Problem 155 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Which of the following statement is false? (a) Dimethyl amine as well as trimethyl amine are soluble in water. (b) Trimethyl amine forms hydrogen bond neither with itself nor with water. (c) Trimethyl amine can act as hydrogen bond acceptor only, while dimethyl amine can serve as both a hydrogen bond donor and acceptor. (d) All the three statements are false.

Step-by-Step Solution

Answer

Statement (b) is false.

Step 1: Understand Hydrogen Bonding in Amines

Amines such as dimethyl amine (\(\text{(CH}_3)_2\text{NH)}\) and trimethyl amine (\(\text{(CH}_3)_3\text{N)}\) differ in their ability to form hydrogen bonds. Dimethyl amine can both donate and accept hydrogen bonds due to the presence of an \(\text{NH}\) group. Trimethyl amine, however, lacks an \(\text{NH}\) group, so it cannot donate hydrogen bonds but can accept them.

Step 2: Analyzing Solubility and Hydrogen Bonding

Let's consider statement (a): Dimethyl amine and trimethyl amine are soluble in water. Both amines are indeed soluble due to their ability to interact with water molecules through hydrogen bonds—dimethyl amine as both donor and acceptor, and trimethyl amine as an acceptor only.

Step 3: Verifying Statement (b) on Hydrogen Bonds

Statement (b) claims that trimethyl amine does not form hydrogen bonds with water. This statement is inaccurate as trimethyl amine can act as a hydrogen bond acceptor, allowing it to form hydrogen bonds with water.

Step 4: Evaluate Statement (c) on Bonding Roles

Statement (c) asserts that trimethyl amine can only act as an acceptor while dimethyl amine can serve as both a donor and acceptor. This statement is true as trimethyl amine lacks the \(\text{NH}\) group required to donate hydrogen bonds.

Step 5: Conclusion about Statements

Since statement (b) is false as trimethyl amine can form hydrogen bonds with water, the correct answer to which statement is false is statement (b). Statement (d) is incorrect because not all three statements are false—only statement (b) is false.

Key Concepts

Solubility of Amines

Amines are organic compounds that often contain one or more nitrogen atoms. The solubility of these compounds in water largely depends on their ability to interact with water molecules via hydrogen bonding. Dimethyl amine and trimethyl amine, both of which are derivatives of ammonia, display distinct solubility characteristics.
Both dimethyl amine ( (CH_3)_2NH) and trimethyl amine ( (CH_3)_3N) are soluble in water. This solubility arises due to their potential to form interactions with water molecules. While dimethyl amine can act as both a hydrogen bond donor and acceptor, trimethyl amine, lacking an NH group, serves only as a hydrogen bond acceptor.
The partial positive charge on the hydrogen of water can be attracted to the electron-rich nitrogen in both amines, facilitating their solubility in water. Hence, although trimethyl amine cannot donate hydrogen bonds, it still interacts well with water thanks to its acceptor capabilities, ensuring its solubility.

Hydrogen Bond Donors and Acceptors

The concept of hydrogen bonding is central to understanding the interaction of amines with water and other molecules. Hydrogen bonds form when there is an attraction between a hydrogen atom attached to a more electronegative atom, such as nitrogen, and another electronegative atom.
In the case of dimethyl amine, it has an NH group, allowing it to provided hydrogen atoms for bonding, acting as both a hydrogen bond donor and acceptor. This characteristic enables dimethyl amine to form robust hydrogen bonds with other molecules, including water.
On the other hand, trimethyl amine lacks an NH group, which restricts its role to that of a hydrogen bond acceptor only. It cannot donate hydrogen atoms for bonding since all its hydrogen atoms are bonded to carbon, which is less electronegative. However, trimethyl amine can still form hydrogen bonds by allowing water molecules or other donors to interact with its nitrogen atom.

Properties of Dimethyl and Trimethyl Amines

Dimethyl amine and trimethyl amine, despite having similar structural names, have differing chemical properties primarily due to the number of hydrogen atoms bonded to their nitrogen.
**Dimethyl Amine**: This compound ( (CH_3)_2NH) is characterized by its nitrogen atom being bonded to two methyl groups and one hydrogen atom. This allows it to:

• Act as a base due to the lone pair of electrons on nitrogen.
• Serve as both a hydrogen bond donor and acceptor, enabling the formation of strong hydrogen bonds with water molecules.
• Exhibit higher solubility in water compared to some other amines.

**Trimethyl Amine**: Trimethyl amine ( (CH_3)_3N) , in comparison, has its nitrogen atom bonded to three methyl groups, meaning:

• It cannot act as a hydrogen bond donor, limiting its interaction in hydrogen bond formation.
• Only serves as a hydrogen bond acceptor, due to the absence of an NH group.
• Maintains solubility in water by acting as an acceptor and interacting through its nitrogen atom.

These molecular properties profoundly influence how each amine reacts and interacts with their environment, particularly concerning solubility and bonding behavior.