Alkenes

In this article, we will cover the topic on Alkenes. These are the following learning outcomes:

• Describe alkenes as a homologous series of unsaturated hydrocarbons with the general formula C_nH_2n.
• Draw and name the first three alkenes.
• Describe the physical and chemical properties of alkenes.

Alkene Homologous Series

Alkenes are a homologous series of hydrocarbons that contain one or more carbon-carbon double covalent bonds (C = C). 

Recall again that hydrocarbons are organic compounds that contain only hydrogen and carbon atoms.

The general formula of alkenes is C_nH_2n, where n = 2,3,4, etc.

From the general formula, it can be seen that the ratio of carbon to hydrogen in an alkene molecule is always 1:2. In other words, the number of hydrogen atoms will always be twice the number of carbon atoms.

Why can’t n take the value of 1 in the general formula? 

All alkenes must have the same functional group of the C = C double bond. It means that the minimum number of carbon atoms in an alkene molecule must be 2.

Unsaturated Hydrocarbons

Any molecule that contains carbon-carbon C = C double bonds is unsaturated. Hence, alkenes are called unsaturated hydrocarbons.

A double bond is formed by sharing 2 pairs of electrons between the carbon atoms. In an alkene molecule, it does not have the maximum number of atoms for the number of carbon atoms present. 

Here’s the dot-and-cross diagram of an ethene molecule.

Let’s get to know the alkenes!

When naming the alkenes, the prefix tells us the number of carbon atoms in the molecule and the suffix tells us the homologous series the molecule belongs to. The homologous series here is alkene, hence, the suffix is ‘-ene’.

The table below shows the names and molecular formula of the first three alkene molecules in the homologous series.

Observe the molecular formulae of the first three alkenes in the series. Each member of the alkene homologous series differs from the next by a  - CH₂ - unit.

Structural Formulae Of Alkenes

A structural formula shows how atoms are arranged in a molecule. It also shows the bonds between the atoms in a molecule.

The table below shows the structural formulae of the first three alkenes in the homologous series.

Name	Molecular Formula	Structural Formula
ethene	C2H4
propene	C3H6
butene	C4H8

Note: The structural formula can also be displayed in a condensed manner. For instance, the condensed formula of ethene can be written as CH₂CH₂. The condensed formula of propene can be written as CH₃CHCH₂ while that of butene can be written as  CH₃CH₂ CHCH₂.

Test Your Skill

Question 1:

Which of the following statements correctly describes alkenes?
 

1. Alkenes are a homologous series of saturated hydrocarbons that contain only    carbon-carbon double covalent bonds.
2. Alkenes are a homologous series of saturated hydrocarbons that contain only carbon-carbon single covalent bonds.
3. Alkenes are unsaturated hydrocarbons that contain carbon-carbon double covalent bonds.
4. Alkenes are unsaturated hydrocarbons that contain only carbon-carbon single covalent bonds.

Answer: 

(C) Alkenes are unsaturated hydrocarbons that contain carbon-carbon double covalent bonds.

Explanation:

Alkanes are saturated hydrocarbons while alkenes are unsaturated hydrocarbons. Alkenes contain carbon-carbon double bonds.

Question 2:

Which of the following molecules belongs to the alkene homologous series?

1. C₂₅H₅₀
2. C₃₀H₆₄
3. C₅₀H₈₀
4. C₃₅H₆₀

Answer: 

(A) C₂₅H₅₀

Explanation:

The general formula of alkenes is C_nH_2n. This means that the number of hydrogen atoms will always be twice the number of carbon atoms.

Physical Properties Of Alkenes

Similar to alkanes, as the number of carbon atoms increases,

1. boiling points increase
2. density increases
3. viscosity increases
4. flammability decreases

Characteristics Of Alkene Homologous Series

Alkenes are a homologous series containing unsaturated hydrocarbon with:

• same functional group (C = C)
• similar chemical properties
• same general formula (C_nH_2n)
• a gradual change in their physical properties.
• successive members differ by a –CH₂– unit.

Question 3:

Which of the following correctly describes alkenes?
 

1. Alkenes are saturated hydrocarbons.
2. The boiling point of alkenes increases down the homologous series.
3. The general formula of alkenes is C_nH_2n+2.
4. The flammability of alkenes increases down the homologous series.

Answer:

(B) The boiling point of alkenes increases down the homologous series.

Explanation:

As the molecular sizes of the alkene molecules increase, the forces of attraction between the molecules also increase. More energy is needed to overcome these stronger forces of attraction.

Question 4:

The table shows the boiling points of three members of the alkene homologous series.

Which of the following represents X?
 

1. CH₄
2. C₂H₄
3. C₅H₁₀
4. C₅H₁₂

Answer:

(C) C₅H₁₀

Explanation:

The boiling point of an alkene increases as the number of carbon atoms in the molecule increases.

