Home Notes Papers

Alkanes

Paper 1Paper 2Paper 3Paper 4

This topic is examined in Paper 1, Paper 2, Paper 3, and Paper 4.

Structure and Bonding of Alkanes

Learning Objective 1: State that the bonding in alkanes is single covalent and that alkanes are saturated hydrocarbons.

Alkanes are a homologous series of hydrocarbons. A hydrocarbon is a compound containing only carbon and hydrogen atoms. It is critical to specify 'only'; if other elements are present, it is not a hydrocarbon.

The bonding in alkanes consists entirely of single covalent bonds. Each carbon atom forms four single covalent bonds (with other carbon or hydrogen atoms), and each hydrogen atom forms one single covalent bond. Because there are only single bonds between the carbon atoms, no more hydrogen atoms can be added to the chain. Therefore, alkanes are described as saturated hydrocarbons.

Term Definition
Single Covalent Bond A shared pair of electrons between two atoms.
Saturated Containing only single bonds; the maximum possible number of hydrogen atoms per carbon atom.
Homologous Series A family of organic compounds with the same general formula and similar chemical properties, differing by a -CH_2- unit.
General Formula

The general formula for alkanes is C_nH_{2n+2}, where n is the number of carbon atoms.

For example:

  • Methane (n=1): CH_4
  • Ethane (n=2): C_2H_6
  • Propane (n=3): C_3H_8
  • Butane (n=4): C_4H_{10}
Chemical Properties and Reactivity

Learning Objective 2: Describe the properties of alkanes as being generally unreactive, except in terms of combustion and substitution by chlorine.

Alkanes are chemically unreactive (inert) under normal conditions. This is because:

  1. The C-C and C-H bonds are strong covalent bonds.
  2. There is little polarity in the molecule, so they do not attract electrophiles or nucleophiles easily.

However, alkanes undergo two main types of reactions:

  1. Combustion: Burning in oxygen.
  2. Substitution: Reaction with halogens (like chlorine) under specific conditions.
Combustion of Ethane
Question: Write the balanced symbol equation for the complete combustion of ethane (C_2H_6).

Step 1: Identify products. Complete combustion always yields CO_2 and H_2O.
Step 2: Balance Carbon. Ethane has 2 C, so we need 2 CO_2.
C_2H_6 + O_2 \rightarrow 2CO_2 + H_2O
Step 3: Balance Hydrogen. Ethane has 6 H, so we need 3 H_2O (since 3 \times 2 = 6).
C_2H_6 + O_2 \rightarrow 2CO_2 + 3H_2O
Step 4: Balance Oxygen. Right side has (2 \times 2) + (3 \times 1) = 7 oxygen atoms. Left side needs 7 oxygen atoms, so 3.5 O_2.

Final Answer:
C_2H_6 + 3.5O_2 \rightarrow 2CO_2 + 3H_2O
(Or multiplied by 2: 2C_2H_6 + 7O_2 \rightarrow 4CO_2 + 6H_2O)

Substitution Reaction
Learning Objective 3: State that in a substitution reaction one atom or group of atoms is replaced by another atom or group of atoms.

In the context of alkanes, substitution means that one (or more) hydrogen atom(s) in the alkane molecule is replaced by a chlorine atom.

Learning Objective 4: Describe the substitution reaction of alkanes with chlorine as a photochemical reaction, with ultraviolet light providing the activation energy, E_a, and draw the structural or displayed formulae of the products, limited to monosubstitution.

Conditions:

  • The reactant is chlorine (Cl_2).
  • The condition is ultraviolet (UV) light (sunlight). This makes the reaction photochemical.
  • The UV light provides the activation energy (E_a) required to break the Cl-Cl bond and initiate the reaction.

Monosubstitution:
This means only one hydrogen atom is replaced by one chlorine atom. The products are a chloroalkane (organic product) and hydrogen chloride (inorganic by-product).

General Equation for Monosubstitution:
C_nH_{2n+2} + Cl_2 \xrightarrow{UV} C_nH_{2n+1}Cl + HCl

Substitution of Propane with Chlorine
Question: Write the equation for the monosubstitution reaction of propane (C_3H_8) with chlorine in UV light.

