Alkanes Revision Notes

Alkanes are organic compounds made up solely of carbon (C) and hydrogen (H) atoms. Each carbon atom forms four single covalent bonds, either with other carbon atoms or with hydrogen atoms. A single covalent bond involves the sharing of one pair of electrons between two atoms, resulting in a strong and stable bond.

In alkanes, carbon atoms are bonded to each other by single C–C bonds, and each carbon atom is bonded to enough hydrogen atoms to complete its four bonds. For example, in methane (CH₄), the simplest alkane, one carbon atom forms four single bonds with four hydrogen atoms.

The bonding can be represented by structural formulae showing all bonds, for example: CH₃–CH₂–CH₃ for propane. Alternatively, displayed formulae show the arrangement of atoms and bonds explicitly.

• Carbon atoms form four single covalent bonds in alkanes.
• Bonds are either C–C or C–H single bonds.
• Single covalent bonds involve sharing one pair of electrons.

Alkanes are classified as saturated hydrocarbons because all carbon–carbon bonds are single bonds, and the carbon atoms have the maximum number of hydrogen atoms attached. Saturation means no more hydrogen atoms can be added without breaking existing bonds.

This contrasts with unsaturated hydrocarbons, such as alkenes and alkynes, which contain double or triple bonds and therefore fewer hydrogen atoms per carbon atom.

The general formula for alkanes is CₙH₂ₙ₊₂, where n is the number of carbon atoms. This formula reflects the saturation, as each carbon is bonded to as many hydrogens as possible given the carbon skeleton.

• Saturated hydrocarbons contain only single bonds.
• Alkanes have the general formula CₙH₂ₙ₊₂.
• Saturation means maximum hydrogen content per carbon atom.

The first four alkanes are: methane (CH₄), ethane (C₂H₆), propane (C₃H₈), and butane (C₄H₁₀). Each successive alkane adds one carbon and two hydrogen atoms, consistent with the general formula.

Structural formulae illustrate the bonding: for example, ethane is CH₃–CH₃, showing two carbon atoms each bonded to three hydrogens and to each other by a single bond.

These examples demonstrate the pattern of single bonds and saturation in alkanes.

• Methane: CH₄
• Ethane: C₂H₆ (CH₃–CH₃)
• Propane: C₃H₈ (CH₃–CH₂–CH₃)
• Butane: C₄H₁₀ (CH₃–(CH₂)₂–CH₃)

Given the molecular formula C₅H₁₂, determine if the compound is an alkane and saturated.

Step 1: Use the general alkane formula CₙH₂ₙ₊₂. For n=5, calculate 2(5)+2 = 12 hydrogens.

Step 2: The given formula matches C₅H₁₂, so the compound fits the alkane formula and is saturated, containing only single bonds and the maximum hydrogen atoms possible.

• Check molecular formula against CₙH₂ₙ₊₂.
• Matching formula indicates saturation and alkane classification.

Alkanes differ from alkenes and alkynes in their bonding and hydrogen content. Alkenes contain at least one carbon–carbon double bond (C=C), and alkynes contain at least one triple bond (C≡C). These multiple bonds reduce the number of hydrogen atoms attached to the carbon skeleton.

For example, ethene (C₂H₄) has a double bond and fewer hydrogens than ethane (C₂H₆). This difference in bonding affects chemical reactivity and physical properties.

Recognising the presence of only single bonds and the maximum hydrogen content confirms a hydrocarbon as an alkane.

• Alkanes have only single C–C bonds; alkenes and alkynes have double or triple bonds.
• Alkanes have more hydrogen atoms per carbon than unsaturated hydrocarbons.
• Bond type affects chemical properties and classification.

Alkanes are saturated hydrocarbons composed solely of carbon and hydrogen atoms connected by single covalent bonds (C–C and C–H). These bonds are relatively strong and non-polar, making alkanes chemically stable and largely unreactive under normal conditions.

The lack of functional groups and the strong sigma bonds mean alkanes do not readily participate in addition or other common organic reactions. This chemical inertness is a defining characteristic of alkanes, distinguishing them from more reactive organic compounds such as alkenes or alcohols.

• Alkanes contain only single C–C and C–H bonds.
• These bonds are strong and non-polar, contributing to low reactivity.
• Alkanes do not easily undergo addition or other typical organic reactions.

Combustion is the primary reaction in which alkanes readily participate. When an alkane burns in the presence of excess oxygen, it undergoes complete combustion, producing carbon dioxide and water.

