Chemistry · Ch 12 — Aldehydes, Ketones and Carboxylic Acids
Nature of intermolecular forces
Nature of intermolecular forces
The carbonyl bond, C=O, present in both aldehydes and ketones, is a POLAR covalent bond (oxygen being considerably more electronegative than carbon). As a direct result, aldehyde and ketone molecules experience DIPOLE-DIPOLE forces of attraction between one another: neighbouring carbonyl-containing molecules orient themselves so that their oppositely-polarised atoms face each other -- the partially-positive carbonyl carbon of one molecule lining up opposite the partially-negative carbonyl oxygen of its neighbour (Fig. 12.1). This dipole-dipole attraction is the starting point for understanding the boiling-point trends worked out in the next two sections, and it is later explicitly ranked (in 12.6.4) as intermediate in strength -- weaker than hydrog …
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What this figure shows. Two carbonyl (C=O) groups drawn facing each other, each carbon marked delta-plus and each oxygen marked delta-minus, oriented so the delta-plus carbon of one molecule faces the delta-minus oxygen of the neighbouring molecule -- illustrating how oppositely-polarised ends of neighbouring polar carbonyl groups line up to attract one another (dipole-dipole intermolecular attraction), the weakest of the three intermolecular forces discu …