Question

Write the structural reactions for the laboratory preparation of methane and ethane with a brief description.

Short answer

Answer: Methane can be prepared in the laboratory using wood distillation (by reacting sodium acetate, sodium hydroxide, and quicklime) and Grignard reagent methods (by reacting a Grignard reagent such as methyl magnesium iodide with water). The structural reactions for methane preparation involve the formation of sodium methanoate and methane gas. Ethane can be prepared using the Wurtz reaction method, which involves sodium metal reduction of alkyl halides (such as ethyl bromide). The structural reaction for ethane preparation includes the formation of an ethane molecule and sodium bromide.

Step-by-step solution

Methane Preparation: Wood Distillation Method

In the laboratory, methane can be prepared by the wood distillation method. In this method, a mixture containing sodium acetate (\(CH_3COONa\)), sodium hydroxide (\(NaOH\)), and quicklime (\(CaO\)) is heated. Sodium methanoate is formed in the reaction between sodium acetate and sodium hydroxide: \(CH_3COONa + NaOH → CH_3OONa + H_2O\) Methane gas is then formed upon the reaction of sodium methanoate with quicklime: \(CH_3OONa + CaO → CH_4 + Na_2CaO_2\)

Methane Preparation: Grignard Reagent Method

Another method to prepare methane in the laboratory is by using a Grignard reagent, such as methyl magnesium iodide (\(CH_3MgI\)). The Grignard reagent is allowed to react with water to form methane gas and a magnesium hydroxide salt: \(CH_3MgI + H_2O → CH_4 + Mg(OH)I\)

Ethane Preparation: Wurtz Reaction Method

Ethane can be prepared in the laboratory using the Wurtz reaction method. This reaction involves the formation of ethane via sodium metal reduction of alkyl halides, such as ethyl bromide (\(C_2H_5Br\)). The Wurtz reaction typically takes place in a solvent mixture that consists of boiling anhydrous \(C_2H_5\cdot Br+C_2H_5\cdot Br+2Na→ C_2H_6+2NaBr\). In summary, methane can be prepared in the laboratory by wood distillation and Grignard reagent methods, while ethane can be prepared using the Wurtz reaction method. The structural reactions for methane preparation involve the formation of sodium methanoate and methane gas, while the reaction for ethane preparation includes the formation of an ethane molecule and sodium bromide.

Key concepts

Methane Preparation

Methane is the simplest hydrocarbon and has various laboratory preparation methods. One of the classic techniques involves the wood distillation method, where a mixture of sodium acetate (CH_3COONa), sodium hydroxide (NaOH), and quicklime (CaO) is heated. The process initially produces sodium methanoate through a reaction between sodium acetate and sodium hydroxide. Subsequently, methane (CH_4) is liberated from sodium methanoate when reacted with quicklime, leaving behind sodium calcium oxide (Na_2CaO_2).

The Grignard reagent method is another technique whereby methane is synthesised from a methyl magnesium iodide (CH_3MgI). When the Grignard reagent reacts with water, it forms methane gas and a byproduct of magnesium hydroxide iodide (Mg(OH)I). These processes are instrumental in creating methane for various research purposes in a controlled laboratory setting.

Ethane Preparation

To prepare ethane in a laboratory environment, the Wurtz reaction serves as a fundamental method. This reaction specifically uses the sodium metal reduction of alkyl halides, like ethyl bromide (C_2H_5Br). Conducted in a solvent, typically under boiling conditions, sodium (Na) acts on the ethyl bromide, yielding ethane (C_2H_6) and sodium bromide (NaBr).

Understanding this reaction requires acknowledging the crucial role of sodium in eliminating halogen atoms and facilitating the coupling of alkyl groups, which forms a higher alkane. The Wurtz reaction not only forms ethane but can also be extended to synthesize higher-order hydrocarbons through similar mechanisms.

Wurtz Reaction

The Wurtz reaction is a cornerstone of synthetic organic chemistry and is used for preparing various hydrocarbons. It involves the coupling of alkyl halides in the presence of sodium metal. The choice of solvent and temperature conditions is crucial for the success of this reaction.

It's known for its simplicity and, despite some limitations regarding the types of alkyl halides and possible side reactions, it remains a valuable tool in the laboratory, especially for constructing simple hydrocarbons like ethane.

Grignard Reagent

Delving into the realm of organic synthesis, the Grignard reagent emerges as an indispensable tool. These reagents, which are organomagnesium halides (RMgX), are named after the French chemist Victor Grignard, who received the Nobel Prize for his work on these compounds. They are highly reactive, allowing them to participate in a variety of reactions that form new carbon-carbon bonds.

Their reactivity with water to form hydrocarbons like methane underscores their utility in creating complex organic molecules, and they serve as building blocks in various synthetic processes within the chemical industry and academic research laboratories.

Wood Distillation Method

The wood distillation method is a traditional technique not just limited to methane production but also for acquiring various chemicals from wood. It entails heating wood in the absence of air to decompose it into charcoal and volatile compounds. These volatile compounds can then be condensed and collected. Methane preparation via wood distillation harkens back to classic chemistry, where a complex mixture of chemicals would be simplified and separated to yield pure substances. This method serves educational purposes as well, offering students a look into the historical practices of chemical production.