l05 - CH 203 O R G A N I C C H E M I S T R Y I Resonance...

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Resonance © Bruno I. Rubio 1 CH 203 O R G A N I C C H E M I S T R Y I Resonance The meaning of resonance structures Sometimes it is possible to write more than one valid Lewis structure for the same molecule. Such structures are called resonance structures. The thiocy- anate ion (SCN ), for example, can be represented by the two different Lewis structures S C N S C N and Both Lewis structures conform to the rules of writing Lewis structures: each atom brings in a number of electrons equal to its group number; all atoms have octets; an extra electron is pitched in to account for the negative charge, etc. The description of bonding in SCN offered by the two Lewis structures is clearly different. One structure says that C and N are linked by a double bond, whereas the other says that C and N are linked by a triple bond; one structure says that N bears a negative charge, whereas the other says that S bears a negative charge; one structure predicts that the N is sp 2 , whereas the other predicts that the N is sp. So, which Lewis structure of SCN is correct? Paradoxically, both are right and both are wrong! The actual structure of SCN is a composite of its two resonance structures. The carbon–sulfur bond in SCN is something in-between a single bond and a double bond. A true carbon–sulfur single bond measures 1.8 × 10 –10 m in length, but the carbon–sulfur bond in SCN measures about 1.7 × 10 –10 m in length, reflecting the fact that it has some double-bond character (recall that double bonds are shorter than single bonds). The car- bon–nitrogen bond in SCN (1.2 × 10 –10 m) is intermediate in length between a carbon–nitrogen double bond (1.3 × 10 –10 m) and a carbon–nitrogen triple bond (1.16 × 10 –10 m). Furthermore, experiment shows that the negative charge on SCN is smeared out in such a way that there is some negative charge on the nitrogen atom and some negative charge on the sulfur atom. A more accurate, but also more confusing, Lewis structure of SCN is
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Resonance © Bruno I. Rubio 2 S C N !" !" in which the dashes represent partial bonds and the !" represent partial nega- tive charges. The structure given above is called a resonance hybrid. We have encountered the word “hybrid” before: you will recall that a hybrid combines the characteristics of two entities (our previously cited example was that of a mule, which combines the characteristics of a horse and of a donkey, but is neither a horse nor a donkey). In like manner, the resonance hybrid of SCN combines the characteristics of its two resonance structures: S C N S C N S C N !" !" = The double-headed arrow linking the two resonance structures indicates that they are alternative descriptions of the bonding in SCN ; neither resonance structure, however, is a completely correct description of the bonding in that molecule.
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