Problem 5 - A Complete Resource Book in Chemistry for JEE Main

Question

An ether which is liquid at room temperature is (a) \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) (b) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OC}_{2} \mathrm{H}_{5}\) (c) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OCH}_{3}\) (d) none

Step-by-Step Solution

Answer

(b) \\(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OC}_{2} \mathrm{H}_{5}\\) is the liquid ether at room temperature.

Step 1: Understand the Physical States

First, recognize that ethers are organic compounds characterized by an oxygen atom connected to two alkyl or aryl groups. Most simple ethers are gases at room temperature, but higher ethers can be liquids. Room temperature is considered to be around 25°C (77°F).

Step 2: Investigate Each Option

Analyze each ether provided: - (a) Dimethyl ether \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) is a gas at room temperature due to its low molecular weight and weak intermolecular forces. - (b) Diethyl ether \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OC}_{2} \mathrm{H}_{5}\) is a liquid at room temperature because it has a higher molecular weight and slightly stronger van der Waals forces. - (c) Methyl ethyl ether \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OCH}_{3}\) is also a gas at room temperature. - (d) The option 'none' implies none of the ethers is liquid.

Step 3: Determine the Correct Answer

Based on the analysis, diethyl ether \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OC}_{2} \mathrm{H}_{5}\) is a known liquid at room temperature. The other provided ethers are gases.

Key Concepts

Physical States of Ethers

Ethers are an interesting group of organic compounds. They are characterized by an oxygen atom bound to two carbon-containing groups, often referred to as alkyl or aryl groups. The physical state of an ether at room temperature, which is around 25°C or 77°F, is highly dependent on its molecular structure. While most simple ethers are gases at room temperature, ethers with larger molecular structures can exist as liquids.

• For instance, dimethyl ether (\(\mathrm{CH}_{3} \mathrm{OCH}_{3}\)) is a gas at room temperature.
• This happens because of its simple structure and lower molecular weight.
• On the other hand, diethyl ether (\(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OC}_{2} \mathrm{H}_{5}\)) is a liquid due to a greater molecular weight and increased van der Waals forces.

Understanding these physical states is crucial when predicting the behavior of ethers in various environments, especially in chemistry labs and industrial applications.

Intermolecular Forces in Ethers

Intermolecular forces play a significant role in determining the properties of ethers, including their physical state. These forces are interactions between molecules, which can be weaker or stronger based on molecule type and structure.

• In ethers, the primary intermolecular force is van der Waals forces (also known as London dispersion forces).
• These forces are relatively weak in comparison to other types of molecular interactions like hydrogen bonds.
• The size of the molecule influences the strength of these forces. Larger molecules with more surface area can exert greater van der Waals forces.

For example, diethyl ether, with a larger molecular structure compared to dimethyl ether, exhibits enhanced van der Waals forces. This increase in intermolecular attraction allows diethyl ether to remain a liquid at room temperature, whereas smaller ethers are more likely to be gases.

Molecular Weight and Boiling Point of Ethers

The molecular weight of an ether is a major determinant of its boiling point. As the molecular weight increases, so does the boiling point. This relation is because higher molecular weight typically leads to stronger intermolecular forces, which require more energy (higher temperatures) to overcome.

• Dimethyl ether, with a low molecular weight, has a low boiling point and remains a gas at room temperature.
• Conversely, diethyl ether has a comparatively higher molecular weight, leading to a higher boiling point that allows it to be a liquid at room temperature.

This pattern is observed across many organic compounds and is particularly important when selecting ethers for specific uses, such as solvents in chemical reactions where maintaining a liquid state can be crucial.