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Detailed Explanation of Reactions of Alkenes ⚛️

Alkenes are a very important group of hydrocarbons in Chemistry, especially in the Year 10 curriculum. They differ from alkanes because they have a double bond between two carbon atoms. This double bond affects how alkenes react, making them more reactive. Understanding the reactions of alkenes is essential for your studies in chemistry at Key Stage 4.

Types of Reactions Alkenes Undergo ➕

Alkenes mainly undergo addition reactions because the carbon-carbon double bond is reactive and can open up to allow other atoms to join. Here are the common types of addition reactions for alkenes:

  1. Hydrogenation
  2. Halogenation
  3. Hydration
  4. Addition of Hydrogen Halides

Let’s look at each one in detail with examples.

1. Hydrogenation of Alkenes 🔥

Hydrogenation is when hydrogen (H₂) is added to the alkene. The double bond breaks, and hydrogen atoms add to each carbon atom. This changes an alkene into an alkane.

  • Conditions: Usually requires a catalyst like nickel (Ni) and heat.
  • Reaction example:
    Ethene (C₂H₄) + Hydrogen (H₂) → Ethane (C₂H₆)
    C₂H₄ + H₂ → C₂H₆

2. Halogenation of Alkenes 🧪

Halogenation involves adding halogen atoms like bromine (Br₂) or chlorine (Cl₂) to the alkene. The double bond breaks and each carbon gets one halogen atom.

  • Reaction with bromine:
    Ethene + Bromine → 1,2-dibromoethane
    C₂H₄ + Br₂ → C₂H₄Br₂
  • This reaction is often tested using bromine water; the bromine water decolourises from orange to colourless showing that an addition reaction has happened.

3. Hydration of Alkenes 💧

Hydration is when water (H₂O) is added across the double bond, turning the alkene into an alcohol.

  • Conditions: Acid catalyst (like phosphoric acid) and heat.
  • Reaction example:
    Ethene + Water → Ethanol
    C₂H₄ + H₂O → C₂H₅OH

4. Addition of Hydrogen Halides 🧬

Hydrogen halides like hydrogen chloride (HCl) or hydrogen bromide (HBr) add to alkenes. This forms haloalkanes (alkanes with a halogen atom).

  • Example:
    Ethene + Hydrogen chloride → Chloroethane
    C₂H₄ + HCl → C₂H₅Cl

Mechanism of Addition Reactions ⚙️

The addition reactions of alkenes happen because the double bond has a lot of electrons, which makes it easy to break and attach to other atoms. The reaction involves:

  • The double bond opens up.
  • New atoms attach to the two carbons that were previously joined by the double bond.
  • This turns the alkene into a more stable compound, usually an alkane, alcohol, or haloalkane.

Summary 📚

  • Alkenes are unsaturated hydrocarbons with a double bond.
  • They mainly react through addition reactions, breaking the double bond.
  • The main types include hydrogenation, halogenation, hydration, and addition of hydrogen halides.
  • These reactions are important because they form many useful products like fuels, plastics, and alcohols in real life.

Study Tips 📝

  • Remember that the double bond is key to alkene reactions.
  • Use molecular models or drawings to visualise how atoms add across the double bond.
  • Practice writing balanced chemical equations for these reactions.
  • Try the bromine water test for alkenes in practical lessons to see the reaction happen.

10 Examination-style 1-Mark Questions with 1-Word Answers on Reactions of Alkenes ❓

  1. What type of reaction is the addition of bromine to an alkene?
    Answer: Addition
  2. Which gas is used to hydrogenate an alkene?
    Answer: Hydrogen
  3. What is the general name for compounds formed when alkenes react with halogens?
    Answer: Dihalides
  4. Which catalyst is commonly used in the hydrogenation of alkenes?
    Answer: Nickel
  5. What colour does bromine water turn when it reacts with an alkene?
    Answer: Colourless
  6. What type of bond in alkenes reacts in addition reactions?
    Answer: Double
  7. When an alkene reacts with steam, what type of compound is formed?
    Answer: Alcohol
  8. What is the process called when an alkene reacts with hydrogen?
    Answer: Hydrogenation
  9. What kind of reaction involves breaking the double bond of an alkene to form a single bond?
    Answer: Addition
  10. What is added across the double bond of an alkene during an addition reaction with hydrogen halides?
    Answer: HX

