Resonance structure atom electronegative

The structure of the conjugate base is more like resonance structure B than A because the nega tive charge is on the more electronegative atom (O versus S)... 

Aldiough diese structures have a positive charge on a more electronegative atom, diey benefit from an additional bond which satisfies file octet requirement of the tricoordinate carbon. These carbocations are well represented by file doubly bonded resonance structures. One indication of file participation of adjacent oxygen substituents is file existence of a barrier to rotation about the C—O bonds in this type of carbocation. 

Both compounds have a pi bond between a carbon atom and an electronegative atom (C=0), and both compounds have a lone pair next to the pi bond. So we would expect their resonance structures to be similar, and we would expect these compounds to have the same number of significant resonance structures. But they do not. Let s see why. Let s start by drawing the resonance structures of the first compound ... 

When a resonance structure has a positive charge on an electronegative atom, that resonance structure wiU only be significant if the electronegative atom has an octet. If it does not have an octet, the resonance structure will not be significant. For example, consider the following ... 

Multiple bonds between atoms with different electronegativities such as the C=0 and S=0 bonds, are also necessarily polar. This polarity is frequently represented by resonance structures such as... 

Structures such as these imply that the bonds are polar, which is consistent with ligands that are more electronegative than the central atom, as is often the case. When resonance structures are written in this way, it is assumed that the bond lines represent fully covalent... 

Idle exponent, m, in the electrostatic attraction term, was taken as unity This value of m is in agreement with, resonance structure II where the two electronegative atoms carry partial charges. The two remaining parameters A and b can be evaluated using Eq. (24) which defines A in terms of b and i 0. The hydrogen bond potential function... 

As another example, the resonance structure for N2O that places the formal negative charge on the more electronegative oxygen atom rather than on the less electronegative nitrogen atom is probably a more accurate representation of the molecule. 

Mulliken proposed (1934) that electronegativities could be obtained from ionisation potentials and electron affinities (we use here the terms appropriate for atoms, rather than atomic substances). If the bonding in a diatomic molecule AB can be represented by the resonance structures ... 

Because the enolate ion is the preferred resonance structure so a better mechanism for the acid base reaction shows the enolate ion being formed simultaneously as the acidic proton is lost (Following fig.). As the hydroxide ion forms its bond to the acidic proton, the C-H bond breaks, and the electrons in that bond form a rbond to the carbonyl carbon atom. Simultaneously, the carbonyl n bond breaks in such a way that both electrons move onto the oxygen. The electronegative oxygen is responsible for making the a proton acidic. 

These atoms are more electronegative than nitrogen and less able to stabilise a positive charge. These resonance structures might occur to a small extent with esters and acid anhydrides, but are far less likely in acid chlorides. This tend also matches the trend in reactivity. 

A resonance structure such as q. produced by moving the electrons to the less electronegative atom so that there is negative formal charge on the less electronegative atom and a positive formal charge on the more electronegative atom, is not important and should not be shown. 

This is rather high for an atom that has an electronegativity of 2.6. It is possible to draw resonance structures that give a more equitable distribution of electrons, and one of them is shown here ... 

An examination of structure III shows that it is not likely to contribute very much to the true structure of OCN- because it violates some of the principles related to resonance structures. Structure III has a positive formal charge on the atom of highest electronegativity, and it has higher overall formal charges. Both of the other structures contribute significantly to the structure OCN-, and a reasonable estimate is that their contributions should be approximately equal. 

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