Rank the compounds below in order of increasing C=O absorption frequency in IR spectrum (top - lowest wavenumber, bottom - highest wavenumber): enter image description here

Question: Would I count compound #1's cyclopentane or all 11 carbons because wavenumber increases with 5C or smaller rings.

If I count #1 as a cyclopentane for this purpose then #3 has the lowest wavenumber and #2 has the highest.


2 Answers 2


Here is nice comparison of all 3 ring sizes. Octahydro-1H-inden-1-one (either cis- or trans-) carbonyl strechings fall between cyclohexanone and cyclobutanone, thus, fused ring is not a factor (it is to some extend because of sunstitution).

IR image


#1 Octahydro-1H-inden-1-one C=O stretch from reference

cis 1730 $\text{cm}^{-1}$
enter image description here

trans 1754 $\text{cm}^{-1}$
enter image description here

#2 - Cyclobutanone NIST Listing C=O stretch at 1790 $\text{cm}^{-1}$ from spectra

#3 - Cyclohexanone NIST Listing C=O stretch at 1740 $\text{cm}^{-1}$ from spectra

  • $\begingroup$ #1 doesn't have stereochemistry here @MaxW $\endgroup$
    – user41987
    Commented Mar 12, 2017 at 21:56
  • 2
    $\begingroup$ interesting, but unhelpful and unpedagogic. $\endgroup$
    – mykhal
    Commented May 11, 2017 at 20:16

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