10 Examination-style 2-Mark Questions with 1-Sentence Answers on Reactions of Alkenes 🧠

  1. Question: What type of reaction occurs when an alkene reacts with bromine water?
    Answer: An alkene undergoes an addition reaction with bromine water, decolourising the bromine.
  2. Question: How can ethene be converted into ethanol?
    Answer: Ethene reacts with steam in the presence of a catalyst to form ethanol by an addition reaction.
  3. Question: What is the product when propene reacts with hydrogen in the presence of a nickel catalyst?
    Answer: Propene reacts with hydrogen to form propane in a hydrogenation addition reaction.
  4. Question: Describe the reaction of an alkene with hydrogen halides like HCl.
    Answer: An alkene reacts with hydrogen halides to form a haloalkane by electrophilic addition.
  5. Question: What test can be used to distinguish an alkene from an alkane based on their reactions?
    Answer: Bromine water test distinguishes alkenes by turning from orange to colourless, while alkanes show no change.
  6. Question: What happens to the double bond in an alkene during an addition reaction?
    Answer: The double bond in the alkene breaks to allow atoms to add across the carbon atoms.
  7. Question: Why do alkenes undergo addition reactions rather than substitution reactions?
    Answer: Alkenes have a reactive double bond that is easily broken to add atoms, unlike alkanes which favour substitution.
  8. Question: What product is formed when ethene reacts with concentrated sulfuric acid?
    Answer: Ethene reacts with concentrated sulfuric acid to form ethyl hydrogen sulfate in an addition reaction.
  9. Question: How can the reaction of an alkene with bromine water be used to test for unsaturation?
    Answer: The decolourisation of bromine water indicates the presence of a carbon–carbon double bond (unsaturation) in alkenes.
  10. Question: What role does a catalyst play in the hydrogenation of alkenes?
    Answer: A catalyst like nickel speeds up the addition of hydrogen to the alkene without being consumed.

10 Examination-style 4-Mark Questions with 6-Sentence Answers on Reactions of Alkenes 🧑‍🎓

Question 1

Describe the reaction of ethene with bromine water. What changes occur, and what does this show about the alkene?

Answer: When ethene reacts with bromine water, the orange-brown colour of bromine water fades and becomes colourless. This happens because the double bond in ethene breaks and bonds with bromine atoms in an addition reaction. The bromine atoms add across the double bond, creating dibromoethane. This test shows that alkenes have a double bond that can break to form new bonds. It is a way to distinguish alkenes from alkanes, which do not decolourise bromine water. The reaction proves that alkenes are more reactive than alkanes.

Question 2

Explain the addition reaction between an alkene and hydrogen. Why is a catalyst needed?

Answer: Alkenes react with hydrogen in an addition reaction called hydrogenation, where the double bond breaks, and hydrogen atoms add to the carbon atoms. This reaction converts the alkene into an alkane. A catalyst, usually nickel, is needed to speed up the reaction by lowering the activation energy. The reaction requires heating so the alkene and hydrogen gas can react on the catalyst’s surface. Hydrogenation is important in the food industry for turning liquid vegetable oils into solid fats. Without the catalyst, the reaction would be too slow or may not occur at all.

Question 3

What happens when an alkene reacts with steam? Describe the conditions and the product formed.

Answer: When an alkene reacts with steam, it undergoes an addition reaction where water is added across the double bond. The double bond breaks, and a hydroxyl (-OH) group and a hydrogen atom attach to the carbon atoms. This forms an alcohol. The reaction requires a catalyst such as phosphoric acid and is carried out at a high temperature and pressure. This process is used industrially to make alcohols from alkenes. The reaction demonstrates how alkenes can be used to produce useful chemicals like ethanol.

