Hydrocarbons are organic compounds made up of carbon and hydrogen atoms. They are classified into alkanes, alkenes, and alkynes, based on the types of bonds between carbon atoms. Understanding these hydrocarbons is essential in organic chemistry, as they form the basis of fuels, plastics, and many industrial chemicals.

Alkanes

Alkanes are saturated hydrocarbons because they contain only single bonds between carbon atoms. They follow the general formula:

C_nH_2n + 2

• Example: Methane (CH₄), Ethane (C₂H₆)
• Properties:
  • Relatively unreactive
  • Burn in oxygen to produce carbon dioxide and water
  • Used as fuels and lubricants

Alkenes

Alkenes are unsaturated hydrocarbons that contain at least one carbon-carbon double bond. They have the general formula:

C_nH_2n

• Example: Ethene (C₂H₄), Propene (C₃H₆)
• Properties:
  • More reactive than alkanes due to the presence of a double bond
  • Undergo addition reactions
  • Used in plastics and chemical synthesis

Alkynes

Alkynes are unsaturated hydrocarbons that contain at least one carbon-carbon triple bond. Their general formula is:

C_nH_2n−2

• Example: Ethyne (C₂H₂), Propyne (C₃H₄)
• Properties:
  • Highly reactive compared to alkanes and alkenes
  • Used in welding (acetylene gas) and organic synthesis

How Do Alkanes, Alkenes, and Alkynes Differ in Structure?

The difference between alkanes, alkenes, and alkynes lies in the types of bonds between carbon atoms, which affect their stability and reactivity.

Hydrocarbon Type	Bond Type	Reactivity	Example
Alkanes	Single bonds	Least reactive	Methane (CH₄)
Alkenes	One or more double bonds	More reactive	Ethene (C₂H₄)
Alkynes	One or more triple bonds	Most reactive	Ethyne (C₂H₂)

Single, Double, and Triple Bonds

• Single bond (Alkanes): Stable, allows free rotation.
• Double bond (Alkenes): Stronger than a single bond but more reactive.
• Triple bond (Alkynes): Strongest bond, making alkynes highly reactive.

The double or triple bond in unsaturated hydrocarbons makes them reactive in chemical reactions such as hydrogenation and polymerisation.

Properties and Uses of Alkanes, Alkenes, and Alkynes

Alkanes, alkenes, and alkynes are important types of hydrocarbons in organic chemistry. They have different physical and chemical properties due to the type of bonds between carbon atoms. These compounds are widely used in fuels, plastics, and industrial applications.

Physical Properties

Property	Alkanes	Alkenes	Alkynes
Bond Type	Single bonds (saturated)	Double bonds (unsaturated)	Triple bonds (unsaturated)
Boiling Point	Increases with chain length	Lower than alkanes of similar size	Higher than alkenes and alkanes
Solubility	Insoluble in water but dissolves in organic solvents	Similar to alkanes	Similar to alkanes
State at Room Temp	Smaller alkanes are gases, larger ones are liquids or solids	Gases or liquids	Gases or liquids

Chemical Properties

Property	Alkanes	Alkenes	Alkynes
Combustion	Burns with a clean flame, producing CO₂ and H₂O	Burns with a smokier flame due to high carbon content	Produces high energy but burns with a sooty flame
Reactivity	Least reactive due to strong C-C single bonds	More reactive due to C=C double bonds	Most reactive due to C≡C triple bonds
Addition Reactions	Does not undergo addition reactions	Reacts with hydrogen, halogens, and water	Reacts with halogens and hydrogen faster than alkenes

How Are Alkanes, Alkenes, and Alkynes Used in Daily Life?

Common Applications

Use	Alkanes	Alkenes	Alkynes
Fuels	Petrol, diesel, natural gas	Used in high-energy fuels	Acetylene (ethyne) used in welding torches
Plastics	Used in paraffin wax and lubricants	Ethene used in making plastics (polyethene)	Not used in plastics
Pharmaceuticals	Used as solvents in medicines	Used in synthesis of drugs	Used in some organic reactions

Industrial Significance

• Alkanes: Used as fuels and lubricants due to their stability.
• Alkenes: Important in plastic production, such as polyethene and polypropene.
• Alkynes: Used in chemical synthesis and high-temperature welding.

