Acrolein electron delocalization

Figure 2.10 Schematic picture of ft-electron delocalization in acrolein (CH2=CH-CHO).

B-0 length is extremely short (1.451 A) for a dative bond, approaching the value found for the covalent B-O bond of Na[( / -borabenzene-OMe)Cr(CO)3J (1.400 A). The C=0 and Ccarbonyi-C bonds in 15 have comparable lengths, indicating that the n electrons of 3-(dimethylamino)acrolein are delocalized effectively upon complexation to [( / -borabenzene)Cr(CO)3] (Table 1-8) [29],... 

Acrolein can be used as an example O ig. 13.1). The major structure is evidently stmcture I, and stmcture II is a reasonable stmcture involved in the electronic delocalization because of the oxygen s larger electronegativity compared to carbons. For the same reason, stmcture III has a priori less chemical relevance. The Huckel-Lewis methods can compute the weights of each stmcture and help to determine if a stmcture is important. 

Ketones and Aldehydes The R2C=0 and the RCH=0 carbon atoms absorb in a characteristic region. Acetone absorbs at 203.3 ppm, and acetaldehyde at 199.3 ppm. Alkyl substitution on the a-carbon causes a shift to the left of the C=0 absorption of 2-3 ppm until steric effects supervene. Replacement of the CH3 of acetone or acetaldehyde by a phenyl group causes a shift to the right of the C=0 absorption (acetophenone, 195.7 ppm benzaldehyde. 190.7 ppm) similarly, a,/3-unsaturation causes shifts to the right (acrolein, 192.1 ppm, compared with propionaldehyde, 201.5 ppm). Presumably, charge delocalization by the benzene ring or the double bond makes the carbonyl carbon less electron deficient. 

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