Question 4

Outline the difference in products between the reaction of ethene with bromine and with hydrogen bromide.

Answer: When ethene reacts with bromine, an addition reaction occurs where bromine atoms add across the double bond to form dibromoethane. This product contains two bromine atoms attached to adjacent carbons. In contrast, ethene reacts with hydrogen bromide to form bromoethane, where one hydrogen atom and one bromine atom add across the double bond. The key difference is that bromine adds symmetrically in the first reaction, but in the second, the hydrogen and bromine add in a way that produces a single bromo-substituted alkane. Both reactions involve breaking the double bond, but the reagents and products differ.

Question 5

Explain why alkenes undergo addition reactions, focusing on the role of the double bond.

Answer: Alkenes undergo addition reactions because they have a carbon-carbon double bond, which contains a pi bond that is weaker than a single sigma bond. This double bond is reactive and can break easily to form two new single bonds with other atoms. During an addition reaction, the pi bond breaks, and atoms or groups add to each carbon atom involved in the double bond. This makes the alkene more stable by converting it into an alkane or derivative with only single bonds. The double bond is the site of reactivity in alkenes, distinguishing them from alkanes. Thus, the presence of the double bond allows alkenes to participate in various addition reactions.

Question 6

Describe how you would test for an alkene in the laboratory.

Answer: To test for an alkene, you would add bromine water to the compound and observe any colour change. Bromine water is an orange-brown liquid. If the compound is an alkene, the bromine water will rapidly lose its colour and become colourless. This happens because the double bond reacts with bromine, causing an addition reaction. If there is no colour change, the compound is likely an alkane or another substance that does not react with bromine. This simple test is useful for identifying the presence of a carbon-carbon double bond.

Question 7

What type of reaction occurs when an alkene reacts with a halogen, and what is the general formula of the product?

Answer: When an alkene reacts with a halogen such as bromine or chlorine, an addition reaction occurs. The halogen atoms add across the double bond, breaking it and forming a dihaloalkane. The general formula of the product is CnH2nX2, where X represents the halogen atoms. For example, ethene reacting with bromine forms dibromoethane. This reaction demonstrates the alkene’s double bond breaking to add halogen atoms. The addition reaction is important in organic chemistry for synthesising various compounds.

Question 8

Explain why the reaction of alkenes with hydrogen halides is called electrophilic addition.

Answer: The reaction of alkenes with hydrogen halides is called electrophilic addition because the double bond in the alkene acts as a nucleophile and attacks the electrophile. The hydrogen atom from the hydrogen halide is positively charged or electron-deficient, making it an electrophile. The alkene’s electrons in the double bond are attracted to this electrophile, forming a carbocation intermediate. Then, the halide ion bonds to the carbocation, completing the addition. This two-step mechanism involves the double bond donating electrons to an electrophile, hence the name electrophilic addition. This reaction is characteristic of the chemical behaviour of alkenes.

Question 9

How can the addition of hydrogen bromide to propene lead to two different products? Explain using carbocation stability.

Answer: When hydrogen bromide adds to propene, the hydrogen atom can attach to either carbon atom of the double bond, leading to two possible carbocations. The more stable carbocation forms when the positive charge is on the central carbon, which is tertiary or secondary, due to greater alkyl group electron donation. This more stable carbocation favours the formation of 2-bromopropane. The less stable carbocation would lead to 1-bromopropane, but it is less favoured. This outcome follows Markovnikov’s rule, where the hydrogen attaches to the carbon with more hydrogens, and the halide attaches to the more substituted carbon. Therefore, addition is regioselective because of carbocation stability.

Question 10

Give two reasons why alkenes are more reactive than alkanes.

Answer: Alkenes are more reactive than alkanes primarily because they contain a carbon-carbon double bond, which includes a pi bond. This pi bond is weaker and more exposed than the single sigma bonds in alkanes, making it easier to break. Secondly, the double bond allows alkenes to undergo addition reactions, where atoms add across the bond, increasing their chemical reactivity. Alkanes only undergo substitution or combustion and are less reactive under normal conditions. The electron-rich double bond in alkenes attracts electrophiles, promoting reactions. These factors make alkenes significantly more reactive than alkanes in the same conditions.

