Question

To prepare a pure sample of n-hexane using sodium metal as one reactant, the other reactants will be (1) ethyl chloride and \(\mathrm{n}\) -butyl chloride (2) methyl bromide and n-pentyl bromide (3) n-propyl bromide (4) ethyl bromide and \(\mathrm{n}\) -butyl bromide

Short answer

Options (1), (2), and (4) are all correct.

Step-by-step solution

- Understand the Reaction Mechanism

To prepare n-hexane using sodium metal as a reactant, the reaction involved is the Wurtz reaction. In the Wurtz reaction, two alkyl halides react with sodium to form a new alkane.

- Identify the Required Carbon Chain Length

n-Hexane has a carbon chain length of 6. Therefore, the total number of carbons contributed by the two alkyl halides must add up to 6.

- Check Each Option for the Right Reactant Combination

Review the options to find a pair of alkyl halides that, when combined, give a total of 6 carbon atoms:(1) Ethyl chloride (C2) + n-butyl chloride (C4) → Total carbons = 2 + 4 = 6(2) Methyl bromide (C1) + n-pentyl bromide (C5) → Total carbons = 1 + 5 = 6(3) n-propyl bromide (C3) alone does not make the correct product.(4) Ethyl bromide (C2) + n-butyl bromide (C4) → Total carbons = 2 + 4 = 6.

- Verify the Correct Combinations

The pairs from options (1), (2), and (4) have the correct carbon totals to form n-hexane.

- Select the Best Pair

The best pairs are those that naturally fit the Wurtz reaction. All pairs provided fit the reaction so the final answer should be an option that includes these valid pairs.

Key concepts

n-hexane preparation

n-Hexane is a simple hydrocarbon with six carbon atoms arranged in a straight chain. It's essential in applications such as gasoline formulation and as an industrial solvent. To prepare n-hexane, the Wurtz reaction is commonly used due to its efficiency in forming new carbon-carbon bonds.

The Wurtz reaction involves reacting alkyl halides with sodium metal. By selecting appropriate alkyl halides, we can sum the carbon atoms to get n-hexane. In this exercise, you have to identify pairs of alkyl halides whose total carbon numbers equal six:

• Ethyl chloride (C2) and n-butyl chloride (C4) → n-hexane (C6)
• Methyl bromide (C1) and n-pentyl bromide (C5) → n-hexane (C6)
• Ethyl bromide (C2) and n-butyl bromide (C4) → n-hexane (C6)

This step-by-step approach ensures you choose reactants that meet the carbon chain length requirement for n-hexane.

alkyl halides

Alkyl halides are compounds where a halogen atom (Cl, Br, I) is bonded to an alkyl group. These are fundamental in organic reactions, particularly for generating new bonds in processes like the Wurtz reaction.

Alkyl halides can be classified based on the type of carbon to which the halogen is attached:

• Primary (1°) alkyl halides: Halogen is attached to a carbon atom bonded to one other carbon atom, e.g., ethyl bromide
• Secondary (2°) alkyl halides: Halogen is attached to a carbon atom bonded to two other carbon atoms
• Tertiary (3°) alkyl halides: Halogen is attached to a carbon atom bonded to three other carbon atoms

In this exercise, specific primary alkyl halides such as ethyl chloride, methyl bromide, and n-butyl chloride are reactants.

Understanding this classification helps choose the correct alkyl halides for reactions like the Wurtz reaction, where primary halides are typically more reactive and form fewer side products.

reaction mechanism

The reaction mechanism of the Wurtz reaction involves a series of steps where sodium metal helps in dehalogenation, ultimately forming a new carbon-carbon bond.

The primary steps in the Wurtz reaction mechanism are:

• Initiation: Two sodium atoms donate their valence electrons to create two alkyl radicals from alkyl halides.
• Propagation: These radicals pair to form a new C-C bond.
• Termination: The final product is an alkane, achieved by linking the carbon atoms from the alkyl halides chosen.

To better understand, consider ethyl chloride (C2H5Cl) and n-butyl chloride (C4H9Cl):

• When reacted with sodium, both C2H5• and C4H9• radicals form.
• These radicals combine to produce C6H14 (n-hexane).

This bimolecular radical formation is the fundamental principle behind why careful selection of alkyl halides based on carbon count is vital.