Question

Furan, \(\mathrm{C}_{4} \mathrm{H}_{4} \mathrm{O},\) is a substance derivable from oat hulls, corn cobs, and other cellulosic waste. It is a starting material for the synthesis of other chemicals used as pharmaceuticals and herbicides. The furan molecule is planar and the \(\mathrm{C}\) and \(\mathrm{O}\) atoms are bonded into a fivemembered pentagonal ring. The H atoms are attached to the C atoms. The chemical behavior of the molecule suggests that it is a resonance hybrid of several contributing structures. These structures show that the double bond character is associated with the entire ring in the form of a \(\pi\) electron cloud. (a) Draw Lewis structures for the several contributing structures to the resonance hybrid mentioned above. (b) Draw orbital diagrams to show the orbitals that are involved in the \(\sigma\) and \(\pi\) bonding in furan. [Hint: You need use only one of the contributing structures, such as the one with no formal charges.] (c) How many \(\pi\) electrons are there in the furan molecule? Show that this number of \(\pi\) electrons is the same, regardless of the contributing structure you use for this assessment.

Short answer

There are five Lewis structures for furan, all contributing to the resonance hybrid. sigma and pi bonding in Furan involves hybridized 2s and 2p orbitals. Furan molecule contains 10 pi electrons which is the same for any of the contributing structures.

Step-by-step solution

Lewis Structures

Furan follows what is known as the Huckel's rule (4n+2 rule) which indicates that the cyclic planar molecule is aromatic. Accordingly, the resonance structures for Furan can be drawn as follows:\[ \text{Structure 1: O=C-C=C-C-H \to H-C=C-C=C-O \] \[ \text{Structure 2: O=C-C=C-C-H \to C=C-O-C=C-H \] \[ \text{Structure 3: O=C-C=C-C-H \to C=C-C=C-O-H \] \[ \text{Structure 4: O=C-C=C-C-H \to C=C-C-O-C=C-H \] \[ \text{Structure 5: O=C-C=C-C-H \to C-C=C-C=C-O \] All these 5 structures contribute to the resonance hybrid.

Orbital Diagrams for sigma and pi bonding in Furan

For each carbon atom in furan, the 2s orbital and two of the 2p orbitals are hybridized, forming three planar sp2 hybrid orbitals that participate in sigma (σ) bonding with hydrogen and carbon atoms. The remaining 2p orbital is perpendicular to the plane of these three orbitals and participates in pi (π) bonding. For the oxygen atom, same process is followed except there's no bonding with hydrogen but with two carbon atoms. As a result it forms two single bonds and one double bond, with the double bond involving a π orbital.

Number of pi Electrons in Furan

In Furan molecule, there are two electrons each in the five C-O π bonds, and that makes a total of 10 π electrons. This is consistent for all the contributing structures.

Key concepts

Resonance Structures

Resonance structures are essential concepts in understanding the chemical behavior of certain molecules, particularly in aromatic compounds like furan. In resonance, we depict a molecule using multiple structures that differ only in the positioning of electrons, not atoms. This helps in visualizing the delocalization of electrons across the entire molecule.
These structures are fictional representations rather than exact configurations but together form a resonance hybrid – the true nature of the molecule. The resonance hybrid is typically more stable than any single structure. In furan, the resonance contributors reveal that the pi (\(\pi\)) electrons are spread over the entire ring, indicating a shared double-bond character throughout.

Aromatic Compounds

Aromatic compounds are a special class of cyclic, planar molecules characterized by substantial stability due to resonance energy and delocalized \(\pi\) electrons. They follow Huckel's rule (4n + 2 \(\pi\) electrons), which furan exemplifies with its ten \(\pi\) electrons.
These are the criteria for aromaticity:

• The molecule is cyclic and planar.
• The \(\pi\) electrons must be delocalized over the cyclic structure.
• It must have (4n + 2) \(\pi\) electrons, where n is a non-negative integer.

This complex arrangement results in the molecule having a unique electronic configuration contributing to its chemical properties, such as low reactivity in typical addition reactions and higher resonance energy.

Lewis Structures

Lewis structures are visual representations that show the arrangement of atoms, electrons, and some bonds within molecules. In furan, Lewis structures help to illustrate the distribution and connectivity of atoms within the compound’s cyclic structure. They are crucial in depicting all potential resonance contributors.
When drawing Lewis structures for furan, we represent all five resonance forms, highlighting the shifting of double bonds and lone pairs across the oxygen and carbon atoms. However, bear in mind that these are only conventional illustrations to show electron placement, and do not individually depict the real electron distribution in the hybrid.

Orbital Diagrams

Orbital diagrams are utilized to show the distribution of electrons within an atom's orbitals and depict the interaction of orbitals during bond formation. For furan, understanding orbital configurations offers insights into how atoms participate in \(\sigma\) and \(\pi\) bonding.
In furan, each carbon atom uses its 2s and two 2p orbitals to form three sp² hybrid orbitals involved in \(\sigma\) bonding with hydrogens and neighboring carbons. The remaining unhybridized 2p orbitals are oriented perpendicularly, contributing to \(\pi\) bonds via the overlap, essential for the aromatic delocalization within the ring. Oxygen shares a similar orbital arrangement, engaging in both \(\sigma\) and \(\pi\) bonding, forming a stable ring configuration.