Alkenes & Their Reactions (Triple only)

Alkenes are a group of hydrocarbons that belong to a family called a homologous series. Every alkene contains:

• Only carbon and hydrogen atoms

• A carbon–carbon double bond somewhere in its structure

This double bond is the key feature that makes alkenes much more reactive than alkanes.

The diagram above shows an example hydrocarbon chain with one double bond. The diagram shows that every carbon still forms four total bonds, while hydrogens form only one.

Why the double bond matters

• The double bond has one “normal” bond + one weaker bond layered on top.

• That weaker bond can break open, allowing new atoms to attach easily.

• This makes alkenes important starting materials in industry for solvents, plastics, and many other chemicals.

Why Alkenes Are Called Unsaturated

Alkenes are said to be unsaturated because the presence of a bond means the molecule contains fewer hydrogen atoms than the equivalent alkane.

• Saturated = full of hydrogen (no double bonds)

• Unsaturated = fewer hydrogens because of a double bond

If the double bond were “opened up,” the carbon atoms could hold additional hydrogen atoms. This is why alkenes readily take part in addition reactions.

General Formula & Recognising Alkenes

All straight-chain alkenes follow the formula:

Here are verbal descriptions (instead of diagrams) using different examples from the original source:

1. Ethene ()

Two carbons double-bonded, with each carbon holding two hydrogens.

2. Propene ()

Three-carbon chain, double bond between carbon 1 and carbon 2.

3. butene ()

Four-carbon chain with the double bond starting at carbon 2.

4. pentene ()

Five-carbon chain; double bond position can vary (e.g. carbon 3).

For butene and pentene the double bond can appear in different places.

Distinguishing Alkenes From Alkanes (Bromine Water Test)

Because alkenes react easily at the bond, they can be identified using bromine water.

Procedure:

• Add a small amount of orange bromine water to the substance being tested.

• If the solution quickly loses its orange colour, an alkene is present.

Why this happens

The double bond reacts with bromine, forming a colourless product.

Alkanes do not do this - they leave the orange colour unchanged.

The diagram above shows two small containers side by side: one showing the starting orange bromine solution, and another showing the mixture turning colourless after contacting an alkene.

Combustion of Alkenes

Complete combustion

With plenty of oxygen, alkenes burn to form:

• Carbon dioxide

• Water

Incomplete combustion

When oxygen supply is limited, alkenes burn with a smoky flame, producing:

• Carbon monoxide

• Solid carbon (soot)

• Possibly some unburned hydrocarbons

This is typical for all hydrocarbons with high carbon content.

Reactivity of Alkenes: The Double Bond in Action

Most reactions of alkenes involve addition reactions.

An addition reaction occurs when:

The bond opens → two new atoms/groups attach to the formerly double-bonded carbons.

This mechanism is the foundation for:

• Hydrogenation

• Hydration

• Addition of halogens

• Many polymerisation reactions

Addition Reactions in Detail

Hydrogenation (Adding Hydrogen)

When an alkene reacts with hydrogen:

• The double bond opens

• Two hydrogen atoms are added

• A saturated alkane is formed

Typical conditions

• Hydrogen gas

• Metal catalyst (nickel, platinum, or palladium)

• Warm conditions

Hydration (Adding Water to Form Alcohols)

Steam can be added across the double bond to produce an alcohol.

Conditions

• Steam

• Catalyst (often phosphoric acid)

• High temperature and pressure

Addition of Halogens

Alkenes react rapidly with halogens such as:

• Chlorine

• Bromine

• Iodine

What happens

• The double bond breaks

• Each carbon attaches to a halogen atom

• A saturated dihaloalkane is formed