Substitution Reactions

What are Substitution Reactions?

Substitution reactions are organic reactions where one atom or group of atoms in a molecule is replaced by another atom or group.

• Alkanes commonly undergo substitution (not addition) reactions.
• Esters can undergo hydrolysis (sometimes described in substitution terms, but usually classified under ester reactions).

Halogenation of Alkanes

Alkanes, which are saturated hydrocarbons, generally undergo substitution reactions rather than addition reactions.

• When alkanes react with halogens (such as chlorine or bromine), a hydrogen atom in the alkane is replaced by a halogen atom.
• This process is called halogenation and typically occurs under ultraviolet (UV) light, which provides the energy needed to initiate the reaction.

Mechanism of Free Radical Substitution

The halogenation of alkanes occurs via a free radical substitution mechanism, which involves three key stages:

1. Initiation

UV light provides the energy needed to break the chlorine molecule ($Cl₂$) by homolytic fission, producing two chlorine radicals ($Cl•$), each containing an unpaired electron.

$$\text{Cl₂} \xrightarrow{\text{UV light}} 2 \text{Cl•}$$

2. Propagation

A chlorine radical ($Cl•$) reacts with methane ($CH₄$), removing one hydrogen atom and creating a methyl radical ($CH₃•$) and hydrogen chloride ($HCl$).

$$\text{CH₄} + \text{Cl•} → \text{CH₃•} + \text{HCl}$$

The methyl radical ($CH₃•$) then reacts with another chlorine molecule ($Cl₂$), forming chloromethane ($CH₃Cl$) and regenerating another chlorine radical ($Cl•$), which continues the chain reaction.

$$\text{CH₃•} + \text{Cl₂} → \text{CH₃Cl} + \text{Cl•}$$

3. Termination

The chain reaction ends when two radicals combine to form a stable molecule, stopping the reaction.

Examples of termination steps include:

• Two chlorine radicals combining: $$2 \text{Cl•} → \text{Cl₂}$$
• A chlorine radical and a methyl radical combined: $$\text{Cl•} + \text{CH₃•} → \text{CH₃Cl}$$

Evidence for the Free Radical Mechanism

The free radical substitution mechanism is supported by several pieces of experimental evidence:

1. Use of Ultraviolet Light: The reaction does not proceed without UV light, showing that light is necessary to break the $Cl₂$ bond and produce radicals.
2. Chain Reaction: Once initiated, the reaction continues as long as chlorine and methane are present, which is characteristic of a radical chain reaction.
3. Formation of Ethane and Butane: In the monochlorination of methane, trace amounts of ethane ($C₂H₆$) are found, while monochlorination of ethane produces small amounts of butane ($C₄H₁₀$). This happens because two methyl radicals ($CH₃•$) or two ethyl radicals ($C₂H₅•$) can combine.
4. Radical Promoters: Adding substances like tetraethyllead, which can generate free radicals, increases the reaction rate, further supporting the radical nature of the reaction.

Esterification

Esterification is the process by which a carboxylic acid reacts with an alcohol to form an ester and water. This reaction is typically catalysed by an acid, such as sulfuric acid ($H₂SO₄$).

Base Hydrolysis of Esters

The reverse of esterification is hydrolysis, where an ester reacts with water to produce a carboxylic acid and an alcohol.

When this reaction is catalysed by a base (usually sodium hydroxide, $NaOH$), it is called saponification.

Soap Manufacture

The production of soap involves the base hydrolysis (saponification) of natural fats and oils, which are triglycerides (esters of fatty acids and glycerol).

Reaction in Soap Making

In the presence of sodium hydroxide ($NaOH$), triglycerides (fats) are broken down into glycerol and the sodium salts of fatty acids, which are the soap molecules.

$$\text{Triglyceride} + \text{3 NaOH} → \text{Glycerol} + \text{3 Soap molecules (fatty acid salts)}$$

Structure of Reactants and Products

• Reactants: Triglycerides consist of a glycerol backbone attached to three fatty acid chains.
• Products: The fatty acid chains (now in the form of sodium salts) are the soap molecules, and glycerol is a byproduct.