Question

An unknown compound dissolves in sulphuric acid, but does not decolourize bromine water and does not react with sodium. Which of the following classes of molecules behave in this manner? (a) phenol (b) alkene (c) alcohol (d) ether

Short answer

The unknown compound is likely an ether.

Step-by-step solution

Identify the Molecular Behavior

To determine which class of molecules an unknown compound belongs to, we need to consider its chemical behavior. The compound dissolves in sulfuric acid, does not decolorize bromine water, and does not react with sodium. This suggests it does not have a double bond (as alkenes would decolorize bromine water), and does not have readily ionizable hydrogen atoms that react with sodium, such as in phenols or alcohols.

Evaluate Alkenes

Alkenes typically react with bromine, leading to decolorization. Since the unknown compound does not decolorize bromine water, it is unlikely to be an alkene. Therefore, option (b) is an incorrect choice.

Evaluate Phenols and Alcohols

Phenols dissolve in sulfuric acid and react with sodium, producing hydrogen gas. Similarly, alcohols can react with sodium under specific conditions to form hydrogen gas. Since the unknown compound does not react with sodium, it is unlikely to be either a phenol or alcohol. Therefore, options (a) and (c) can be ruled out.

Evaluate Ethers

Ethers generally dissolve in strong acids like sulfuric acid, and do not react with sodium or decolorize bromine water. They are relatively inert compared to other classes mentioned. Given these characteristics, the behavior of the unknown compound aligns with ethers.

Key concepts

Solubility in Sulfuric Acid

Understanding how different substances dissolve in sulfuric acid is key to identifying molecular characteristics. Sulfuric acid is a strong acid, capable of donating protons (H⁺ ions) which helps break down various compounds.

Some molecules with polar groups, such as ethers, are soluble in sulfuric acid due to their ability to form intermolecular bonds with the acid. The lone pairs on the oxygen in an ether can interact with the hydrogens from sulfuric acid, allowing them to dissolve.

In comparison, non-polar compounds, or compounds resistant to ionization, may not dissolve as easily. The solubility of ethers in sulfuric acid without further reactive behavior can be a clue in identifying their presence.

Reaction with Bromine Water

The use of bromine water as a test is a classic way to detect unsaturation in organic compounds. This test relies on the brown color of bromine disappearing when added to substances with double or triple bonds.

Alkenes, for example, are known to readily react with bromine water, causing decolorization as bromine breaks the double bond and forms a new product. Since the compound in questions does not cause bromine water to change color, it indicates the absence of double bonds, ruling out alkenes.

Ethers, on the other hand, are generally unreactive when it comes to bromine water. Their lack of double bond structure means they do not interact with bromine, maintaining the bromine's color and confirming the suspected class of the compound.

Reaction with Sodium

Reactions with sodium metal are informative, especially for spotting the presence of reactive hydrogen atoms. When compounds like alcohols or phenols react with sodium, they produce hydrogen gas, evident through bubbling.

Alcohols possess a hydroxyl group (OH) that can release a hydrogen ion, leading to the reaction with sodium. Phenols react similarly due to their acidic hydroxyl group.

The tested compound does not react with sodium, meaning it likely lacks the reactive hydrogen characteristic of alcohols and phenols. Ethers, lacking such an ionizable hydrogen, do not engage in this reaction with sodium, aligning with the characteristics observed.

Characteristics of Ethers

Ethers are interesting compounds with specific traits. They have an oxygen atom connected to two alkyl or aryl groups, represented generally as R-O-R'. This configuration contributes to their chemical stability.

Ethers are typically unreactive; they don't react with acids, bases, or water under normal conditions. Their stability in reactions makes them good solvents for many organic reactions.

• Lack of a hydroxyl group prevents reactions with sodium.
• Absence of unsaturation explains why they do not decolorize bromine water.
• Solubility in strong acids like sulfuric acid is due to the ether oxygen's ability to interact with protons.

These characteristics help identify ethers as the probable class of the unknown compound.