Structure and bonding of Aldehydes
Chemically, an aldehyde is a compound containing a functional group with the structure −CHO, consisting of a carbonyl centre (a carbon double-bonded to oxygen) with the carbon atom also bonded to hydrogen and to any generic alkyl or side chain R group. The functional group itself (i.e. without the "R" side chain) is known as an aldehyde or formyl group.
Aldehydes feature a Sp2-hybridized, planar carbon centre that is connected by a double bond to oxygen and a single bond to hydrogen. The C–H bond is not ordinarily acidic. Because of resonance stabilization of the conjugate base, an α-hydrogen in an aldehyde (not shown in the picture above) is far more acidic, with a pKa near 17, compared to the acidity of a typical alkane (pKa about 50). This acidification is attributed to (i) the electron-withdrawing quality of the formyl center and (ii) the fact that the conjugate base, an enolate anion, delocalizes its negative charge. Related to (i), the aldehyde group is somewhat polar. The formyl proton itself does not readily undergo deprotonation. The anionic species formally derived from deprotonation of an aldehyde proton, known as an acyl anion, is highly unstable and must be kept at low temperatures. In fact, with the exception of certain hindered dialkylformamides, the synthesis of acyl anions by direct deprotonation is not a feasible route, since the deprotonated species will almost immediately add to the highly reactive carbonyl of the starting material to form an acyloin compound. For this reason, the acidity of the formyl proton is difficult to measure. In the case of HCONiPr2, the acidity of the formyl group was found to be very close to that of diisopropylamine (pKa~36). The gas-phase acidity of aldehyde was found to be 1,640 kJ/mol (393 kcal/mol), making it more acidic than hydrogen (1,700 kJ/mol, 400 kcal/mol) and ammonia (1,680 kJ/mol, 402 kcal/mol), but less acidic than water (1,600 kJ/mol, 390 kcal/mol) in the gas phase.
Aldehydes (except those without an alpha carbon, or without protons on the alpha carbon, such as formaldehyde and benzaldehyde) can exist in either the keto or the enol tautomer. Keto–enol tautomerism is catalyzed by either acid or base. Usually the enol is the minority tautomer, but it is more reactive. At around 360 kJ/mol (85 kcal/mol), the formyl C-H bond is weaker than that of a typical bond between hydrogen and a sp2-hybridized carbon. Thus aldehydes are prone to undergo hydrogen-atom abstraction in the presence of free radicals, a fact accounts for the ease with which aldehydes undergo autoxidation.
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