What is the general formula of alkanes?

The general formula for alkanes is CnH2n+2.

Why are alkanes called saturated hydrocarbons?

They are saturated because each carbon atom forms single bonds only, and the molecule contains the maximum number of hydrogen atoms possible for the carbon skeleton.

How do alkanes undergo halogenation?

Halogenation of alkanes occurs via a free radical substitution mechanism under UV light, producing alkyl halides and hydrogen halide byproducts.

Alkanes — Saturated Hydrocarbons

Introduction

Alkanes are the simplest class of hydrocarbons consisting entirely of single bonds between carbon atoms and saturated with hydrogen atoms. Their general formula is C_nH_2n+2. They are also called saturated hydrocarbons because they contain the maximum possible number of hydrogen atoms per carbon.

Classification

Linear alkanes: Carbon atoms connected in a straight chain (n-alkanes).
Branched alkanes: Chains with alkyl substituents (iso- and neo- structures).
Cycloalkanes: Ring-shaped saturated hydrocarbons (C_nH_2n); often discussed with alkanes but are technically cycloalkanes.

Nomenclature

Alkanes are named by selecting the longest carbon chain, numbering it for the lowest set of locants, naming substituents, and using the suffix -ane. Examples:

Methane (CH₄) – 1 carbon
Ethane (C₂H₆) – 2 carbons
Propane (C₃H₈) – 3 carbons
Butane (C₄H₁₀) – 4 carbons

Physical Properties

Non-polar molecules: insoluble in water but soluble in organic solvents (e.g., hexane, benzene).
Boiling and melting points increase with molecular weight (due to increased van der Waals forces).
Lower alkanes (C1–C4) are gases at room temperature; medium alkanes are liquids; very high alkanes are waxy solids.

Chemical Properties

1. Combustion

Alkanes burn in oxygen to produce carbon dioxide and water, releasing energy. Example (general):

C_nH_2n+2 + (3n+1)/2 O₂ → n CO₂ + (n+1) H₂O

2. Free Radical Substitution (Halogenation)

Under UV light, halogens react with alkanes by a chain mechanism producing haloalkanes and hydrogen halides. The three main steps are initiation, propagation, and termination.

3. Stability

Alkanes are relatively inert toward many reagents because of strong C–C and C–H bonds and lack of polarity. They are commonly used as solvents and fuels but require energetic conditions for many reactions.

Uses

Fuels (natural gas — methane, petrol/diesel — mixtures of alkanes).
Feedstock in the petrochemical industry to make plastics, detergents, solvents and more.
Lubricants, waxes, and paraffin products.

Summary:
Alkanes are fundamental hydrocarbons with saturated carbon chains. Their chemical inertness and combustion properties make them vital fuels and industrial feedstocks.

Multiple Choice Questions (MCQs)

General formula of alkanes is: A. C_nH_2n B. C_nH_2n+2 C. C_nH_2n-2 D. C_nH_n Answer: B. Explanation: Alkanes have the general formula C_nH_2n+2.
Which reaction mechanism is involved in halogenation of alkanes? A. Electrophilic addition B. Nucleophilic substitution C. Free radical substitution D. Elimination Answer: C. Explanation: Halogenation of alkanes proceeds via a free radical chain mechanism.
Which of the following alkanes is a gas at room temperature? A. Nonane B. Octane C. Methane D. Octadecane Answer: C. Explanation: Methane (CH₄) is a gas at room temperature.

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Fill in the blanks

Alkanes are also called __________ hydrocarbons. Answer: saturated
The IUPAC suffix for alkanes is __________. Answer: -ane
General molecular formula of alkanes is __________. Answer: CnH2n+2

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Quick Quiz — Auto grade (3 questions)

1. What is the IUPAC name for CH₃–CH₂–CH₂–CH₃? Propane Butane Pentane
2. Which statement is TRUE about alkanes? They contain double bonds. They are highly polar. They are relatively inert and nonpolar.
3. During complete combustion of propane (C₃H₈), main gaseous products are: CO + H₂ CO₂ + H₂O C + H₂O

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Explanations

Butane is CH₃–CH₂–CH₂–CH₃ (4 carbons). IUPAC name: butane.
Alkanes are relatively inert and nonpolar due to strong C–C and C–H bonds.
Complete combustion produces carbon dioxide (CO₂) and water (H₂O).

Frequently Asked Questions (FAQ)

Q1: What is an alkane?

A: An alkane is a saturated hydrocarbon with only single C–C bonds and general formula C_nH_2n+2.

Q2: Why are alkanes less reactive?

A: Because C–H and C–C bonds are relatively strong and nonpolar; reactions often need heat, light (UV) or catalysts.

Q3: How are alkanes named when branched?

A: Find the longest chain as parent, number for lowest substituent locants, name alkyl groups and assemble the name using commas and hyphens per IUPAC rules.

References & links

External: IUPAC — International Union of Pure and Applied Chemistry

Author: CHEMASH
Date: Sep 03, 2025