Problem 36 - Chemistry The Molecular Nature of Matter

Question

Hydrazine is a Brønsted base but urea does not exhibit basic properties. Offer an explanation.

Step-by-Step Solution

Answer

Hydrazine has nitrogen atoms with available lone pairs to accept protons and exhibit basic properties, while urea's nitrogen lone pairs are involved in resonance with the carbonyl group, lessening its ability to accept protons and thus not exhibiting basic properties.

Step 1: Understanding Brønsted base concepts

Acknowledge that Brønsted bases are substances which can accept protons (H+ ions). In this context, it is important to understand that a molecule's ability to act as a Brønsted base is often associated with the presence of lone pair electrons that can be donated to a proton.

Step 2: Structure evaluation of hydrazine

Examine the molecular structure of hydrazine (N2H4). Note that the nitrogen atoms in hydrazine have lone pairs that are not involved in bonding, which makes these nitrogen atoms potential sites for accepting protons, hence displaying basic properties.

Step 3: Structure evaluation of urea

Analyze the molecular structure of urea (NH2CONH2). Although urea contains nitrogen atoms with lone pairs, these pairs are involved in resonance with the carbonyl group (C=O) which delocalizes the lone pair electrons, making them less available for bonding with protons. This delocalization due to resonance diminishes the basic properties of the nitrogen atoms in urea.

Step 4: Comparing hydrazine and urea

Compare the ability of the two substances to accept protons. Hydrazine can act as a Brønsted base because its structure allows the nitrogen's lone pairs to easily accept protons. In contrast, urea's nitrogen's lone pairs are less available due to resonance with the carbonyl group, which decreases its ability to act as a Brønsted base.

Key Concepts

Proton Acceptance in Bases

In chemistry, a Brønsted base is recognized by its capability to accept protons (H+ ions). This process is fundamental to acid-base chemistry and hinges on the presence of lone pair electrons. These electrons are not involved in bonding and can be shared with hydrogen ions during the process of proton acceptance.

When a molecule acts as a Brønsted base, it temporarily gains a hydrogen atom, forming a bond by donating its lone pair to the proton. The strength of a base therefore heavily relies on the availability of these lone pairs. As the lone pairs are more readily available for bonding, the molecule's basicity increases. This explains why certain molecules with apparent lone pairs might not exhibit basic properties if these pairs are less accessible for forming bonds with protons.

Hydrazine Structure

Hydrazine (N₂H₄), often used as a rocket fuel, is a clear example of molecule structure impacting basicity. It consists of two nitrogen atoms bonded together, each with a pair of hydrogen atoms. A key feature in the structure of hydrazine is the lone pairs on the nitrogen atoms. These lone pairs remain uninvolved in any bonding and are effectively 'free' to accept protons, thus endowing hydrazine with its basic characteristics. The straightforward, symmetrical arrangement of its molecule aids in the accessibility of these lone pairs for accepting protons, making hydrazine a good Brønsted base.

Urea and Resonance Effect

Urea (NH₂CONH₂), a common component in fertilizers and animal feed, also features nitrogen atoms with lone pairs. However, unlike hydrazine, the nitrogen's lone pairs in urea participate in a resonance effect with the carbonyl (C=O) group.

This resonance creates a delocalization of the lone pair electrons over the entire molecule, making them less accessible for proton bonding. Because the lone pairs are essentially 'shared' with other atoms in the molecule, they cannot readily bond to a proton. Thus, despite having nitrogen atoms equipped with lone pairs, urea is not significantly basic due to this resonance effect that impacts electron availability.

Molecular Structure Impact on Basicity

The basicity of a molecule is not solely determined by the presence of nitrogen atoms or lone pairs; it's profoundly influenced by the molecule's overall structure. The positioning of atoms within a molecule and the presence of resonance structures affect the distribution of electrons and, hence, their availability for proton acceptance.

Hydrazine’s nitrogen atoms have ready-to-bond lone pairs because of the simple, symmetrical structure lacking resonance, while urea’s basicity is impaired by the resonance effect which ties up the nitrogen's lone pairs. In summary, a molecule with a structure that allows its lone pairs to remain local and unshared will generally have greater basicity than one with a complex structure where electron density is distributed through resonance or other electronic effects.