Chapter 25 Aromatic compounds

Question 1

what are the 3 limitations of Kekulē’s benzene

Answer 1

1. lack of reactivity-unlike alkenes doesn’t decolourise bromine water at rtp and is resistant to electrophilic attack
2. Enthalpy of hydrogenation is less exothermic than expected- expected to be 3 x -120 which is one double bond but it is actually -208 kJ mol^-1
3. intermediate bond length of 0.139 nm - all c-c bonds are equal, double bond should’ve meant that c=c is longer than c-c

Question 2

what is benzenes molecular formula?

Answer 2

C₆H₆

Question 3

benzene is flat so it can easily stack meaning its ____ _____ forces are ____ to overcome.

Answer 3

weak London forces

hard to overcome

Question 4

how many sigma bonds does each carbon atom make?

Answer 4

3 C-C, C-C and C-H

Question 5

Each carbon atom has __ electron/s in a p-orbital

Answer 5

1

Question 6

Describe the arrangement of p-orbitals in benzene.

Answer 6

• adjacent p-orbital electrons overlap sideways, above and below the plane forming a ring of electron density
• the 6 ∏-bonds are spread across the six C atoms
• the 6 electrons occupying the system of ∏-bonds are delocalised

Question 7

benzene has a ________ ring of __ _____ with ____ e- ______.

Answer 7

delocalised

∏ electrons

LOW e- density

Question 8

benzene cannot induce a dipole in Br₂ and undergo electrophilic addition because of …

Answer 8

• Higher stability due to delocalised ∏ e-
• its lower electrons density making Br LESS ATTRACTED TO BENZENE

Question 9

name 5 properties of benzene:

Answer 9

1. carcinogenic
2. good solvent
3. highly flammable
4. colourless
5. sweet smelling

Question 10

When benzene is the parent chain. Name the prefix used for the following:

Alkyl groups, halogens and nitro groups.

Answer 10

1. ethyl-, methyl-, propyl- (usual)
2. chloro-, bromo- (usual)
3. nitro-

Question 11

When benzene is a substituent, a prefix is used instead PHENYL- with the following suffixes:

CO-

CHO-

COOH-

NH₂-

Answer 11

• one
• al
• zoic acid
• amine

Question 12

what is the name of this reaction?_____ ______

Answer 12

electrophilic substitution

Question 13

what are the conditions for bromination (electrophilic substitution)?

Answer 13

FeBr₃ Br₂

_{halogen carrier also needed}

Question 14

what are the conditions for nitration (electrophilic substitution)?

Answer 14

conc. HNO₃ and conc. H₂SO₄

50°C - 60°C

Question 15

what are the conditions for alkylation (electrophilic substitution)?

Answer 15

AlCl₃

halogen carrier made from cutting branch and adding Cl

eg. CH₃Cl

Question 16

what are the conditions for acylation (electrophilic substitution)?

Answer 16

AlCl₃

halogen carrier made from cutting branch and adding Cl

eg Cl-C=OCH₃

_{remember it is added to benzene by the C with the O}

Question 17

what reaction is important in aromatic synthesis? why?

Answer 17

Friedel-Crafts

formation od C bonds is possible

Question 18

what are the three steps of electrophilic substitution:

Answer 18

1. generation of electrophile
2. addition of electrophile to benzene ring
3. regeneration of catalyst using leftover H⁺

Question 19

what happens during nitration above 60°C ?

Answer 19

dinitrobenzene will form

Question 20

how many arrows are involved in the whole three steps?

Answer 20

2

Question 21

what are the conditions for nitration of phenol?

Answer 21

doesn’t require conc. HNO₃ or the presence of H₂SO₄ catalyst

Question 22

phenol + bromine → ?

Answer 22

2,4,6-tribromophenol + 3HBr

Question 23

phenol + dilute. HNO₃ → ?

Answer 23

mixture of 2-nitrophenol and 4-nitrophenol + H₂O

Question 24

what is the number for withdrawing groups?

Answer 24

3

Question 25

what is the number for electron donating groups?

Answer 25

2- 4-

Question 26

list some 3-directing groups.

Answer 26

NO2

Question 27

2-4- directing groups ________ electrons into the ring and reactivity \_\_\_\_\_\_\_\_\_

Answer 27

DONATES INCREASES

Question 28

3- directing groups ________ electrons into the ring and reactivity \_\_\_\_\_\_\_\_\_

Answer 28

WITHDRAW DECREASES

Question 29

list some 2-4- directing groups

Answer 29

NH2 OH