Between the three alkenes, C₃H₆ has the lowest boiling point, followed by C₄H₈, with X having the highest boiling point. Therefore, X must have a greater number of carbon atoms than C₄H₈. 

Chemical Properties Of Alkenes

Alkenes undergo two main reactions – combustion and addition.

Combustion

Alkenes burn in excess air (oxygen), undergoing complete combustion, to produce carbon dioxide and water vapour.

Take the alkene, propene, for instance.

Word Equation:

propene + oxygen  \(\xrightarrow{\quad}\)  carbon dioxide + water vapour

Chemical Equation:

2C₃H₆ + 9O₂  \(\xrightarrow{\quad}\)  6CO₂ + 6H₂O

Alkenes burn with a sootier flame than alkanes with a similar number of carbon atoms. This is because, for the same number of carbon atoms, the percentage of carbon by mass in alkene molecules is higher than that in alkane molecules.

Let us do a comparison of the percentage by mass of carbon in ethane and in ethene.

% of C in ethane, C₂H₆\(\begin{align}\\[2ex] &= \frac{2 \times 12}{2 \times 12 + 6 \times 1} \times 100\% \\[2ex] &= 80\% \end{align}\)a
% of C in ethene, C₂H₄\(\begin{align}\\[2ex] &= \frac{2 \times 12}{2 \times 12 + 4 \times 1} \times 100\% \\[2ex] &= 85.7\% \end{align}\)

Due to the greater percentage by mass of carbon in ethene, more oxygen is needed to react with the carbon atoms to form carbon dioxide for complete combustion. 

Therefore, the chances of alkenes undergoing incomplete combustion will be higher, as there might be insufficient oxygen to react. Due to incomplete combustion, carbon (soot) or carbon monoxide will be produced, which makes the flame sootier.

Addition

An addition reaction is a reaction in which an unsaturated organic compound, such as alkene, can combine with another substance to form a single new compound.

The carbon-carbon double bond in an alkene breaks to form new single bonds. New atoms can be added to each carbon atom of the double bond to form a single covalent bond product.

In other words, after the addition reaction, the unsaturated alkene becomes a saturated product.

Examples of substances used in the addition reaction with alkenes:

1. hydrogen
2. bromine
3. steam

Addition Of Hydrogen (Hydrogenation)

The addition of hydrogen is also known as hydrogenation. Alkenes can react with hydrogen to form alkanes.

For Example:

ethene + hydrogen  \(\xrightarrow{\quad}\)   ethane

C₂H₄ + H₂  \(\xrightarrow{\quad}\)  C₂H₆

Conditions Needed:

• Temperature: 200ºC
• Catalyst: Nickel

Application Of Hydrogenation

We can use the hydrogenation process in the manufacturing of margarine from vegetable oil.

Addition Of Bromine (Bromination)

If an alkene is added to a solution of bromine, the reddish-brown colour disappears (decolorises) immediately.

For Example:

ethene + bromine  \(\xrightarrow{\quad}\)  1,2-dibromoethane

C₂H₄ + Br₂  \(\xrightarrow{\quad}\)  C₂H₄Br₂

The final product in this bromination reaction is 1,2-dibromoethane. The numbers 1 and 2 in the name tell the positions of the bromine atoms on the main carbon chain.

The bromine solution is reddish-brown, but after bromination, the bromine is used up in the reaction, thus the reddish-brown colour disappears and a colorless liquid (1,2-dibromoethane) is formed. 

The bromination reaction, thus allows us to distinguish saturated hydrocarbons (alkanes) from unsaturated hydrocarbons (alkenes). Alkanes do not undergo bromination under normal conditions, hence alkanes do not decolourise bromine solution.

Addition Of Steam (Hydration)

Alkenes can also undergo an addition reaction with steam, also known as hydration, to produce alcohols.

For Example:

ethene + steam  \(\xrightarrow{\quad}\)  ethanol

C₂H₄ + H₂O  \(\xrightarrow{\quad}\)  C₂H₅OH

Conditions Needed:

• Temperature: 300ºC
• Pressure: 60 atm
• Catalyst: Phosphoric(V) acid, H₃PO₄

Question 5:

The structures of four organic compounds are shown below.

Which compounds decolourise bromine water?
 

1. 1 and 2
2. 1, 2 and 4
3. 2 and 4
4. 3 and 4

Answer:

(C) 2 and 4

Explanation:

The bromination reaction serves as a chemical test for the presence of unsaturated hydrocarbons. It is used to distinguish between an alkane and an alkene. Alkanes do not decolourise bromine solution under normal conditions, but alkenes decolourise bromine solution rapidly.

Compounds 2 and 4 are alkenes. Both of them have a C = C double bond each. The bromine atoms in the molecule will add to each carbon atom of the double bond.