Unit 3: Section 6 - Aromatic Compounds and Amines

Question 1

How does the delocalised electron ring system arise in benzene?

Answer 1

1. Benzene is a planar cyclic structure with the formula C6H6
2. Each carbon atom forms single covalent bonds to the carbons on either side of it and to one hydrogen atom
3. The final unpaired electron on each carbon atom is loacated in a p-orbital that sticks out above and below the plane of the ring
4. The p-orbitals on each carbon atom combine to form a ring of delocalised electrons
5. All the carbon bonds in the ring are the same, so they’re the same length - 140pm which lies between the C-C and C=C

Question 2

How do we know benzene is more stable than cyclohexa-1,3,5-triene?

Answer 2

1. Cyclohexene has one double bond - when it’s hydrogenated, the enthalpy change is -120kJmole^-1, if benzene had 3 double bonds, you’d expect it to have an enthalpy of hydrogenation of -360kJmol^-1
2. The experimental enthalpy of hydrogenation of benzene is -208kJmol^-1 - far less exothermic than expected
3. More energy must have been put in to break the bonds in benzene than would be needed to break the bonds in a theoretical cyclohexa-1,3,5-triene molecule
4. This difference indicates benzene is more stable - which is thought to be due to the delocalised electron ring system

Question 3

What is an arene?
(aromatic compound)

Answer 3

Compounds containing a benzene ring

Question 4

What mechanism do arenes undergo and why?

Answer 4

Electrophilic substitution
* As the benzene ring is a region of high electron density, it attracts electrophiles
* As the benzene ring’s so stable, it doesn’t undergo electrophilic addition reactions, which would destroy the delocalised ring of electrons
* Instead it undergoes electrophilic substitution reactions where one of the hydrogen atoms (of the other functional group) is substituted for the electron

Question 5

Why is Friedel-Crafts acylation used?

Answer 5

1. Useful chemicals such as dyes and pharmaceuticals contain benzene rings but because benzene is so stable, it’s fairly unreactive - it can be difficult to make chemicals that contain benzene
2. Friedel-Crafts acylation rections are used to add an acyl group to the benzene ring
3. Once an acyl group has been added, the side chains can be modified using further reactions to make useful products

Question 6

What is the equation for Friedel-Crafts acylation?

Answer 6

Acyl chloride + benzene -> phenylketone + HCl

Question 7

What electrophile is used for Friedel-Crafts acylation?

Answer 7

• CH3C=O where the C has a positive charge
• Acyl chloride + AlCl3 -> Ch3C=Om + AlCl4-

Question 8

How is an acyl chloride substituted into the benzene ring?

Answer 8

1. Electrons in the benzene ring are attracted to the positively charged carbocation.
2. 2 electrons from the benzene bond with the carbocation, which partially breaks the delocalised ring and gives it a positive charge
3. The negatively charged AlCl4- ion is attracted to the positively charged ring
4. One chloride ion breaks away from the aluminium chloride ion and bonds with the hydrogen ion - this removes the hydrogen from the ring forming HCl - allows the catalyst to reform
5. A phenylketone is formed

Question 9

What conditions are needed for Friedel-Crafts acylation to occur?

Answer 9

The reactants need to be heated under reflux in a non-aqueous solvent (dry ether) for the reaction to occur

Question 10

How is the catalyst reformed in Friedel-Crafts acylation?

Answer 10

AlCl4- + H+ -> AlCl3 + HCl
This acylation should take place in a fume cupboard as HCl is a toxic gas

Question 11

How do you form the electrophile for a nitration reaction?

Answer 11

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

Question 12

How is the electrophile substituted into the benzene ring?

Answer 12

1. The nitronium ion attacks the benzene ring
2. An unstable intermediate forms
3. The H+ ion is lost - reacts with the HSO4- to reform the catalyst

Question 13

How do you form only one NO2 group (mononitration)?

Answer 13

Keep the temperature below 55°C otherwise you’ll get lots of substitutions

Question 14

What are nitration reactions used for?

