AS/A-level Chemistry - Halogenoalkanes

Halogenoalkanes (Haloalkanes)

• Structure: Contain the functional group C-X, where X is a halogen (F, Cl, Br, I)
• Types:
  • Halogenoalkanes- Halogen is attached to an aliphatic skeleton - alkyl group
  • Haloarenes- Halogen is attached directly to a benzene (aromatic) ring

• Names: Based on the original alkane with a prefix indicating halogens and their position. 

Physical properties

• Boiling points
  • Boling point increases with mass
  • for isomeric compounds the greater the branching, the lower the boiling point
• Solubility
  • Halogenoalkanes are soluble in organic solvents but soluble in water, they are not polar enough and don't exhibit hydrogen bonding. 

Nucleophilic Substitution Reactions 

Nucleophilic substitution mechanism is one of the most important reaction in AS/A-level Chemistry! Make sure you know it!👨‍🏫

• Halogens have a greater electronegativity than carbon
• A dipole is induced in the C-X bond and it becomes polar
• The carbon is thus open to attack by nucleophiles

Nucleophiles:

• examples are OH ̄, CN ̄, NH3and H2O
• possess at least one LONE PAIR of electrons
• are attracted to the slightly positive (electron deficient) carbon

• the nucleophile uses its lone pair to provide the electrons for a new bond
• as carbon can only have 8 electrons in its outer shell a halide ion is displaced
• the result is substitution following attack by a nucleophile
• the mechanism is therefore known as - NUCLEOPHILIC SUBSTITUTION

Rate of reaction

• the rate of reaction depends on the strength not the polarity of the C-X bond

Practical investigation 

• The time taken for a precipitate of silver halide is measured. The faster the precipitate forms, the faster the hydrolysis and the weaker the C-X bond.
• warm equal amounts of each halogenoalkane in a water bath
• add a solution of ethanol, water and aqueous silver nitrate to each
• record the time it takes for a precipitate to appear

AgCl (white), AgBr (cream), AgI (yellow), AgF is soluble

Example of Nucleophilic substitution

• Reagent: Aqueous sodium (or potassium) hydroxide
• Conditions: Reflux in aqueous solution 
• Product: Alcohol
• Nucleophile: Hydroxide ion (OH^-)

• This reaction is sometimes known as HYROLYSIS.

Uses of Halogenoalkanes

• Synthetic: The reactivity of the C-X bond means that halogenoalkanes play an important partin synthetic organic chemistry. The halogen can be replaced by a variety of groupsvia a nucleophilic substitution mechanism.
• Monomers: Chloroethene, tetrafluoroethen
• Polymers: PVC, PTFE
• CFC's: dichlorofluoromethane (refrigerant), trichlorofluoromethane (aerosol propellant blowing agent), bromochlorodifluoromethane (fire extinguishers)

Problems with CFC's

• Ozone layer:
  • CFC’s have been blamed for environmental damage by thinning the ozone layer 
  • Ozone absorbs a lot of harmful UV radiation

Do you remember the content in AS/A-level Chemistry regarding Free Radical substitution? 👩‍🏫 

• CFC’s break up in the atmosphere to form free radicals 

CF₂Cl₂ ➔ CF₂Cl• + Cl• 

• the free radicals catalyse the breaking up of ozone

2O₃ ➔ 3O₂

• Solution
  • CFC’s were designed by chemists to help people
  • chemists now synthesise alternatives to CFC’s to protect the environment such as hydrocarbons and HCFC’s
  • CO₂ can be use as an alternative blowing agent
  • this will allow the reversal of the ozone layer problem

Reference:

https://getrevising.co.uk/files/documents/halogenoalkanes/Halogenoalkanes.pdf

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