3.10.1

Substitution Reactions

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Halogenoalkanes

A halogenoalkane is a halogenated alkane. This means that one (or more) of the hydrogen atoms in an alkane has been replaced by a halogen atom.

Names of halogenoalkanes

Names of halogenoalkanes

  • The names of halogenoalkanes use the halogen names at the start.
    • E.g. Fluoromethane.
    • E.g. Chloromethane.
    • E.g. Bromomethane.
    • E.g. Iodomethane.
Properties of halogenoalkanes

Properties of halogenoalkanes

  • Halogenoalkanes are polar molecules and contain polar bonds.
  • The polar bonds are important in their reactions.
  • Because halogens are more electronegative than carbon, the carbon atoms carry a partial positive charge and the halogen atoms carry a partial negative charge.
  • Halogenoalkanes undergo substitution reactions with ammonia, hydroxide ions, and cyanide ions.
Nucleophiles and electrophiles

Nucleophiles and electrophiles

  • A nucleophile is a molecule that is attracted to a nucleus.
    • Nuclei have positive charges, so nucleophiles tend to be negatively charged.
    • Molecules with lone pairs are also nucleophilic.
  • An electrophile is a molecule that is attracted to a pair of electrons.
    • Electrons have negative charges, so electrophiles tend to be positively charged.

Nucleophilic Substitution

Halogenoalkanes react with nucleophiles in a nucleophilic substitution reaction. This mechanism is a one-step process.

Nucleophilic substitution

Nucleophilic substitution

  • The mechanism for nucleophilic substitution with hydroxide ions is shown above.
    • This makes an alcohol.
With ammonia

With ammonia

  • Nucleophilic substitution can be used to make amines. This is shown above.
With cyanide ions

With cyanide ions

  • Nucleophilic substitution can be used to make nitriles. This is shown above.
Reactivity

Reactivity

  • The reactivity of halogenoalkanes decreases in the order I>Br>Cl>F.
  • This is because the C-I bond is weakest and the C-F bond is strongest.
  • For the halogenoalkane to react, the C-X bond must break (X means any halogen).
  • This means that the activation energy is highest for a halogenoalkane with a carbon-fluorine bond and the reaction is therefore slowest.
Jump to other topics
1

Structure - Models of the Particulate of Matter

2

Structure - Models of Bonding & Structure

3

Structure - Classification of Matter

3.1

The Periodic Table: Classification of Elements

3.2

Periodic Trends

3.3

Group 1 Alkali Metals

3.4

Halogens

3.5

Noble gases, group 18

3.6

Functional Groups: Classification of Organic

3.7

Functional Group Chemistry

3.8

Alkanes

3.9

Alcohols

3.10

Halogenoalkanes

4

Reactivity - What Drives Chemical Reaction?

5

Reactivity - How Much, How Fast & How Far?

6

Reactivity - The Mechanisms of Chemical Change

7

Measurement, Data Processing & Analysis

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