Polar Covalent Bonds Acids and Bases

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Dipole moment () - Net molecular polarity, due to difference in summed charges

 - magnitude of charge Q at end of molecular dipole times distance r between charges

 = Q  r, in debyes (D), 1 D = 3.336  1030 coulomb meter

length of an average covalent bond, the dipole moment would be 1.60  1029 Cm, or 4.80 D.

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Dipole Moments in Water and Ammonia

Large dipole moments

EN of O and N > H

Both O and N have lone-pair electrons oriented away from all nuclei

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Absence of Dipole Moments

In symmetrical molecules, the dipole moments of each bond has one in the opposite direction

The effects of the local dipoles cancel each other

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Formal Charges

Sometimes it is necessary to have structures with formal charges on individual atoms

We compare the bonding of the atom in the molecule to the valence electron structure

If the atom has one more electron in the molecule, it is shown with a “-” charge

If the atom has one less electron, it is shown with a “+” charge

Neutral molecules with both a “+” and a “-” are dipolar

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• Atomic sulfur has 6 valence electrons.

Dimethyl suloxide sulfur has only 5.

• It has lost an electron and has positive charge.

• Oxygen atom in DMSO has gained electron and has (-) charge.

Formal Charge for Dimethyl Sulfoxide

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Resonance

Some molecules are have structures that cannot be shown with a single representation

In these cases we draw structures that contribute to the final structure but which differ in the position of the  bond(s) or lone pair(s)

Such a structure is delocalized and is represented by resonance forms

The resonance forms are connected by a double-headed arrow

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Resonance Hybrids

A structure with resonance forms does not alternate between the forms

Instead, it is a hybrid of the two resonance forms, so the structure is called a resonance hybrid

For example, benzene (C6H6) has two resonance forms with alternating double and single bonds

In the resonance hybrid, the actual structure, all its C-C bonds are equivalent, midway between double and single

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