Animated Solution for Chemistry - Organic Chemistry: Arrange the following compounds in increasing order of $C-OH$ bond length : methanol, phenol, p-ethoxyphenol

Bond Order and Bond Length

When comparing the lengths of chemical bonds, the most crucial factor to consider is the bond order. A pure single bond is the longest, a double bond is shorter, and a triple bond is the shortest.

However, in organic chemistry, resonance often blurs these lines. When lone pairs of electrons delocalize into a conjugated system (like a benzene ring), a single bond can acquire partial double bond character. This delocalization pulls the atoms closer together, effectively shortening the bond length. Let's apply this principle to methanol, phenol, and p-ethoxyphenol.

The Baseline

Let's start with methanol ($C{H}_3-OH$). In this aliphatic alcohol, the oxygen atom is attached to an $s{p}^3$ hybridized carbon. The lone pairs on the oxygen atom are localized; they have no adjacent $\pi$ system to interact with.

Because there is absolutely no resonance, the $C-O$ bond in methanol is a pure single bond. Consequently, it has the lowest bond order and the maximum bond length among the three compounds.

The Power of Resonance

Now, consider phenol ($C_6{H}_5-OH$). Here, the oxygen atom is directly attached to an $s{p}^2$ hybridized carbon of the benzene ring. The lone pairs on the oxygen atom are in perfect alignment to conjugate with the $\pi$ electron cloud of the ring.

Through the $+R$ effect (positive resonance effect), the oxygen donates electron density into the ring. This delocalization creates a partial double bond character between the carbon and oxygen atoms. Because a partial double bond is stronger and shorter than a pure single bond, the $C-O$ bond in phenol is significantly shorter than that in methanol.

The Tug of War

Finally, let's examine p-ethoxyphenol. This molecule has an $-OH$ group at one end of the benzene ring and an ethoxy group ($-O{C}_2{H}_5$) at the para position.

Just like in phenol, the $-OH$ group wants to donate its lone pair into the ring via the $+R$ effect. However, the $-O{C}_2{H}_5$ group also possesses lone pairs on its oxygen atom and exerts its own strong $+R$ effect. Because these two electron-donating groups are para to each other, they end up pumping electron density into the same positions within the ring.

This creates a scenario akin to cross-conjugation. The $+R$ effect of the ethoxy group opposes and diminishes the extent to which the $-OH$ group can delocalize its lone pair. As a result, the $C-O$ bond of the hydroxyl group in p-ethoxyphenol has less partial double bond character compared to phenol.

Less double bond character translates to a slightly longer bond. Therefore, the $C-O$ bond in p-ethoxyphenol is longer than in phenol, but it is still shorter than the pure single bond in methanol.

The correct increasing order of $C-OH$ bond length is: Phenol < p-ethoxyphenol < Methanol