unsaturationnumber - CH 310M/318M Dr. Brian M. Bocknack...

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CH 310M/318M Dr. Brian M. Bocknack Unsaturation Number When we first started to look at the structures of organic molecules, we began by considering alkanes. An alkane is a saturated hydrocarbon. When an organic molecule is “saturated”, it has the maximum number of hydrogen atoms possible for the number of carbon atoms present in the structure. For an acyclic alkane, the general molecular formula is C n H 2n+2 . In other words, the maximum number of H atoms possible is 2n + 2 if there are n C atoms present in an organic molecule. When a hydrocarbon has n C atoms, but fewer than 2n + 2 H atoms, it is said to be unsaturated . When we discussed cycloalkanes, we saw that closure of a ring requires that 2 H atoms be removed from the structure of an acyclic alkane: An acyclic alkane (0 rings) has molecular formula C n H 2n+2 A monocycloalkane (1 ring) has molecular formula C n H 2n A bicycloalkane (2 rings) has molecular formula C n H 2n–2 A tricycloalkane (3 rings) has molecular formula C n H 2n–4 etc… Another way to introduce unsaturation into a hydrocarbon is to incorporate pi bonds into the structure. Suppose we wanted to convert ethane into ethylene. Since carbon can never be surrounded by more than 8 electrons, the only way to form a second bond between the carbon atoms is to remove 1 H atom from each carbon atom. If this were to happen, each C atom would be left with an unsatisfied bond. Formation of a second carbon–carbon bond, a pi bond, would allow each C atom to achieve its typical valency (4 bonds, 0 lone pairs) again. This “thought experiment” is outlined schematically below. Please note that ethylene cannot be formed directly from ethane in this way – the point of this exercise is simply to illustrate how formation of a pi bond must reduce the number of H atoms present in the structure! Ethane is saturated, and has molecular formula C 2 H 6 . Ethylene is unsaturated, and has molecular formula C 2 H 4 . H H H H H H Ethane (C 2 H 6 ) (saturated) remove 2 H H H H H (2 unsatisfied bonds) form new π bond H H H H Ethylene (C 2 H 4 ) (unsaturated) H H If we wanted to introduce a second pi bond into the structure of ethylene, we would need to remove 2 more H atoms, 1 from each carbon. Once again, removal of a hydrogen atom from each carbon leaves behind an unsatisfied bond. Each C atom can return to its typical valency (4 bonds, 0 lone pairs) if a third bond forms between them. Ethylene, with one pi bond, has molecular formula C 2 H 4 . Acetylene, which has two pi bonds, has molecular formula C 2 H 2 . This process is shown schematically at the top of the next page.
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unsaturationnumber - CH 310M/318M Dr. Brian M. Bocknack...

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