Reactions of alkenes

Welcome to the reactions of alkenes! If you have already studied alkanes, you will know that they are relatively unreactive. Alkenes are the complete opposite. Because they have a "spare" bond waiting to be used, they are highly reactive and form the building blocks for many everyday chemicals.

What you'll learn:

What a functional group is and why it controls how a molecule behaves.
Why alkenes burn differently from alkanes.
How to draw and predict the products when alkenes react with hydrogen, halogens, and water.

The alkene functional group

Alkenes belong to a homologous series (a family of compounds) characterised by having at least one carbon-carbon double bond (C=C).

Definition

Functional group

A functional group is a specific atom, or group of atoms, in a molecule that determines how that molecule will react chemically. For alkenes, the functional group is the C=C double bond.

Because the reactions of organic compounds are determined almost entirely by their functional groups, all alkenes react in very similar ways.

You need to know the names and be able to draw the displayed formulae of the first four alkenes:

Ethene ( $C2H4\text{C}_2\text{H}_4$ )
Propene ( $C3H6\text{C}_3\text{H}_6$ )
Butene ( $C4H8\text{C}_4\text{H}_8$ )
Pentene ( $C5H10\text{C}_5\text{H}_{10}$ )

Tip

Where is methene?

You might wonder why the list starts at ethene instead of "methene". Because the functional group is a carbon-carbon double bond, you need at least two carbon atoms in the molecule to form it!

Combustion of alkenes

Just like other hydrocarbons (such as alkanes), alkenes can react with oxygen in combustion reactions. If there is a massive excess of oxygen, they will undergo complete combustion to produce carbon dioxide and water.

However, in standard air, alkenes almost always undergo incomplete combustion. This happens because they have a higher ratio of carbon to hydrogen compared to alkanes. As a result, when alkenes burn in air, they tend to produce a smoky yellow flame and leave behind unburnt carbon (soot), alongside carbon monoxide and water.

Addition reactions

Because alkenes are unsaturated (they contain a double bond), they can "open up" that bond to attach new atoms. This is called an addition reaction. In an addition reaction, the C=C double bond breaks to become a C-C single bond, and new atoms are added across the two carbon atoms.

Analogy

Opening a gate

Think of the two carbon atoms in a double bond like two people holding both of each other's hands. To grab a new object, they don't have to walk away from each other completely. They just let go of one hand each (breaking the double bond into a single bond). Now, they remain linked by one hand, but they both have a "spare" hand to grab something new!

1. Adding halogens (Halogenation)

Alkenes react readily with halogens — such as chlorine ( $Cl2\text{Cl}_2$ ), bromine ( $Br2\text{Br}_2$ ), and iodine ( $I2\text{I}_2$ ) — to form dihaloalkanes. This reaction happens spontaneously at room temperature; no catalyst is needed.

For example, when ethene reacts with bromine, the double bond breaks and a bromine atom attaches to each carbon, forming 1,2-dibromoethane.

alt text

\text{C}_2\text{H}_4\text{(g)} + \text{Br}_2\text{(l)} \rightarrow \text{C}_2\text{H}_4\text{Br}_2\text{(l)}

Key Idea

The bromine water test

This exact reaction is the standard test for unsaturation (checking if a compound is an alkene). If you shake an alkene with orange bromine water, the bromine reacts and adds across the double bond. The solution quickly turns colourless. If you do this with an alkane, nothing happens and the water stays orange.

2. Adding hydrogen (Hydrogenation)

You can turn an unsaturated alkene back into a saturated alkane by reacting it with hydrogen gas ( $H2\text{H}_2$ ).

Because hydrogen is less reactive than the halogens, this addition reaction requires specific conditions to work:

A temperature of around 150 °C.
A nickel catalyst.

For example, reacting propene ( $C3H6\text{C}_3\text{H}_6$ ) with hydrogen produces propane ( $C3H8\text{C}_3\text{H}_8$ ). This process is heavily used in the food industry to turn liquid vegetable oils (which contain double bonds) into solid spreads like margarine.

3. Adding water (Hydration)

Alkenes can react with water to form alcohols (molecules containing an -OH functional group). For this to happen, the water must be in the form of steam.

The conditions required for hydration are quite extreme:

Very high temperature (around 300 °C).
High pressure (around 70 atmospheres).
A solid phosphoric acid catalyst.

Unlike the other addition reactions, hydration is reversible. When ethene reacts with steam, it forms ethanol ( $C2H5OH\text{C}_2\text{H}_5\text{OH}$ ). Unreacted ethene and steam are simply recycled back through the system.

alt text

\text{C}_2\text{H}_4\text{(g)} + \text{H}_2\text{O(g)} \rightleftharpoons \text{C}_2\text{H}_5\text{OH(g)}

Example

Predicting the product of an addition reaction

You must be able to draw the displayed formulae of the products when alkenes react. Let's predict and draw the product of the reaction between butene and chlorine.

Draw the starting alkene: Butene has four carbon atoms. Draw four carbons in a row, place a double bond between the first two, and single bonds between the rest. Add hydrogen atoms so that every carbon makes exactly 4 bonds in total.
Break the double bond: Redraw the exact same carbon skeleton, but replace the C=C double bond with a C-C single bond. Ensure all the original hydrogen atoms stay exactly where they were.
Add the new atoms: The chlorine molecule ( $Cl2\text{Cl}_2$ ) provides two chlorine atoms. Look at the two specific carbon atoms that used to share the double bond — each now only has 3 bonds. Attach one chlorine atom to the first carbon, and the second chlorine atom to the second carbon.

Common Mistake

Adding to the wrong carbons

When drawing the product of an addition reaction on longer chains (like butene or pentene), students often accidentally attach the new atoms to the end carbons, regardless of where the double bond was. Only add the new atoms to the two specific carbon atoms that originally held the double bond!

Exam technique

In the exam

Check the reagent: If the question asks for the conditions of an addition reaction, look closely at what is being added. Halogens need no special conditions, hydrogen needs 150 °C + a nickel catalyst, and water needs 300 °C + high pressure + a phosphoric acid catalyst.
Count your bonds: After drawing any organic molecule, double-check that absolutely every single carbon atom has exactly four bonds touching it.
Use the exact words: If asked what happens when bromine water is added to an alkene, state that it goes from "orange" to "colourless". Do not use the word "clear" (water is clear, but so is apple juice — "colourless" proves there is no pigment left).

Self review

Check yourself

What is the functional group of an alkene?
Why do alkenes often burn with a smoky flame?
What are the required conditions for the hydrogenation of an alkene?
If propene reacts with steam in the presence of a catalyst, what family of compounds is the product?
How would you chemically test a liquid to see if it is an alkene, and what would you observe?