Question

Rank the polarity of the following compounds: ferrocenecarboxylic acid, acetylferrocene, ferrocenecarboxaldehyde, ferrocene, (dimethylaminomethyl) ferrocene, and ferrocenemethanol and give a detailed explanation why.

Answer 1

Ferrocene< dimethylaminomethyl) ferrocene< acetylferrocene, <ferrocenecarboxaldehyde< ferrocenemethanol< ferrocenecarboxylic acid

(η5-C5H5)2Fe is organometallic complex with each cyclopentadienyl (Cp) ring pie- bonded to the Fe atom , that is, the five C atoms of each Cp aromatic ring are equidistant to the Fe atom with the Fe atom lying between the two Cp rings.The pie-electrons are dispersed due to ring resonance.The primary reason of non-polarity is the rotation of the Cp rings about the Fe atoms being immeasurably fast. The molecule is symmetric and consequently nonpolar.

Dimethyl amino methyl ferrocene has amino N atom attached to C atom of methylene CH2 and CH3.As the electronegativity difference between C and N is less than that between O and H or C and O so the bond between N and C is less polar consequently.

[More electronegative atoms pulls electronic cloud between the atoms more strongly ,hence more strong inductive effect results in development of high partial charges on the atoms.Electronegative atom acquires partial negative charge ,while less electronegative atom acquires partial positive charge.]

Acetyl ferrocene has polar CO group but less polar than OH (due to less electronegativity differences between atoms) while most polar ferrocenecarboxylic acid has both Co and OH group with CO group strongly pulling the electron density from O of OH group.As a result H+ is loosely attached and is acidic in nature.Ionic character of COO- makes it most polar.

Ferrocenecarboxaldehyde has CO group but acetylferrocene has CO-CH3 group with CH3 group pushing electron towards C of CO thereby reducing polarity of CO