Chemistry » Organic Molecules » Addition, Elimination And Substitution Reactions

### Fact:

Remember from the section on saturated and unsaturated structures that you can perform tests to determine if a compound is saturated or not. If bromine water or potassium permanganate are decolourised by a compound the compound is unsaturated, if they are not decolourised the compound is saturated.

## Hydrohalogenation

Hydrohalogenation involves the addition of a hydrogen atom and a halogen atom to an unsaturated compound (containing a carbon-carbon double bond). An example is given in the figure below. X can be fluorine (F), chlorine (Cl), bromine (Br) or iodine (I).

The hydrohalogenation of ethene to a haloethane.

If more than one product is possible the major product will be the compound where:

#### Tip:

Remember that a $$\color{red}{\textbf{tertiary carbon atom}}$$ is bonded to three other carbon atoms, a $$\color{orange}{\textbf{secondary carbon atom}}$$ is bonded to two other carbon atoms, and a $$\color{blue}{\textbf{primary carbon atom}}$$ is bonded to one other carbon atom. So tertiary is the most substituted, secondary is less substituted than tertiary and more substituted than primary, and primary is the least substituted.

• the $$\color{green}{\textbf{hydrogen atom}}$$ is added to $$\color{blue}{\textbf{least substituted carbon atom}}$$

• i.e. the carbon atom with the least number of carbon atoms bonded to it

• the $$\color{purple}{\textbf{halogen atom}}$$ is added to the $$\color{red}{\textbf{more substituted carbon atom}}$$

• i.e. the carbon atom with the most number of carbon atoms bonded to it

This is shown in the figure below.

### Definition: Major and minor products

The major product of a reaction is the product that is most likely to form. Minor products are those that are less likely to form.

The hydrohalogenation of 2-methylpropene to form 2-fluoro-2-methylpropane (major product) and 1-fluoro-2-methylpropane (minor product).

Reaction conditions:

• no water present in the reaction

## Halogenation

Halogenation is very similar to hydrohalogenation but a diatomic halogen molecule is added across the double bond. An example is given in the figure below.

The reaction between ethene and bromine to form 1,2-dibromoethane.

## Hydration

A hydration reaction involves the addition of water ($$\text{H}_{2}\text{O}$$) to an unsaturated compound. This is one way of preparing an alcohol from the corresponding alkene (see figure below).

The hydration of ethene to ethanol.

If more than one product is possible the major product will be the compound where:

• the $$\color{green}{\textbf{hydrogen atom}}$$ is added to the $$\color{blue}{\textbf{least substituted carbon atom}}$$

• the $$\color{purple}{\textbf{hydroxyl anion}}$$ ($$\text{OH}^{-}$$) is added to the $$\color{red}{\textbf{more substituted carbon atom}}$$

The hydration of 2-methylpropene to form 2-methylpropan-2-one (major product) and 2-methyl-propan-1-one (minor product).

Reaction conditions:

• water must be present in excess

• an acid catalyst is needed for this reaction to take place

• the catalyst that is most commonly used is phosphoric acid ($$\text{H}_{3}\text{PO}_{4}$$)

### Fact:

Fermentation can refer to the conversion of sugar to alcohol using yeast (a fungus). The process of fermentation produces items such as wine, beer and yogurt. To make wine, grape juice is fermented to produce alcohol. This reaction is shown below:

$$\text{C}_{6}\text{H}_{12}\text{O}_{6}$$ $$\to$$ $$2\text{CO}_{2} + 2\text{C}_{2}\text{H}_{5}\text{OH}+$$ energy

The learners should focus their research on the chemistry behind these fermentation processes. They can either present their findings as a discussion to the class, or in a short (1-page) report format. The research should be on local-level fermentation processes, not industrial scale breweries.

## Hydrogenation

Hydrogenation involves adding hydrogen $$(\text{H}_{2})$$ to an alkene. During hydrogenation the double bond is broken (as with hydrohalogenation and halogenation) and more hydrogen atoms are added to the molecule. A specific example is shown in the figure below.

The hydrogenation of ethene to ethane.

Reaction conditions:

• a catalyst such as platinum ($$\text{Pt}$$), palladium ($$\text{Pd}$$) or nickel ($$\text{Ni}$$) is needed for these reactions

• heating is required

• the reaction must be done under an inert atmosphere, not air (e.g. $$\text{N}_{2}(\text{g})$$ atmosphere)

The hydrogenation of vegetable oils to form margarine is another example of this addition reaction (see Figure 4.82).

The hydrogenation of sunflower oil to make margarine.

## Polymerisation Reactions

A polymer is made up of lots of smaller units called monomers. When these monomers are added together, they form a polymer. One way for polymerisation to occur is through an addition reaction. More details are given later on.

The polymerisation of vinyl chloride monomers to form a polyvinyl chloride polymer.

Polyvinyl chloride (see figure above) is used in construction and in clothing, as well as having many other uses.