Addition Reactions

students, in organic chemistry, one small change in a molecule can create a completely new substance 🌱. Addition reactions are one of the main ways this happens. In an addition reaction, atoms or groups of atoms are added across a multiple bond, usually a carbon-carbon double bond $\left(\mathrm{C=C}\right)$ or triple bond $\left(\mathrm{C\equiv C}\right)$. The result is a molecule with fewer unsaturated bonds and more single bonds.

What you will learn

By the end of this lesson, you should be able to:

• explain what an addition reaction is and identify its key features,
• describe addition reactions of alkenes and alkynes using correct chemical language,
• predict products from simple addition reactions,
• connect addition reactions to mechanism thinking in Reactivity 3,
• use examples to show why addition reactions matter in industry and everyday life.

Addition reactions are important because they show how organic molecules can be transformed in a controlled way. They also connect to broader chemistry ideas in Reactivity 3, where you study how substances change through mechanisms, electron movement, and reaction pathways.

What makes a reaction an addition reaction?

An addition reaction happens when two reactants combine to form one product, and the product contains atoms from both reactants. In organic chemistry, addition usually happens when a molecule has a multiple bond. The $\pi$ bond in a double or triple bond is weaker and more reactive than a $\sigma$ bond, so it can break and allow new atoms to attach.

For example, ethene reacts with bromine:

$$\mathrm{C_2H_4 + Br_2 \rightarrow C_2H_4Br_2}$$

This is an addition reaction because bromine adds across the $\mathrm{C=C}$ bond to form 1,2-dibromoethane.

A useful idea is that addition reactions reduce unsaturation. An alkene has one degree of unsaturation from the double bond, while the product is usually more saturated. This is why addition reactions are often used to turn reactive small molecules into more stable compounds.

Why double bonds react so easily

A carbon-carbon double bond consists of one $\sigma$ bond and one $\pi$ bond. The $\pi$ bond is above and below the plane of the molecule, so it is easier to break than a $\sigma$ bond. When the $\pi$ bond breaks, the two carbon atoms can form new single bonds with incoming atoms or groups.

This is why alkenes are much more reactive than alkanes. Alkanes have only $\sigma$ bonds, so they do not usually undergo addition reactions under normal conditions. students, this is a key comparison to remember: double bonds invite addition, single bonds usually do not.

Common addition reactions of alkenes

Alkenes are the classic examples used in IB Chemistry HL. Several important addition reactions can occur.

1. Hydrogenation

Hydrogenation is the addition of hydrogen across a double bond.

$$\mathrm{C_2H_4 + H_2 \rightarrow C_2H_6}$$

Ethene becomes ethane. This reaction usually needs a metal catalyst such as nickel, platinum, or palladium. Hydrogenation is used in food production to make vegetable oils more saturated, which changes their texture. This is a real-world example of how chemistry changes material properties 🍞.

2. Halogenation

Halogens such as bromine or chlorine can add across an alkene.

$$\mathrm{C_2H_4 + Br_2 \rightarrow C_2H_4Br_2}$$

This reaction is useful as a test for unsaturation. Bromine water is orange-brown, and when it reacts with an alkene, the colour disappears because bromine is used up. This colour change is evidence that an addition reaction has occurred.

3. Hydrohalogenation

A hydrogen halide such as hydrogen bromide can add across a double bond.

$$\mathrm{C_3H_6 + HBr \rightarrow C_3H_7Br}$$

For propene, the major product is usually 2-bromopropane. This happens because the reaction often follows Markovnikov’s rule: the hydrogen adds to the carbon already carrying more hydrogens, while the halogen adds to the more substituted carbon. This is tied to the stability of the intermediate formed during the mechanism.

4. Hydration

Water can be added across a double bond to form an alcohol.

$$\mathrm{C_2H_4 + H_2O \rightarrow C_2H_5OH}$$

This reaction requires an acid catalyst and high temperature and pressure in industrial settings. It is a major method for producing ethanol. students, this is a helpful example because it connects addition reactions to fuels, solvents, and the chemical industry.

