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- 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, also known as olefins, are unsaturated hydrocarbons.
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 CnH2n. Examples of alkenes include ethene (C2H4), butene (C4H8) and decene (C10H20).
Ethene. StudySmarter Originals
Alkene electrophilic addition reactions
Alkenes react in various different ways, but most commonly in electrophilic addition reactions.
An addition reaction is a reaction that joins together two smaller molecules to make one larger molecule.
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.
An electrophile is an electron pair acceptor.
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.
- The electrophile is attracted to the high electron density of the alkene's C=C double bond.
- 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.
- The negative ion then forms a bond with the carbocation, neutralising the molecule.
The general mechanism of alkene electrophilic addition reactions. StudySmarter 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.
Remember, halogenoalkanes are organic molecules containing at least one carbon-halogen bond, represented as C-X. For more information, check out Halogenoalkanes.
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. StudySmarter 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.
You might have noticed something. Which carbon atom does the positive section of the electrophile add to? With symmetrical alkenes like ethene, it doesn't really matter - the electrophile is as likely to bond with one carbon atom as the other. But things get a little more tricky when we consider larger, asymmetric alkenes. Find out more in Products of Electrophilic Addition Reactions.
Reaction with halogens
Alkenes react with halogen molecules at room temperature to form dihalogenoalkanes, in another form of halogenation.
As you might be able to guess from the name, a dihalogenoalkane is simply a halogenoalkane with two halogen atoms.
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. StudySmarter 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.
A catalyst is a substance that speeds up the rate of a reaction, without being used up in the process.
The reaction between sulphuric acid and ethene is shown below:
The mechanism for the electrophilic addition reaction between sulphuric acid and ethene. StudySmarter 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.StudySmarter Originals
You can find the mechanism for this reaction in Production of Ethanol. You should also note that here, we've shown the hydration of ethene as going to completion. In fact, it is actually a reversible reaction. The article linked above will explain the conditions we choose to maximise the yield of ethanol.
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:
The mechanism for the electrophilic addition reaction between hydrogen and ethene. StudySmarter Originals
Hydrogenation is commonly used in margarine production as it ‘hardens’ vegetable oils, raising their melting point so that they are solid at room temperature. In particular, it involves partial hydrogenation, in which some of the C=C double cis-bonds in oils are hydrogenated and some are turned into trans-bonds (check out Isomerism to find out more about cis-/trans- isomerism). Partial hydrogenation helps turn oils into a smooth, creamy, spreadable product with a luxurious mouth feel.
Although trans fats are cheap to produce, easy to use, and help give some foods a desirable taste and texture, they have been linked to raised cholesterol levels and an increased risk of heart disease, thanks to their impact on levels of 'bad cholesterol', LDL. Are they really worth the taste?
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. StudySmarter 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.
Although these reactions produce a distinctive colour change, they are not a useful indicator of the presence of alkenes, as potassium permanganate is able to oxidise a variety of molecules. Electrophilic addition with bromine water is a more useful test, as explored above.
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. StudySmarter Originals
Polymerisation reactions
Finally, we'll take a brief dive into alkene polymerisation reactions. Multiple alkenes can join together to form addition polymers.
Polymers are large molecules made up of repeating units called monomers. Addition polymers are a particular type of polymer formed without creating any other byproducts.
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.
We use addition polymers such as polyalkenes to make things like plastics, fabric and building materials. All sorts of monomers can be used, provided they contain a C=C double bond. The exact properties of polymers depend on the monomer and its structure, which you can explore in Properties of Polymers.
There is also another type of polymerisation: condensation polymerisation. It uses monomers with two different functional groups. Find out more in Polymerisation Reactions.
Reactions of Alkenes - Key takeaways
- Alkenes are saturated hydrocarbons with one or more C=C double bonds.
- Alkenes react in electrophilic addition reactions. In these reactions, an electrophile combines with an alkene to form a larger molecule, breaking the alkene's C=C double bond.
- Examples of alkene electrophilic addition reactions include:
- Alkenes also react with manganate(VII) solution in an oxidation reaction. The products depend on the conditions.
- Alkenes take part in polymerisation reactions, forming addition polymers.
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Frequently Asked Questions about Reactions of Alkenes
What type of reaction do alkenes undergo?
Alkenes typically undergo electrophilic addition reactions.
. What are the four reactions of alkenes?
Four common reactions involving alkenes are halogenation, hydrogenation, oxidation, and hydration.
Which reagents do alkenes typically react with?
Alkenes typically react with halogens, hydrogen halides, and steam if in the presence of an acid catalyst.
What are some examples of alkenes?
The simplest alkene is ethene, which has just two carbon atoms and four hydrogen atoms. Other alkenes include propene and butene.
How do you write an addition reaction involving an alkene?
To represent addition reactions, you can write equations as with any other reaction. Remember to write out your products and reactants using structural formulae to help you identify the different functional groups and show the changes in the molecules.
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