[3.3.3] Halogenoalkanes

Question 1

How do you name halogenoalkanes?

Answer 1

• Based on original alkane, with a prefix indicating halogen atom:
  
  • Fluoro for F; Chloro for Cl; Bromo for Br; Iodo for I.
    
    • Substituents are listed alphabetically.

Question 2

Name this molecule.

Answer 2

1-bromopropane.

Question 3

Name this molecule.

Answer 3

2-chloro-2-methylbutane.

Question 4

What is a primary halogenoalkane?

Answer 4

A halogenoalkane with only one carbon attached to the carbon atom adjoining the halogen.

Question 5

What is a secondary halogenoalkane?

Answer 5

A halogenoalkane with two carbon atoms attached to the carbon atom adjoining the halogen.

Question 6

What is a tertiary halogenoalkane?

Answer 6

A halogenoalkane with three carbon atoms attached to the carbon atom adjoining the halogen.

Question 7

Classify each of the molecules below as either being: a primary, secondary or tertiary halogenoalkane.

Answer 7

Question 8

What reactions do halogenoalkanes undergo?

Answer 8

• Nucleophilic substitution.
• Elimination.

Question 9

Which structure of halogenoalkane favours nucleophilic substitution reactions?

Answer 9

Primary halogenoalkanes.

Question 10

Which structure of halogenoalkane favours elimination reactions?

Answer 10

Tertiary halogenoalkanes.

Question 11

What is a nucleophile?

Answer 11

• An electron pair donator - they always have a lone pair.
  
  • e.g. :OH⁻, :NH₃, CN⁻.

Question 12

Explain why the carbon-halogen bond enthalpy influences the rate of reaction.

Answer 12

• The rate of substitution reactions depends on the strength of the C-X bonds (X = halogen atom).
• The weaker the bond (i.e. the lower its bond enthalpy) the easier it is to break and the faster the reaction.
• The iodoalkanes are the fastest to substitute, then the bromoalkanes, the the chloroalkanes and finally the slowest of the four being the fluoroalkanes.

Question 13

Describe and explain a method by which you could carry out test-tube hydrolysis of halogenoalkanes to compare their relative rates of reaction.

Answer 13

1. Aqueous silver nitrate is added to a halogenoalkane and the halide leaving group combines with a silver ion to form a silver halide precipitate.
   
   • e.g. Ag⁺ (aq) + I⁻ (aq) -> AgI (s)
2. The precipitate only forms when the halide ion has left the halogenoalkane and so the rate of formation of the precipitate can be used to compare the reactivity of the different halogenoalkanes.
3. The quicker the precipitate is formed, the faster the substitution reaction and the more reactive the halogenoalkane.
   
   • The rate of substitution reactions depends on the strength of the C-X bond (X being the halogen atom). The weaker the bond, the easier it is to break and the faster the reaction.
4. As you go down group 7, the formation of the precipitate increases as the strength of the C-X bond becomes weaker so:
   
   • AgI, a yellow precipiate, forms the fastest
   • Then AgBr, a cream precipitate
   • And AgCl, a white precipitate forms the slowest

Question 14

What is the change in functional group, reagent, role of reagent & conditions when a halogenoalkane undergoes a nucleophilic substitution reaction with hydroxide ions ?

Answer 14

CHANGE IN FUNCTIONAL GROUP

• Halogenoalkane -> alcohol.

REAGENT

• Potassium (or sodium) hydroxide

ROLE OF REAGENT

• Nucleophile, :OH⁻

CONDITIONS

• In aqueous solution; Heat under reflux
  
  • The aqueous conditions needed are important. If the solvent changed to ethanol, an elimination reaction would occur.

Question 15

Draw and state the mechanism when bromoethane reacts with aqueous potassium hydroxide.

Draw the overall equation for this reaction too.

Name the product.

Answer 15

• MECHANISM: Nucleophilic substitution.
• PRODUCT: Ethanol.

(The carbon has a small positive charge because of the electronegativity difference between the carbon and the halogen)

Question 16

What is the change in functional group, reagent, role of reagent & conditions when a halogenoalkane undergoes a nucleophilic substitution reaction with cyanide ions?

Answer 16

CHANGE IN FUNCTIONAL GROUP

• Halogenoalkane -> nitrile.

REAGENT

• Potassium cyanide/KCN dissolved in ethanol/watermixture

ROLE OF REAGENT

• Nucleophile, :CN⁻

CONDITIONS

• Heating under reflux.

Question 17

Draw and state the mechanism when bromoethane reacts with potassium cyanide/KCN.

Draw the overall equation for this reaction too.

Name the product.

Answer 17

• MECHANISM: Nucleophilic substitution.
• PRODUCT: Propanenitrile.
  
  • This reaction increases the length of the carbon chain which is reflected in the name.

