or both! Thus you get bonding and antibonding orbitals (red and green respectively). They are waves in that when electrons come together to form, they can either participate in constructive, destructive interference. In order to form molecular orbitals, you have to think about electrons as both particles and waves. I'm only using methane as an example because I can't find any better pictures. "Wait," you might say, "Methane doesn't have resonance." Well, it actually sort of does. So you see the funny little blobs at the bottom? Those are the MOs of methane. See this picture first:, the accompanying lecture with relavent section is marked here: I'm gonna attempt to explain it using molecular orbital theory. Try moving the lone pair around (subject to the limit of four bonds on carbon - in the examples I sketched, note how the nitrogen lone pair "displaced" a bond) and you can often discover explanations for why a conjugated or aromatic system reacts preferentially in certain places. C=C-N) you're likely to see resonance structures that make a significant contribution to the true structure of the system. C=C-C=C) or an atom with lone pairs adjacent to a double bond (e.g. Typically, whenever you have conjugated double bonds (i.e. This explains why aniline directs electrophilic substitution to those two positions preferentially - you can't build a resonance structure involving the nitrogen lone pair that causes a charge buildup on the meta- positions.
#Meaning of resonance in chemistry full#
higher at the ortho- and para- positions, but not to the extent of a full charge. The true electron distribution in the molecule is somewhere in between these three i.e. For example I've sketched here some possible resonance structures of aniline. Formal treatments of the orbitals of benzene show that the electrons are distributed evenly around the ring rather than hopping from place to place.ĭespite this slight trap, resonance structures are often helpful in understanding reactivity patterns in molecules. The word "resonance" is slightly misleading because it gives the impression that the molecule is rapidly flipping between the two different configurations of double bonds, but this isn't the case. If you number the atoms around the ring, what's to stop the bonds swapping from 1-2, 3-4 and 5-6 to 2-3, 4-5 and 6-1? The answer is nothing - all the C-C bonds in benzene are identical, with character somewhere between single and double. The classic example is benzene, which is usually represented as having three double bonds and three single bonds. Resonance is the name given to the observed phenomenon that in certain systems, two or more different "ordinary" structures could all be considered valid for a molecule.
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