Chapter 25 - Aromatic chemistry

Question 1

What is benzene?

Answer 1

Aromatic hydrocarbon compound - C₆H₆

(Aromatic - has one or more benzene rings)

Question 2

Define arene

Answer 2

Aromatic hydrocarbon containing one or more benzene rings

Question 3

What was kekulé’s model of benzene?

Answer 3

suggested that structure of benzene was a 6 carbon ring joined by alternate single and double bonds

Question 4

What were the 3 pieces of evidence to disprove kekule’s model?

Answer 4

1. ) Lack of reactivity of benzene (electrophillic addition)
2. ) Lengths of carbon-carbon bonds in benzene
3. ) Hydrogenation enthalpies

Question 5

What was the lack of reactivity argument against Kekule’s model?

Answer 5

• Kekule’s structure has C=C bonds so it should decolourise bromine in an electrophile reaction
• However, this doesn’t happen and benzene doesn’t decolourise bromine under normal conditions. Therefore, benzene can’t have C=C bonds

Question 6

What is the length of carbon-carbon bond argument against kekule’s model?

Answer 6

using X-ray diffraction all the bonds in benzene were found to be 0.139 nm in length - in between the length of a single bond (0.153nm) and a double bond (0.134nm)

Question 7

What is the hydrogenation enthalpies argument against kekule’s model?

Answer 7

enthalpy change of hydrogenation should be 3x that of cyclohexene (-120kJmol^-1)

Actual value found to be -208kJmol^-1 instead of -360kJmol^-1. 152kJmol^-1 less energy is produced than expected, so benzene more stable than expected.

Question 8

Describe the delocalised model of benzene proposed after kekule’s model?

Answer 8

• Each carbon has one electron in a p-orbital which overlap sideways forming a ring of π-bonds above and below the plane of carbon atoms. Electrons in system of π-bonds are said to be delocalised.

Question 9

What type of reaction is nitration of benzene?

Answer 9

electrophillic substitution

Question 10

What are the conditoins for nitration of benzene?

Answer 10

benzene + HNO₃ with H₂SO₄ at 50ºC

Question 11

What happens when the reaction temp rises above 50ºC?

Answer 11

further substitution may occur leading to the production of dinitrobenzene

Question 12

What are some uses of nitrobenzene?

Answer 12

dyes, pharamceuticals and pesticides

Question 13

Write the 3 steps in the mechanism for nitration of benzene?

Answer 13

step 1: HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄^- + H₂O

step 2:

step 3 : H⁺ + HSO₄^- → H₂SO₄

Question 14

As halogens can’t react with benzene on their own, what is required? Give examples of this

Answer 14

halogen carrier

e.g. FeCl3, AlBr₃, FeBr₃, AlCl₃

Question 15

What are the conditions for bromination of benzene? (hydrogen substituted by bromine)

Answer 15

Room temperature and pressure in presence of halogen carrier.

Question 16

What is formed as a by product of bromination?

Answer 16

Hx where x is a halogen

Question 17

*Write the 3 steps of the mechanism for bromination of benzene? (would be the same for chlorination)

Answer 17

step 1: Br₂ + FeBr₃ → FeBr₄^- + Br⁺

step 2:

step 3: H⁺ + FeBr₄^- → FeBr₃ + HBr

Question 18

What happens in alkylation of benzene?

Answer 18

substitution of a hydrogen atom in the benzene ring by an alkyl group.

• benzene reacted with haloalkane in presence of AlCl₃
• AKA friede-crafts alkylation

Question 19

Write the equation for alkylation of benzene with C₂H₅Cl

Answer 19

Question 20

What happens in acylation of benzene?

Answer 20

Benzene + acyl chloride in presence of AlCl₃ forms aromatic ketone (phenyl ketone)

Question 21

Write equation for acylation of benzene with ethanoyl chloride and AlCl₃

Answer 21

Question 22

Why doesn’t benzene react with bromine?

Answer 22

• Benzene has delocalised π electrons spread above and below the plane of carbon atoms in the ring structure. Therefore, electron density less than C=C bond in an alkene.
• insufficient electron density to polarise bromine molecules

Alkenes can react with bromine as π bond electrons are localised in alkenes

Question 23

What’s a phenol?

Answer 23

Organic chemical containing hydroxyl -OH functional group directly bonded to an aromatic ring

Question 24

Why is a phenol classed as a weak acid?

Answer 24

Due to ability to partially dissociate to produce protons and a phenoxide ion

Question 25

Why are pehnols lesss soluble in water than alcohols?

Answer 25

presence of non-polar benzene ring

Question 26

Are phenols more acidic than alcohols and carboxylic acids?

Answer 26

more acidic than alcohols but less acidic than carboxylic acids

Question 27

What can be used to distinguish between a phenol and a carboxylic acid?

Answer 27

Reaction with sodium carbonate - carboxylic acid produces CO₂ - Phenols don't react

Question 28

What happens in reaction of phenol with sodium hydroxide? Write an equation

Answer 28

Phenol reacts with sodium hydroxide to form a salt, sodium phenoxide and water in a neutralisation reaction.

Question 29

Write equation for phenol with bromine

Answer 29

Question 30

Write equation for nitration of phenols

Answer 30

Question 31

What is the increased reactivity of phenols compared to benzene caused by?

Answer 31

Lone pair of electrons from the oxygen p-orbital of the -OH group being donated into the π-system of phenol electron density increased, therefore phenol more susceptable to attack from electrophiles. Sufficient to polarise molecules e.g. bromine

Question 32

What does an activating group do?

Answer 32

Makes aromatic ring that the group joined to react more readily with electrophiles

Question 33

What does a deactivating group do?

Answer 33

Makes aromatic ring that the group joined to react less readily with electrophiles

Question 34

What is a directing effect?

Answer 34

Different groups have different directing effects on any second substituent on the benzene ring. Determines which carbons the second substitutent group adds to

Question 35

What directing effect do activting groups have?

Answer 35

2- and 4- directing groups

Question 36

what directing effect do deactivating groups have?

Answer 36

3-directing groups with the expception of the halogens - which are 2.4 directing

Question 37

Give some examples of 2- and 4- directing groups and 3-directing groups?

Answer 37