  1. Reactants: Propane (C_3H_8) and Chlorine (Cl_2).
  2. Condition: UV light (write above the arrow).
  3. Products:
    • One H is removed from propane, leaving a C_3H_7 group.
    • One Cl replaces that H, forming chloropropane (C_3H_7Cl).
    • The removed H combines with the other Cl to form hydrogen chloride (HCl).

Symbol Equation:
C_3H_8 + Cl_2 \xrightarrow{UV} C_3H_7Cl + HCl

Structural Formula of Chloropropane:
The structural formula shows how atoms are arranged. For monosubstitution, the chlorine can attach to any carbon, but typically we show it at the end for simplicity unless specified otherwise.
Condensed Structural Formula: CH_3CH_2CH_2Cl

Displayed Formula of Chloropropane:
A displayed formula shows all covalent bonds. For chloropropane (C_3H_7Cl):

  • Carbon 1 is bonded to 3 Hydrogens and Carbon 2.
  • Carbon 2 is bonded to 2 Hydrogens, Carbon 1, and Carbon 3.
  • Carbon 3 is bonded to 2 Hydrogens, Carbon 2, and 1 Chlorine.

Visual representation of bonds:
H H H
| | |
H - C - C - C - Cl
| | |
H H H

Note: Ensure you draw the bond between the last Carbon and the Chlorine atom. Do not forget the HCl product.

⚠︎ Hydrocarbon Definition and Substitution Products

Mistake 1: Incomplete Hydrocarbon Definition

  • Error: Stating 'Alkanes contain carbon and hydrogen.'
  • Correction: You must state 'Alkanes contain only carbon and hydrogen.' The word 'only' is essential to distinguish them from other organic compounds like alcohols or carboxylic acids.

Mistake 2: Incorrect Substitution Products

  • Error: Thinking the products are a chloroalkane and hydrogen gas (H_2).
  • Correction: The removed hydrogen atom combines with the remaining chlorine atom to form hydrogen chloride (HCl), not H_2. The equation must show both C_nH_{2n+1}Cl and HCl.

Mistake 3: Missing Condition

  • Error: Writing the substitution equation without specifying UV light.
  • Correction: Alkanes do not react with chlorine in the dark. You must include UV light (or sunlight) above the reaction arrow to show the photochemical condition.
Writing Equations and Definitions
Tip 1: Balancing Combustion Equations
When asked to write the equation for complete combustion, examiners accept fractional coefficients (e.g., 3.5 O_2) as long as the equation is balanced. However, if you prefer integers, multiply all coefficients by 2. Always check that the number of atoms of each element is equal on both sides.

Tip 2: Describing Substitution
When asked to describe substitution, use the phrase 'one hydrogen atom is replaced by one chlorine atom'. This directly addresses the definition of substitution. Do not just say 'chlorine reacts with alkane'; you must specify the replacement mechanism.

Tip 3: Role of UV Light
If asked for the role of UV light, state: 'UV light provides the activation energy (E_a) for the reaction.' This is the precise markscheme phrase. Saying 'it starts the reaction' is too vague and may not gain full marks.

Practice Questions
Q:
State two conditions required for methane to react with chlorine.
A:
  1. Chlorine (or Cl_2)
    2. Ultraviolet light (or UV light / sunlight)
Q:
What type of reaction occurs when ethane reacts with chlorine in the presence of UV light?
A:
Substitution
Q:
Write the balanced symbol equation for the complete combustion of propane (C_3H_8).
A:
C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O
Q:
Explain why alkanes are described as saturated hydrocarbons.
A:
Because they contain only single bonds between carbon atoms (and thus hold the maximum number of hydrogen atoms).
Q:
Draw the displayed formula of chloromethane (CH_3Cl).
A:
A central Carbon atom bonded to three Hydrogen atoms and one Chlorine atom. All bonds must be shown as lines.
H
|
H - C - Cl
|
H
Beta v0.7.8 Free while we're in beta — it transitions to paid post launch. Thank you for supporting us at this stage!