How addition reactions work: the mechanism idea

In Reactivity 3, it is not enough to say “the reactants mix and a product forms.” You should explain how the reaction happens. That is where mechanisms matter.

A mechanism is a step-by-step description of how bonds break and form. In addition reactions of alkenes, the $\pi$ bond acts as an electron-rich region. It can attract electrophiles, which are electron-pair acceptors.

A common mechanism idea is electrophilic addition. The alkene donates electron density from the $\pi$ bond to an electrophile. Then the rest of the molecule adds on in a later step.

For example, when bromine reacts with ethene, the $\pi$ bond polarizes the $\mathrm{Br_2}$ molecule. One bromine atom becomes slightly positive and can be attacked by the alkene. The result is the formation of new bonds and a final dibromo product.

This is important because it shows that addition reactions are not random. They follow patterns based on electron movement, bond strengths, and stability of intermediates. That is exactly the kind of reasoning IB Chemistry HL expects.

Product prediction and exam reasoning

To predict the product of an addition reaction, students, follow these steps:

1. Identify the multiple bond.
2. Decide what is being added, such as $\mathrm{H_2}$, $\mathrm{Br_2}$, $\mathrm{HBr}$, or $\mathrm{H_2O}$.
3. Replace the $\pi$ bond with new single bonds to the added atoms.
4. Check whether the product should follow a rule such as Markovnikov’s rule.

Example: propene plus hydrogen bromide.

$$\mathrm{CH_3CH=CH_2 + HBr \rightarrow CH_3CHBrCH_3}$$

The major product is 2-bromopropane, not 1-bromopropane. Why? The mechanism leads to the more stable intermediate and therefore the more stable product pathway.

Another example: but-2-ene plus bromine.

$$\mathrm{CH_3CH=CHCH_3 + Br_2 \rightarrow CH_3CHBrCHBrCH_3}$$

Here, bromine adds across the double bond to give 2,3-dibromobutane.

When writing answers, make sure you include correct structural formulas and names. Many marks in IB come from accurate communication, not just the final product.

Evidence, observations, and applications

Addition reactions can be identified through evidence from experiments. A classic observation is the decolourisation of bromine water. Another is the disappearance of the alkene starting material and appearance of a new compound with different physical properties.

In industry, addition reactions are used to make polymers, alcohols, and halogenoalkanes. For example, addition polymerisation involves many alkene molecules joining together by opening the double bond. Although addition polymerisation is a separate subtopic, it uses the same central idea: the reactive $\pi$ bond allows new bonds to form.

Addition reactions also fit into the wider Reactivity 3 theme because they show how structure controls reactivity. The presence of a multiple bond changes electron distribution, which changes the reaction pathway. That is the same type of reasoning you use in acid-base chemistry, redox chemistry, and organic mechanisms: what species is electron-rich, what species is electron-poor, and what bonds are likely to change?

Conclusion

Addition reactions are a major pattern in organic chemistry. They usually involve alkenes or alkynes, where a $\pi$ bond breaks and new atoms are added across the multiple bond. These reactions are important in laboratories, industry, and everyday materials. They also show how chemists explain change using mechanisms, not just equations.

If you can identify the bond being attacked, predict the added atoms, and explain the electron movement, you are using strong IB Chemistry HL thinking. students, that skill will help you in many parts of Reactivity 3 and beyond 🔬.

Study Notes

• Addition reactions join atoms across a multiple bond, usually $\mathrm{C=C}$ or $\mathrm{C\equiv C}$.
• The $\pi$ bond is more reactive than a $\sigma$ bond, so it breaks more easily.
• Common alkene addition reactions include hydrogenation, halogenation, hydrohalogenation, and hydration.
• Bromine water is a test for unsaturation because its colour disappears when it reacts with an alkene.
• Addition reactions often follow electrophilic addition mechanisms.
• Markovnikov’s rule helps predict the major product in some unsymmetrical addition reactions.
• Addition reactions connect structure, bonding, mechanism, and industrial chemistry.
• Always write balanced equations and correct structural products in exam answers.