Krische, 310N Lecture 4: Tu - 01/24/08, Ch. 16 We have now completed chapters covering 1H and 13C NMR spectroscopy and IR spectroscopy. We now begin the chapter covering aldehydes and ketones. Structure begets reactivity: C=O double bond is polarized such that the oxygen bears a partial negative charge and the carbon bears a partial positive charge. This naturally follows from the respective electronegativities of carbon and oxygen (the latter being more electronegative). The polarity/dipole of the C=O double bond is reflected by a zwitterionic resonance structure. In accordance with the polarization of the C=O double bond, electrophiles react at oxygen and nucleophiles react at carbon. From the standpoint of mechanism, it is important to recognize that in reactions with electrophiles (Brnsted acids or Lewis acids), oxygen reacts using its lone pairs (not the C=O pi-bond). Based on our understanding of the structure of the carbonyl group, we began to explore its reactivity with oxygen nucleophiles. Specifically, acid catalyzed hydration was discussed and a comparison was made with the reactivity of alkenes under similar conditions. It was shown that protonation activates the carbonyl group as an electrophile, rendering it highly susceptible to nucleophilic addition. This activation is manifested by an increase in the C-O bond length of the carbonyl group upon protonation, i.e. 1.20 angstroms vs. 1.27 angstroms. This is substantial considering a normal C-O single bond has a length of 1.43 angstroms. This lengthening owes to the contribution the of non-octet resonance structure of the protonated carbonyl. We observed that formaldehyde hydrates more readily than acetaldehyde, and that acetaldehyde hydrates more readily than acetone. These effects were rationalized on the basis of steric and electronic
effects (CH3 groups of acetone bump into each other upon hydration, more substituted double bond is more stable). Electron withdrawing groups promote addition of nucleophiles (e.g. H2O) to the carbonyl group. This was illustrated by Keq for hydration of the following aldehydes Cl3CCHO>ClCH2CHO>CH3CHO. Through a mechanism strictly analogous to the acid catalyzed hydration of an aldehyde or ketone, alcohols will condense with aldehydes and ketones to form hemi-acetals and hemi-ketals, respectively. Unless cyclic, hemi-acetals and hemi-ketals are typically unstable and under acidic conditions and will react further to give acetals and ketals. Cyclic hemi-acetals a...