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.

Step by step solution

01

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.

02

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.

03

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.

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