Question

A hydrocarbon \(\mathrm{C}_{5} \mathrm{H}_{12}\) (A) on treatment with chlorine undergoes monochlorination by the replacement of the only \(1^{\circ}\) hydrogen to give(B) \(\mathrm{C}_{5} \mathrm{H}_{11} \mathrm{Cl}\). When (B) is treated with Na metal in ether, the product obtained could be (a) \(\quad\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}-\mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}\) (b) \(\mathrm{CH}_{3}-\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}-\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}-\mathrm{CH}_{3}\) (c) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{C}\left(\mathrm{CH}_{3}\right)_{3}\) (d) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{CH}_{2} \mathrm{Cl}\)

Short answer

Answer: The product obtained is (b) \(\mathrm{CH}_{3}-\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}-\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}-\mathrm{CH}_{3}\).

Step-by-step solution

Determine the structure of the hydrocarbon (A)

Since the compound A has a molecular formula of \(\mathrm{C}_{5} \mathrm{H}_{12}\), it must be an alkane. The only possible alkane with this molecular formula is pentane, which has the structure: \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{3}\).

Find the structure of compound (B) after monochlorination

We are given that only one primary hydrogen is replaced during the monochlorination reaction. From the structure of pentane, there are two primary carbons: \(\mathrm{CH}_{3}\) at both ends. Therefore, the monochlorination can result in the formation of 1-chloropentane or 5-chloropentane, which have the same structure: \(\mathrm{CH}_{2}\mathrm{Cl}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{3}\).

Analyze the reaction of compound (B) with Na metal in ether

When an alkyl halide reacts with Na metal in ether, it undergoes the Wurtz reaction. The Wurtz reaction is a coupling reaction that forms a new carbon-carbon bond by combining two molecules of the alkyl halide. In this case, two molecules of 1-chloropentane will react with two equivalents of Na metal in ether, leading to the formation of a new carbon-carbon bond between the two \(\mathrm{CH}_{2}\mathrm{Cl}\) sites.

Identify the product obtained after the reaction between compound (B) and Na metal in ether

The Wurtz reaction of 1-chloropentane will produce the following product: \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{3}\). Comparing this product with the given options, we see that the correct product matches option (b): \(\mathrm{CH}_{3}-\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}-\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}-\mathrm{CH}_{3}\). So, the product obtained after compound B is treated with Na metal in ether is (b) \(\mathrm{CH}_{3}-\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}-\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}-\mathrm{CH}_{3}\).

Key concepts

Monochlorination of Alkanes

Monochlorination of alkanes is an important reaction in organic chemistry, particularly for IIT JEE aspirants studying hydrocarbons. It involves the substitution of one hydrogen atom in an alkane with a chlorine atom.

This reaction is initiated by light or heat, which causes the homolytic cleavage of chlorine molecules to form chlorine radicals. These radicals are highly reactive and can abstract hydrogen atoms from the alkane, resulting in the formation of alkyl radicals. Subsequently, the alkyl radicals combine with chlorine radicals to produce alkyl chlorides.

The selectivity of the monochlorination process depends on the type of hydrogen atoms present in the alkane. Primary hydrogens are less likely to be substituted compared to secondary or tertiary hydrogens due to their bond dissociation energies. However, when the exercise specifies that a primary hydrogen is replaced, it is essential to consider the structure of the alkane and identify all the primary hydrogens to determine the possible monochlorinated products.

Wurtz Reaction

The Wurtz reaction is a classical method used in organic synthesis for coupling two alkyl halides to form a longer carbon chain.

The reaction takes place in the presence of metallic sodium (Na) in a dry ether solvent. When two molecules of an alkyl halide come in contact with sodium, the metal donates an electron to each halogen, forming two molecules of sodium halide and creating a free radical intermediate. These free radicals then combine to form a new carbon-carbon bond.

Limitations of the Wurtz Reaction

Despite its usefulness, the Wurtz reaction has limitations. It sometimes produces a mixture of products due to multiple combinations of radicals, and it works best with primary and secondary alkyl halides; tertiary alkyl halides often lead to eliminations. Moreover, it cannot be applied to the synthesis of certain types of structures, such as those with functional groups sensitive to strong bases or metals like sodium.

Structure of Hydrocarbons

Understanding the structure of hydrocarbons is fundamental in organic chemistry. Hydrocarbons are compounds composed exclusively of carbon (C) and hydrogen (H) atoms.

Alkanes are the simplest class of hydrocarbons, characterized by single bonds between carbon atoms. They follow the general formula (CnH2n+2). Alkanes can be straight-chained or branched, and the complexity of their structure increases with the number of carbon atoms.

Isomerism in Hydrocarbons

The structure of hydrocarbons is also significant because it introduces the concept of isomerism. Isomers are compounds with the same molecular formula but different structural arrangements. In alkanes, isomerism can occur due to branching, resulting in different physical and chemical properties.

For example, the hydrocarbon in the exercise, pentane (C5H12), can exist in several isomeric forms, which will react differently under various conditions, such as monochlorination.