Why Are Alkanes Saturated and Alkenes/Alkynes Unsaturated?

Hydrocarbons, including alkanes, alkenes, and alkynes, are organic compounds made up of carbon and hydrogen atoms. They are classified based on the type of bonds between carbon atoms.

• Alkanes are saturated hydrocarbons because they contain only single bonds between carbon atoms.
• Alkenes and alkynes are unsaturated hydrocarbons because they contain at least one double or triple bond, respectively.

Understanding this difference helps explain their reactivity, chemical properties, and uses.

Explanation of Saturation in Hydrocarbons

Alkanes: Saturated Hydrocarbons

Alkanes are considered saturated because they have only single bonds between carbon atoms. This means:

• Each carbon atom forms four single bonds, either with other carbon atoms or hydrogen atoms.
• Their general formula is CₙH₂ₙ₊₂, where n is the number of carbon atoms.
• Since they have no double or triple bonds, they cannot undergo addition reactions.

Example: Methane (CH₄) and Ethane (C₂H₆).

Alkenes and Alkynes: Unsaturated Hydrocarbons

• Alkenes contain at least one double bond between carbon atoms. Their general formula is CₙH₂ₙ.
• Alkynes contain at least one triple bond and have the general formula CₙH₂ₙ₋₂.

Because of their double or triple bonds, alkenes and alkynes are more reactive than alkanes and readily undergo addition reactions.

Example: Ethene (C₂H₄) and Ethyne (C₂H₂).

Impact on Chemical Behaviour

Property	Alkanes (Saturated)	Alkenes & Alkynes (Unsaturated)
Bond Type	Single bonds	At least one double or triple bond
Reactivity	Low reactivity	More reactive due to double/triple bonds
Common Reactions	Substitution reactions	Addition reactions
Use in Industry	Fuels and lubricants	Plastics and chemical synthesis

Since alkenes and alkynes have weaker π-bonds in their double and triple bonds, they are more likely to react than alkanes.

How Do Alkanes, Alkenes, and Alkynes React in Chemical Reactions?

Combustion and Oxidation

All hydrocarbons undergo combustion when burned in oxygen, forming carbon dioxide and water.

Hydrocarbon + O₂ → CO₂ + H₂O

• Complete combustion produces CO₂ and H₂O.
• Incomplete combustion (limited oxygen) produces carbon monoxide (CO) or carbon (soot).

Example:

Methane combustion: CH4+2O2→CO2+2H2O

Addition Reactions of Alkenes and Alkynes

Since alkenes and alkynes are unsaturated, they react easily with hydrogen, halogens, and water in addition reactions.

• Hydrogenation: Converts alkenes to alkanes by adding hydrogen.

C2H4+H2→C2H6

• Halogenation: Reaction with halogens (e.g., bromine water test for alkenes).

C2H4 + Br2 → C2H4Br2

Alkynes also undergo addition reactions, but in two steps, first forming an alkene, then an alkane.

Substitution Reactions in Alkanes

Since alkanes are saturated hydrocarbons, they do not readily undergo addition reactions. Instead, they react via substitution reactions, where one hydrogen atom is replaced by another functional group.

Example:

Methane reacts with chlorine under,

UV light:

CH4+Cl2→CH3Cl+HCl

This reaction continues, forming multiple substituted products like dichloromethane (CH₂Cl₂).

What Are the Differences Between Alkenes and Alkynes?

Alkenes and alkynes are both unsaturated hydrocarbons, meaning they contain carbon-carbon multiple bonds. However, they differ in structure, reactivity, and physical properties.

Structural Variations

• Alkenes contain at least one double bond between carbon atoms.
• Alkynes contain at least one triple bond between carbon atoms.

Hydrocarbon Type	General Formula	Bond Type	Example
Alkene	CnH2n	One or more double bonds	Ethene (C2H4)
Alkyne	CnH2n−2​	One or more triple bonds	Ethyne (C2H2​)

Reactivity Differences

• Alkenes are more reactive than alkanes due to the presence of a double bond. They undergo addition reactions with hydrogen, halogens, and acids.
• Alkynes are even more reactive than alkenes due to the presence of a triple bond, making them highly reactive in addition reactions.

