Aromatic Compounds and Amines

Question 1

What is the formula and structure of benzene?

Answer 1

Benzene has the formula of C6H6 and it has a planar cyclic structure (flat ring)

Question 2

What forms the ring of delocalized electrons in benzene?

Answer 2

Each carbon in the ring makes 3 bonds to two carbon atoms and one hydrogen atom. The final delocalized electron is located in the p orbital that sticks out above and below the plane of the ring. The p orbitals on each carbon atom combine into a ring of delocalized electrons.

Question 3

What is constant in a benzene ring?

Answer 3

All of the carbon-carbon bonds in the ring are the same, so they are the same length.

Question 4

Why is benzene more stable than cyclohexa-1,3,5-triene? (proof)

Answer 4

Enthalpy of hydrogenation of cylohexene to cyclohexane is -120Kj mol-1 so you would expect cyclohexa-1,3,5-triene to be -360Kj mol-1. In reality, it is much less exothermic than expected as more energy has been put into breaking benzene bonds than cyclohexane-1,3,5-triene, so benzene is more stable due to the delocalized ring of electrons.

Question 5

What are arenes/aromatic compounds?

Answer 5

Arenes/aromatic compounds are compounds that contain a benzene ring.

Question 6

Why does benzene undergo electrophilic substitution not electrophilic addition?

Answer 6

The benzene ring is an area of high electron density so it attracts electrophiles. As the benzene ring is so stable it doesn’t undergo electrophilic addition which would destroy the ring so it undergoes electriphilic substitution instead.

Question 7

What are some examples of compounds with a benzene ring?

Answer 7

Many useful chemicals such as dyes and pharmaceuticals contain benzene rings.

Question 8

Why can only some electrophiles attack benzene ring?

Answer 8

An electrophile has to have a strong positive charge to be able to attack the stable benzene ring- most aren’t polarised enough. But some can be made into stronger electrophiles using a catalyst called a halogen carrier.

Question 8

What are friedel-crafts acylation reactions?

Answer 8

Friedel-Crafts acylation reactions are used to add an acyl group to the benzene ring. Once an acyl group has been added, the side chains can be modified using further reactions to make useful products.

Question 9

How is an acyl chloride electrophile substituted into a benzene ring?

Answer 9

• Electrons in the benzene ring are attracted to the positively charged carbocation. Two electrons from the benzene bond with the carbocation. This partially breaks the delocalized ring and gives it a positive charge
• The negatively charged AlCl4- ion is attracted to the positively charged ring. One chloride ion breaks away from the aluminum chloride ion and bonds with the hydrogen ion. This removes the hydrogen from the ring forming HCl. It also allows the catalyst to form.

Question 10

HOw does AlCl3 make the acyl chloride electrophile stronger?

Answer 10

AlCl3 accepts a lone pair of electrons from the acyl chloride. As the lone pair of electrons are pulled away, the polarisation in the acyl chloride increases and it forms a carbocation. This makes it a much stronger electrophile and gives it a strong enough charger to react with the benzene ring.

Question 11

What conditions are needed in Friedel-Crafts Acylation reactions to produce phenylketones?

Answer 11

The reactants need to be heated under reflux in a non-aqueous solvent for the reaction to occur.

Question 12

How do you get nitrobenzene?

Answer 12

To get nitrobenzene you need to warm benzene with concentrated nitric and sulphuric acids, you get nitrobenzene. Sulfuric acid acts as a catalyst- it helps to make the nitronium ion which is the electrophile.

Question 13

What conditions do you need for mononitration?

Answer 13

You need to keep the temperature below 55 degrees.

Question 14

Why are nitration reactions helpful?

Answer 14

• Nitro compounds can be reduced to form aromatic amines which are used to manufacture dyes and pharmaceuticals.
• Some nitro compounds can be used as explosives like trinitrotoluene which is TNT

Question 15

How are amines formed?

Answer 15

Amines are formed if one or more hydrogens in ammonia are replaced with an organic group

Question 16

What defines a primary, secondary, and tertiary amine?

Answer 16

If one of the hydrogens in ammonia is replaced with an organic group it is a primary. If there are two replaced then it is secondary etc.

Question 17

Why are quaternary ammonium salts formed?

Answer 17

Because quaternary ammonium ions are positively charged, they will hang around with any negative ions that are near. The complexes formed are called quaternary ammonium salts.

Question 18

How are cationic surfactants formed?

Answer 18

Quaternary ammonium salts with at least one long hydrocarbon chain are used as cationic surfactants. THe hydrocarbon tail will bind to nonpolar substances such as grease, whilst the cationic head will dissolve in water, so they are useful in things like fabric cleaners and hair products.

Question 19

Why are cationic surfactants often used as fabric conditioners?

Answer 19

The positively charged part will bind to negatively charged surfaces such as hair and fibre. This gets rid of static, so they are often used in fabric conditioners.

Question 20

Why are amines weak bases

Answer 20

Amines act as weak bases because they accept protons. There’s a lone pair of electrons on the nitrogen atom that can form a dative covalent bond with a H+ ion.

Question 21

How is the strength of an amine base dependent of?

Answer 21

The strength of the base depends on how available the nitrogen’s lone pair of electrons is. The more available the lone pair is, the more likely the amine is to accept a proton, and the stronger a base it will be. A lone pair of electrons will be more available if its electron density is higher.

Question 22

Why are primary amines stronger bases than ammonia and aromatic compounds?

Answer 22

• the benzene ring draws electrons towards itself and the nitrogen lone pair gets partially delocalized into the ring. So the electron density on the nitrogen decreases making the lone pair much less available
• alkyl groups push electrons onto attached groups. so the electron density on the nitrogen atom increases. This makes the lone pair more available.

Question 23

Why are amines also considered nucleophiles?

Answer 23

They react with halogen alkanes in a nucleophilic substitution reaction, or with acyl chlorides and acid anhydrides in nucleophilic addition-elimination reactions.

Question 24

How do you produce aliphatic amines?

Answer 24

• halogenoalkanes
• reducing nitriles

Question 25

How do you produce an amine using a halogenoalkane?

Answer 25

You can produce an amine by heating a halogenoalkane with exvess ammonia.
produces:
- mixture of primary, secondary, and tertiary amines and quaternary ammonium salts. because the primary amine that you produce girst has a lone pari fo felectrons so it can react with the reamining halogenoalkanes as a nucleophile in a nucelophillic substitution reaction.

Question 26

How do you produce a primary amine using a nitrile?

Answer 26

You can produce a primary amine by reducing a nitrile.
- use lithium aluminum hydride in a non-aqueous solvent, followed by some dilute acid.
(LiAlH4 is too expensive for industrial use) in industry, it is replaced with hydrogen gas with a metal catalyst like platinum and nickel at high temperatures and pressure,

Question 27

How are aromatic amines produced?

Answer 27

Aromatic amines are produced by reducing nitro compounds such as nitrobenzene
- heat a mixture of the compound, tin metal, and conc HCl under reflux to make a salt.
- add an alkali like NaOH solution to make an aromatic amine

Question 28

What are amides?

Answer 28

Amides are carboxylic acid derivatives. THey contain the functional group -CONH2. The carbonyl group pulls electrons away from the NH2 group, so amides behave differently from amines.