3.3 Halogenoalkanes

Question 1

What type of bond is a C-Hal bond

Answer 1

• Polar
• The halogen atom is more electronegative than the carbon atom, producing a delta positive carbon atom and a delta negative halogen atom

Question 2

What is the weakest carbon-halogen bond and why

Answer 2

• Carbon-Iodine is the weakest
• Iodine is the biggest halogen atom, so it has the weakest attraction between its nucleus and shared pair of electrons (in the covalent bond) so the least amount of energy is required to break it.

Question 3

How does the C-I bond break

Answer 3

Heterolytically

Question 4

What is used to test for the presence of halogens

Answer 4

Aqueous silver nitrate

Question 5

What happens when you react halogenoalkanes with aqueous silver nitrate

Answer 5

• It produces a precipitate (AgHal)
• The colour of the precipitate is dependent on the halogen present

Question 6

What colour precipitate does Chlorine produce (+ chemical name)

Answer 6

White precipitate (silver chloride)

Question 7

What colour precipitate does Bromine produce (+ chemical name)

Answer 7

Cream precipitate (silver bromide)

Question 8

What colour precipitate does Iodine produce

Answer 8

Pale yellow (silver iodide)

Question 9

What is the name of the reaction between halogen ions and aqueous silver nitrate

Answer 9

Nucleophilic substitution

Question 10

What are the 2 types of reactions halogenoalkanes undergo

Answer 10

• Nucleophilic substitution
• Nucleophilic elimination

Question 11

What is a nucleophile

Answer 11

A species with a lone pair of electrons that reacts with a delta positive carbon atom.

Question 12

What are 3 common examples of nucleophiles

Answer 12

• CN (cyanide) with lone pair on the carbon
• NH3 (ammonia) with lone pair on the nitrogen
• OH (hydroxide) with lone pair on the oxygen

Question 13

What are the products formed from nucleophilic substitution

Answer 13

Alcohols, amines and nitriles.

Question 14

What are the products formed from nucleophilic elimination

Answer 14

Alkenes and water

Question 15

How are alcohols formed from halogenoalkanes

Answer 15

• An aqueous solution of sodium hydroxide or potassium hydroxide with ethanol is used
• The halogen atom is substituted by an OH- ion, forming an alcohol and a halide ion

Question 16

What conditions are needed to form alcohols from halogenoalkanes

Answer 16

• Aqueous solution of sodium hydroxide or potassium hydroxide with ethanol is used
• This reaction is very slow at room temperature, so the reaction mixture is warmed

Question 17

Why is it quicker to use an OH- ion to produce an alcohol than water

Answer 17

The hydroxide ion has a full negative charge, whereas the oxygen atom in water only has a partial negative charge.

Question 18

How are nitriles formed

Answer 18

• CN- nucleophile used
• Halogen atom replaced with a CN- ion, producing a nitrile and halide ion
• The nitrile has one more carbon atom than the halogenoalkane

Question 19

How are amines formed

Answer 19

• NH3 nucleophile used
• Halogen atom replaced by NH3- ion, causing the N atom to have a positive charge
• A second NH3 molecule is used (acts as a base) which removes a H atom, forming an amine and an ammonium ion which reacts with the halogen ion to form an ammonium halide (e.g. ammonium chloride)

Question 20

What is the difference in conditions for using an OH- ion to produce an alcohol vs to produce an alkene

Answer 20

• Nucleophilic substitution: NaOH (warm, aqueous) => produces an alcohol
• Nucleophilic elimination: NaOH (hot, in ethanol) => produces an alkene

Question 21

How are alkenes formed (nucleophilic elimination)

Answer 21

• OH- acts as a base and reacts with a hydrogen atom to produce a water molecule (C-H bond broken)
• This causes a double bond to form between the adjacent carbon atom
• The Halogen atom is then removed (forms a halide ion)

Question 22

What type of bond is between a carbon and nitrogen atom

Answer 22

Triple bond

Question 23

Which is more reactive: Halogenoalkanes or alkanes

Answer 23

Halogenoalkanes

Question 24

Is the C-Hal bond polar or non-polar and why

Answer 24

Polar as there is a big enough difference in electronegativity between the Carbon and Halogen atom.

Question 25

Describe the solubility of halogenoalkanes

Answer 25

They are generally insoluble in water as they have large regions that are non-polar

Question 26

How does chain length affect the boiling point

Answer 26

As chain length increases, boiling point increases (more van der waals forces between larger molecules).

Question 27

Which has a higher boiling point: CH3Cl or CH3I (chloromethane or iodomethane)

Answer 27

- Iodomethane has a higher boiling point (boiling point increases down the group) as Iodine is a larger atom, so there are more electrons which means the van der waals forces between molecules are stronger.

Question 28

What does the reactivity of halogenoalkanes depend on

Answer 28

How easy it is to break the C-Hal bond

Question 29

Which is more reactive: C-Cl or C-I

Answer 29

- The C-I bond is more reactive (Iodoalkanes) as the C-I bond has a lower bond enthalpy, meaning less energy is required to breal it.

Question 30

What does CFC stand for

Answer 30

Chlorofluorocarbons

Question 31

What is the formula for ozone

Answer 31

O3

Question 32

What is the role of ozone

Answer 32

It absorbs UV light which is useful as UV light can cause cancer in humans.

Question 33

How do CFCs cause the break down of ozone

Answer 33

- UV light breaks down the C-Cl bond, forming Cl free radicals - The Cl free radicals can react with ozone, causing it to break down

Question 34

What type of reaction is this

Answer 34

Free radical substitution (chain reaction)

Question 35

What are the propagation steps in ozone breakdown (* means free radical)

Answer 35

Cl* + O3 => ClO* + O2 ClO* + O3 => 2O2 + Cl*

Question 36

What is the overall reaction for ozone breakdown

Answer 36

2(O3) => 3(O2)