Question

Many of the beautiful colors of butterflies' wings, autumn leaves, and flowers are due to the presence of flavone derivatives called anthocyanins. When anthocyanins are hydrolyzed in hydrochloric acid (to remove sugar residues), they produce aromatic oxygen-containing salts. One of these is delphinidin chloride. Explain why delphinidin chloride is aromatic. What is the hybridization of the heterocyclic oxygen in this compound?

Short answer

Delphinidin chloride is aromatic because it meets all four criteria of aromaticity: it is cyclic, planar, contains 10 π-electrons (which fits the 4n+2 rule with n=2), and all atoms in the ring system are conjugated. The heterocyclic oxygen in delphinidin chloride is sp2 hybridized as it forms a double bond and a single bond within the conjugated ring system.

Step-by-step solution

Determine if delphinidin chloride is cyclic

Delphinidin chloride, with the chemical formula C_15H_11O_6Cl, contains a cyclic structure with double bonds and a heterocyclic oxygen atom. This satisfies the first criterion for aromaticity.

Determine if delphinidin chloride is planar

In delphinidin chloride, all the carbon and oxygen atoms in the ring are sp2 hybridized, which means they all have planar geometry. This satisfies the second criterion for aromaticity.

Determine if delphinidin chloride contains (4n + 2) π-electrons

Counting the number of π-electrons in the delphinidin chloride cyclic structure, there are a total of 10 π-electrons, which can be represented as 4n + 2 when n=2. This satisfies the third criterion for aromaticity.

Check for conjugation

All atoms in the ring system have p-orbitals overlapping by a conjugation system allowing for electron delocalization. This satisfies the fourth criterion for aromaticity. Having satisfied all four criteria of aromaticity, it can be concluded that delphinidin chloride is aromatic.

Determine the hybridization of the heterocyclic oxygen

In delphinidin chloride, the heterocyclic oxygen is involved in the conjugated ring system and forms a double bond as well as a single bond. Thus, the heterocyclic oxygen is sp2 hybridized. In conclusion, delphinidin chloride is an aromatic compound, and the heterocyclic oxygen in this compound is sp2 hybridized.

Key concepts

Hybridization

In understanding the concept of hybridization, we look at how atomic orbitals mix to form new hybrid orbitals suitable for pairing electrons to form chemical bonds. For example, in delphinidin chloride, the heterocyclic oxygen is part of a ring structure and is involved in bonding that requires overlapping of p-orbitals with adjacent atoms. This type of hybridization is known as sp2 hybridization.
This specific hybridization means:

• The oxygen atom forms one double bond and one single bond in the ring system.
• It involves the mixing of one s-orbital and two p-orbitals from the oxygen atom.
• This configuration results in three sp2 hybrid orbitals, which align in a trigonal planar shape, ideal for conjugation with the aromatic ring.

This hybridization is crucial for allowing the ring to maintain its aromatic properties, as the planar structure permits effective overlap of p-orbitals.

Aromaticity Criteria

Aromatic compounds, like delphinidin chloride, exhibit unique stability due to their structure and electron distribution. To be classified as aromatic, a compound must satisfy several criteria:

• Cyclic Structure: The compound should form a closed, ring-shaped cyclic structure.
• Planarity: All atoms in the ring must be perfectly aligned in one plane. In delphinidin chloride, the sp2 hybridization ensures that the atoms adopt a planar structure.
• (4n + 2) π-Electrons Rule: The ring system must house a total of 4n + 2 π-electrons. For delphinidin chloride, this means having 10 π-electrons, adhering to this rule with n being 2.
• Conjugation: There should be a continuous overlap of p-orbitals, creating a fully conjugated system.

Meeting all these criteria confirms the aromatic nature of delphinidin chloride, thereby explaining its unique stability and chemical properties.

Conjugation

Conjugation refers to a system where electrons can move freely across adjacent overlapping p-orbitals, enabling electron delocalization. This is critical in aromatic compounds, including delphinidin chloride, where conjugation stabilizes the molecular structure.
Specific features of conjugation include:

• Electron Delocalization: Electrons are not confined to one pair of atoms but can move across the entire ring.
• P-Orbital Overlap: Continuous overlapping of p-orbitals across the ring provides the necessary path for electron movement.
• Planarity: A planar geometry is required to maintain effective p-orbital overlap.

For delphinidin chloride, each atom contributes to the conjugated system, thus facilitating a stable aromatic structure. The essence of conjugation in delphinidin chloride is seen in its aromatic character, contributing to the chemical properties observed in its fascinating structure.