Question

RECALL How do the resonance structures of the peptide group contribute to the planar arrangement of this group of atoms?

Short answer

Resonance structures create a partial double bond character, leading to a planar arrangement.

Step-by-step solution

Understand Resonance Structures

Resonance structures are different ways of representing a molecule where the connectivity is the same but the distribution of electrons is different. They help in delocalizing electrons across the molecule.

Examine the Peptide Bond

In a peptide bond, the resonance occurs between the carbonyl oxygen, the carbonyl carbon, and the nitrogen. The lone pair of electrons on the nitrogen can delocalize to form a double bond with the carbon, while the double bond of the carbonyl group can shift to become a lone pair on the oxygen.

Identify Resonance Structures

The primary resonance forms for the peptide bond are: 1. The natural form: N−C=O 2. The resonance form: N=C−O−

Analyze Electron Delocalization

Due to the delocalization of electrons between the nitrogen, carbon, and oxygen, a partial double bond character is established between the carbonyl carbon and nitrogen. This results in restricted rotation around the C—N bond.

Assess Planarity

The partial double bond character forces the peptide bond and associated atoms to adopt a planar configuration. The sp2 hybridization of the carbon and nitrogen atoms also supports this planar arrangement, minimizing repulsion and allowing for resonance stabilization.

Conclusion

The resonance structures contribute to a partial double bond character between C and N, resulting in a planar arrangement of the peptide group due to restricted rotation and sp2 hybridization.

Key concepts

Resonance Structures

Resonance structures are different ways to represent the same molecule. They show various possible distributions of electrons, even though the arrangement of atoms remains the same. This is crucial because real molecules don't always fit neatly into a single structure. Rather, they are often a blend of several resonance forms. This blending or 'resonance' stabilizes the molecule, making it more stable than any single structure alone.

Planar Configuration

A planar configuration means that the atoms making up a molecule lie in the same plane. For peptide bonds, this planarity is very important. Due to the partial double bond character from resonance, rotation around the C—N bond is restricted. This ensures the peptide bond and the atoms connected to it stay in the same flat plane. This configuration minimizes steric hindrance and maximizes stability.

Electron Delocalization

Electron delocalization occurs when electrons are spread over several atoms, rather than confined to a single bond or atom. In peptide bonds, the lone pair of electrons on the nitrogen atom can spread onto the carbon-oxygen bond. This creates a partial double bond. This delocalization decreases the energy and increases the stability of the molecule. It is a key reason why the peptide bond has such unique properties.

sp2 Hybridization

The concept of sp2 hybridization involves the mixing of one s orbital and two p orbitals from the same atom to form three equivalent hybrid orbitals. In the context of peptide bonds, both the carbonyl carbon and nitrogen undergo sp2 hybridization. This results in bond angles of approximately 120 degrees and contributes to the planar arrangement of the molecule. The sp2 hybridization ensures that the electrons are placed in a way that maximizes overlap and stability.