Why are alkanes generally unreactive toward acids and bases at room temperature?

Alkanes consist of non-polar C-C and C-H sigma bonds which lack electron-rich or electron-poor sites. Consequently, they do not attract electrophilic or nucleophilic reagents that typically drive acid-base reactions.

What is the difference between homolytic and heterolytic fission in the context of alkane reactions?

Homolytic fission occurs when a bond breaks and each atom retains one electron, forming radicals (common in alkane halogenation). Heterolytic fission occurs when one atom takes both electrons, forming ions, which is rare for alkanes due to their non-polar nature.

When should you expect carbon monoxide ($CO$) as a product of alkane combustion instead of carbon dioxide ($CO_2$)?

Carbon monoxide is produced during incomplete combustion, which occurs when the oxygen supply is insufficient to fully oxidize the carbon atoms in the alkane chain.

Define the 'Initiation' step in a free radical substitution mechanism.

Initiation is the first step where a halogen molecule absorbs UV light energy to undergo homolytic fission, resulting in the creation of two reactive halogen free radicals.

How does the stability of a tertiary radical compare to a primary radical, and why?

A tertiary radical is more stable than a primary radical because it has three electron-donating alkyl groups that help delocalize the unpaired electron through the inductive effect and hyperconjugation.

What is a common error when writing the termination steps for the chlorination of methane?

A common error is forgetting that two methyl radicals can combine to form ethane ($C_2H_6$). Students often only consider the combination of a methyl radical with a chlorine radical.

Why is UV light necessary for the chlorination of alkanes?

UV light provides the specific quantum of energy required to break the $Cl-Cl$ bond homolytically. Without this energy, the initiation step cannot occur, and the chain reaction cannot begin.

What is the general formula for the complete combustion of an alkane?

The general formula is $C_nH_{2n+2} + (\frac{3n+1}{2})O_2 \rightarrow nCO_2 + (n+1)H_2O$. It shows that the amount of oxygen required scales linearly with the number of carbons.

Compare the selectivity of Bromination vs. Chlorination of alkanes.

Bromination is highly selective and primarily targets the most stable radical site (e.g., tertiary), whereas chlorination is less selective and produces a more varied mixture of isomers due to its more exothermic and faster propagation steps.

What happens to the 'radical' in the propagation step of halogenation?

The radical is 'recycled.' In the first propagation step, a halogen radical is consumed to make an alkyl radical; in the second step, the alkyl radical reacts to produce the haloalkane and regenerate a new halogen radical to continue the cycle.

Chemical Reactivity of Alkanes | Cambridge International Examinations AS-Level Chemistry

1. Foundational Inertness of Alkanes

Saturated Nature: Alkanes contain only single sigma ( $\sigma$ ) bonds, meaning every carbon atom is bonded to the maximum number of hydrogen atoms possible. This saturation leaves no 'sites of unsaturation' (like double or triple bonds) for addition reactions to occur.
Bond Strength and Polarity: The C-C and C-H bonds are exceptionally strong, requiring significant energy to break. Furthermore, the electronegativity difference between Carbon (2.5) and Hydrogen (2.1) is minimal, resulting in non-polar molecules that do not attract electrophiles or nucleophiles.
Paraffinic Character: Historically termed 'paraffins' (from Latin parum affinis, meaning 'little affinity'), alkanes do not react with common laboratory reagents like acids, bases, or oxidizing agents at room temperature.

2. Combustion: Oxidation of Alkanes

Complete Combustion: In the presence of excess oxygen, alkanes undergo highly exothermic oxidation to produce carbon dioxide and water. This reaction is the basis for the use of alkanes as global energy sources.

General Equation: $C_nH_{2n+2} + (\frac{3n+1}{2})O_2 \rightarrow nCO_2 + (n+1)H_2O + \text{Energy}$

Incomplete Combustion: When oxygen supply is limited, the reaction produces carbon monoxide ( $CO$ ) or solid carbon (soot, $C$ ). Carbon monoxide is a dangerous silent toxin because it binds irreversibly to hemoglobin, preventing oxygen transport in the blood.
Energy Density: As the carbon chain length increases, the heat of combustion per mole increases, though the heat of combustion per gram remains relatively constant across the homologous series.

