Bromine water (Br₂) is a rich orange-brown colour. But add a few drops of an alkene, and it turns colourless. This simple test-tube reaction is a useful way of testing for an alkene's characteristic C=C double bond, and is just one example of reactions of alkenes.

• This article is about reactions of alkenes.
• We'll first define alkene before looking at their most common type of reaction: electrophilic addition reactions.
• We'll focus on how they react in addition reactions with hydrogen halides, halogens, hydrogen and steam.
• After that, we'll explore other reactions of alkenes, including their reaction with manganate(VII) solution.

What are alkenes?

Alkenes are organic molecules made of just carbon and hydrogen atoms. They are characterised by one or more carbon-carbon double bond (C=C). The presence of this double bond means that they are unsaturated.

Alkenes have the general formula C_nH_2n. Examples of alkenes include ethene (C₂H₄), butene (C₄H₈) and decene (C₁₀H₂₀).

Ethene. Vaia Originals

Alkene electrophilic addition reactions

Alkenes react in various different ways, but most commonly in electrophilic addition reactions.

Addition reactions always involve breaking a double or triple bond, creating two new single bonds.

In electrophilic addition reactions, one of the small molecules involved is an electrophile.

Electrophiles are electron deficient. They contain a positive ion or partially positive atom with a vacant orbital, and are attracted to areas of high electron density, such as a C=C double bond.

Examples of electrophiles are:

• The hydrogen ion, H⁺.
• δ+ hydrogen atoms, such as in sulfuric acid, water, or hydrogen halides.
• Chlorine cations, Cl⁺.

To summarise, in electrophilic addition reactions, an electron-deficient molecule (an electrophile) is attracted to an area of high electron density in another organic molecule. The electrophile forms a covalent bond with the organic molecule, forming one overall larger molecule. During the process, a double or triple covalent bond is broken and two new single bonds are formed.

As we explore above, alkenes all contain a C=C double bond. This contains two pairs of electrons, and so is an area of high electron density. As a result, alkenes frequently react in electrophilic addition reactions, including with:

• Hydrogen halides, HX.
• Halogens, X₂.
• Sulphuric acid, H₂SO₄.
• Steam, H₂O(g).
• Hydrogen, H₂.

But before we look at each of these reactions in more detail, let's first learn the mechanism for alkene electrophilic addition reactions.

Alkene addition reaction mechanism

Alkene electrophilic addition reactions all follow the same general mechanism.

1. The electrophile is attracted to the high electron density of the alkene's C=C double bond.
2. The electrophile accepts a pair of electrons from the C=C double bond and forms a bond with the alkene. This breaks a bond in the electrophile heterolitically. Overall, this step creates a positive carbon ion, called a carbocation, and a negative ion.
3. The negative ion then forms a bond with the carbocation, neutralising the molecule.

The general mechanism of alkene electrophilic addition reactions. Vaia Originals

Let's now apply that mechanism to specific alkene electrophilic addition reactions.

Examples of alkene addition reactions

First up: the reaction of alkenes with hydrogen halides, HX.

Reaction with hydrogen halides

Hydrogen halides, such as HBr and HCl, add across the C=C double bond in alkenes to form a halogenoalkane. This is an electrophilic addition reaction known as halogenation. It takes place at room temperature.

Because halogens are more electronegative than hydrogen, the H-X bond is polar and the hydrogen atom has a partial positive charge. This enables it to act as an electrophile.

The mechanism for the reaction between hydrogen bromide (HBr) and ethene (C₂H₄) is shown below. This produces bromoethane (CH₃CH₂Br).

The mechanism for the electrophilic addition reaction between hydrogen bromide and ethene. Vaia Originals

Note the following:

• The partially positive H atom in HBr is attracted to the C=C double bond in ethene.
• The H atom forms a bond with ethene, causing the H-Br bond to break. Overall, this forms a positive carbocation and a negative Br^- ion.
• The Br^- ion forms a bond with the positive carbocation, resulting in bromoethane.

Reaction with halogens

Alkenes react with halogen molecules at room temperature to form dihalogenoalkanes, in another form of halogenation.

Although halogen molecules are not initially electrophiles, when they approach the alkene’s electron-rich C=C bond, a dipole is induced. The halogen atom nearest to the C=C bond becomes partially positively charged and can act as an electrophile. The reaction then follows the general mechanism we explored earlier.