This reaction is highly exothermic and is the basis for the use of alkanes as fuels.

The general balanced equation for the complete combustion of an alkane (using methane, CH₄, as an example) is:

• CH₄ + 2O₂ → CO₂ + 2H₂O (balanced equation for methane combustion)
• Conditions: Sufficient oxygen supply and ignition source (spark or flame).
• Observation: A blue flame with heat and light is produced during complete combustion.

If oxygen supply is limited, incomplete combustion occurs, producing carbon monoxide (CO) or carbon (soot) alongside water.

This is less efficient and produces toxic gases, which is hazardous.

Incomplete combustion can be represented by the equation:

• 2CH₄ + 3O₂ → 2CO + 4H₂O (carbon monoxide formation)
• CH₄ + O₂ → C + 2H₂O (soot formation)
• Observation: A yellow, smoky flame indicates incomplete combustion.

Alkanes react with chlorine in a substitution reaction, where one or more hydrogen atoms are replaced by chlorine atoms.

This reaction requires ultraviolet (UV) light to provide the activation energy needed to break the Cl–Cl bond, initiating a chain reaction.

The reaction proceeds via a free radical mechanism involving initiation, propagation, and termination steps.

• Initiation: Cl₂ → 2Cl• (UV light breaks Cl–Cl bond forming chlorine radicals)
• Propagation: Cl• + CH₄ → HCl + CH₃•
• CH₃• + Cl₂ → CH₃Cl + Cl•
• Termination: Two radicals combine to form a stable molecule (e.g., Cl• + Cl• → Cl₂)
• The overall reaction for monosubstitution of methane is: CH₄ + Cl₂ → CH₃Cl + HCl

The substitution reaction occurs only in the presence of UV light; without it, the reaction is very slow or does not proceed.

The products are a mixture of chloromethane and hydrogen chloride gas.

This reaction is important in organic synthesis and demonstrates the reactivity of alkanes under specific conditions.

• UV light is essential to initiate the reaction.
• Reaction mixture may become acidic due to HCl formation.
• Multiple substitutions can occur if excess chlorine is present, producing dichloromethane, chloroform, etc.
• The reaction is a typical example of a free radical substitution mechanism.

A substitution reaction is a chemical reaction in which one atom or a group of atoms in a molecule is replaced by a different atom or group of atoms. This process results in the formation of a new compound where the original atom or group has been substituted.

In organic chemistry, substitution reactions are common and important for modifying molecules to create derivatives with different properties. The replaced atom or group is often a hydrogen atom or a functional group, and the substituting species can be an atom, ion, or radical.

• Substitution involves replacement of one atom/group by another.
• Occurs in organic molecules such as alkanes and aromatic compounds.
• Leads to formation of new compounds with altered chemical properties.

Alkanes are saturated hydrocarbons containing only single covalent bonds. They are generally unreactive but can undergo substitution reactions, especially with halogens such as chlorine or bromine.

In these reactions, a hydrogen atom in the alkane is replaced by a halogen atom. For example, methane (CH₄) reacts with chlorine (Cl₂) under ultraviolet (UV) light to form chloromethane (CH₃Cl) and hydrogen chloride (HCl).

The reaction is initiated by UV light which provides the activation energy to break the Cl–Cl bond, producing chlorine radicals that propagate the substitution process.

• Alkanes undergo substitution mainly with halogens.
• UV light initiates the reaction by breaking halogen bonds.
• Example: CH₄ + Cl₂ → CH₃Cl + HCl (in presence of UV light).

The substitution reaction of alkanes with halogens proceeds via a free radical mechanism involving three main steps: initiation, propagation, and termination.

1. Initiation: UV light breaks the halogen molecule into two halogen radicals.

2. Propagation: The halogen radical abstracts a hydrogen atom from the alkane, forming a hydrocarbon radical and hydrogen halide. The hydrocarbon radical then reacts with another halogen molecule to form the substituted product and regenerate a halogen radical, continuing the chain reaction. 3. Termination: Two radicals combine to form a stable molecule, ending the chain reaction.

• Initiation: Cl₂ → 2Cl• (UV light required).
• Propagation: Cl• + CH₄ → HCl + CH₃•; CH₃• + Cl₂ → CH₃Cl + Cl•.
• Termination: Combination of radicals to form stable molecules.

Substitution reactions can be represented using balanced chemical equations and structural formulae to show the change in the molecule.