(The carbon has a small positive charge because of the electronegativity difference between the carbon and the halogen)

Question 18

How do you name nitriles?

Answer 18

1. The suffix -nitrile is used.
2. Nitrile groups have to be at the end of the chain.
3. Start numbering the chain from the C in the CN.
4. Nitrile groups increase the length of the carbon chain which needs to be reflected in the name.

Question 19

Name this molecule.

Answer 19

Propanenitrile.

Question 20

Name this molecule.

Answer 20

3-methylbutanenitrile.

(Note the naming: butanenitrile and not butannitrile)

Question 21

What is the change in functional group, reagent, role of reagent & conditions when a halogenoalkane undergoes a nucleophilic substitution reaction with ammonia?

Answer 21

CHANGE IN FUNCTIONAL GROUP

• Halogenoalkane -> amine.

REAGENT

• NH₃ dissolved in ethanol.

ROLE OF REAGENT

• Nucleophile, :NH₃

CONDITIONS

• Heating under pressure (in a sealed tube).

Question 22

Draw and state the mechanism when 1-bromopropane reacts with ammonia.

Draw the overall equation for this reaction too.

Name the product.

Answer 22

• MECHANISM: Nucleophilic substitution.
• PRODUCT: Propan-1-amine.

(The carbon has a small positive charge because of the electronegativity difference between the carbon and the halogen)

Question 23

What is elimination?

Answer 23

Removal of small molecule (often water) from an organic molecule.

Question 24

What is the change in functional group, reagent, role of reagent & conditions when a halogenoalkane undergoes an elimination reaction with hydroxide ions?

Answer 24

CHANGE IN FUNCTIONAL GROUP

• Halogenoalkane -> alkene.

REAGENT

• Potassium (or sodium) hydroxide

ROLE OF REAGENT

• Base, :OH⁻

CONDITIONS

• In ethanol ; heat.

Question 25

Draw and state the mechanism when 2-bromopropane reacts with alcoholic potassium hydroxide. Draw the overall equation for this reaction too. Name the product.

Answer 25

- MECHANISM: **Elimination**. - PRODUCT: **Propene**.

Question 26

Often a mixture of different structural isomers forms as a result of elimination and nucleophilic substitution reactions. What types of halogenoalkane structures allow for this to happen?

Answer 26

With **unsymmetrical secondary and tertiary halogenoalkanes**, two or sometimes three different structural isomers can be formed.

Question 27

Name the products formed from the elimination of 2-methyl-2-chlorobutane.

Answer 27

Question 28

Describe the uses of halogenoalkanes. Why have many of these uses now been stopped?

Answer 28

_USES OF HALOGENOALKANES_ - Chloroalkanes and chlorofluoroalkanes can be used as solvents. - CH₃CCl₃ was used as the solvent in dry cleaning. - Halogenoalkanes have been used as refrigerants, pesticides and aersol propellants. _WHY HAVE MANY OF THESE USES STOPPED?_ - Many of these uses have now been stopped due to the **toxicity** of halogenoalkanes and also their **detrimental effect on the atmosphere**. - Man-made chlorofluorocarbons (CFCs) can cause a **hole to form in the ozone layer**.

Question 29

How is ozone beneficial?

Answer 29

- Naturally occurring ozone (**O₃**) layer in the **upper atmosphere** is beneficial as it **filters out** much of the sun's harmful **UV radiation**.

Question 30

What is the issue with ozone in the lower atmosphere?

Answer 30

A **pollutant** and contributes towards the **formation of smog**.

Question 31

How do chlorine free radicals form in the upper atmosphere?

Answer 31

- Chlorine radicals are formed in the upper atmosphere when energy from **ultra-violet radiation** causes **C-Cl bonds** in chlorofluorocarbons (CFCs) to **break**. - **CF₂Cl₂ -> CF₂Cl· + Cl·**

Question 32

Explain, using equations, how chlorine free radical atoms catalyse the decomposition of ozone and contribute to the hole in the ozone layer.

Answer 32

1. **Cl· + O₃ -> ClO· + O₂** 2. **ClO· + O₃ -> 2O₂ + Cl·** - The **regenerated Cl radical** means that one Cl radical could destroy many thousands of ozone molecules which can lead to a hole in the ozone layer. _OVERALL EQUATION_ - **2O₃ -> 3O₂**

Question 33

Legislation to ban the use of CFCs was supported by chemists and they have now developed alternative chlorine-free compounds. What alternatives have been developed?

Answer 33

- HFCs (hydrofluorocarbons) e.g. CH₂FCF₃ are now used for refrigerators and air conditioners. - These are safer as they do **not** contain the C-Cl bond. - The C-F bond is **stronger** than the C-Cl bond and is **not affected by UV**.