Question

Use resonance structures to explain why urea, \(\left(\mathrm{NH}_{2}\right)_{2}\) \(\mathrm{C}=\mathrm{O},\) is not basic like typical amines. Without using resonance structures, use electronegativities to explain the same fact.

Short answer

Resonance structures show the delocalization of electron density away from the nitrogen atoms, making the lone pairs less available to act as a base. Additionally, the higher electronegativity of oxygen draws electron density towards itself, further decreasing the basicity of urea compared to typical amines.

Step-by-step solution

Draw the Lewis Structure of Urea

First, draw the Lewis structure of urea. It has a central carbon atom double-bonded to an oxygen atom and singly bonded to two nitrogen atoms, each of which has two hydrogen atoms attached.

Identify Possible Resonance Structures

Look for possible resonance structures by considering the movement of electrons. In the case of urea, the lone pair of electrons on the nitrogen can participate in resonance by forming a double bond with the carbon atom, which pushes the double bond between the carbon and oxygen into becoming a lone pair on the oxygen.

Draw Resonance Structures

Draw the resonance structures for urea, where the lone pairs of nitrogen form a double bond with the carbon atom, and the carbon-oxygen double bond becomes a single bond with a lone pair of electrons on oxygen.

Explain the Basicity of Amines

Amines are typically basic due to the presence of a lone pair of electrons on the nitrogen atom, which can be donated to a proton (H+).

Explain the Decreased Basicity of Urea Using Resonance Structures

In urea, the lone pairs on the nitrogen are delocalized in the resonance structures, which means they are less available to bond with a proton. This delocalization of electron density decreases the basicity of urea compared to typical amines.

Explain the Role of Electronegativity

Electronegativity can also explain the decreased basicity. Oxygen is more electronegative than nitrogen and carbon, which means it can draw electrons towards itself more effectively. The carbon to oxygen double bond means the carbon is somewhat positively charged, pulling electron density away from the nitrogen atoms and decreasing their ability to donate a lone pair to a proton.

Key concepts

Understanding Resonance Structures

Resonance structures are a fundamental concept in chemistry that help us understand the delocalization of electrons within a molecule. Delocalized electrons are those that can be shared among more than two atoms; they move within a molecule to generate different but related structures called resonance structures. These are not real structures but rather an averaged picture of all the possibilities.

For instance, urea has resonance structures due to the nitrogen atoms' lone pairs, which can contribute to forming a double bond with the central carbon. This electron shift results in an alternative structure, where the carbon-to-oxygen bond changes. Because of resonance, nitrogen's lone pairs are not as freely available for bonding with protons, which is why urea displays lower basicity compared to typical amines.

Grasping the concept of resonance is key because it provides insights into the stability, reactivity, and electronic distribution of molecules such as urea, affecting their chemical behavior.

The Significance of Lewis Structures

The Lewis structure is a simple representation of the bonding between atoms and the lone pair of electrons in a molecule. It helps visualize how the valence electrons are distributed amongst the atoms. When drawing the Lewis structure of urea, begin by noting that the central carbon atom is double-bonded to an oxygen atom and singly bonded to two nitrogen atoms, each with two hydrogens.

Adding to the Lewis structure are the lone pairs on nitrogen and oxygen, which play a crucial role in the molecule's overall electron distribution and reactivity. Understanding the Lewis structure is essential as it sets the stage for identifying potential resonance structures, revealing how the molecule can redistribute its electrons in different patterns. Moreover, the Lewis structure of urea allows us to observe that due to electron delocalization, the nitrogen atoms are less nucleophilic and hence, urea exhibits less basicity than expected.

Electronegativity and Its Effects on Basicity

Electronegativity is a property that indicates how strongly atoms attract electrons in a bond. In a molecule like urea, oxygen is more electronegative than carbon and nitrogen, which means it pulls shared electrons more towards itself. This creates a partial negative charge on the oxygen and a partial positive charge on the carbon.

As the electrons are pulled towards oxygen, the electron density around the nitrogen atoms decreases, making it less likely for them to donate electrons to form bonds with protons (H+). This effect of electronegativity on electron distribution is another reason why urea's basicity is lower than other amines, which lack this strong electronegative influence.

Understanding electronegativity can help predict molecule behavior - it explains why the available electron pairs on urea's nitrogen are not as ready to engage in bonding, which contributes to urea's unique chemical properties.