Question

Thiophene can be removed from commercial benzene by: (a) Steam distillation (b) Shaking with conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (c) Shaking with \(\mathrm{NaOH}\) solution (d) Shaking with ether

Short answer

Shaking with conc. H_{2}SO_{4} , answer (b), can react with thiophene.

Step-by-step solution

Understand Thiophene's Properties

Thiophene is a sulfur-containing heterocyclic compound with aromatic properties. It's chemically resistant, making it difficult to interact with many reagents.

Analyze Steam Distillation

Steam distillation is often used to separate components based on differences in their boiling points. Thiophene and benzene have similar boiling points, making steam distillation an ineffective method for separation.

Evaluate Reaction with H2SO4

Concentrated sulfuric acid ( H_{2}SO_{4} ) can sulfonate or react with thiophene due to its action as a sulfonating agent and strong acid characteristics. However, this reaction is complex and not typically used for purification from benzene.

Consider Reaction with NaOH

Sodium hydroxide ( NaOH ) solution is a strong base, and thiophene would not readily react with it since thiophene is aromatic and lacks acidic protons or groups that could react with NaOH.

Evaluate Shaking with Ether

Ether is an organic solvent that can dissolve many organic compounds, including both thiophene and benzene. Shaking with ether does not separate thiophene from benzene.

Key concepts

Aromatic Compounds

Aromatic compounds are a class of molecules known for their stability and unique chemical properties. Their characteristic structure features a ring with alternating double and single bonds forming a conjugated system. This arrangement allows for the electrons to delocalize across the ring, resulting in a resonance that gives these compounds their aromatic nature.

Some key features of aromatic compounds include:

• High stability due to resonance, making them less reactive than other unsaturated compounds.
• Planarity, meaning all the atoms in the aromatic ring are on the same plane, which helps in the delocalization of electrons.
• They often undergo substitution reactions rather than addition reactions, preserving the aromatic system. This is because addition would disrupt the conjugated system.

Thiophene, which is part of our original exercise, is an aromatic compound that includes a sulfur atom within its ring. Despite the presence of sulfur, it maintains the stability typical of other aromatic compounds thanks to its electron-rich ring system.

Heterocyclic Chemistry

Heterocyclic chemistry deals with the study and synthesis of heterocyclic compounds, which are rings that include a different atom besides carbon, such as nitrogen, oxygen, or sulfur. Thiophene is a classic example of a heterocyclic compound, where the five-membered ring contains four carbon atoms and one sulfur atom.

Thiophene's importance in heterocyclic chemistry comes from its combination of aromaticity with the presence of a heteroatom (sulfur). This creates unique chemical behaviors:

• The sulfur atom contributes to the aromatic stability while introducing slight polarity to the molecule.
• Thiophene can engage in both nucleophilic and electrophilic substitution reactions, similar to benzene, but with sulfur providing distinct reaction pathways.
• Its aromatic stability is comparable to benzene, making it a subject of interest in synthetic chemistry due to its resistance to oxidation and various chemical reactions.

In separation processes such as the one discussed in the exercise, the unique heterocyclic structure and aromatic nature of thiophene make it challenging to separate from other aromatic compounds like benzene using simple methods.

Organic Solvents

Organic solvents are crucial tools in chemistry, enabling the dissolution, mixing, or separation of substances without causing chemical reactions with the solute. They are diverse in their polarities, boiling points, and chemical structures.

When considering solvents for separation purposes, like in the original thiophene problem, several factors come into play:

• Solubility: The solvent should dissolve the desired compounds efficiently. For thiophene and benzene, common organic solvents like ethers can dissolve both, making them unsuitable for separation purposes.
• Polarity: The polarity of the solvent should interact with either the solute's chemical makeup or any added reagents to achieve separation.
• Boiling point: Solvents with a lower boiling point may be ideal for processes like distillation if there's a significant difference between solutes' boiling points. However, this doesn't apply to thiophene and benzene, which boil at similar temperatures.

In our exercise, organic solvents were considered for their ability to potentially dissolve different substances, but the similarity in chemical properties between thiophene and benzene rendered common solvents ineffective for their separation.