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Hyperconjugation with Lone Electron Pairs

Calculations have shown that the rotational barrier of the C-O bond in methanol (1.1 kcal/mol) is significantly lower than the corresponding rotational barrier of methyl hypofluorite (MeOF, 3.7 kcal/mol) or methyl hypochlorite (MeOCl, 3.5 kcal/mol), in which a strong tc h — o o Hai hyperconjugation is possible [9]. Similarly, in 1,2-dihaloethenes such as 1,2-difluoroethene, l-chloro-2-fluoroethene, or 1,2-dichloroethene the cis isomers are more stable than the corresponding trans isomers [10,11]. [Pg.19]


The weakening of a bonds by negative hyperconjugation with lone electron pairs also reveals itself in IR and NMR spectra. Thus, C-H, N-H, or O-H bonds oriented trans or antiperiplanar to an unshared, vicinal electron pair are weakened and have therefore a significantly reduced IR vibrational frequency [17]. The C-H vibrational frequency in aldehydes is, for example, lower than that in alkenes (Scheme 2.7). Polycyclic amines with at least two hydrogen atoms antiperiplanar to the lone pair on nitrogen have characteristic absorption bands at 2800-2700 cm-1 which have been used to infer the relative configuration of such amines [18]. [Pg.21]

If an electron pair is present on metal, its direction should be such that a negative hyperconjugation of this lone pair with the a (Si-X) antibonding... [Pg.291]

The interaction of the lone-pair electrons on an amine nitrogen with adjacent C—H bonds is another example of a hyperconjugative effect that can be described in MO language. The lone-pair electrons, when properly aligned with the C—H bond, lead to a... [Pg.56]


See other pages where Hyperconjugation with Lone Electron Pairs is mentioned: [Pg.19]    [Pg.19]    [Pg.21]    [Pg.19]    [Pg.19]    [Pg.21]    [Pg.284]    [Pg.17]    [Pg.19]    [Pg.22]    [Pg.156]    [Pg.164]    [Pg.180]    [Pg.186]    [Pg.270]    [Pg.286]    [Pg.318]    [Pg.144]    [Pg.377]    [Pg.219]    [Pg.252]    [Pg.267]    [Pg.189]    [Pg.290]    [Pg.65]    [Pg.11]    [Pg.1032]    [Pg.488]    [Pg.488]    [Pg.75]    [Pg.330]    [Pg.488]    [Pg.488]    [Pg.56]    [Pg.51]    [Pg.284]    [Pg.279]    [Pg.279]    [Pg.1435]    [Pg.280]    [Pg.280]    [Pg.189]    [Pg.13]    [Pg.132]    [Pg.76]    [Pg.325]    [Pg.50]    [Pg.29]    [Pg.22]   


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