For example, bromine (Br₂) reacts with ethene to form 1,2-dibromoethane (CH₂BrCH₂Br), as shown below:

The mechanism for the electrophilic addition reaction between bromine and ethene. Vaia Originals

As we mentioned in the introduction, This reaction enables bromine to be used as a test for the alkene functional group. Orange-brown bromine water will be decolourised if added to a solution containing an alkene. This is because the bromine adds onto the C=C double bond, forming a dibromoalkane.

Reaction with sulphuric acid

Alkenes react with concentrated sulfuric acid (H₂SO₄) at room temperature in a highly exothermic reaction. The electrophile is one of the hydrogen atoms in the acid molecule.

Sulphuric acid, shown with some of its partial charges.commons.wikimedia.org

If water is then added, the end product is an alcohol and the sulphuric acid reforms. This means the sulphuric acid acts as a catalyst.

The reaction between sulphuric acid and ethene is shown below:

The mechanism for the electrophilic addition reaction between sulphuric acid and ethene. Vaia Originals

Reaction with steam

Another acid, phosphoric acid (H, is commonly used in industry as a catalyst for the reaction between ethene and steam to form ethanol (CH₃CH₂OH). It has a slightly different, more complicated mechanism. This is a hydration reaction and takes place under a pressure of 60 atm and a temperature of 300℃. Here's the reaction:

The electrophilic addition reaction between ethene and water produces ethanol.Vaia Originals

Reaction with hydrogen

Alkenes react with hydrogen in a hydrogenation reaction to form alkanes. The reaction occurs at 140℃ in the presence of a nickel catalyst.

For example, ethene can be hydrogenated into ethane:

Other reactions of alkenes

Alkenes can also take place in oxidation reactions with potassium manganate(VII) solution (KMnO₄) and polymerisation reactions.

Reaction with manganate solution

Alkenes react with potassium manganate(VII) solution in an oxidation reaction, in which manganate(VII) ions act as an oxidising agent. It is characterised by a dramatic colour change. The exact products vary depending on the conditions.

Cold dilute acidic manganate solution

At room temperature, alkenes react with dilute, acidic manganate(VII) solution to form a diol. The reaction also releases Mn²⁺ ions, which turn the purple manganate solution colourless.

For example, reacting ethene with cold, dilute, acidic manganate solution to form ethane-1,2-diol. Here, we've represented the oxidising agent (manganate(VII) ions) as [O].

The reaction between cold, dilute, acidic manganate(VII) solution and ethene. Vaia Originals

Cold dilute alkaline manganate solution

The reaction above took place in acidic conditions. However, if the manganate solution is instead alkaline, we get a slightly different result. Once again, the alkene is oxidised into a diol, but this time the manganate ions are reduced into Mn⁴⁺ ions. These turn the purple solution dark green before producing a dark brown precipitate.

Hot concentrated acidic manganate solution

Heating an alkene with concentrated, acidic manganate solution doesn't just end with a diol. The harsh conditions mean that the alcohol is oxidised further. In essence, the alkene's C=C double bond is completely broken and the alkene is split in two. Each of the halved molecules forms a C=O double bond with the oxidising agent. We form either carbon dioxide (CO₂), an aldehyde (RCHO), a carboxylic acid (RCOOH) or a ketone (RCOR').

How do we know which products we'll end up with? We take each carbon atom in the C=C double bond separatel, and consider the two groups attached to them.

• If the carbon atom is attached to two R groups, we form a ketone.
• If the carbon atom is attached to one R group and one hydrogen atom, we form an aldehyde, which then gets oxidised into a carboxylic acid.
• If the carbon atom is attached to two hydrogen atoms, we form carbon dioxide.

Here's a diagram showing the three possible outcomes.

The possible products of the reaction of hot, concentrated, acidic manganate(VII) solution with an alkene. Vaia Originals

Polymerisation reactions

Finally, we'll take a brief dive into alkene polymerisation reactions. Multiple alkenes can join together to form addition polymers.

Alkene polymers, called polyalkenes, consist of lots of alkene monomers connected by single covalent bonds. The C=C double bond in each alkene opens up and is used to connect to the adjacent alkene, forming one long polymer chain.