Chapter 15 - haloalkanes

Question 1

What are haloalkanes?

Answer 1

Compounds containing the elements carbon, hydrogen and at least one halogen

Question 2

What is a nucleophile?

Answer 2

An atom or group of atoms that is attracted to an electron deficient area, where it donates a pair of electrons to form a new covalent bond.

Question 3

Gives examples of nucleophiles

Answer 3

• Hydroxide ions - OH-
• water molecules - H20
• Ammonia molecules - NH3

Question 4

What is the reaction mechanism called where a nucleophile replaces the halogen in a haloalkane

Answer 4

Nucleophilic substitution

Question 5

What is the reaction called when the halogen atom is replaced by an -OH group in a haloalkane? (by nucleophillic substitution)

Answer 5

Hydrolysis

Question 6

What is hydrolysis?

Answer 6

Chemical reaction involving water or an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule

Question 7

Write the equation for hydrolysis of bromoethane with sodium hydroxide

Answer 7

CH3CH2Br + OH- -> (reflux) CH3CH2OH + Br-

Question 8

What is the equation for hydrolysis of bromoethane with water?

Answer 8

CH3CH2Br + H2O -> CH3CH2OH + H+ +Br-

Question 9

What does the rate of hydrolysis depend on?

Answer 9

• Strength of carbon - halogen bond

- Rate of hydrolysis increases as the strength of the carbon- -halogen bond decreases

Question 10

Briefly describe experiment to test rates of hydrolysis

Answer 10

Add 1-chlorobutane, 1-bromobutane and 1-iodobutane to separate test tubes of ethanol and add aqueous silver nitrate. Observe and record what time each precipitate forms at. 1-chlorobutane forms a white precipitate, 1-bromobutane forms a cream precipitate, 1-iodobutane forms a yellow precipitate.

Question 11

What is the trend in bond enthalpies down group 7 of their carbon bonds?

Answer 11

Decreases

C-F bond is the strongest

Question 12

Which haloalkane is hydrolysed fastest: primary, secondary or tertiary?

Answer 12

Tertiary
most stable
reacts by two step mechanism

Question 13

What are CFCs

Answer 13

Chlorofluoro-carbon
Haloalkane that contains chlorine, fluorine and carbon
all the hydrogens have been replaced

Question 14

What are the properties of CFCs? What were they used for?

Answer 14

• Very stable (due to carbon - halogen and bond strength), volatile, non- flammeable and non-toxic
• Used in fridges, aerosols cans, dry cleaning and air conditioning

Question 15

How do CFCs deplete the ozone layer?

Answer 15

UV radiation in stratosphere provides energy to break a carbon-halogen bond in CFCs, breaking C-Cl bond (lowest bond enthalpy) (photodissociation) forming a chlorine radical which reacts with ozone molecules to form oxygen - breaking it down.

Question 16

Write down the equations showing how CFCs break down the ozone layer

Answer 16

CF2Cl2 -> CF2Cl. + Cl.
propagation
Cl. + O3 -> ClO. + O2 
ClO. + O -> Cl. + O2
chlorine radical regenerated cycle.

Question 17

Why do CFCs take years to reach ozone layer?

Answer 17

Very stable

Question 18

What other radicals cause ozone depletion?

Answer 18

Nitrogen oxide radicals

Formed during lightning strikes and aircraft travel

Question 19

When were CFCs removed?

Answer 19

1987 Montreal protocol

Question 20

Write 2 equations to show how NO catalyses the ozone breakdown?

Answer 20

NO + O3 -> NO2 + O2

NO2 + O -> NO + O2