10 Examination-Style 6-Mark Questions with 10-Sentence Answers on Reactions of Alkenes 🏆

Question 1: Describe the reaction of alkenes with bromine water and explain how it can be used to test for the presence of an alkene.

Alkenes react with bromine water in an addition reaction. Bromine water is orange-brown in colour. When you add bromine water to an alkene, the bromine gets added across the double bond. This breaks the double bond and forms a colourless dibromoalkane. Because of this, the orange-brown colour disappears. Alkanes do not react with bromine water, so their colour stays the same. This reaction is called the bromine water test for unsaturation. By adding bromine water to a sample, you can tell if it contains an alkene if the colour disappears. This reaction is useful in identifying unknown compounds in a lab. The reaction shows the alkene’s double bond is more reactive than a single bond in alkanes. This is due to the electron-rich double bond attracting the bromine molecule.

Question 2: Explain the mechanism of electrophilic addition in the reaction between ethene and hydrogen bromide.

In electrophilic addition, ethene reacts with hydrogen bromide (HBr). Ethene has a double bond which is rich in electrons and acts as a nucleophile. The HBr molecule is polar; the hydrogen end is slightly positive. The double bond attacks the hydrogen atom first, breaking the H-Br bond. This forms a carbocation intermediate, where the positive charge is on one carbon atom. The bromide ion (Br-) then quickly bonds to the positively charged carbon. The double bond is replaced by two single bonds, one with hydrogen and one with bromine. This full reaction converts ethene into bromoethane. The reaction happens very fast because the carbocation is unstable. This mechanism explains why alkenes react quickly with polar molecules like HBr.

Question 3: Outline the addition polymerisation of ethene and describe the structure of the polymer formed.

Addition polymerisation is a reaction where many small alkene molecules join together to form a long chain polymer. Ethene is an alkene with a double bond between two carbon atoms. During polymerisation, the double bonds open up. This allows each ethene molecule to connect to the next one by single bonds. The result is a long chain called poly(ethene) or polyethylene. The polymer consists only of carbon and hydrogen atoms arranged in a repeating pattern. The physical properties of polyethylene depend on the length of these chains. Polyethylene can be flexible or rigid and is commonly used in plastic bags and bottles. This process does not form any by-products because it’s an addition reaction. It is an important industrial reaction for making many useful plastics. The reaction happens under pressure with a catalyst to speed it up.

Question 4: Compare the products formed when propene reacts with water in an acid-catalysed hydration reaction.

When propene reacts with water in the presence of an acid catalyst, an addition reaction occurs. The double bond in propene breaks and water adds across it. The reaction follows Markovnikov’s rule: the hydrogen from water adds to the carbon with more hydrogen atoms already. The hydroxyl (OH) group attaches to the carbon with fewer hydrogens, forming an alcohol. The major product is propan-2-ol (an alcohol). Sometimes, a minor product of propan-1-ol can form if the reaction is not perfectly controlled. The acid catalyst used is usually sulfuric acid, which helps to protonate the alkene first. This reaction is reversible and must be heated under controlled conditions. The hydration reaction is important for producing industrial alcohols from alkenes. It illustrates the addition reaction mechanism where water adds in two steps.

Question 5: Explain why alkenes are more reactive than alkanes, using the example of their reaction with hydrogen.

Alkenes are more reactive than alkanes because of their double bonds. Alkenes have a carbon-carbon double bond, while alkanes only have single bonds. The double bond contains a pi bond that is weaker and more reactive than the single sigma bonds. This means alkenes can break the double bond easily to react with other substances. For example, in the reaction with hydrogen (hydrogenation), hydrogen adds across the double bond. This breaks the double bond and forms an alkane. Alkanes do not react with hydrogen under normal conditions because their bonds are strong and stable. The double bond in alkenes gives them a higher electron density, making them more attractive to reagents like hydrogen. This makes alkenes useful starting materials in chemical synthesis. Their reactivity comes from the presence of the double bond which is ready to open up in reactions.

