Question

Alkyl halides react with dialkyl copper reagents to give (a) alkenes (b) alkyl copper halides (c) alkanes (d) alkenyl halides

Short answer

The correct product is (c) alkanes.

Step-by-step solution

Understanding the Reaction

Alkyl halides react with dialkyl copper reagents, typically with a reaction known as the Gilman reaction or a Corey-House synthesis. This involves the replacement of the halide in the alkyl halide with an alkyl group from the copper reagent.

Analyzing the Product Type

The reaction between an alkyl halide, R-X, and a dialkyl copper reagent, R'_2CuLi, results in the formation of an alkane, R-R'. This occurs because the copper reagent substitutes one of its alkyl groups for the halide.

Selecting the Correct Answer

Given that the reaction produces an alkane by substituting the halide group with an alkyl group from the dialkyl copper reagent, the correct answer is (c) alkanes.

Key concepts

Alkyl Halides

Alkyl halides, also known as haloalkanes, are organic compounds where a halogen atom is bonded to an sp³ hybridized carbon atom. They play a crucial role in synthetic organic chemistry due to their reactivity. The carbon-halogen bond is polarized, making the carbon atom electrophilic, which means it can be attacked by nucleophiles. This characteristic paves the way for various substitution and elimination reactions. The type of halide (fluoride, chloride, bromide, or iodide) influences the reactivity and the mechanism of the reaction involved.

• Primary Alkyl Halides: These have the halogen atom attached to a carbon atom that is connected to only one other carbon.
• Secondary Alkyl Halides: The halogen is attached to a carbon atom bonded to two other carbons.
• Tertiary Alkyl Halides: Here, the carbon with the halogen is bound to three other carbons, which often affects the reaction mechanism, favoring elimination over substitution due to steric hindrance.

Alkyl halides serve as essential starting materials in many synthesis reactions, including the creation of alkanes through the Gilman reaction.

Dialkyl Copper Reagents

Dialkyl copper reagents, often referred to as Gilman reagents, have the general formula R₂CuLi where R represents an alkyl group. These reagents are organocopper compounds formed by the reaction of a copper(I) salt with organolithium reagents. As a type of organometallic compound, they are significant in organic synthesis for forming carbon-carbon bonds.

They are uniquely effective at

• Carrying Alkyl Groups: Dialkyl copper reagents can transfer alkyl groups to suitable electrophiles such as alkyl halides, facilitating the substitution reaction.
• Adding Milder Conditions: Compared to other organometallic reagents like Grignard reagents, they are less reactive and often more selective, reducing the chances of multiple reaction pathways.

By replacing a halogen in an alkyl halide with an alkyl group, dialkyl copper reagents provide a versatile route in synthesizing alkanes.

Corey-House Synthesis

The Corey-House synthesis is a powerful method that utilizes dialkyl copper reagents to construct carbon-carbon bonds. Named after E.J. Corey and W.S. House, this synthesis offers a strategic approach to formulating complex organic molecules from simpler ones.

The process involves several key steps:

• Preparation of Dialkyl Copper Reagents: Initial formation of the organocopper compound from organolithium precursors and copper(I) halides.
• Reaction with Alkyl Halides: Subsequent reaction of these reagents with alkyl halides to yield alkanes, as the copper reagent displaces the halide group.

The Corey-House synthesis is valued for its efficiency and ability to proceed under mild conditions, which is beneficial in multi-step synthesis processes where reaction conditions can affect other functional groups present in the molecule.

Alkanes

Alkanes, the simplest type of hydrocarbons, consist solely of carbon and hydrogen atoms connected by single bonds, making them saturated hydrocarbons. These compounds can be represented by the general formula \[ C_nH_{2n+2} \] where \( n \) is the number of carbon atoms.

• Properties: Alkanes are known for being relatively unreactive due to the strength of the carbon-carbon and carbon-hydrogen bonds, making them stable under most conditions.
• Uses: Due to their stability, alkanes are commonly used as fuels (e.g., methane, propane) and lubricants. Their inert nature also serves as a framework in more complex organic molecules in synthetic applications.
• Synthesis: They can be synthesized through various methods, one of the primary ways being through reactions involving organometallic compounds like in the Corey-House synthesis, where dialkyl copper reagents react with alkyl halides to form new carbon-carbon bonds.

Understanding alkanes is integral to grasping broader organic chemical reactions and their applications in industry and research.