3.3.10 - aromatic chemistry

Question 1

benzene is a _____, ________ molecule with the formula __________

Answer 1

benzene is a cyclic, planar molecule with the formula C6h6

Question 2

carbon has __ valent electrons

Answer 2

carbon has 4 valent electrons

Question 3

each carbon is bonded to __ other carbons and __ hydrogen atom.

Answer 3

each carbon is bonded to 2 other carbons and one hydrogen atom

Question 4

where is the final lone electron

Answer 4

in a p orbital which sticks out above and below the planar ring

Question 5

the lone electrons in the p orbital combine to form a ….

Answer 5

delocalised ring of electrons

Question 6

What’s important about bond lengths of c-c bonds and why does this occur

Answer 6

All C-C bonds are of the same bond length
Due to the delocalised electron structure

Question 7

Kekules structure of benzene

Answer 7

Benzene has double bonds
He thought there was alternating single and double bonds

Question 8

Why is kekules structure wrong?

Answer 8

Electrons are delocalised

Question 9

Used structure benzene?

Answer 9

Hexagon with circle inside, the circle contains the delocalised electrons

Question 10

Kekules theoretical structure with alternating single and double bonds can also be called

Answer 10

Cyclohexa - 1,3,5 - triene

Question 11

3 ways of looking at benzenes stability

Question 12

Enthalpy change of hydrogenation benzene vs its theoretical

Answer 12

Enthalpy of hydrogenation of cyclohexene is 120kjmol-1- also has one double bond
The theoretical version with 3 double bonds has an enthalpy change of hydrogenation of 3x-120 = -360kjmol-1
However,the actual /experimental value of enthalpy of hydrogenation of benzene is less exothermic than the predicted -360kjmol-1

Question 13

The actual enthalpy hydrogenation at benzene is far less exothermic than the predicted -360kjmol
What does this tell us

Answer 13

Energy is required to break bonds and energy is released to make bonds
This suggests mare energy is required to break the bonds in benzene verses cyclohexa - 1,3,5 - triene thus benzene is more stable than the theoretical cyclo… with 3 double bonds
This stability is due to the delocalised electron structure

Question 14

What are arenes

Answer 14

Aromatic compounds are molecules that contain a benzene ring

Question 15

2 ways of naming arenes

Answer 15

benzene at the end eg bromobenzene, nitrobenzen, 1,2 - dimethyl benzene
or
phenol (benzene w hyroxyl), phenyl amine

Question 16

arenes undergo what type of reaction and why

Answer 16

electrophillic sub
benzene ring has high electron density so electrophiles are attracted to it

Question 17

is benzene stable or unstable and what does this mean for the kind of reaction it wont undergo

Answer 17

benzene is stable so unlike traditional alkenes they dont undergo electrophillic addition as it would distrupt stable ring of electrons

Question 18

2 mechanisms of electrophilic sub that benzene undergoes

Answer 18

friedel-crafts acylation
nitration reaction

Question 19

what must the electrophile added on to the benzene ring have that acyl groups dont have

Answer 19

a very strong positive charger

Question 20

in order to make acyl groups have more of a positive charge we…

Answer 20

use a halogen carrier to act as a catalyst
(AlCl3)
producing a much stronger electrophile

Question 21

equation to form stronger electrophile

Answer 21

RCOCl + AlCl3 —> RC(+)O + AlCl4-

Question 22

Conditions for electrophilic sub

Answer 22

Under reflux
Dry ether solvent

Question 23

how to form nitrobenzene (2 STEPS)

Answer 23

1) we make the electrophile by reacting sulfuric acid with nitric acid
HNO3 + H2so4 –> H2no3+ + HSO4-
H2no+ —-> NO2+ + H2O (forms nitronium ion)

Question 24

what else is formed when nitrobenzene is formed in an electrophilic sub reaction and what does it do

Answer 24

a H+ ion is formed which reacts with HSO4- formed in the first step - making h2so4 again.
hence h2so4 is a catalyst

Question 25

a temp below ____ will ensure ____ no2 sub
above this will result in _____

Answer 25

a temp below 55 will ensure one no2 sub
above this will result in multiple sub