2. Arrange the following compounds in increasing order of C-OH bond length: Methanol, phenol, p
ethoxyphenol
a. Phenol
The correct increasing order of C-OH bond length is phenol < p-ethoxyphenol < methanol (option a), as phenol’s resonance gives maximum C-OH shortening, p-ethoxyphenol shows reduced resonance due to competing +M effect of ethoxy group, and methanol lacks resonance entirely.
Resonance Effects Analysis
Bond length decreases with partial double bond character from resonance.
Phenol: Oxygen lone pair delocalizes fully into benzene ring, creating significant C=O partial double bond character (shortest C-OH ~136 pm vs. methanol’s longer bond).
p-Ethoxyphenol: Para-ethoxy (-OCH₂CH₃) group donates electrons (+M effect), competing with -OH resonance and reducing C-OH double bond character, lengthening bond relative to phenol.
Methanol: Pure sp³ C-O single bond with no resonance (longest C-OH bond).
Option Analysis
Options test resonance hierarchy understanding.
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a. Phenol < p-ethoxyphenol < methanol: Correct – maximum resonance (phenol) to minimum (methanol).
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b. Phenol < methanol < p-ethoxyphenol: Wrong – ignores ethoxy’s resonance-reducing effect on phenolic OH.
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c. p-ethoxyphenol < methanol < phenol: Wrong – p-ethoxyphenol cannot have shorter bond than phenol.
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d. Methanol < phenol < p-ethoxyphenol: Wrong – reverses actual resonance order.
Bond Length Order Table
| Compound | Resonance Effect | C-OH Bond Length | Order |
|---|---|---|---|
| Phenol | Full delocalization into ring | Shortest (~136 pm) | 1st |
| p-Ethoxyphenol | Reduced by competing +M ethoxy | Intermediate | 2nd |
| Methanol | None (sp³ single bond) | Longest | 3rd |