For example, the substitution of one hydrogen atom in methane by chlorine is shown as: CH₄ + Cl₂ → CH₃Cl + HCl Structural formulae illustrate the replacement of a hydrogen atom by a chlorine atom in the methane molecule.

Monosubstitution refers to the replacement of only one hydrogen atom in the alkane molecule, producing a single substituted product. Further substitution can occur but leads to multiple substituted products.

• Balanced equation: CH₄ + Cl₂ → CH₃Cl + HCl.
• Structural formula shows substitution of H by Cl.
• Monosubstitution produces one substituted product.

Substitution reactions of alkanes with halogens require ultraviolet light to initiate the reaction by breaking the halogen-halogen bond.

Observations during the reaction may include the formation of a colourless gas (hydrogen halide) and changes in the halogen colour intensity (e.g., fading of chlorine's greenish-yellow colour).

The reaction is typically carried out in the absence of air to prevent unwanted side reactions and under controlled conditions to favour monosubstitution.

• UV light is necessary to start the reaction.
• Hydrogen halide gas (e.g., HCl) is produced.
• Halogen colour fades as it reacts.
• Controlled conditions favour monosubstitution.

Alkanes are saturated hydrocarbons containing only single covalent bonds. They are generally unreactive but can undergo substitution reactions with halogens such as chlorine under specific conditions.

When alkanes react with chlorine, one or more hydrogen atoms can be replaced by chlorine atoms. This process is called a substitution reaction because one atom (hydrogen) is substituted by another (chlorine).

This reaction requires ultraviolet (UV) light to provide the activation energy necessary to break bonds and initiate the reaction. Without UV light, the reaction proceeds very slowly or not at all.

• Alkanes react with chlorine in the presence of UV light.
• The reaction replaces hydrogen atoms with chlorine atoms (substitution).
• UV light supplies the activation energy for the reaction.

Ultraviolet light provides the energy needed to start the substitution reaction between alkanes and chlorine. This energy breaks the Cl–Cl bond in chlorine molecules, allowing chlorine atoms to react with the alkane.

The reaction is photochemical, meaning it depends on light energy. UV light has sufficient energy to initiate the reaction by breaking bonds, which thermal energy alone cannot achieve efficiently at room temperature.

This activation energy is essential to overcome the energy barrier for the reaction to proceed.

• UV light breaks the Cl–Cl bond in chlorine molecules.
• The reaction is photochemical, requiring light energy.
• Activation energy from UV light initiates substitution.

The simplest substitution involves replacing one hydrogen atom in an alkane molecule with one chlorine atom, producing a monosubstituted chloroalkane and hydrogen chloride (HCl).

For example, methane (CH₄) reacts with chlorine (Cl₂) under UV light to form chloromethane (CH₃Cl) and hydrogen chloride (HCl).

The balanced chemical equation for this monosubstitution is: CH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g)

• Monosubstitution replaces one hydrogen atom with chlorine.
• Example: methane + chlorine → chloromethane + hydrogen chloride.
• Balanced equation: CH₄ + Cl₂ → CH₃Cl + HCl

Structural formulae show how atoms are bonded in a molecule, while displayed formulae show all bonds as lines between atoms.

For methane (CH₄), the structural formula is CH₄, and the displayed formula shows a central carbon atom bonded to four hydrogen atoms.

For chloromethane (CH₃Cl), the structural formula is CH₃Cl, indicating one hydrogen replaced by chlorine. The displayed formula shows the carbon atom bonded to three hydrogen atoms and one chlorine atom, with all bonds represented by lines.

• Structural formula: CH₄ and CH₃Cl.
• Displayed formula shows all bonds explicitly.
• Monosubstitution replaces one H with Cl in the displayed formula.

Consider ethane (C₂H₆) reacting with chlorine under UV light to form chloroethane (C₂H₅Cl) and hydrogen chloride.

The balanced equation is: C₂H₆(g) + Cl₂(g) → C₂H₅Cl(g) + HCl(g)

The structural formula of ethane is CH₃–CH₃. After substitution, one hydrogen atom is replaced by chlorine, giving CH₃–CH₂Cl as the structural formula of chloroethane. The displayed formula shows all bonds explicitly, with chlorine bonded to one of the carbon atoms instead of hydrogen.

• Ethane reacts with chlorine under UV light.
• Balanced equation: C₂H₆ + Cl₂ → C₂H₅Cl + HCl.
• Structural formula changes from CH₃–CH₃ to CH₃–CH₂Cl.
• Displayed formula shows chlorine replacing one hydrogen bond.