4.2.2 - Haloalkanes

Question 1

How are haloalkanes named?

Answer 1

Using the alkane name as the base, with prefixes like fluoro-, chloro-, bromo-, or iodo- listed alphabetically.

Question 2

What is a primary haloalkane?

Answer 2

The carbon attached to the halogen is bonded to only one other carbon.

Question 3

What is a secondary haloalkane?

Answer 3

The carbon attached to the halogen is bonded to two other carbons.

Question 4

What is a tertiary haloalkane?

Answer 4

The carbon attached to the halogen is bonded to three other carbons.

Question 5

What is a nucleophile?

Answer 5

An electron pair donor (e.g. :OH⁻, :NH₃, CN⁻).

Question 6

Why does nucleophilic substitution occur in haloalkanes?

Answer 6

Because the C–X bond is polar, with δ⁺ on carbon, which attracts nucleophiles.

Question 7

How does bond strength affect rate of substitution?

Answer 7

Weaker C–X bonds break more easily, so substitution occurs faster.

Question 8

Which C–X bond is the weakest?

Answer 8

C–I (238 kJ/mol)

Question 9

Which haloalkane reacts fastest in substitution?

Answer 9

Iodoalkanes

Question 10

Which haloalkane reacts slowest?

Answer 10

Fluoroalkanes (very unreactive due to strong C–F bond).

Question 11

What mechanism is used in haloalkane substitution?

Answer 11

Nucleophilic substitution with curly arrows showing electron movement.

Question 12

What is the role of a curly arrow?

Answer 12

Shows the movement of a pair of electrons — always starts from a lone pair or a bond.

Question 13

Draw what the nucleophilic substitution would look like in general.

Question 14

What is the product of the hydrolysis of haloalkanes?

Answer 14

An alcohol

Question 14

What is the reagent used for hydrolysis of haloalkanes?

Answer 14

Aqueous KOH or NaOH

Question 14

What are the conditions for hydrolysis of haloalkanes?

Answer 14

Heat under reflux, aqueous solvent.

Question 15

What functional group change occurs in the hydrolysis of haloalkanes?

Answer 15

Haloalkane → Alcohol

Question 16

What happens if ethanol is used instead of water, in the hydrolysis of haloalkanes?

Answer 16

An elimination reaction occurs instead of substitution.

Question 17

Draw the mechanism where CH3Br reacts with OH-.

Question 17

What is hydrolysis?

Answer 17

Splitting a molecule by reaction with water.

Question 18

Why are primary haloalkanes more reactive in nucleophilic substitution reactions?

Answer 18

Because the carbon atom bonded to the halogen is less hindered, making it more accessible to nucleophiles.

Question 19

What is the definition of hydrolysis in the context of haloalkanes?

Answer 19

Hydrolysis is a substitution reaction where the halogen atom in a haloalkane is replaced by a hydroxyl group, forming an alcohol.

Question 20

What is the rate of hydrolysis determined by?

Answer 20

The rate of hydrolysis is determined by the bond enthalpy of the C – X Bond
–> The greater the difference in electronegativity, the greater the bond enthalpy.

Question 21

Why does the rate of hydrolysis of haloalkanes not depend on electronegativity alone?

Answer 21

Because the key factor is bond enthalpy, not polarity — C–F is most polar, but hardest to break.

Question 22

How can the rate of hydrolysis of different haloalkanes be compared experimentally?

Answer 22

By reacting them with water in the presence of aqueous silver nitrate and ethanol, and measuring the time taken for the formation of a silver halide precipitate.

Question 23

How can the rate of hydrolysis of haloalkanes be measured?

Answer 23

By timing the formation of a silver halide precipitate after adding aqueous silver nitrate.

Question 24

What causes the precipitate to form in this test?

Answer 24

The halide ion leaves the haloalkane and reacts with Ag⁺ to form AgX.

Question 25

What are the precipitate colours for each halide?

Answer 25

AgCl – white, AgBr – cream, AgI – yellow

Question 26

Which haloalkane forms a precipitate the slowest?

Answer 26

Flouroalkane (due to strongest C–F bond)

Question 27

Which haloalkane forms a precipitate fastest?

Answer 27

Iodoalkane (due to weakest C–I bond)

Question 28

Why is ethanol used as a solvent in the silver nitrate hydrolysis experiment?

Answer 28

To allow both water and haloalkane (which are immiscible) to dissolve and react.

Question 29

What is an elimination reaction in the context of haloalkanes?

Answer 29

A reaction where a hydrogen atom and a halogen atom (H and X) are removed from adjacent carbon atoms to form an alkene.

Question 30

What conditions favour elimination over nucleophilic substitution?

Answer 30

Using hot, ethanolic potassium hydroxide (KOH) solution.

Question 31

What is the role of the OH⁻ ion in elimination?

Answer 31

It acts as a base, removing a proton (H⁺) from the carbon adjacent to the one bonded to the halogen.

Question 32

What is the role of the OH⁻ ion in nucleophilic substitution?

Answer 32

It acts as a nucleophile, donating a lone pair to the carbon and displacing the halide ion.

Question 33

What is the organic product of an elimination reaction of a haloalkane?

Answer 33

An alkene.

Question 34

What two atoms are removed in elimination?

Answer 34

A hydrogen atom and a halogen atom (from adjacent carbon atoms).

Question 35

Water a poor nucleophile, but how can it react?

Answer 35

Water is a poor nucleophile, but it can react slowly with haloalkanes in a substitution reaction. Use reflux OR heat for more than 20 minutes.

