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Reactions of Alkenes

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Chemistry

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.

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

An electrophile is an electron pair acceptor.

Electrophiles 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, .
  • δ+ hydrogen atoms, such as in sulfuric acid or hydrogen halides.
  • Chlorine cations, .

Remember, alkenes are unsaturated hydrocarbons with one or more C=C double bond. For a quick recap, check out Alkenes.

Addition reactions of alkenes

As we mentioned above, electrophiles react with alkenes in addition reactions. They all follow the same general mechanism.

  1. The electrophile accepts a pair of electrons from the alkene’s C=C double bond and is added onto the alkene by forming a bond with one of its carbon atoms. This creates a positive carbon ion, called a carbocation.
  2. A negative ion then adds onto the carbocation, neutralising the molecule.

Reactions of Alkenes, general mechanism electrophilic addition, StudySmarterIn the first step, the electrophile adds onto the alkene, forming a positive carbocation. Here we've shown the carbocation circled in red. In the second step, a negative ion adds onto the carbocation. Anna Brewer, StudySmarter Originals

Reaction with hydrogen halides

Hydrogen halides, such as and , add across the C=C double bond in an alkene to form a halogenoalkane. This is an electrophilic addition reaction known as halogenation.

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 and ethene is shown below:

Reactions of alkenes, hydrogen bromide reaction ethene, StudySmarterThe reaction between hydrogen bromide and ethene. Anna Brewer, StudySmarter Originals

Reaction with halogens

Alkenes react with halogen molecules to form dihalogenoalkanes in another form of halogenation. As you might be able to guess from the name, a dihalogenoalkane is just an alkane with two halogen atoms attached.

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 is able to act as an electrophile. The reaction then follows the general mechanism we explored earlier.

For example, bromine can react with the alkene ethene to form 1,2-dibromoethane, as shown below:

Reactions of alkenes, bromine reaction ethene, StudySmarterThe reaction between bromine and ethene. Anna Brewer, StudySmarter Originals

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, bromine water test alkenes, StudySmarterLeft: Orange-brown bromine water. Right: Decolourised bromine water after the addition of an alkene. Anna Brewer, StudySmarter Originals

Reaction with acids

Alkenes react with concentrated sulfuric acid 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, StudySmarterSulfuric acid, shown with some of its partial charges.commons.wikimedia.org

If water is then added, the end product is an alcohol, and the sulfuric acid reforms. This means the sulfuric 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 sulfuric acid and ethene is shown below:

Reactions of alkenes, sulfuric acid reaction ethene, StudySmarterThe reaction between sulfuric acid and ethene. Anna Brewer, StudySmarter Originals

Another acid, phosphoric acid, is commonly used in industry as a catalyst for the reaction between ethene and steam to form ethanol. 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℃.

Reactions of alkenes, phosphoric acid reaction ethene, StudySmarterThe reaction between phosphoric acid and ethene. Anna Brewer, StudySmarter Originals

How else can alkenes react?

Alkenes can also take place in oxidation reactions to form alcohols, and hydrogenation reactions to form alkanes.

Reaction with potassium permanganate solution

Alkenes react with cold, dilute potassium permanganate solution to form alcohols. This is an oxidation reaction and is characterised by a dramatic colour change.

  • Under acidic conditions, the permanganate solution changes from purple to colourless in the presence of an alkene.
  • Under alkaline conditions, the presence of an alkene causes the permanganate solution to change from purple to dark green, before producing a dark brown precipitate.

A diol is formed, and potassium permanganate acts as the oxidising agent. For example, reacting the alkene ethene with cold potassium permanganate solution will result in the production of ethane-1,2-diol, as shown by the equation below:

Reactions of alkenes, potassium permanganate oxidation reaction ethene, StudySmarterThe reaction between potassium permanganate, represented by [O], and ethene. Anna Brewer, StudySmarter Originals

Although this reaction produces a distinctive colour change, it is 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.

The above reaction uses cold, dilute potassium permanganate solution. However, if you use hot, concentrated, acidic potassium permanganate solution, further oxidation reactions occur. The diol splits up into two smaller molecules which are eventually oxidised into carboxylic acids.

To learn about a different type of alcohol oxidation, take a look at Oxidation of Alcohols.

Reaction with hydrogen

Alkenes react with hydrogen in a hydrogenation reaction to form alkanes. A nickel catalyst is used. For example, ethene can be hydrogenated into ethane:

Reactions of alkenes, hydrogen reaction ethene, StudySmarterThe reaction between ethene and hydrogen.Anna Brewer, 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. A certain proportion of the C=C double bonds in vegetable oils in margarines are hydrogenated to achieve the product's desired properties. This is known as partial hydrogenation, and the hydrogenated oils are known as trans fats.