3. Free Radical Substitution (Halogenation)

Reaction Conditions: Alkanes react with halogens (typically $Cl_2$ or $Br_2$ ) only in the presence of Ultraviolet (UV) light or high temperatures ( $250-400^{\circ}C$ ). This energy is required to break the halogen-halogen bond.
The Mechanism: The reaction proceeds via a three-stage free radical mechanism: Initiation, Propagation, and Termination.

The Three Stages

Initiation: UV light causes the homolytic fission of the halogen molecule, creating two highly reactive halogen radicals ( $X\cdot$ ).
Propagation: A halogen radical abstracts a hydrogen atom from the alkane to form $HX$ and an alkyl radical. The alkyl radical then reacts with another halogen molecule to form the haloalkane and regenerate the halogen radical.
Termination: Two radicals collide and combine to form a stable molecule, effectively ending the chain reaction.
Product Mixtures: Because propagation can continue as long as there are hydrogen atoms available, halogenation often results in a complex mixture of mono-, di-, and poly-substituted products.

Energy profile diagram of a free radical substitution reaction showing the activation energy required to reach the transition state.

4. Selectivity and Radical Stability

Radical Stability Hierarchy: Not all hydrogen atoms in an alkane are equally likely to be replaced. The stability of the resulting intermediate radical determines the major product. Stability follows the order: Tertiary ( $3^{\circ}$ ) > Secondary ( $2^{\circ}$ ) > Primary ( $1^{\circ}$ ) > Methyl.
Inductive Effect: Alkyl groups are electron-donating. More alkyl groups attached to the radical carbon help stabilize the electron deficiency through the inductive effect and hyperconjugation.
Bromination vs. Chlorination: Bromine is more selective than chlorine. Because the abstraction of hydrogen by a bromine radical is endothermic, the transition state resembles the radical intermediate, making the stability of that radical the dominant factor in product distribution.

5. Key Distinctions in Alkane Reactions

Understanding the differences between reaction types is crucial for predicting outcomes in organic synthesis.

Feature	Combustion	Halogenation	Cracking
Reagent	Oxygen ( $O_2$ )	Halogen ( $X_2$ )	Heat/Catalyst
Energy Source	Spark/Flame	UV Light	High Temp
Main Product	$CO_2$ and $H_2O$	Haloalkanes	Alkenes + Alkanes
Mechanism	Radical/Redox	Free Radical Substitution	Thermal Decomposition

Cracking: This process breaks long-chain alkanes into smaller, more useful alkanes and alkenes. Thermal cracking uses high pressure and temperature to produce high yields of alkenes, while catalytic cracking uses lower temperatures and a zeolite catalyst to produce branched alkanes and aromatic compounds.

6. Exam Strategy & Common Pitfalls

Identify the Conditions: If you see 'UV light' or ' $h\nu$ ' over a reaction arrow with an alkane and a halogen, immediately think Free Radical Substitution.
Predicting Major Products: In exams, always look for the most substituted carbon (tertiary if available) to place the halogen atom, as this corresponds to the most stable radical intermediate.
Stoichiometry Matters: If the question specifies 'excess alkane', the major product is the mono-substituted haloalkane. If 'excess halogen' is used, expect poly-substituted products like $CCl_4$ .
Common Mistake: Forgetting that termination steps can produce unexpected side products, such as ethane forming during the chlorination of methane due to two methyl radicals colliding.

Chemical Reactivity of Alkanes

Summary

1. Foundational Inertness of Alkanes

2. Combustion: Oxidation of Alkanes

3. Free Radical Substitution (Halogenation)

4. Selectivity and Radical Stability

5. Key Distinctions in Alkane Reactions

6. Exam Strategy & Common Pitfalls

Chemical Reactivity of Alkanes

Summary

1. Foundational Inertness of Alkanes

2. Combustion: Oxidation of Alkanes

3. Free Radical Substitution (Halogenation)

The Three Stages

4. Selectivity and Radical Stability

5. Key Distinctions in Alkane Reactions

6. Exam Strategy & Common Pitfalls

The Three Stages