Problem 159 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Number of hydrocarbons formed when \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Br}\) and \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{Br}\) are treated with \(\mathrm{Na}\) in the presence of dry ether is:

Step-by-Step Solution

Answer

Three hydrocarbons: butane, pentane, and hexane.

Step 1: Identify the Reactants

The compounds given are ethyl bromide \( (\mathrm{C}_2\mathrm{H}_5\mathrm{Br}) \) and propyl bromide \( (\mathrm{CH}_3\! -\mathrm{CH}_2\! -\mathrm{CH}_2\mathrm{Br}) \). Both will undergo the Wurtz reaction in the presence of sodium \( (\mathrm{Na}) \) and dry ether.

Step 2: Understand the Wurtz Reaction

The Wurtz reaction involves the coupling of two alkyl halides in the presence of sodium, resulting in the formation of a longer alkane and sodium bromide as a byproduct. The general reaction is \( \mathrm{R-Br} + \mathrm{R'-Br} + 2\mathrm{Na} \rightarrow \mathrm{R-R'} + 2\mathrm{NaBr} \).

Step 3: Identify Possible Hydrocarbon Products

From ethyl bromide \( (\mathrm{C}_2\mathrm{H}_5\mathrm{Br}) \) and propyl bromide \( (\mathrm{CH}_3\! -\mathrm{CH}_2\! -\mathrm{CH}_2\mathrm{Br}) \), possible hydrocarbons formed are (1) butane \( \mathrm{C}_4\mathrm{H}_{10} \) from coupling two ethyl groups, (2) hexane \( \mathrm{C}_6\mathrm{H}_{14} \) from coupling two propyl groups, and (3) pentane \( \mathrm{C}_5\mathrm{H}_{12} \) from coupling ethyl and propyl groups.

Step 4: List All Products Formed

List all hydrocarbons formed: butane \( (\mathrm{C}_4\mathrm{H}_{10}) \), hexane \( (\mathrm{C}_6\mathrm{H}_{14}) \), and pentane \( (\mathrm{C}_5\mathrm{H}_{12}) \). Thus, there are three different hydrocarbons that can be formed from this reaction.

Key Concepts

Alkyl Halides

Alkyl halides, also known as haloalkanes, are organic compounds where a halogen atom is bonded to an alkyl group. In simpler terms, think of them as hydrocarbons where one or more hydrogen atoms have been replaced by a halogen like chlorine, bromine, or iodine.
These compounds play a crucial role in organic chemistry due to their reactivity and versatility in forming other compounds.

• The carbon-halogen bond in alkyl halides is polar because halogens are more electronegative than carbon. This means the halogen pulls electron density towards itself, making it partially negative and the carbon partially positive.
• This bond polarity is a key reason why alkyl halides are so reactive, especially in substitution and elimination reactions.

When used in reactions like the Wurtz reaction, alkyl halides can help build larger molecules from smaller ones, making them vital in synthesizing complex organic compounds.

Organic Chemistry Reactions

Organic chemistry features a fascinating array of reactions that transform simple molecules into complex structures. These reactions typically involve changes in bonds and functional groups, driven by the need to either form or break chemical bonds. The Wurtz reaction is a classic example showcasing the transformation of alkyl halides into alkanes.

• The Wurtz reaction is specifically a coupling reaction. It involves the reaction of two molecules of an alkyl halide with sodium metal, resulting in the formation of a new carbon-carbon bond.
• This reaction requires dry ether as a solvent, to prevent the sodium from reacting with water, which would lead to undesirable byproducts.
• Such reactions are valuable because they allow chemists to construct long carbon chains, integral to many synthetic procedures in organic chemistry.

Understanding organic reactions like the Wurtz reaction helps in visualizing how molecules interact and transform, playing a crucial role in drug development and materials science.

Hydrocarbon Formation

Hydrocarbon formation through chemical reactions is a pivotal process in the field of organic chemistry. Hydrocarbons are compounds comprising solely of carbon and hydrogen, and they serve as the backbone for forming more complex organic structures.
In reactions like the Wurtz reaction, the conversion of small molecules to larger hydrocarbons is achieved.

• In our example, ethyl bromide and propyl bromide undergo a Wurtz reaction to form products like butane, hexane, and pentane.
• These products indicate the possible combinations of carbon chains that can be formed when smaller alkyl groups are linked together.

By forming these hydrocarbons, the reaction illustrates not just synthesis but also concepts like chain branching and isomer formation, which are pivotal in understanding more sophisticated organic synthesis pathways.