Question 6: Describe the reaction between an alkene and hydrogen in the presence of a nickel catalyst and its industrial significance.

Alkenes react with hydrogen in an addition reaction called hydrogenation. This reaction requires a nickel catalyst and high temperature. The double bond in the alkene breaks and hydrogen atoms add to each carbon. For example, ethene reacts with hydrogen to form ethane. The catalyst provides a surface for the reaction, helping break the double bond and allowing hydrogen to bond. This reaction is used in the food industry to convert unsaturated vegetable oils into saturated fats. Hydrogenation improves the texture and shelf life of products like margarine. The reaction is important because it changes liquid oils into solid fats at room temperature. It also removes double bonds, making the molecule less reactive. This process shows how alkenes can be chemically changed to suit industrial needs.

Question 7: What happens during the reaction of alkenes with concentrated sulfuric acid, and how can the product be used to make alcohols?

When alkenes react with concentrated sulfuric acid, they undergo an electrophilic addition reaction. The double bond in the alkene attacks the hydrogen ion from the acid, creating a carbocation intermediate. The hydrogen attaches to one carbon, and the sulfuric acid’s bisulfate ion (HSO4-) bonds to the positively charged carbon. The product formed is an alkyl hydrogen sulfate compound. This compound can be reacted with water to hydrolyse the sulfuric acid group, forming an alcohol. For example, reacting ethene with sulfuric acid, then with water, produces ethanol. This two-step process is a key industrial method for making alcohols from alkenes. The reaction is initially reversible, so careful conditions are required. This reaction highlights the use of electrophilic addition to form useful products from alkenes. It demonstrates how alkenes can be converted into different functional groups.

Question 8: Explain how you can distinguish an alkene from an alkane using a chemical test and why this test works.

You can distinguish an alkene from an alkane using the bromine water test. Add bromine water, which is orange-brown, to the sample. If the sample contains an alkene, the bromine water’s colour will disappear and turn colourless. This happens because the alkene’s double bond reacts with bromine in an addition reaction. The bromine atoms add across the double bond, forming a colourless dibromoalkane. Alkanes do not undergo this reaction because they only have single bonds which are less reactive. Therefore, the bromine water remains orange-brown with alkanes. This test works because alkenes are unsaturated hydrocarbons with double bonds, which are more reactive. It is a simple and effective way to tell apart these two types of hydrocarbons. The disappearance of the bromine colour is a visible sign of an alkene’s presence.

Question 9: Describe what happens to the double bond in alkenes during the addition of chlorine, and name the product formed.

When chlorine reacts with an alkene, it adds across the double bond in an addition reaction. The double bond breaks and each carbon atom bonds to one chlorine atom. This forms a product called a dichloroalkane. For example, when ethene reacts with chlorine, 1,2-dichloroethane is produced. The reaction causes the orange colour of chlorine gas to disappear. This shows that chlorine has added to the alkene. The process is similar to the reaction with bromine but uses chlorine atoms instead. The reaction is used in industry to produce chemicals like solvents. The double bond in the alkene is important because it allows the addition to happen. This reaction changes the alkene into a saturated compound with two halogen atoms.

Question 10: How do the physical properties of alkenes compare to alkanes, and what effect does the double bond have on these properties?

Alkenes and alkanes have similar physical properties because they are both hydrocarbons. They are non-polar, so they do not dissolve in water but dissolve in organic solvents. Both have fairly low melting and boiling points, which increase with the size of the molecule. However, alkenes often have slightly lower melting points than corresponding alkanes because the double bond affects their shape. The double bond makes alkenes less symmetrical than alkanes, reducing how well molecules pack together. This results in weaker intermolecular forces and slightly lower boiling points. Alkenes also tend to be more reactive because of the double bond. This reactivity influences their chemical properties but does not greatly change physical properties like density. Understanding the effect of the double bond helps explain why alkenes behave differently in reactions compared to alkanes.

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