Convince yourself that each of the following hydrocarbons fits the formula C_nH_2n+2.

When a hydrocarbon has the formula C_nH_2n, it is two hydrogen atoms shy of being completely saturated. Its degree of unsaturation is 1, which is determined by subtracting H_2n from H_2n+2 and dividing the difference by 2. A hydrocarbon with a degree of unsaturation of 1 is either an acyclic (non-cyclic) alkene or a cycloalkane. You can imagine taking n-pentane (above left) and mentally removing a hydrogen atom from C₁ and C₂. What you have left, in addition to H₂, is 1-pentene. If the same mental exercise were conducted at C₁ and C₅ of n-pentane and the two radicals were joined, a cyclopentane ring would be formed.

Convince yourself that each of the following hydrocarbons fits the formula C_nH_2n..

If a compound fits the formula C_nH_2n-2 it has two degrees of unsaturation. Do the math. This compound may be have a triple bond (alkyne), two double bonds (diene), a double bond and a ring (cycloalkene), or two ring (bicyclic alkane).

Convince yourself that each of the following hydrocarbons fits the formula C_nH_2n.-2.

The degree of unsaturation may also be employed in another way. Given a particular hydrocarbon structure for which you know the number of carbons and the degree of unsaturation, you can calculate the number of hydrogen atoms. Benzene has 6 carbon atoms and 4 degrees of unsaturation (1 ring and 3 double bonds). If you work backwards and double the degrees of unsaturation you have 8.

The maximum number of hydrogens for a C₆ compound is 14. The difference between 14 and 8 is 6. Benzene is C₆H₆.

Hydrocarbons must have an even number of hydrogen atoms. If the number of hydrogens is odd, you have a radical.

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