Chemistry » Organic Molecules » Addition, Elimination And Substitution Reactions

# Substitution Reactions

## Substitution reactions

Some simple examples of substitution reactions are shown below:

$$\text{CH}_{4} + \text{Cl}_{2}$$ $$\to$$ $$\text{CH}_{3}\text{Cl} + \text{HCl}$$

A chlorine atom (from $$\text{Cl}_{2}$$) and a hydrogen atom (from $$\text{CH}_{4}$$) are exchanged to create new products ($$\text{CH}_{3}\text{Cl}$$ and $$\text{HCl}$$).

1. Formation of haloalkanes

Haloalkanes can be formed when the hydroxyl ($$-\text{OH}$$) group of an alcohol is replaced by a halogen atom (X = Cl, Br). This reaction works best with tertiary alcohols where it can occur at room temperature (see figure below)

The formation of a tertiary haloalkane from a tertiary alcohol by a substitution with HX (X = Cl, Br).

A tertiary carbon atom is attached to three other carbon atoms. A secondary carbon atom is attached to two other carbon atoms while a primary carbon atom is attached to one other carbon atom. In the figure above, the carbon is attached to the functional group as well as three other carbon atoms. This makes it a tertiary alcohol on the left of the arrow and a tertiary haloalkane on the right.

It is possible for a secondary or primary alcohol to form a secondary or primary haloalkane as well (see figure below) but this requires high temperatures and the reaction is very slow.

The formation of a primary haloethane from ethanol by a substitution with $$\text{HX}$$ (X = Cl, Br).

2. Hydrolysis

In a previous section we saw the hydration of an ethene to form an alcohol. That is an addition reaction. Alcohols can also be formed through a substitution reaction with a haloalkane. In the example given in the figure below the haloalkane would be dissolved in water.

The formation of ethanol from chloroethane through a substitution reaction with water.

Using a base (e.g. $$\text{KOH}$$, $$\text{NaOH}$$) dissolved in water and warming the solution would increase the rate of the reaction (see figure below). However, in order for substitution to occur the following reaction conditions must be used:

• low temperatures (around room temperature)

• a dilute solution of a strong base (e.g. $$\text{NaOH}$$)

• the solution must be aqueous (in water)

The formation of ethanol from bromoethane through a substitution reaction with potassium hydroxide ($$\text{KOH}$$).

If the concentration of the base is too high then the halogen atom and a hydrogen atom could be eliminated (dehydrohalogenation).

3. Formation of haloalkanes from alkanes

Another way of forming a haloalkane involves the removal of a hydrogen atom from a saturated compound. The hydrogen atom is replaced by a halogen ($$\text{F}$$, $$\text{Cl}$$, $$\text{Br}$$ or $$\text{I}$$) to form a haloalkane (see figure below). As alkanes are not very reactive light is needed for this reaction to take place.

The formation of bromoethane from ethane through a substitution reaction with $$\text{Br}_{2}$$.

#### Fact:

Refer to the section on saturated and unsaturated structures for more information on why alkanes are not very reactive.

Reaction conditions:

• energy in the form of light

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