D. A. Evans The Anomeric Effect: Carbonyl Groups Chem 206 Do the following valence bond resonance structures have meaning? hyde C-H Infrared Stretching Frequencies Prediction: The IR C-H stretching frequency for aldehydes is lower than the closely related olefin C-H stretching frequency For years this observation has gone unexplained R Prediction: As X becomes more electronegative, the IR frequency should increase VC-H=2730 cm-1 vC-H= 3050 cm CH3 Me CBr3 MeCF Sigma conjugation of the lone pair anti to the H will weaken the bond UC=o(cm1)1720 1750 This will result in a low frequency shift 1780 Infrared evidence for lone pair delocalization into vicinal antibonding orbitals The N-H stretching frequency of cis-methyl diazene is 200 cm"lower Prediction: As the indicated pi-bonding increases, the X-C-0 than the trans isomer bond angle should decrease. This distortion improves overlap antibonding N-H=2188cm1 antibond d*CX→ o lone pair Evidence for this distortion has been obtained by X-ray crystallography , filled vN-H=2317 cm-1 Corey Tetrahedron Lett. 1992, 33, 7103-7106 a The low-frequency shift of the cis isomer is a result of N-H bond weakening due to the anti lone pair on the adjacent(vicinal) nitrogen which is interacting with the N-H antibonding orbital. Note that the orbital overlap is not nearly as good from the trans isomer Craig co-workers JACS 1979, 101, 2480.D. A. Evans The Anomeric Effect: Carbonyl Groups Chem 206 Do the following valence bond resonance structures have meaning? n C–H = 3050 cm -1 n C–H = 2730 cm -1 Aldehyde C–H Infrared Stretching Frequencies Prediction: The IR C–H stretching frequency for aldehydes is lower than the closely related olefin C–H stretching frequency. For years this observation has gone unexplained. C H C R O H C R R R ●● ●● C R O X ●● ●● C R O X ●● ●● – + Prediction: As X becomes more electronegative, the IR frequency should increase uC=O (cm 1720 1750 1780 -1) Me CH3 O Me CBr3 O Me CF3 O Prediction: As the indicated pi-bonding increases, the X–C–O bond angle should decrease. This distortion improves overlap. C R O X ●● s* C–X ®O lone pair C R O X ●● Evidence for this distortion has been obtained by X-ray crystallography Corey, Tetrahedron Lett. 1992, 33, 7103-7106 Sigma conjugation of the lone pair anti to the H will weaken the bond. This will result in a low frequency shift. filled N-SP2 Infrared evidence for lone pair delocalization into vicinal antibonding orbitals. n N–H = 2188 cm -1 n N–H = 2317 cm -1 filled N-SP2 antibonding s* N–H .. antibonding s* N–H The N–H stretching frequency of cis-methyl diazene is 200 cm-1 lower than the trans isomer. N N Me H N H N Me N N Me N N Me ●● ●● ●● ●● ■ The low-frequency shift of the cis isomer is a result of N–H bond weakening due to the anti lone pair on the adjacent (vicinal) nitrogen which is interacting with the N–H antibonding orbital. Note that the orbital overlap is not nearly as good from the trans isomer. N. C. Craig & co-workers JACS 1979, 101, 2480. H H