Question 36

What were CFCs used for?

Answer 36

Refrigerants, aerosol propellants, air conditioners.

Question 37

Why were CFCs ideal for those uses?

Answer 37

They were non-toxic, non-flammable, and volatile.

Question 38

Why are CFCs harmful?

Answer 38

TThey catalyse the breakdown of ozone in the upper atmosphere.

Question 39

What is the beneficial role of the ozone layer?

Answer 39

It absorbs harmful UV radiation from the sun.

Question 40

How is ozone formed naturally?

Answer 40

O₂ + UV → 2O* → O* + O₂ → O₃

Question 41

How is ozone broken down naturally?

Answer 41

O₃ + UV → O₂ + O*

Question 42

What is the net reaction catalysed by Cl* radicals?

Answer 42

O₃ + O* → 2O₂

Question 43

How are chlorine radicals formed from CFCs?

Answer 43

C–Cl bonds break under UV light: CF₂Cl₂ → CF₂Cl* + Cl*

Question 44

Why are Cl* radicals dangerous?

Answer 44

They are regenerated and can destroy thousands of ozone molecules.

Question 45

What are NOx compounds and how do they affect ozone?

Answer 45

Produced by aircraft/lightning; they also catalyse ozone breakdown via NO and NO₂ radicals.

Question 46

What safer alternative is used today instead of CFCs?

Answer 46

HFCs (hydrofluorocarbons), which do not contain C–Cl bonds.

Question 47

Why is CO₂ used as a blowing agent now?

Answer 47

To replace CFCs and reduce environmental damage.

Question 48

Why is CFC use still a concern?

Answer 48

They persist in the atmosphere and are still in use in some countries/products.

Question 49

How did chemists help reduce CFC damage?

Answer 49

They supported legislation to ban them and developed safer alternatives.

Question 50

What are the environmental concerns associated with organo-halogen compounds?

Answer 50

Compounds like chlorofluorocarbons (CFCs) can lead to ozone layer depletion, increasing the risk of harmful UV radiation reaching the Earth's surface.

Question 51

How do chlorine radicals catalyse the breakdown of ozone?

Answer 51

Chlorine radicals react with ozone molecules to form chlorine monoxide and oxygen. The chlorine monoxide can then react with an oxygen atom to regenerate the chlorine radical, allowing the cycle to continue and deplete more ozone.

Question 52

What makes haloalkanes susceptible to attack?

Answer 52

The polarity makes haloalkanes susceptible to nucleophilic attack by nucleophiles.

Question 53

What safety hazard is associated with haloalkanes?

Answer 53

They are often toxic and volatile — reactions should be done in a fume hood.

Question 54

Why is substitution with cyanide (KCN) particularly useful in synthesis?

Answer 54

It extends the carbon chain by 1 carbon, forming a nitrile group, useful for further synthesis.

Question 55

What type of mechanism occurs when haloalkanes react with KCN or NH₃?

Answer 55

Nucleophilic substitution (SN2 for primary haloalkanes; SN1 may occur for tertiary).

Question 56

What’s the environmental concern of nitriles formed from KCN reactions?

Answer 56

Nitriles can be toxic and require careful handling/disposal.

Question 57

What are the 4 nucleophiles you need to know?

Answer 57

* :OH- * :CN- * :H- * :NH3

Question 58

Draw the mechanism where CH3CH2Br reacts with :OH-.

Question 59

Draw the mechanism where CH3CH2Br reacts with :CN-.

Question 60

Draw the mechanism where CH3CH2Br reacts with :NH3.

Question 61

Draw the mechanism of the elimination reaction using KOH, of CH3CHICH3.

Question 62

What are the conditions needed for the nucleophilic substitution with warm, aqueous NaOH?

Answer 62

Aqueous, warm

Question 63

What happens during the nucleophilic substitution with warm, aqueous NaOH?

Answer 63

Halogen atom is replaced by an OH group.

Question 64

What is the overall equation of the nucleophilic substitution with warm, aqueous NaOH?

Answer 64

R – X + NaOH → R – OH + NaX

Question 65

What are the conditions needed for the nucleophilic substitution with KCN?

Answer 65

Ethanolic, warm

Question 66

What happens during the nucleophilic substitution with KCN?

Answer 66

Halogen atom is replaced a CN group.

Question 67

What is the overall equation of the nucleophilic substitution with KCN?

Answer 67

R – X + KCN → R – CN + KX

Question 67

What are the conditions needed for the nucleophilic substitution with NH3?

Answer 67

Excess concentrated Ammonia dissolved in ethanol at pressure in a sealed container.

Question 68

What happens during the nucleophilic substitution with NH3?

Answer 68

First molecule of NH3: halogen atom is replaced by NH2 group; second molecule of NH3: leads to the formation of NH4X.

Question 69

What is the overall equation of the nucleophilic substitution with NH3?

Answer 69

R – X + 2 NH3 → R – NH2 + NH4X

Question 70

What are the conditions needed for the elimination reaction with KOH?

Answer 70

Ethanolic, hot

Question 71

What happens during the elimination reaction with KOH?

Answer 71

The halogen atom and one H atom from an adjacent C atom is removed giving an alkene (note that elimination cannot happen if there is no H on an adjacent C atom). A mixture of alkenes could be formed depending on which of the adjacent C atoms the H is lost from.

Question 72

What is the overall equation of the elimination reaction with KOH? Use the example of CH3CH2X as your haloalkane.

Answer 72

CH3CH2X + KOH --> ethene + KX + H2O