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. Are they really worth the taste?

Major and minor products of reactions with alkenes

If the alkene used in an electrophilic addition reaction is asymmetric, multiple products can be formed. This is because the electrophile can add onto either carbon in the alkene’s C=C bond. The result is that two different structural isomers are produced.

Structural isomers are molecules with the same molecular formula but different structural formulae.

For example, when propene is attacked by hydrogen bromide, the hydrogen electrophile can add on to either carbon 1 or carbon 2, as labelled in the diagram. If the hydrogen adds on to carbon 1, the bromide ion must add onto carbon 2 and 2-bromopropane will be formed. If the hydrogen instead adds onto carbon 2, the bromide ion will add on to carbon 1 and 1-bromopropane will be formed. The reactions are shown below.

Reactions of alkenes, propene reaction, StudySmarterTop: Propene. Left: 1-bromopropane. Right: 2-bromopropane Anna Brewer, StudySmarter Originals

However, one of the isomers will be produced more than the other. To understand why this is the case, we need to look at the inductive effect, different types of carbocations, and carbocation relative stability.

The inductive effect

Alkyl groups, such as and , have a tendency to release electrons. This can be shown by an arrow on the covalent bond pointing away from the alkyl group. This releasing of electrons, called the positive inductive effect, stabilises the positive charge on any carbocation.

The positive inductive effect. Anna Brewer, StudySmarter Originals

Types of carbocations

Carbocations are classified as primary, secondary, or tertiary, depending on the number of alkyl groups attached to the positive carbon atom. For example, a primary carbocation has one alkyl group attached to it whereas a tertiary has three.

The stability of carbocations

Because alkyl groups tend to release electrons, which stabilise and reduce the charge on the carbocation, the more alkyl groups a carbocation has, the more stable it will be. Therefore, tertiary carbocations will be more stable than secondary carbocations, which in turn will be more stable than primary carbocations.

Reactions of alkenes, carbocation stability primary secondary tertiary, StudySmarterThe stability of different types of carbocations. Anna Brewer, StudySmarter Originals

It is easier to form a more stable molecule than a less stable one. This means that the major product of an electrophilic addition reaction will be one involving a more stable carbocation, such as a tertiary carbocation.

In our example using propene and hydrogen bromide, , the hydrogen will tend to add on to carbon 1 of the alkene as this will result in the formation of a secondary carbocation. This means that the major product will be 2-bromopropane. If the hydrogen instead added onto carbon 2, a less stable primary carbocation would be formed. Although this reaction does occur, it is less common than the major reaction.

This follows Markovnikov's Rule, which states that ‘when a compound HX is added to an unsymmetrical alkene, the hydrogen becomes attached to the carbon with the most hydrogens attached to it already’. This is because the carbon atom with the most hydrogen atoms attached forms the least stable carbocation.

Reactions of alkenes, carbocation stability Markovnikov's rule, StudySmarterA table showing how Markovnikov's Rule relates to carbocation stability, Anna Brewer, StudySmarter Originals

Another example is the reaction between hydrogen bromide and 2-methylbut-2-ene. Here, the major product will be 2-bromo-2-methylbutane. The major product contains the more stable carbocation intermediate due to the positive inductive effect of alkyl groups.

Reactions of alkenes, 2-methylbut-2-ene reaction, StudySmarterThe reaction between hydrogen bromide and 2-methylbut-2-ene. Anna Brewer, StudySmarter Originals

Reactions of Alkenes - Key takeaways

  • An electrophile is an electron pair acceptor.
  • Alkenes can react in electrophilic addition reactions to form halogenoalkanes, dihalogenoalkanes and alcohols.
  • Alkenes can undergo oxidation and hydrogenation reactions to form alcohols and alkanes respectively.
  • If the alkene used in electrophilic addition reactions is asymmetric, different isomeric products will be formed.

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?

Show answer

Answer

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

Show question

Question

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

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Answer

Chloroethene.

Show question

Question

What is a carbocation?


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Answer

A positively charged carbon atom.

Show question

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.

Show question

Question

What is the colour change seen when cold acidic potassium permanganate solution reacts with propene?


Show answer

Answer

Purple to colourless.

Show question

Question

What is the colour change seen when cold alkaline potassium permanganate solution reacts with propene?

Show answer

Answer

Purple to dark green.

Show question

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.


Show question

Question

Describe the test for alkenes.

Show answer

Answer

Orange-brown bromine water decolourises.

Show question

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.

Show question

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

Show question

Question

Which type of carbocation is the most stable?

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Answer

Tertiary carbocations.

Show question

Question

Alkyl groups are electron _____.

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Answer

​Releasing.

Show question

Question

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.

Show question

Question

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


Show answer

Answer

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

Show question

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