l02 - CH 203 O R G A N I C C H E M I S T R Y I...

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Hybridization © Bruno I. Rubio 1 CH 203 O R G A N I C C H E M I S T R Y I Hybridization We pointed out in an earlier lecture that there are two theories of bonding: the classical and the molecular orbital theory. We mentioned that the Lewis structure is the most important articulation of the classical theory. Now we turn to the molecular orbital theory, which views a chemical bond as the overlap of atomic orbitals to form molecular orbitals. We need the molecular orbital theory of bonding because Lewis structures, for all their usefulness, frequently misrepresent a molecule’s shape. Consider the Lewis structures of methane (CH 4 ): H C H H H The Lewis structure might suggest that the molecule is flat and cross-shaped and that the hydrogen–carbon–hydrogen bond angles equal 90°. But it is known from experiment that methane is neither flat nor cross-shaped nor are its bond angles equal to 90° -- they are actually equal to 109.5°. The molecular orbital theory of bonding explains these observations about methane. Hybrid atomic orbitals Carbon doesn’t employ its 1s, 2s and 2p atomic orbitals (AOs) when forming bonds to hydrogen in methane. We know this because the use of these AOs leads to an incorrect description of the shape of the CH 4 molecule. The resulting (incorrect) structure of CH 4 might look something like this: H H H H C x y z 90° 90° 90°
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Hybridization © Bruno I. Rubio 2 The hydrogen 1s AOs are shown interacting with the three carbon 2p AOs. Such a structure would exhibit several hydrogen–carbon–hydrogen bond angles of 90°, but it is known from experiment that there are no 90° bond angles in CH 4 . Actually, carbon uses “hybrid” AOs to form bonds rather than the “pure” AOs we have discussed so far. We need to pause to examine the meaning of the word “hybrid” because we will use this term in several contexts during our study of organic chemistry. An example of a hybrid is a mule, which results when a horse and a donkey mate. A mule is neither a horse nor a donkey: it is a dis- tinct creature that is a mixture of those two animals and has characteristics of both -- the imposing stature of a horse and the long ears of a donkey. So it is with hybrid AOs: we shall see that a pure 2s AO combines with one or more pure 2p AOs to create new and distinct hybrid AOs that are neither 2s nor 2p AOs, but have characteristics of both. There are three ways in which pure AOs hybridize: sp 3 , sp 2 and sp hybridiza- tion. It is possible in many cases to predict an atom’s hybridization state by determining that atom’s steric number (SN) according to the following for- mula: SN of an atom = the number of atoms bonded to that atom + the number of unshared electron pairs on that atom When an atom’s steric number is known, the (usually correct) prediction of that atom’s hybridization state is sp SN–1 . We have been careful to qualify the soundness of the prediction of an atom’s hybridization state from its steric number because there is one major exception to this rule: it doesn’t always work when resonance is involved. We will deal with this issue later.
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