2. Arrange the following compounds in increasing order of C-OH bond length: Methanol, phenol, p 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. 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). Options test resonance hierarchy understanding. a. Phenol < p-ethoxyphenol < methanol: Correct – maximum resonance (phenol) to minimum (methanol). b. Phenol < methanol < p-ethoxyphenol: Wrong – ignores ethoxy’s resonance-reducing effect on phenolic OH. c. p-ethoxyphenol < methanol < phenol: Wrong – p-ethoxyphenol cannot have shorter bond than phenol. d. Methanol < phenol < p-ethoxyphenol: Wrong – reverses actual resonance order. This resonance competition pattern is key for CSIR NET organic chemistry bond length questions.
ethoxyphenol
a. PhenolResonance Effects Analysis
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
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
