Organic Chemistry: Arenes

Question 1

What is an addition reaction?

Answer 1

An addition reaction occurs when two substances combine to form a single substance. For example, when hydrogen bromide is added to ethene:

HBr + C₂H₄ ⇔ C₂H₅Br

Question 2

What is a substitution reaction?

Answer 2

A substitution reaction is whereby an atom or a group in a compound is replaced by another atom or group from another compound.

For example, when the hydroxide group in sodium hydroxide replaces the chlorine in chloroethane.

C₂H₅Cl + NaOH ⇔ C₂H₅OH + NaCl

Question 3

What is an elimination reaction?

Answer 3

An elimination reaction is when the components of a single molecule are removed from an organic molecule and are not replaced by other atoms or groups.

For instance, a hydrogen and a halogen group are removed by an alkali, forming a C=C group. This happens with 2 bromo-propane in the presence of hot potassium hydroxide in ethanol.

CH₃CHBrCH₃ + KOH ⇔ CH₂=CHCH₃ + KBr + H₂O

Question 4

What is an electrophile?

Answer 4

An electrophile is an atom, an ion or a group that, when forming a covalent bond, attacks an electron rich site and accepts a pair of electrons from that site.

Examples include:

HBr, NO₂⁺, CH₃⁺ and CH₃C⁺O

Question 5

What is a nucleophile?

Answer 5

A nucleophile is an atom, an ion or a group that attacks a lone σ⁺ site, forming a covalent bond by donating a lone pair of electrons to the atom.

Examples include:

H₂O, NH₃, OH^- and CN^-.

Question 6

What is a free radical?

Answer 6

A free radical is an atom or a group with an unpaired electron.

Examples include:

Cl• and CH₃•

Question 7

What are arenes sometimes known as?

Answer 7

Arenes are sometimes known as aromatic compounds.

Question 8

What is benzene?

Answer 8

Benzene is a six carbon compound (C₆H₆) which is cyclic. The atoms are arranged in a hexagonal ring.

Question 9

What is the Kekulé model?

Answer 9

The Kekulé shows that there is an arrangement of double and single bonds, regularly arranged.

Question 10

What is wrong with the Kekulé model?

Answer 10

The Kekulé suggests that carbon atoms are arranged in a regular double and single bond structure.

If this was the case, benzene woudl react in certain ways, however, it doesn’t.

Double and single bonding suggests that bond lengths in benzene vary, however, bond lengths between the benzene atoms are pretty much the same length, so they can’t be double or single bonds.

Question 11

Why is real benzene more stable than the Kekulé structure gives it credit for?

Answer 11

Real benzene is more stable because it doesnt ahve double bonds that can be attacked.

Enthalpy change calculations can be wrong when you use the Kekulé structure, so it cannot be correct.

Question 12

What is the name of the other benzene structure?

Answer 12

The other benzene structure is known as the delocalised electron structure.

Question 13

What does the delocalised electron structure look like?

Answer 13

Question 14

Which is more stable, benzne or cyclohexatriene?

Answer 14

Benzene is more stable than cyclohexatriene.

Question 15

What thermochemical evidence is there for benzene being more stable than the Kekulé model?

How can the amount by which it can be stabilised be calculated?

Answer 15

The amount it can be stabilised can be calculated by is from the enthalpy of hydrogenation.

cyclohexene + hydrogen ⇒ cyclohexane

∆H = -119 kJ mol^-1

^{In theory, cyclohexene should have an enthalpy change of -357kJ mol-1, so the structure cannot be as the Kekule model dictates.}

benzene + hydrogen ⇒ cyclohexane

∆H = -207 kJ mol-1

Question 16

What is benzene combustion reaction?

Answer 16

Benzene combusts in limited air to produce a smoky orange falem. Combustion is incomplete and particles of carbon form.

Question 17

What addition reactions does benzene undergo?

Answer 17

Benzene undergoes reactions with hydrogen in the presence of a hot nickel catalyst.

Question 18

Why will benzene not undergo addition reactions with halogens?

Answer 18

Benzene will not undergo reactions with halogens because the double bonds is not as susceptible as in alkenes.

Question 19

When will benzene react bromine?

Answer 19

Dry benzene will react with liquid bromine in the presence of iron.

Question 20

What is given off when dry benzene reacts with liquid bromine?

Answer 20

When dry benzene reacts with liquid bromine, steamy fumes of hydrogen bromide are given off and bromobenzene is formed (C₆H₅Br).

Question 21

What is the mechanism for the reaction of benzene with bromine?

Answer 21

Question 22

What happens when benzene is warmed with a mixture of sulfuric and nitric acid?

Answer 22

The nitro-group replaces a hydrogen atom in the benzene ring to form nitrobenzene and water.

Question 23

What must react with the nitric acid to make the electrophile NO₂⁺?

Answer 23

Sulfuric acid reacts with the nitric acid to form the electrophile, as well as water and a hydrogen sulphate ion (HSO₄^-).

H₂SO₄ + HNO₃ ⇔ NO₂⁺ + H₂O + HSO₄^-

Question 24

What is the reaction mechanism for the reaction of benzene and nitric acid?

Answer 24

Question 25

What is the name of the reaction when benzene is combined with fuming sulfuric acid?

Answer 25

When benzene is combined with fuming sulfuric acid, it is known as sulfonation.

Question 26

What is produced when benzene is warmed with fuming sulfuric acid?

Answer 26

When benzene is warmed with fuming sulfuric acid, benzene sulfonicacid is produced.

Question 27

What is the mechanism for the reaction of benzene with fuming sulfuric acid?

Answer 27