Arenes - Benzene

Question 1

Aliphatic

Answer 1

Straight or Branches chain organic substances

Question 2

Aromatic/Arenee

Answer 2

One or more rings of six carbon atoms that contain delocalised electrons

Question 3

Why is the Kekule Model incorrect?

Answer 3

The c-c and c=c bonds are different lengths.

In addition when measuring enthaply change of hydrogenation benzene is more stable then the model suggests.

Question 4

Delocalized model

Answer 4

Each carbon forms 3 sigma bonds (1 to hydrogen and 1 to each carbon). This leaves one electron free on each p orbital of the carbons, forming a ring of Pi bonds above and below the carbon plane as the p orbitals overlap sideways

Question 5

Enthaply change of hydrogenation

Answer 5

React an alkene with hydrogen gas to add two atoms of hydrogen across the double bond.

Question 6

Evidence for delocaised model

Answer 6

Benzene is more resistance to hydrogenation then expected, due to the delocalised ring of electrons. The value is less exothermic, hence more energy was required to break the bonds then Kekule. (Delocalisation energy)

Question 7

Alkenes + Bromine water

Answer 7

Electrophilic Addition

Bromine water discolourises

Question 8

Why is benzene resistant to electrophillic addition

Answer 8

The delocalized Pi-bonds spread out the negative charge = Less electron dense

Question 9

Toxicity of benzne

Answer 9

Its a carcinogen

Question 10

Benzene + Oxygen

Answer 10

Combustion
Smoky flame due to low C:H ratio
C6H6 + 7.5O2 -> 6Co2 + 3H2O

Question 11

Benzene + Bromine

Answer 11

Electrophillic Substitution

Condition: Iron(III) bromide catalyst

Question 12

Halogen Carrier

Answer 12

Accepts a lone pair of electrons from a halogen on electophile, increasing the polarisation of the molecule making the electorphile stronger

Question 13

Formation of electorphiles

Answer 13

AlCl3 + Cl2 -> AlCl4- + Cl+

Question 14

Nitration of Benzene

Answer 14

Electorphilic Substitution
Regents: Conc Nitric acid + Conc Sulfuric Acid (Cat)
Electrophile: NO2 +
Conditions: Below 55 (For mononitration)

Question 15

Formation of NO2 + Electrophile

Answer 15

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

Make sure to show reformation of catalyst H2SO4!

Question 16

Why is NO3- least likely to be in nitrating mixture?

Answer 16

React with the catalyst to form another species

Question 17

Alkylation of Benzene

Answer 17

Electrophillic Substitution

1. R+ attacks Benzene
2. Intermediate forms
3. Halogen carrier attracted to H, removing it
4. R group is left on benzne + HX + AlX3

Show reformation of catalyst!
Heat under reflux

Question 18

Acylation of Benzne

Answer 18

Electrophillic Substitution

1. R-C+=O attacks Benzene
2. Intermeidate forms
3. Halogen carrier attracted to H, removing it
4. Acyl group left + HX + AlX3

Heat under reflux in non-aqueous solvent
Produces Phenylketones

Question 19

Hydrogenation of Benzene

Answer 19

Electrophillic Subsitution
Nickle Catalyst
200 degress
30 atm

Addition and reduction

Question 20

Effects of side groups (With no lone pairs) on Benzene

Answer 20

Electron releasing groups (Alkyl/phenols/amines) release electrons into delocalised system increasing electron density hence more attractive to electrophiles.

Question 21

Effects of side groups with lone pairs on Benzne

Answer 21

Extends the delocalisation of benzne to the lone pair on the side group.

Cl - Causes ring to repel nucelophiles, and C-Cl bond becomes stronger

NH2-Less basic as is delocalised and will accept protons

OH- Delocalisation makes C-O bond stronger O-H weaker, its more acidic then alcohols and doesn’t oxidise.

Question 22

When is phenol more reactive then benzene?

Answer 22

Phenol will decolourise bromine water, as the OH group makes the ring more attracted to electrophiles.

Increases chances for electrophilic subisition as the lone pair in the p orbital of oxygen overlap with the delocalised elecetrons in the Pi orbital of benzene, becoming partially delocaised into the Pi system itself increasing the electron density.

Question 23

Bromination of Phenol

Answer 23

Reacts more then once, as bromine is more polarised
2,4,6-Tribromophenol
White solid

Question 24

Properties of Phenol

Answer 24

Slightly acidic.

Will react with sodium metal and sodium hydroxide, but not sodium carbonate as isn’t strong enough of an acid.