3.3 Halogenoalkanes

Question 1

bond between carbon and halogen

Answer 1

polar bond

This is because the halogen is more electronegative than the carbon. It also means that the ∂+ carbon is susceptible to nucleophilic attack which leads to substitution reactions.

Question 2

polar bond

Answer 2

Question 3

nucleophilic attack

Answer 3

Question 4

Answer 4

Question 5

elimination reaction

Answer 5

is one that involves the loss of a small molecule to produce a double bond.

Question 6

halogenoalkanes elimination reactions

Answer 6

Halogenoalkanes can undergo elimination reactions to form alkenes. For example, HBr can be eliminated from 1-bromopropane to form propene.

Question 7

Hydrogen halides - elimination

Answer 7

Hydrogen halides, such as HBr, are acidic and they need to be removed with an alkali, such as sodium hydroxide, NaOH. This must be dissolved in ethanol to avoid a substitution reaction occurring.

Question 8

Answer 8

Question 9

Elimination with unsymmetrical halogenoalkanes

Answer 9

With an unsymmetrical halogenoalkane, more than one alkene can be produced during an elimination reaction.

Question 10

With an unsymmetrical halogenoalkane, more than one alkene can be produced during an elimination reaction.

2-bromobutane

Answer 10

With 2-bromobutane both but-2-ene and but-1-ene can be formed. (You do not need to know which one is more likely to form, just that both are a possibility.)

Question 11

A nucleophile is

Answer 11

a species that has a lone pair of electrons that can be donated to an electron-deficient species. The nucleophile in this reaction is provided by the hydroxide ion, OH-, in aqueous sodium hydroxide, NaOH(aq).

Question 12

eg hydrolysis of chlorobutane

Answer 12

Question 13

Answer 13

X NUCLEI

right = electrons

Question 14

Answer 14

X ethanolic
right = aqueous

Question 15

Answer 15

X IONIC

right= polar

Question 16

Answer 16

X HALOGEN

right = carbon

Question 17

Answer 17

X HYDROGEN

right = halogen

Question 18

nucleophilic substitution of alcohols

conditions and why these are used

Answer 18

conditions:
reflux - continuous evaporation and condensation of reactions

Reflux is used when the reaction is slow, therefore, the reaction needs to be heated for a significant amount of time. This process also maximises yield.

heated with aqueous NaOH for 15 minutes, preferably using a hot plate or heating mantle. This is because alcohol is flammable and could ignite with a naked flame. The product is then distilled off.

Question 19

reflux apparatus

Answer 19

Question 19

distillation apparatus

Answer 19

Question 20

Changing the halogen in the bond affects the

Answer 20

electronegativity and the bond strength of the carbon-halogen bond in the halogenoalkane.

Question 21

What happens to electronegativity as the size of the halogen increases in halogenoalkanes?

Answer 21

Electronegativity decreases as the size of the halogen increases.
(Cl > Br > I)

Question 22

Which carbon-halogen bond is the most polar and why?

Answer 22

The C–Cl bond is the most polar because chlorine is the most electronegative, making the carbon atom most δ⁺.

Question 23

Based only on bond polarity, what would the predicted order of hydrolysis rates be?

Answer 23

Chloroalkane > Bromoalkane > Iodoalkane (based on increasing bond polarity).

Question 24

Why is the C–Cl bond the strongest among C–Cl, C–Br, and C–I?

Answer 24

Chlorine is the smallest halogen and gets closest to the carbon, so its bonding electrons are more strongly attracted to the carbon nucleus.

Question 25

What is the correct order of halogenoalkane reactivity based on bond strength?

Answer 25

Iodoalkane > Bromoalkane > Chloroalkane (Because the C–I bond is weakest and easiest to break.)

Question 26

Which factor has a greater influence on the rate of substitution: bond polarity or bond strength?

Answer 26

Bond strength is more important than bond polarity.

Question 27

How can the rate of hydrolysis of halogenoalkanes be measured?

Answer 27

By measuring the rate at which halide ions (X⁻) form precipitates (AgX) with Ag⁺ ions using a colorimeter.

Question 28

Answer 28

Question 29

Answer 29

Question 30

What is the first step in testing a halogenoalkane for hydrolysis products?

Answer 30

Add aqueous sodium hydroxide (NaOH) and heat to hydrolyse the halogenoalkane.

Question 31

Why is dilute nitric acid (HNO₃) added after hydrolysis with NaOH?

Answer 31

To neutralise any excess NaOH so it doesn't interfere with the silver nitrate test.

Question 32

What reagent is added to detect the halide ion after hydrolysis and neutralisation?

Answer 32

Aqueous silver nitrate (AgNO₃).

Question 33

What is the ionic equation for the formation of a silver halide precipitate?

Answer 33

X⁻ + Ag⁺ → AgX (s), where X⁻ is the halide ion.

Question 34

Why might ammonia (NH₃) be used after silver nitrate is added?

Answer 34

To help distinguish between different silver halide precipitates, as their colours can be unclear.

Question 35

Answer 35

Question 36

Why are halogenoalkanes effective as solvents?

Answer 36

They are polar compounds that can mix with both polar and non-polar organic compounds but are insoluble in water, making them useful in dry cleaning and degreasing.

Question 37

Why are halogenoalkanes used as refrigerants?

Answer 37

They have boiling points close to room temperature, allowing easy vaporisation and cooling by evaporation; they are also non-toxic and non-flammable.

Question 38

What are CFCs and why were they used in refrigeration?

Answer 38

Chlorofluorocarbons (CFCs) are halogenoalkanes once widely used as refrigerants due to their stability and safety, but they are now avoided due to environmental harm (e.g., ozone depletion).

Question 39

Which halogenoalkane was one of the first anaesthetics used in surgery?

Answer 39

CHCl₃ (chloroform).

Question 40

Why is chloroform no longer widely used as an anaesthetic?

Answer 40

Other halogenoalkanes are now considered more effective and safer.

Question 41

Why are CFCs harmful to the ozone layer?

Answer 41

They release chlorine free radicals (Cl*) in the upper atmosphere when UV light breaks the C–Cl bond.

Question 41

What environmental issue is associated with CFCs?

Answer 41

Depletion of the ozone (O₃) layer, allowing harmful UV rays to reach Earth's surface and increasing skin cancer rates.

Question 41

What were CFCs commonly used for before their environmental impact was known?

Answer 41

As refrigerants and in aerosol sprays.

Question 42

What is used instead of CFCs and why?

Answer 42

Hydrofluorocarbons (HFCs), because they don’t contain chlorine and thus don’t produce ozone-damaging free radicals.

Question 43

Why does the C–Cl bond in CFCs break more easily than C–F or C–H bonds?

Answer 43

It is weaker and requires less energy to break, making it susceptible to photodissociation by UV light.

Question 44

What happens in the initiation step of ozone depletion with trichlorofluoromethane (CFCl₃)?

Answer 44

CFCl₃ → Cl* + *CFCl₂ (UV light breaks the C–Cl bond, forming chlorine free radicals.)

Question 45

What are two key propagation steps in the ozone depletion mechanism?

Answer 45

Cl* + O₃ → ClO* + O₂ ClO* + O₃ → Cl* + 2O₂

Question 46

What is an example of a termination step in the ozone depletion reaction?

Answer 46

Cl* + ClO* → Cl₂O (Two free radicals combine to form a stable product.)