Question

The best method of preparation of an alkane with odd number of carbons from two alkylhalides is (a) Wurtz reaction (b) Kolbe's electrolysis (c) Corey-House synthesis (d) Ullmann reaction

Short answer

A. Wurtz reaction B. Kolbe's electrolysis C. Corey-House synthesis D. Ullmann reaction Answer: C. Corey-House synthesis Explanation: The Corey-House synthesis is the best method for preparing an alkane with an odd number of carbons from two alkylhalides, as it allows the formation of both symmetric and asymmetric alkanes. In contrast, the Wurtz reaction and Kolbe's electrolysis typically yield alkanes with an even number of carbons, and the Ullmann reaction produces biaryl compounds instead of alkanes.

Step-by-step solution

1. Understand the four reaction methods

Before finding the best method, we need to understand each of the four given methods: (a) Wurtz reaction: A coupling reaction where two alkylhalides react in the presence of sodium metal to form a symmetrical alkane with an even number of carbons. (b) Kolbe's electrolysis: An electrochemical decarboxylation reaction involving the conversion of a carboxylic acid (or its salts) into an alkane, usually with an even number of carbons. (c) Corey-House synthesis: A two-step reaction involving organocuprates to form new carbon-carbon bonds, allowing the preparation of both symmetric and asymmetric alkanes. (d) Ullmann reaction: A copper-catalyzed coupling reaction involving the reaction between an aryl halide and an amine to form a biaryl compound.

2. Compare the reaction methods for odd-numbered alkanes

Comparing these methods: - The Wurtz reaction and Kolbe's electrolysis generally yield alkanes with an even number of carbons, which does not fulfill our requirement. - The Ullmann reaction yields biaryl compounds and not alkanes, so it is not suitable for the given condition. However, the Corey-House synthesis has the flexibility to create both symmetric and asymmetric alkanes, which means it can be used to prepare an alkane with an odd number of carbons.

3. Determine the best method

Based on the analysis above, the best method of preparation of an alkane with an odd number of carbons from two alkylhalides is: (c) Corey-House synthesis

Key concepts

Alkane Preparation

Alkanes are a fundamental type of hydrocarbon composed solely of single-bonded carbon and hydrogen atoms. When preparing alkanes, particularly those with a specified number of carbon atoms, chemists can use various synthesis techniques. The goal is to form stable carbon-carbon bonds, assembling molecules with the desired structure and properties.

There are different methods available for alkane preparation depending on the desired carbon chain length and structure. The reaction techniques must be chosen carefully to ensure the creation of the correct alkane, whether symmetric or asymmetric. For instance, the Corey-House synthesis stands out for its capability to form alkanes with odd or even numbers of carbon atoms. Other methods, like the Wurtz reaction, tend to produce even numbered carbon chains, thereby limiting their utility in some contexts.

Alkylhalides

Alkylhalides, or haloalkanes, are a crucial class of compounds in organic chemistry. These molecules consist of an alkane backbone with one or more halogen atoms (such as fluorine, chlorine, bromine, or iodine) replacing hydrogen atoms.

The unique properties of alkylhalides, such as their reactivity and solubility, make them excellent starting points for various organic reactions. For example, they are commonly used in substitution and elimination reactions, where the halogen atom is replaced by another atom or group.

Notably, in processes like the Corey-House synthesis, alkylhalides play a vital role as reactants in forming new carbon-carbon bonds. The reactivity of alkylhalides can significantly influence the course and outcome of the reaction, enabling the synthesis of alkanes with the desired carbon number and arrangement.

Wurtz Reaction

The Wurtz reaction is a well-known method used for synthesizing alkanes through a coupling process. This reaction typically involves the treatment of two alkylhalides with sodium metal in a suitable solvent, like dry ether.

The primary result of the Wurtz reaction is the formation of symmetrical alkanes with even numbers of carbon atoms. For example, reacting two identical ethyl chloride molecules can yield butane, with a total of four carbon atoms.

While the Wurtz reaction is efficient for producing even-numbered alkanes, it does not easily accommodate the creation of alkanes with an odd number of carbon atoms. This limitation may render it less suitable in cases where an odd-numbered alkane is desired, such as in various advanced organic synthesis applications.

Kolbe's Electrolysis

Kolbe's electrolysis is an interesting electrochemical technique that transforms carboxylic acids into hydrocarbons, typically yielding alkanes. The method involves the decarboxylation of a carboxylic acid's salt under an electric current, converting it into its corresponding alkane.

A significant aspect of Kolbe's electrolysis is that it mainly produces saturated hydrocarbons with even numbers of carbon atoms. This constraint arises because the process combines two carboxylic acid molecules, effectively doubling the carbon chain length.

As it tends to produce even-numbered alkanes, Kolbe's electrolysis is not typically the ideal choice for synthesizing odd-numbered alkanes. Alternatives, like the Corey-House synthesis, may be more suitable when seeking to create alkanes with singular carbon chain lengths.

Ullmann Reaction

The Ullmann reaction is a classic technique primarily used for forming biaryl compounds. It involves the coupling of aryl halides in the presence of copper catalysts, effectively creating complex aromatic structures.

While the Ullmann reaction is pivotal in creating carbon-carbon bonds, it is not used for forming simple alkane chains. Instead, it is more commonly applied in synthesizing substances like biphenyls and other multi-ring aromatic systems.

Due to its focus on aromatic compounds, the Ullmann reaction is not relevant when the goal is to synthesize an alkane, whether odd or even in carbon number. Its application is specific to aromatic chemistry and does not cater to the direct production of aliphatic hydrocarbons.