further substitutions reactions may occur leading to the production of dinitrobenzene.

Question 4

Uses of nitrobenzene:

Answer 4

Preparation of dyes, pharmaceuticals, pesticides, paracetamol.

Question 5

Electrophile involved in nitration of benzene:

Answer 5

NO2 +

Question 6

Nitration of benzene:

Role of catalyst:

Answer 6

Step 1:
HNO3 + h2so4 -> no2 + hso4- + h2o
{{conc nitric acid & conc. sulphuric acid}}

Step 3:
H+ + hso4- -> h2so4

(Step2: mechanism)

Question 7

Which catalyst is required for reaction of benzene and halogens?

Answer 7

Halogen carrier

E.g. AlCl3, FeCl3, AlBr3, FeBr3 which can be generated “in situ” from the metal &halogen.

Question 8

Bromination of benzene requires:

Answer 8

Room temperature pressure
Halogen carrier
(Electrophilic subs)

Question 9

Bromination of benzene:

Answer 9

C6H6 + Br2 -> C6H5Br + HBr

Question 10

Alkylation of benzene:

{{Friedel-Crafts alkylation}}

Answer 10

Benzene + Haloalkane -> alkyl benzene + hydrogen halide
(AlCl3 halogen carrier cat.)
E.g.
C6H6 + C2H5Cl -> C6H5C2H5 + HCl

Question 11

Acylation of benzene:

Benzene + acyl chloride

Answer 11

Benzene + ethanoyl chloride -> phenylethanone + HCl

AlCl3 cat.

Question 12

Evidence to disprove Kekulé’s model:

Answer 12

Benzene does not undergo electrophilic addition reactions like c=c; does not decolourise bromine (at RTP); bond length between that of a single and double bond; hydrogenation enthalpies: is less than three times the enthalpy of hydrogenation of cyclohexene (enthalpy of stabilisation);

Question 13

Benzene reflux with conc HNO3 (cat: conc H2SO4), 50C:

Answer 13

C6H5NO2 (Nitrobenzene)

[excess HNO3 above 50C to give 1,3-dinitrobenzene (further substitutions) ]

Question 14

C6H5NO2 and Sn, conc HCl

Answer 14

C6H5NH2

Question 15

Benzene, RCl, AlCl3, reflux, anhydrous conditions:

Alkylation/ Friedel-Crafts

Answer 15

C6H5R

Question 16

Benzene, ROCl, AlCl3, reflux, anhydrous:

Answer 16

C6H5COR

Question 17

Benzene, Br2, FeBr3:

Answer 17

C6H5Br (bromobenzene)

Question 18

Benzene vs. hexane:

Answer 18

BP same

MP higher than hexane

Question 19

Effect of side chains on benzene:

Answer 19

Increase bp: larger molecule; more imf;

Decrease mp: reduces symmetry; packs less efficiently;

Question 20

Benzene burns with smoky flame, incomplete combustion because:

Answer 20

High carbon to hydrogen ratio

Question 21

Phenol as a weak acid:

Answer 21

Weak solubility (non-polar benzene ring); partially dissociates in water to give phenoxide ion + proton; more acidic than alcohol; less acidic than RCOOHs; ethanol does not react with any base; phenol + RCOOHs react with strong alkalis; RCOOHs react with weak base (i.e. Na2(CO3)2)

Question 22

Bromination of phenol:

Electrophilic substitution

Answer 22

Identifies; decolorises bromine; white ppt;

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

Question 23

Phenoxide ion is stable:

Answer 23

Negative charge on oxygen can be delocalised over the benzene ring;

Question 24

Uses of phenol:

Answer 24

Antiseptics
Disinfectants
Production of plastics
Resin for paints

Question 25

Electrophilic substitution reactions of phenol:

Answer 25

Bromination;

Nitration;

Question 26

Nitration of phenol:

Answer 26

RTP; mixture of 2-nitrophenol and 4-nitrophenol is formed:

Phenol + HNO3 -> 2 or 4-nitrophenol + water

Question 27

Phenol more reactive than benzene:

Answer 27

Phenol:
Lone pair of electrons from oxygen p-orbital of the -OH group being donated to pi-system; electron density increased; can polarise; attracts nucleophiles; benzene ring in phenol is activated;