Example of Addition Reaction:

• Alkene Addition: Ethene reacts with bromine:

C₂H₄+Br₂→C₂H₄Br₂

• Alkyne Addition: Ethyne reacts with hydrogen:

C₂H₂+2H₂→C₂H₆

How Are Alkanes, Alkenes, and Alkynes Named?

Naming hydrocarbons follows IUPAC naming rules, which standardise how organic molecules are named based on the number of carbon atoms and type of bonds.

IUPAC Naming Rules

1. Find the longest carbon chain containing the functional group (double or triple bond).
2. Number the carbon atoms to give the lowest number to the functional group.
3. Identify and name side chains (if any).
4. Use the correct suffix:
   • “-ane” for alkanes (single bonds).
   • “-ene” for alkenes (double bonds).
   • “-yne” for alkynes (triple bonds).

Common Naming Conventions

Hydrocarbon Type	Example	Molecular Formula
Alkane	Methane	CH4​
Alkene	Ethene	C2H4​
Alkyne	Ethyne	C2H2

How Can You Identify Alkanes, Alkenes, and Alkynes in a Lab?

Alkanes, alkenes, and alkynes are hydrocarbons with different chemical properties. Identifying them in a laboratory setting is crucial for understanding their reactivity and applications. Various chemical tests help distinguish these compounds, with the bromine water test being one of the most commonly used methods.

Bromine Water Test for Unsaturation

What Is the Bromine Water Test?

The bromine water test is used to identify whether a hydrocarbon is saturated or unsaturated. This test distinguishes alkanes (saturated hydrocarbons) from alkenes and alkynes (unsaturated hydrocarbons).

Procedure:

1. Prepare the Test Tubes: Add 2 cm³ of bromine water to separate test tubes.
2. Add the Hydrocarbon Sample: Introduce the alkane, alkene, or alkyne into the test tube.
3. Observe the Colour Change: Shake the solution gently and note any changes.

Results:

Hydrocarbon Type	Reaction with Bromine Water	Explanation
Alkane	No colour change (remains orange-brown)	Alkanes contain only single bonds and do not react with bromine.
Alkene	Decolourises bromine water (turns colourless)	Alkenes contain a double bond, which reacts with bromine.
Alkyne	Decolourises bromine water (turns colourless)	Alkynes contain a triple bond, which also reacts with bromine.

This reaction occurs because alkenes and alkynes undergo addition reactions, breaking the bromine molecule into two bromine atoms, which bond to the carbon-carbon double or triple bond.

Chemical Tests to Identify Alkanes, Alkenes, and Alkynes

1. Potassium Permanganate (KMnO₄) Test

This test, also called the Baeyer’s test, helps confirm the presence of an alkene or alkyne.

Procedure:

1. Add a few drops of dilute potassium permanganate solution to the hydrocarbon sample.
2. Shake the test tube and observe the colour change.

Results:

Hydrocarbon Type	Reaction with KMnO₄	Explanation
Alkane	No reaction, remains purple	Alkanes do not have double or triple bonds to react with KMnO₄.
Alkene	Turns brown, forming a precipitate	The double bond breaks, causing oxidation and forming a brown manganese dioxide precipitate.
Alkyne	Turns brown, forming a precipitate	The triple bond breaks, causing oxidation similar to alkenes.

2. Combustion Test

Burning hydrocarbons in oxygen reveals differences in flame characteristics.

Results:

Hydrocarbon Type	Flame Characteristics	Explanation
Alkane	Clean blue flame	Alkanes burn completely due to their saturated structure.
Alkene	Yellow smoky flame	Alkenes have higher carbon content, leading to incomplete combustion.
Alkyne	Very sooty flame	Alkynes have even higher carbon content, producing more soot.

3. Reaction with Sulfuric Acid (H₂SO₄) Test

This test helps distinguish alkenes from alkanes.

Procedure:

1. Add concentrated sulfuric acid to the hydrocarbon sample.
2. Observe whether the solution forms a layer or dissolves.

Results:

Hydrocarbon Type	Reaction with H₂SO₄	Explanation
Alkane	No reaction, forms a separate layer	Alkanes do not react with sulfuric acid.
Alkene	Dissolves in sulfuric acid	Alkenes form an alkyl hydrogen sulfate compound.
Alkyne	Dissolves in sulfuric acid	Alkynes form a similar reactive intermediate.

Read more Atomic Structure and Periodic table

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