Problem 22 - Chemistry in 30 days

Question

An organic compound of molecular formula \(\mathrm{C}_{4} \mathrm{H}_{10} \mathrm{O}\) does not react with sodium. With excess of \(\mathrm{HI}\), it gives only one type of alkyl halide. The compound is (a) Ethoxyethane (b) 2 -Methoxypropane (c) 1-Methoxypropane (d) 1-Butanol

Step-by-Step Solution

Answer

The compound is (a) Ethoxyethane.

Step 1: Interpret the Compound's Formula

The molecular formula is \( \mathrm{C}_{4} \mathrm{H}_{10} \mathrm{O} \), which corresponds to an ether or an alcohol. The fact that it does not react with sodium suggests it is not an alcohol, as alcohols typically react with sodium to produce hydrogen gas.

Step 2: Analyze Reaction with Excess HI

The compound, when treated with excess \( \mathrm{HI} \), gives only one type of alkyl halide. This indicates that the compound is symmetrical, as an asymmetric ether would yield two different alkyl halides.

Step 3: Deduce the Structure from Options

Of the given options, ethoxyethane (option a) satisfies both criteria. It is an ether, making it non-reactive with sodium, and it is symmetrical, thus forming only one type of alkyl halide with excess \( \mathrm{HI} \).

Key Concepts

Ethers

Ethers are a fascinating class of organic compounds. They feature an oxygen atom bonded to two alkyl or aryl groups, forming the general structure \(R-O-R'\). The presence of an oxygen atom confers unique properties to ethers.
Ethers exhibit relatively low reactivity compared to alcohols or acids. This is primarily because they lack the hydroxyl (OH) group commonly found in alcohols.

• They have higher boiling points than alkanes of comparable molecular weight, but lower than alcohols.
• The oxygen atom allows ethers to form hydrogen bonds with hydrogen bond donors, enhancing solubility in water.

Their inertness often makes ethers excellent solvents, particularly in reactions where other functional groups might interfere.

Alkyl Halides

Alkyl halides form an essential category of organic molecules. In these compounds, one or more hydrogen atoms in an alkane are replaced by a halogen such as fluorine, chlorine, bromine, or iodine. The resulting compound is commonly written in the form \(RX\), where \(R\) represents the alkyl group and \(X\) the halogen.
Alkyl halides are notable for their participation in substitution and elimination reactions. Their reactivity lies in the polar carbon-halogen bond:

• The bond is polarized, with carbon holding a partial positive charge, making it a site for nucleophilic attack.
• The nature of \(X\) influences reactivity; iodine, being the largest, forms the longest and weakest bond, rendering alkyl iodides more reactive than alkyl fluorides.

These compounds are versatile in organic synthesis, forming various products like alcohols, ethers, and amines through different reactions.

Symmetrical Compounds

Symmetry in organic chemistry often refers to molecules having indistinguishable halves. This symmetry can impact chemical reactions significantly. Symmetrical ethers, such as ethoxyethane, are a prime example.
When symmetrical ethers react with excess hydrogen iodide (HI), they break down into identical pieces, yielding only one type of alkyl halide:

• In the case of ethoxyethane, the reaction with HI produces ethyl iodide exclusively.
• Asymmetrical ethers, however, would produce a mixture of alkyl halides.

Recognizing symmetry in compounds is crucial for predicting the products of chemical reactions accurately, simplifying the identification process.

Chemical Reactions with Sodium

Reactions with sodium metal provide insights about different organic compounds. Typically, alcohols react with sodium to form alkoxides, releasing hydrogen gas in the process. This reactivity is a helpful indicator:

• Alcohols exhibit this classic gas evolution upon sodium exposure.
• Ethers, on the other hand, remain inert to sodium, which aligns with their general chemical inertness.

This distinction is important when deducing the functional groups or identities of organic compounds based on their chemical behavior. In the context of the exercise, the lack of reactivity of the ether indicated it could not be an alcohol, thereby guiding the solution process.