BENZENE

Question 1

name some qualities of benzene

Answer 1

colourless
aromatic (sweet smelling)
highly flammable
carcinogen

Question 2

Describe Kekulé’s structure of benzene

Answer 2

Planar
cyclic
alternating double and single bonds

Question 3

what type of reactions would we expect benzene to undergo based on Kekulé’s model

Answer 3

• electrophilic addition w/ alkenes
• decolourise Br water rapidly

*but does not do this

Question 4

describe the thermodynamic evidence for stability that disproved Kekulé’s theory

Answer 4

when unsaturated hydrocarbons are reduced to corresponding saturated compound energy is released
when one C=C is broken, 120 KJ/mol is released
therefore if benzene had 3 separate C=C we would expect (3x120) 360 KJ/mol to be released

benzene actually only released 208 KJ/mol when reduced .:. lower down energy scale

it is 152 KJ more stable than expected
–> this is known as the resonance energy

Question 5

what kind of C to C bods does benzene have

Answer 5

no true C=C as does not readily go under electrophilic addition

all 6 C-C bond lengths are similar
(C=C are shorter than C-C)

Question 6

how much energy per mol would be released if 1 C=C brond where to break in cyclohexane

Answer 6

120 kJ mol-1

Question 7

how much energy per mol would we expect benzene to be release based off kekulé’s model and how much is actually released

Answer 7

kekulé= 360 kJ mol-1

actual= 208 kJ mol-1

Question 8

what is the actual structure of benzene

Answer 8

6 π e-
that are delocalised around the benzene ring
by overlapping P orbitals

no double bonds + all bond lengths are equal

planar structure

• to draw: “squashed benzene” with halo above + below
  halo = 6 overlapping P orbitals above + below the plane (π bond)

Question 9

naming benzene derivatives

Answer 9

monosubstitution of halogen / NO2 / simple alkyl, they become the prefeix (e.g cholor benzene / nitrobenzene / ethylbenzene

if more reactive functional group / large alkyl (+7 C), PHENYL becoms the prefix, e.g, phenylethanone

exlcusions:
benzoic acid (benzene w/ COOH)
phenylamine (benzene w/ NH2)
benzaldehyde (benzene w/ CHO)
namphthalene (two benzenes)

Question 10

halogenation of benzene

- type of reaction

Answer 10

• type of reaction: ELECTROPHILIC SUB

Question 11

halogenation of benzene

- electrophile?

Answer 11

+Cl

Question 12

halogenation of benzene

- reagents + conditions

Answer 12

• reagents: HALOGEN + HALOGEN CARRIER

- conditions: REFLUX W/ HALOGEN CARRIER

Question 13

halogenation of benzene

• equation using Cl2

Answer 13

• equation using Cl2:

C6H6 + Cl2 -(FeCl3)-> C6H5Cl + HCl

Question 14

halogenation of benzene

• draw mechanism

Answer 14

1.
arrow from benzene ring to +Cl

2.
electron ring breaks around the C the CL bonds to
arrow from H+ on benzene –> positive ring

3.
forms halobenzene + H+ (which regenerates the catalyst)

Question 15

halogenation of benzene

• electrophile formation (w/ Cl2 + FeCl3)

Answer 15

Cl2 + FeCl3 –> +Cl + FeCl4-

Question 16

halogenation of benzene

• catalyst regeneration

Answer 16

FeCl4- + H+ –> FeCl3 + HCl

Question 17

Nitration of Benzene

- type of reaction

Answer 17

electrophilic substitution

Question 18

Nitration of Benzene

reagents + conditions

Answer 18

reagents: CONC H2SO4 + CONC HNO3
conditions: reflux @ 50ºC

any higher than 50 –> polysubstitution

Question 19

Nitration of Benzene

equation

Answer 19

C6H6 + HNO3 –> C6H5NO2 + H2O

Question 20

Nitration of Benzene

- mechanism

Answer 20

1.
arrow from benzene ring to +NO2

2.
electron ring breaks around the C the NO2 bonds to
arrow from H+ on benzene –> positive ring

3.
forms nitrobenzene + H+ (which forms the H2O)

Question 21

Nitration of Benzene

- electrophile

Answer 21

+NO2

Question 22

Nitration of Benzene

- electrophile formation

Answer 22

H2SO4 + HNO3 –> (+)NO2 + HSO4(-) +H2O

Question 23

electron density in benzene vs alkene

Answer 23

benzene = delocalised
alkene = localised

Question 24

benzene vs alkene reactivity

Answer 24

benzene = more stable
requires halogen carrier to create an electrophile

alkene = less stable
no halogen carrier required, as it can induce a dipole in the halogen itself

Question 25

which groups are e- donating and how does this affect the e- density of the ring.

when these groups are attached what are the substitution positions

Answer 25

OH, CH3, NH2
increases e- density as they are e- donating

take up C 2 4, 6

*activate benzene

Question 26

which group(s) are e- withdrawing and how does this affect the e- density of the ring.

when these groups are attached what are the substitution positions

Answer 26

NO2
decreases e- desnity as they are e- withdrawing

C 3 + 5

*deactivate benzene

Question 27

FRIEDAL-CRAFT ACYLATION

reagents + conditions

Answer 27

reagents: acyl chloride + anhyderous AlCl3
conditions: dry inert solvent + relfux @ 50ºc

Question 28

FRIEDAL-CRAFT ACYLATION

electrophile

Answer 28

R-C(+)=O

Question 29

FRIEDAL-CRAFT ACYLATION

regenerating the catalyst

Answer 29

H(+) + AlCl4(-) –> AlCl3 + HCl

Question 30

FRIEDAL-CRAFT ACYLATION

generating electrophile

Answer 30

R-C(=O)Cl + AlCl3 –> R-C(+)=O + AlCl4(-)

Question 31

FRIEDAL-CRAFT ALKYLATION

reagents + conditions

Answer 31

reagents: haloalkane + anhydrous AlCl3
conditions: room temp + dry inert solvent

Question 32

FRIEDAL-CRAFT ALKYLATION

electrophile

Answer 32

+carbocation (e.g, +CH3)

Question 33

FRIEDAL-CRAFT ALKYLATION

regenerating catalyst

Answer 33

H(+) + AlCl4(-) –> AlCl3 + HCl

Question 34

FRIEDAL-CRAFT ALKYLATION
generating electrophile
e.g, CH3CH2Cl

Answer 34

CH3CH2Cl + AlCl3 –> CH3C(+)H + AlCl4(-)