Question

(a) What contributing structure(s) would account for the double-bond character of the carbon-nitrogen bond in amides? (b) What does this resonance mean in terms of orbitals?

Short answer

The double-bond character of the carbon-nitrogen bond in amides arises from resonance between two contributing structures: (i) nitrogen with a lone pair and a carbon-oxygen double bond, and (ii) nitrogen double-bonded to carbon, and oxygen with a negative charge and a lone pair. In terms of orbitals, this resonance involves the overlap of the non-bonding orbital on nitrogen with the pi* (antibonding) orbital of the carbon-oxygen double bond, leading to electron delocalization and a shortened, strengthened carbon-nitrogen bond. \[ \mathrm{ R-C(=O)-NH_2 \rightleftharpoons R-C(-O^-)-N(=)H_2 } \]

Step-by-step solution

(a) Contributing structures for amides

Amides contain a carbon-nitrogen bond that has a double-bond character due to the resonance between the amide's lone pair of electrons on nitrogen and the carbonyl group's pi bond. The contributing structures that account for this double-bond character are: (i) The nitrogen atom has a lone pair of electrons, and the carbon atom is double-bonded to the oxygen atom. (ii) The nitrogen atom is double-bonded to the carbon atom, and the oxygen atom has a negative charge and a lone pair of electrons. We can represent these contributing structures using resonance arrows as follows: \[ \mathrm{ R-C(=O)-NH_2 \rightleftharpoons R-C(-O^-)-N(=)H_2 } \] where R represents the rest of the molecule.

(b) Resonance in terms of orbitals

Resonance involves the delocalization of electrons within certain molecules, and in terms of orbitals, this could be explained as the overlap between the atomic orbitals of the atoms involved. In the case of amides: 1. A lone pair (non-bonding) orbital on the nitrogen atom overlaps with the pi* (antibonding) orbital of the carbon-oxygen double bond, which is in a higher energy level. 2. This overlap results in the delocalization of the lone pair of electrons on the nitrogen, creating a partial pi bond between the carbon and nitrogen atoms. 3. Simultaneously, the pi bond between carbon and oxygen weakens due to the presence of the new pi bond between carbon and nitrogen, making it more like a single bond. 4. This combination of electron delocalization and weakened pi bonds gives the carbon-nitrogen bond its double-bond character, making it shorter and stronger than a typical single bond between carbon and nitrogen. In summary, the resonance in amides results from the overlap of atomic orbitals on the nitrogen, carbon, and oxygen atoms, leading to the delocalization of electrons and the double-bond character observed in the carbon-nitrogen bond.

Key concepts

Contributing Structures

In the context of amides, contributing structures play a crucial role in explaining how the carbon-nitrogen bond in these compounds gains a partial double-bond character. This phenomenon results from resonance, an important concept in organic chemistry.
When we look at amides, we find two primary resonance structures that contribute to this double-bond character:

• In the first structure, the nitrogen has a lone pair of electrons, while the carbon remains double-bonded to oxygen.
• In the second structure, the nitrogen forms a double bond with carbon, causing oxygen to carry a negative charge along with a lone pair.

These resonance structures can be represented using arrows that indicate the electron movement from one molecule part to another, contributing equally to the overall structure. The mix creates a hybrid structure where the carbon-nitrogen bond shows characteristics of both single and double bonds.

Delocalization of Electrons

Delocalization of electrons refers to the spreading of electron density across multiple atoms. This concept is pivotal in understanding resonance in amides.
In amides, electrons are not restricted to one atom; instead, they are shared between nitrogen, carbon, and oxygen. This allows these atoms to stabilize the molecule more effectively:

• The nitrogen's lone pair delocalizes, participating in forming a pi bond between carbon and nitrogen.
• This movement means electron distribution is more evenly spread across the molecule, enhancing stability.

This delocalization reduces the energy of the system, leading to greater stability and shorter, stronger bonds in the carbon-nitrogen linkage. This distribution of electron density is essential for explaining the properties of amide molecules.

Atomic Orbitals Overlap

The overlap of atomic orbitals is a fundamental concept explaining the resonance in amides. It is through this overlap that electrons can be shared across different atoms.
In amides, we see specific overlaps contributing to their overall resonant structure:

• The lone pair on nitrogen overlaps with the pi* orbital of the carbon-oxygen bond, indicating a higher energy level.
• This overlapping allows the formation of a partial pi bond between carbon and nitrogen, simultaneously weakening the carbon-oxygen bond.

The idea here is about achieving an energy-efficient state through proper overlap, where the electron pair on nitrogen can bond with carbon. Thus, atomic orbital overlap provides a pathway for electron delocalization and is key to understanding the bonding characteristics in amides.

Carbon-Nitrogen Double Bond Character

The carbon-nitrogen double-bond character in amides is an intriguing feature resulting from resonance. This character gives rise to specific bonding and structural attributes.
The partial double-bond nature in the carbon-nitrogen linkage arises from:

• The delocalization of electrons from nitrogen across to carbon, forming a bond with more strength and decreased length than a regular single bond.
• Simultaneous weakening of the carbon-oxygen double bond as its pi character shifts partially to the carbon-nitrogen bond.

This unique bond setup is what makes amides interesting. The partial double-bond character means that the bond is not as rigid as a full double bond, yet stronger than a single bond, explaining the remarkable stability and lower reactivity typical in amide compounds.