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Why are the HOMO and LUMOs often $\pi$ orbitals in carbon based compounds that contain multiple double bonds or conjugation such as ethene and butadiene?

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  • $\begingroup$ You can visit this page to find out how to make your future posts better.‎ $\endgroup$ – M.A.R. Feb 28 '15 at 17:59
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Why are the HOMO and LUMOs often π orbitals in carbon based compounds

The heat of hydrogenation of ethylene is -32.5 kcal/m

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In other words, ethylene is 32.5 kcal/m less stable (higher in energy) than ethane and this amount of energy (heat) is given off when we hydrogenate ethylene and convert it to ethane.

This tells us that pi bonds are, generally, less stable (higher in energy) than sigma bonds as shown in the following diagram (just forget about the delta bond orbitals, they don't play into this discussion).

enter image description here

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This diagram is a pictorial answer to your question. Since pi bonds are generally higher in energy than sigma bonds, they will become the HOMO in most molecules.

Since bonding and antibonding orbitals are arranged symmetrically about the zero energy point, whatever type of orbital is the HOMO, then the antibonding counterpart will be the LUMO. If a $\ce{\pi}$ orbital is the HOMO, than a $\pi^{*}%$ (pi star) orbital will be the LUMO.

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