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Reactions of Alkenes

Reactions of Alkenes

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, 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).

Reactions of alkenes ethene studysmarterEthene. 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.

  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.

Reactions of alkenes electrophilic addition reaction general mechanism StudySmarterThe 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).

Reactions of alkenes electrophilic addition reaction HBr ethene mechanism StudySmarterThe 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:

Reactions of alkenes electrophilic addition reaction br2 ethene mechanism StudySmarterThe 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.

Reactions of alkenes testing for alkenes bromine water StudySmarter

Testing for alkenes using bromine water. StudySmarter Originals

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.

Reactions of alkenes, sulfuric acid, StudySmarterSulphuric 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:

Reactions of alkenes electrophilic addition reaction sulphuric acid ethene mechanism StudySmarterThe 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:

Reactions of alkenes electrophilic addition reaction steam ethene mechanism StudySmarterThe 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:

Reactions of alkenes electrophilic addition reaction hydrogen ethene mechanism StudySmarterThe 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].

Reactions of alkenes cold dilute acidic manganate(VII) solution etheneThe 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.

Reactions of alkenes hot concentrated acidic manganate(VII) solution ethene products StudySmarterThe 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:
    • The reaction with hydrogen halides to form halogenoalkanes.
    • The reaction with halogens to form dihalogenoalkanes.
    • The reaction with sulphuric acid.
    • The reaction with hydrogen to form alkanes.
    • The reaction with steam to form alcohols.
  • 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.

Frequently Asked Questions about Reactions of Alkenes

Alkenes typically undergo electrophilic addition reactions.

Four common reactions involving alkenes are halogenation, hydrogenation, oxidation, and hydration.

Alkenes typically react with halogens, hydrogen halides, and steam if in the presence of an acid catalyst.

The simplest alkene is ethene, which has just two carbon atoms and four hydrogen atoms. Other alkenes include propene and butene.

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.

Final Reactions of Alkenes Quiz

Question

What is an electrophile?

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Answer

An electron pair acceptor with a vacant orbital and a positive or partially positive charge.

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Question

What is the product of the electrophilic addition reaction involving HCl and ethene?

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Answer

Chloroethene.

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Question

What is a carbocation?


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Answer

A positively charged carbon atom.

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Question

What is the product of the reaction between propene, sulfuric acid, and water?

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Answer

Propanol

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Question

Give the conditions for the reaction between ethene and steam.


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Answer

  • 60 atm.
  • 300 ℃.
  • Phosphoric acid catalyst.

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Question

Why is the reaction with potassium permanganate solution not a useful test for alkenes?


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Answer

Potassium permanganate can oxidise a variety of compounds and so will also change colour in reaction with molecules other than alkenes.


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Question

Describe the test for alkenes.

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Answer

Orange-brown bromine water decolourises.

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Question

What is the product of the reaction between an alkene and hydrogen in the presence of a nickel catalyst?


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Answer

An alkane.

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Question

How many alkyl groups do primary carbocations have attached to the positive carbon atom?

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Answer

1

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Question

How many alkyl groups do tertiary carbocations have attached to the positive carbon atom?


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Answer

3

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Question

Which type of carbocation is the most stable?

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Answer

Tertiary carbocations.

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Question

Alkyl groups are electron _____.

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Answer

​Releasing.

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Why are tertiary carbocations more stable than primary carbocations?

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Answer

  • Tertiary carbocations have three alkyl groups attached to the positive carbon atom whereas primary carbocations only have one.
  • Alkyl groups are electron-releasing. This stabilises the charge of the carbocation.
  • As tertiary carbocations have more alkyl groups, they produce a more stable carbocation.

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Question

In the reaction between 1-chloroprop-1-ene and HBr, what will the products be?


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Answer

The major product will be 2-bromo-1-chloropropane and the minor product will be 1-bromo-1-chloropropane.

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Question

Give the colour change for the reaction between an alkene and cold, dilute, acidic manganate(VII) solution.

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Answer

Purple to colourless

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Question

Give the products of the reaction between an alkene and cold, dilute, acidic manganate(VII) solution.

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Answer

A diol

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Question

Give the colour change and products of the reaction between an alkene and cold, dilute, alkaline manganate(VII) solution.

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Answer

Colour change: purple to dark green, eventually forming a dark brown precipitate. 

Products: a diol.

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Question

Give the possible products of the reaction between an alkene and hot, concentrated, acidic manganate(VII) solution.

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Answer

  • Carbon dioxide
  • Aldehydes
  • Carboxylic acids 
  • Ketones

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Question

What is electrophilic substitution?

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Answer

An electrophile replaces a functional group in a molecule

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Question

What is electrophilic addition?

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Answer

An electrophile is added to the overall compound

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Question

What is an alkene?

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Answer

It has a double carbon to carbon bond

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Question

In electrophilic addition, are double carbon to carbon bonds broken?

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Answer

Yes

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In electrophilic substitution, are double carbon to carbon bonds broken?

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Answer

Yes

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Question

What allows electrophiles to attacked alkenes?

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Answer

The carbon to carbon double bond has high electron density, making it susceptible to an attack

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Question

Does electrophilic addition take place with alkanes or alkenes?

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Answer

Alkenes

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Question

What is Markownikoff’s rule?


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Answer

When there is a compound that includes hydrogen (H), hydrogen attaches to the carbon that has the most hydrogens at the beginning

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Question

According to Markownikoff’s rule, is the major or minor product more favourable?


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Answer

Major product

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Question

In a reaction between hydrogen bromine and propene, what is the major product?

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Answer

1-bromopropene

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In a reaction between hydrogen bromine and propene, what is the minor product?

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Answer

1-bromopropene

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When Brand propene react, what is the product?


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Answer

1,2, dibromopropane

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Question

How many hydrogens does a primary carbocation have?

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Answer

2

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Question

How many hydrogens does a secondary carbocation have?




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Answer

1

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Question

How many hydrogens does a tertiary carbocation have?

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Answer

0

Show question

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