Aromatic compounds

Question 1

Describe Kekule’s benzene

Answer 1

Ring structure made of six carbons with alternating single, double cabron bonds.

Question 2

List the 3 reasons as evidence disproving kekule’s benzene

Answer 2

1. Benzenes lack of reactivity.
2. The lengths of the Carbon-Carbon bonds in benzene. (being the same).
3. Hydrogenation enthalpies.

Question 3

Describe how benzenes lack of reactivity disproves kekule’s benzene structure?

Answer 3

The lack of reactivity can be shown by benzenes lack to:
- decolorise bromine water
- undergo electrophilic addition

If benzene had double C=C bonds like kekule’s model predicted it should react.

Question 4

How does benzene’s X ray diffraction disprove kekule’s benzene model?

Answer 4

Benzene’s X ray diffraction shows that all the carbon-carbon bonds are of the same length. Kekule’s benzene should have two different bond lengths. One smaller for the C=C bond and one larger for the C-C bonds.

Question 5

How does Benzene’s hydrogenation values disprove Kekule’s benzene model?

Answer 5

Benzenes hydrogenation value is much lower than what would be expected for kekule’s model. Expected hydrogenation value = -360kjmol
actual value = -208 kjmol

Benzene is more stable than kekule thought.

Question 6

Describe the delocalised model of benzene

Answer 6

• Benzene is a planar, cyclic, hexagonal hydrocarbon with 6 hydrogens and 6 carbons.
• Each carbon of benzene uses 3 of its 4 available electrons to sigma bond to 2 carbons and 1 hydrogen.
• Each carbon has 1 available electron in a p orbital, at right angles to the plane of bonded carbons and hydrogens.
• Adjacent p orbitals overlap sideways to form a ring of electron density, via pi bonding.
• The electrons in the system of pi bonds are said to be delocalised.

Question 7

How is benzene named when it is bonded to Halogen, alkyl or nitro groups?

Answer 7

Benzene takes the sufix
e.g. nitrobenzene

Question 7

How is benzene named when it is bonded to any functional groups, and what are the exceptions?

Answer 7

Benzene takes the prefix -phenyl.
Execptions:
halogens, akyl or nitro groups.
Benzoic acid
phenylamine
benzaldehyde

Question 8

Name all the electrophilic substitutions of benzene

Answer 8

-Nitration
-Halogenation (chlorination, bromination)
-Alkylation
-Acylation

Question 9

What conditions are needed to produce nitrobenzene

Answer 9

Sulfuric acid, nitric acid and the temperature must be 50 °c

Question 10

What conditions are needed to produce dinitrobenzene

Answer 10

Sulfuric acid, nitric acid, heat must be above 50 °c.

Question 11

Write the equation for the catalytic process in nitration of benzene (including reformation of catalyst)

Answer 11

Step 1: H₂SO₄ + HNO₃ → HSO₄⁻ + NO₂⁺ + H₂O

Step 2: HSO₄⁻ + H⁺ → H₂SO₄

Question 12

What catalysts are used in the halogenation of benzene and what are they also referred as?

Answer 12

AlBr₃
AlCl₃
FeBr₃
FeCl₃
halogen carriers

Question 13

Draw the mechanism for the nitration of benzene

Answer 13

Look it up

Question 14

Draw the mechanism for the bromination of benzene

Answer 14

look it up

Question 15

Why is a catalyst needed for the bromination of benzene

Answer 15

Benzene is too stable to react with the non polar benzene so a halogen carrier is used to generate a bromonium ion Br⁺.

Question 16

Describe what is needed for friedel-crafts alkylation

Answer 16

AlCl3 as the halogen carrier
and a haloalkane (of what alkyl chain you want to add).

Question 17

What is needed for acylation?

Answer 17

Bezene reacts with the halogen carries AlCl₃ and an acyl chloride.

Question 18

Describe in detail what happens when bromine reacts with an alkene

Answer 18

• The alkene contains a double carbon C=C bond which contains two localised electrons form the pi bond.
• The localised electrons in the pi bond induce a dipole in the non polar Br-Br molecule, making one bromine slightly electro positive and the other negative.
• The slight positivity allows the bromine molecule to reacts as an electrophile.

Question 19

Why does benzene not react with bromine compared to alkenes

Answer 19

• Benzene has a delocalised pi system of electrons, spreading the electrons above and below the hexagonal plane of carbons.
• Benzene therefore has a lower electron density compared to the C=C of alkenes.
• Due to the low electron density unlike alkenes benzene cannot induce a dipole/ polarise the bromine molecule, preventing bromine from acting as an electrophile and preventing reaction.

Question 20

Why are phenols weak acids

Answer 20

Phenols are weak acids due to the non polar benzene ring which isnt very soluble in water (polar).
However phenol does partially dissociate to a phenoxide ion and proton making it a weak acid.

Question 21

Why are alcohols less acidic than phenols?

Answer 21

Alcohols don’t react (neutralise) with sodium hydroxide (a strong base) whilst phenols do.

Question 22

Why are carboxylic acids more acidic than phenols?

Answer 22

Carboxylic acids react with sodium carbonate (weak base), whilst phenols do not.

Question 23

What is produce when brominating a phenol

Answer 23

2,4,6-trinitrophenol

Question 24

What is needed to brominate phenol

Answer 24

just aqueous bromine, no halogen carrier needed. At room temp.

Question 25

What is produced in the nitration of phenol?

Answer 25

2-nitrophenol or 4-nitrophenol

Question 26

What is needed for the nitration of phenol

Answer 26

just dilute nitric acid. at room temp.

Question 27

Why are phenols more reactive than benzene?

Answer 27

- The lone pair form the -OH of phenols is donated from the pi orbital of oxygen to the delocalise pi system of the benzene ring in phenol. - The electron density of the benzene ring in phenol is therefore increased, allowing phenols to attract electrophiles more strongly than benzene on its own. - Increasing the susceptability of the ring to attack an electrophile - what spec says.

Question 28

Why do phenols not need a halogen carrier for bromination?

Answer 28

- Phenols have a higher electron density and therefore can induce a dipole on the bromine molecule to make it act as an electrophile whilst benzene cant.

Question 29

Which groups are 2,4 directing (2 groups)

Answer 29

NH₂ and OH

Question 30

Which group is 3 directing?

Answer 30

NO₂

Question 31

Describe what electron donating groups do to the reactivity of benzene

Answer 31

Groups like NH₂ and OH are electron donating groups, they activate the benzene ring (increase electron density), increasing the reacivity of the aromatic ring with electrophiles.

Question 32

Describe what electron withdrawing groups do to the reactivity of benzene

Answer 32

Electron withdrawing groups such as NO₂ deactivate the benzene ring decreasing the reactivity of the aromatic ring with electrophiles.