Answer 14

1. Nitro compounds can be reduced to form aromatic amines which are used to manufacture dyes and pharmaceuticals
2. Some nitro-compounds can be used as explosives - TNT

Question 15

What is an amine?

Answer 15

If one or more of the hydrogens in ammonia is replaced with an organic group
1. If one hydrogen is replaced, you get a 1° amine
2. If two are replaced, you get a 2° amine
3. Three is a 3° amine
4. Four is a quaternery ammonium ion, which can no longer react as there is no longer an available lone pair on the nitrogen

Question 16

What are the uses of quaternary ammonium salts?

Answer 16

1. Cationic surfactants - if they have at least one long hydrocarbon chain which will bind to non polar substances such as grease, whilst the cationic head will dissolvce in water, so they are used in things like fabric cleaners and hair products
2. Fabric conditioners - the positively charged ammonium ion will bind to negatively charged surfaces such as hair and fibre which gets rid of static

Question 17

How do amines act as bases?

Answer 17

1. Act as weak bases because they accept protons - a lone pair of electrons on the nitrogen atom that is available forms a dative covalent bond with an H+ ion
2. The strength depends on how available the lone pair is, the more available, the more likely the amine is to accept a proton and the stronger the base is
3. A lone pair of electrons will be availbale if the electron density is higher

Question 18

What is the order of greater availablity of lone pair of electrons and therefore the stronger base?

Answer 18

1. Primary aromatic amine
2. Ammonia
3. Primary aliphatic amine
4. 2° aliphatic amine
5. 3° aliphatic amine
6. Quaternery ammonium ions

Question 19

Why is the order of availability of lone pair of electrons and therefore stronger the base, the way it is?

Answer 19

1. The benzene ring draws electrons towards itself and the nitrogen lone pair gets partially delocalised onto the ring - the electron density on the nitrogen decreases, making the lone pair on the nirogen less available
2. Alkyl groups push electrons onto attached groups, so the electron denisty on the nitrogen increases - the lone pair on the nitrogen is more available

Question 20

What do amines react with?

Answer 20

• The lone pair makes them nucleophiles
• They react with halogenoalkanes in a nucelophilic subsititution reaction
• They also react with acyl chlorides and acid anhydrides in nucleophilic addition-elimination reactions

Question 21

How can aliphatic amines be made?

Answer 21

• By heating a halogenoalkane with excess ammonia
• By reducing a nitrile

Question 22

How do you make an amine by heating a halogenoalkane with excess ammonia?

Answer 22

• Undergoes nucleophilic substitution
• You end up with a mixture of 1°, 2° and 3° amines and quaternary ammonium salts
• Further substitutions can occur, as the amines produced have a lone pair of electrons, and so will act as a nucleophile

Question 23

What are the 2 ways to reduce a nitrile to form a primary amine?

Answer 23

1. Use LiAlH4 (a strong reducing agent) in a non aqueous solvent (dry ehter), followed by dilute acid
   * R-CH2-C≡N + 6[H] ——> R-CH2-CH2-NH2 + 2H2O
   * LiAlH4 is too expensive for industry
2. Use a nickel catalyst and hydrogen gas at high temperatures and pressure - catalytic hydrogenation
   * R-CH2-C≡N + 2H2 ——> R-CH2-CH2-NH2

Question 24

How are aromatic amines made?

Answer 24

By reducing a nitro compound
1. Heat a mixture of a nitro compound, tin metal and conc HCl under reflux to make a salt
2. Add an alkali such as NaOH

Question 25

What are aromatic amines used for?

Answer 25

Starting molecules for dyes and pharmaceuticals

Question 26

Why do amides behave differently to amines?

Answer 26

• Amides contain the functional group -CONH2
• The carbonyl group pulls electrons away from the NH2 group

Question 27

What happens when an aromatic amine is reacted in an excess of HCl?

Answer 27

The NH2 group becomes +NH3