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In chemistry, the mesomeric effect (or resonance effect) is a property of substituents or functional groups in a chemical compound.It is defined as the polarity produced in the molecule by the interaction of two pi bonds or between a pi bond and lone pair of electrons present on an adjacent atom. [1]
The nitrite ion has a symmetrical structure (C 2v symmetry), with both N–O bonds having equal length and a bond angle of about 115°. In valence bond theory , it is described as a resonance hybrid with equal contributions from two canonical forms that are mirror images of each other.
Ammonia forms 1:1 adducts with a variety of Lewis acids such as I 2, phenol, and Al(CH 3) 3. Ammonia is a hard base (HSAB theory) and its E & C parameters are E B = 2.31 and C B = 2.04. Its relative donor strength toward a series of acids, versus other Lewis bases, can be illustrated by C-B plots .
Expressing resonance when drawing Lewis structures may be done either by drawing each of the possible resonance forms and placing double-headed arrows between them or by using dashed lines to represent the partial bonds (although the latter is a good representation of the resonance hybrid which is not, formally speaking, a Lewis structure ...
The most common Lewis bases are anions. The strength of Lewis basicity correlates with the pK a of the parent acid: acids with high pK a 's give good Lewis bases. As usual, a weaker acid has a stronger conjugate base. Examples of Lewis bases based on the general definition of electron pair donor include: simple anions, such as H − and F −
Formal charges in ozone and the nitrate anion. In chemistry, a formal charge (F.C. or q*), in the covalent view of chemical bonding, is the hypothetical charge assigned to an atom in a molecule, assuming that electrons in all chemical bonds are shared equally between atoms, regardless of relative electronegativity.
The amino group can donate electron density through resonance. Attack occurs at ortho and para positions, because the (partial) formal negative charges at these positions indicate a local electron excess. On the other hand, the nitrobenzene resonance structures have positive charges around the ring system:
Resonance structures of ammonia-borane. The structure of the solid indicates a close association of the NH and the BH centers. The closest H−H distance is 1.990 Å, which can be compared with the H−H bonding distance of 0.74 Å. This interaction is called a dihydrogen bond.