![]() The six delocalized electrons go into three molecular orbitals - two in each. Because the electrons are no longer held between just two carbon atoms, but are spread over the whole ring, the electrons are said to be delocalized. This extensive sideways overlap produces a system of pi bonds which are spread out over the whole carbon ring. Notice that the p electron on each carbon atom is overlapping with those on both sides of it. Only a part of the ring is shown because the diagram gets extremely cluttered if you try to draw any more. The next diagram shows the sigma bonds formed, but for the moment leaves the p orbitals alone. Each carbon atom uses the sp 2 hybrids to form sigma bonds with two other carbons and one hydrogen atom. The difference in benzene is that each carbon atom is joined to two other similar carbon atoms instead of just one. This is all exactly the same as happens in ethene. Each carbon atom now looks like the diagram above. The remaining p orbital is at right angles to them. The three sp 2 hybrid orbitals arrange themselves as far apart as possible - which is at 120° to each other in a plane. The new orbitals formed are called sp 2 hybrids, because they are made by an s orbital and two p orbitals reorganizing themselves. They use the 2s electron and two of the 2p electrons, but leave the other 2p electron unchanged.'įigure 1.2: Step 2: Hydribidization of the atomic orbitals The carbon atom is now said to be in an excited state.īecause each carbon is only joining to three other atoms, when the carbon atoms hybridize their outer orbitals before forming bonds, they only need to hybridise three of the orbitals rather than all four. The extra energy released when these electrons are used for bonding more than compensates for the initial input. There is only a small energy gap between the 2s and 2p orbitals, and an electron is promoted from the 2s to the empty 2p to give 4 unpaired electrons. Each carbon atom has to join to three other atoms (one hydrogen and two carbons) and doesn't have enough unpaired electrons to form the required number of bonds, so it needs to promote one of the 2s 2 pair into the empty 2p z orbital.įigure 1.1: Step 1: Promotion of an electron \)īenzene is built from hydrogen atoms (1s 1) and carbon atoms (1s 22s 22p x 12p y 1).
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