Hydrocarbons: Alkanes, Alkenes, and Alkynes Explained

What Are Hydrocarbons?

Hydrocarbons are organic compounds made up of only carbon and hydrogen atoms. They are the simplest organic molecules and form the foundation of organic chemistry. Every fossil fuel — from natural gas to crude oil — is primarily made of hydrocarbons.

Hydrocarbons are classified by the type of carbon-carbon bonds they contain:

• Alkanes — single bonds only (saturated)
• Alkenes — at least one double bond (unsaturated)
• Alkynes — at least one triple bond (unsaturated)

Learning Goals: By the end of this guide, you should be able to:

1. Define and distinguish alkanes, alkenes, and alkynes.
2. Write general formulas and name simple hydrocarbons using IUPAC rules.
3. Explain trends in physical properties within a homologous series.
4. Describe key reactions of each hydrocarbon type.
5. Apply these concepts to exam-style problems.

Alkanes: Saturated Hydrocarbons

General Formula: $C_nH_{2n+2}$

Alkanes contain only single bonds between carbon atoms. Every carbon has 4 bonds and every hydrogen has 1. Because all carbon-carbon bonds are single bonds, alkanes are described as saturated — they can't take on any more atoms.

The First Six Alkanes

Physical Property Trends

As the carbon chain gets longer:

Key Reactions of Alkanes

1. Combustion (the most important reaction):

Complete combustion:

Incomplete combustion (limited oxygen):

2. Free Radical Substitution (with halogens, UV light):

This proceeds via three stages: initiation → propagation → termination.

Alkenes: Unsaturated Hydrocarbons

General Formula: $C_nH_{2n}$

Alkenes contain at least one carbon-carbon double bond ($C=C$). This double bond consists of one σ (sigma) bond and one π (pi) bond. The π bond makes alkenes much more reactive than alkanes.

Why Are Alkenes "Unsaturated"?

Because the double bond can "open up" to accept more atoms — this is the basis of addition reactions. In contrast, alkanes are saturated because all bonds are single and there's no capacity to add more atoms.

Key Reactions of Alkenes

The bromine test: Alkenes decolourise orange bromine water. Alkanes do not. This is the standard test to distinguish them.

Geometric Isomerism (cis-trans)

Because the double bond prevents free rotation, alkenes can have geometric isomers:

• cis-: same groups on the same side of the double bond
• trans-: same groups on opposite sides

This requires each carbon of the $C=C$ to have two different groups attached.

Alkynes: Triple-Bonded Hydrocarbons

General Formula: $C_nH_{2n-2}$

Alkynes contain at least one carbon-carbon triple bond ($C \equiv C$). This consists of one σ bond and two π bonds. Alkynes are the most unsaturated class of hydrocarbons.

Alkynes undergo similar addition reactions to alkenes but can react twice (because the triple bond can open to a double bond, then to a single bond).

Comparison: Alkanes vs. Alkenes vs. Alkynes

IUPAC Naming Rules

Steps for naming a hydrocarbon:

1. Find the longest continuous carbon chain — this gives the base name (meth-, eth-, prop-, but-, pent-, hex-...).
2. Identify the highest-priority functional group — this gives the suffix (-ane, -ene, -yne).
3. Number the chain so the functional group gets the lowest possible number.
4. Name any substituents (e.g., methyl, ethyl) with their position number.

Examples:

• $CH_3CH_2CH_3$ → propane (3 carbons, all single bonds)
• $CH_2=CHCH_3$ → prop-1-ene (3 carbons, double bond at position 1)
• $CH_3CH(CH_3)CH_3$ → 2-methylpropane (3-carbon main chain + methyl branch at C2)

Worked Examples

Example 1: Drawing Isomers

Question: Draw all structural isomers of $C_4H_{10}$.

Solution: $C_4H_{10}$ is an alkane ($C_nH_{2n+2}$ with n=4). Two isomers exist:

1. Butane: $CH_3CH_2CH_2CH_3$ (straight chain)
2. 2-methylpropane: $CH_3CH(CH_3)CH_3$ (branched)

No other arrangements are possible — any other drawing is just a rotation of one of these two.

Example 2: Predicting Products

Question: What is the product when propene reacts with $HBr$?

Solution: This is an electrophilic addition. $HBr$ adds across the $C=C$ double bond.

By Markovnikov's rule, the H adds to the carbon with more hydrogens:

Product: 2-bromopropane (major product).

Example 3: Combustion Calculation

Question: Calculate the volume of $O_2$ needed for complete combustion of 1 mol of hexane.

Solution:

For 1 mol hexane: $\frac{19}{2} = 9.5$ mol $O_2$

At STP (1 mol gas = 24 dm³): $9.5 \times 24 = 228\ dm^3$

Common Mistakes

1. Confusing saturated and unsaturated — Saturated means all single bonds (alkanes). Unsaturated means at least one double or triple bond (alkenes, alkynes). It does NOT refer to the number of hydrogen atoms.

2. Forgetting that a double bond counts as one domain in VSEPR — For molecular shape prediction, $C=C$ counts as one bonding domain, not two.

3. Drawing impossible isomers — A common mistake is drawing the same molecule in a different orientation and counting it as a new isomer. Always check: is the connectivity actually different?

4. Ignoring Markovnikov's rule — In addition reactions with asymmetric alkenes, the hydrogen adds to the carbon with more H atoms already bonded to it.

5. Not balancing combustion equations — Especially with larger alkanes, students often get the $O_2$ coefficient wrong. Always balance C first, then H, then O.

Exam Tips (A-Level / AP / IB)

• Bromine test: If asked to distinguish an alkane from an alkene, describe the bromine water test. Alkenes decolourise the orange bromine water; alkanes do not.
• For naming questions, always find the longest chain first — don't assume the chain runs left-to-right.
• For combustion calculations, write and balance the equation before doing any mole calculations.
• Know the difference between electrophilic addition (alkenes) and free radical substitution (alkanes) — they are completely different mechanisms.

Frequently Asked Questions

What is the difference between saturated and unsaturated hydrocarbons?

Saturated hydrocarbons (alkanes) contain only single C–C bonds, so they hold the maximum number of hydrogen atoms. Unsaturated hydrocarbons (alkenes, alkynes) have double or triple bonds and can undergo addition reactions.

Why do alkanes have low reactivity?

Alkanes only have strong, non-polar C–C and C–H bonds. These bonds are difficult to break, and the molecule has no region of high electron density to attract electrophiles. They mainly react via free radical substitution under UV light.

What is the bromine water test?

Add orange bromine water ($Br_2(aq)$) to the substance. If the colour disappears (decolourises), the substance contains a $C=C$ double bond (alkene). If the orange colour remains, it is saturated (alkane).

Why do boiling points increase with chain length?

Longer hydrocarbon chains have more electrons and greater surface area, leading to stronger London dispersion forces between molecules. More energy is needed to overcome these forces, resulting in a higher boiling point.

What is Markovnikov's rule?

When $HX$ adds to an asymmetric alkene, the hydrogen atom bonds to the carbon that already has more hydrogen atoms. This forms the more stable carbocation intermediate (the major product).

Related Topics

• Hydrocarbon Derivatives — What happens when you replace hydrogen with functional groups like –OH, –COOH, and –NH₂.
• Structural Isomerism — How molecules with the same formula can have different structures.
• Curly Arrow Mechanisms — Understand the electron-pushing behind addition and substitution reactions.