Question

Propose a synthesis for n-butane from n-butylbromide using the Grignard Reaction.

Short answer

To synthesize n-butane from n-butylbromide using the Grignard Reaction, first form the Grignard reagent by reacting n-butylbromide with magnesium in anhydrous ether, producing n-butylmagnesium bromide: \(C_4H_9Br + Mg → C_4H_9MgBr\). Next, react the n-butylmagnesium bromide Grignard reagent with water to form n-butane and magnesium hydroxide bromide: \(C_4H_9MgBr + H_2O → C_4H_10 + Mg(OH)Br\).

Step-by-step solution

Understand the Grignard Reaction

The Grignard Reaction is a prominent organic chemistry reaction that involves the creation of a Grignard reagent and its subsequent reactions with various electrophiles. The Grignard reagent is formed by the reaction of an alkyl or aryl halide (R-X) with magnesium metal (Mg) in anhydrous ether solvent. This forms an organomagnesium halide (RMgX) called a Grignard reagent. The general equation for this reaction is: R-X + Mg → RMgX Step 2:

Formation of the Grignard Reagent

Since we are given n-butylbromide as the starting material, the first step in the synthesis is to form the Grignard reagent from n-butylbromide. We will react n-butylbromide with magnesium in an anhydrous ether solvent to produce n-butylmagnesium bromide. The equation for this reaction is: C_4H_9Br + Mg → C_4H_9MgBr Step 3:

Choose the Appropriate Electrophile for the Grignard Reagent

To synthesize n-butane from the n-butylmagnesium bromide Grignard reagent, we need a suitable electrophile. Since n-butane has the formula C_4H_10, we want the Grignard reagent to react with an electrophile that can provide a single proton and no additional carbon atoms. The appropriate electrophile in this case is water (H_2O). Step 4:

React the Grignard Reagent with the Electrophile

Now, we will react the n-butylmagnesium bromide Grignard reagent with water to form n-butane. The Grignard reagent is a strong base and will readily react with acidic protons such as those from water. The equation for this reaction is: C_4H_9MgBr + H_2O → C_4H_10 + Mg(OH)Br Step 5:

Overall Synthesis

In summary, the synthesis of n-butane from n-butylbromide through the Grignard Reaction involves two steps: (1) the formation of the Grignard reagent by reacting n-butylbromide with magnesium in anhydrous ether, and (2) the reaction of the Grignard reagent with water to produce n-butane. The overall equation for the synthesis is: C_4H_9Br + Mg + H_2O → C_4H_10 + Mg(OH)Br

Key concepts

n-Butane Synthesis

Creating n-butane through the Grignard reaction is a multi-step process that showcases the versatility of organometallic chemistry. To synthesize n-butane from n-butylbromide, one must first understand the intricacies of the Grignard reaction.

This synthesis requires two key reactions: the creation of the Grignard reagent from n-butylbromide and its reaction with an electrophile, water, to produce the desired n-butane. Step by step, the conversion involves the formation of n-butylmagnesium bromide, which then reacts with the electrophile, yielding not only n-butane but also by-products like magnesium hydroxide bromide. It's a classic example of how a simple proton (hydrogen ion) can be introduced to an organic chain, extending its length and transforming it into a more useful hydrocarbon.

Grignard Reagent Formation

The formation of a Grignard reagent is the cornerstone of the reaction that bears its name. This highly reactive compound is synthesized when an alkyl halide reacts with magnesium in an anhydrous ether environment.

The significance of using an anhydrous ether cannot be overstated as it prevents unwanted reactions that can occur in the presence of moisture. Reacting magnesium with n-butylbromide under these conditions leads to the formation of n-butylmagnesium bromide. This step is critical because it sets the stage for creating the carbon-carbon bonds needed to synthesize more complex organic molecules.

Organomagnesium Halides

Key Characteristics and Reactivity

Organomagnesium halides, often referred to as Grignard reagents, are crucial intermediates in synthetic organic chemistry. They form when a halogenated hydrocarbon reacts with magnesium. These compounds are unique not only for their reactivity but also for their selectivity.

The carbon-magnesium bond in these reagents is highly polarized, with the carbon being nucleophilic. This allows for the addition of carbon atoms to electrophilic centers, facilitating the construction of carbon skeletons that form the backbone of organic chemistry. The reactivity of organomagnesium halides makes them invaluable for synthesizing a wide array of organic compounds, leading from simple hydrocarbons to more elaborate molecules like pharmaceuticals.

Alkyl Halide with Magnesium

The interaction between an alkyl halide and magnesium is the first step in creating a Grignard reagent and, by extension, the synthesis of various organic compounds. It's a fundamental reaction in which the halide, carrying a leaving group, meets the magnesium, yielding an organomagnesium halide.

In ensuring the success of this reaction, the choice of solvent and careful control of conditions like temperature and moisture level are vital. The alkyl halide contributes the carbon chain that will eventually be extended or modified, and the magnesium plays a pivotal role as a connecting agent, facilitating the subsequent addition of various groups to this carbon chain through further reactions.