There are several types of strain in cycloalkanes. There is angle strain as a result of the smaller than desirable angle between carbon atoms. There is also torsional strain, which involves hydrogen atoms being closer together than they would like to be. The combination of angle strain and torsional strain is referred to as ring strain. The ring strain decreases as the size of the ring increases so that, with cyclohexane, there is no ring strain at all. Any rings higher than that will not be seen generally in organic chemistry. Even with no ring strain, cyclohexane is not a straight planar molecule. It can be in two different shapes in nature: the chair conformation and the boat conformation. These are the most desirable shapes. The chair conformation is more desirable than the boat conformation because it has less interference between the hydrogen atoms, called “transannular strain”. Only the chair conformation has no angle strain because it has no “eclipsing strain” and a small amount of “steric strain” (which is crowding of hydrogen atoms). Steric strain occurs when two atoms are in close proximity to one another. A summary of the different types of strain involves the following: •
Transannular strain—this is the crowding of two side groups in a ring.
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Eclipsing strain—this is also called torsional strain, which is strain between molecules because of the bond interaction between two eclipsed atoms or groups.
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Bond angle strain—this is when there is poor overlap between carbon atoms that allows overlapping of the atomic/hybrid orbitals.
ALKENES As the molecular formula grows in size and number of carbon atoms, the number of possible structures increases. For example, with C5H8, what’s clear is that the hydrogen saturation is 4 less than is acceptable. This means that there may be a triple bond, a couple of double bonds, or a ring structure with a double bond, and two rings, among others. This means that there needs to be a consistency in the nomenclature that defines these types of molecules.
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