Review the key concepts, formulae, and examples before starting your quiz.
πConcepts
Alkanes are saturated open-chain hydrocarbons containing carbon-carbon single bonds with the general formula .
Nomenclature: Based on the IUPAC system, the longest continuous carbon chain is identified as the parent alkane. Substituents are named as alkyl groups ( methyl, ethyl) and their positions are indicated by the lowest possible locant numbers.
Isomerism: Alkanes with four or more carbon atoms exhibit chain isomerism. For example, has two isomers: -butane and -methylpropane (isobutane).
Preparation from Alkyl Halides: (a) Wurtz Reaction: (used for preparing symmetrical alkanes). (b) Reduction: .
Preparation from Carboxylic Acids: (a) Decarboxylation: Heating sodium salt of carboxylic acid with soda lime () yields an alkane with one less carbon atom. (b) Kolbe's Electrolytic Method: Electrolysis of aqueous sodium/potassium salts of fatty acids gives alkanes at the anode.
Physical Properties: Boiling points increase with increasing molecular mass due to stronger Van der Waals forces. However, branching decreases the boiling point as the surface area decreases (e.g., -pentane > isopentane > neopentane).
Chemical Properties: Alkanes are relatively unreactive (paraffins) but undergo substitution reactions like halogenation (), combustion, and aromatization (converting alkanes with 6+ carbons into benzene derivatives).
πFormulae
π‘Examples
Problem 1:
Write the IUPAC name of .
Solution:
is incorrect; the correct name is depending on the longest chain.
Explanation:
Identify the longest carbon chain. In this molecule, the longest continuous chain contains 7 carbons (Heptane) or 8 if the ethyl group is part of the main chain. Selecting the longest chain as 7 carbons: if we number from the left, substituents are at . If we number to maximize length, it becomes an octane derivative. Correct selection: ensures the longest chain and lowest locants.
Problem 2:
Why is the Wurtz reaction not preferred for preparing unsymmetrical alkanes like propane?
Solution:
It results in a mixture of alkanes which are difficult to separate.
Explanation:
If we use a mixture of and , three products are formed: (ethane), (butane), and (propane). The boiling points of these alkanes are often close, making separation by fractional distillation very difficult.
Problem 3:
Convert Sodium Ethanoate to Methane.
Solution:
Explanation:
This is a decarboxylation reaction. The sodium salt of the carboxylic acid is heated with soda lime, which removes the group in the form of carbonate, reducing the carbon chain length by one.