Degree of Unsaturation: Worked Solution - Organic Chemistry Quiz 4

This post is the worked solution for organic chemistry Quiz 4 which talks about the degree of unsaturation of organic compounds. If you have not tried this question, you may like to visit this post first.

Saturated vs Unsaturated Compounds

From the alkanes and alkenes chapter, we learn that alkanes have only single bonds and are saturated compounds. On the other hand, alkenes have double bonds, and are unsaturated compounds. A common test for saturation (or to distinguish alkanes from alkenes) is to add aqueous bromine (in the dark). Alkenes will turn reddish brown bromine colorless. For alkanes, bromine will remain reddish brown.
This video from youtube is a good illustration of the test:

Formula of Alkane and Alkene

In O Levels, they generally tell you that alkane has a general molecular formula of C_nH_2n+2, while alkene has a general molecular formula of C_nH_2n. Note that the alkene general molecular formula of C_nH_2n only applies to straight chain alkene with just one double bond. What happens when there is more than one double bond?
Every additional double bond you have will decrease the number of hydrogen atoms by 2.

Worked Solutions to Quiz 4

For a straight chain alkane molecule with 20 carbon atoms, we would expect 42 hydrogen atoms (using the formula of C_nH_2n+2. For every double bond you have, the number of hydrogen atoms will decrease by 2. Since there are 30 hydrogen atoms in this hydrocarbon, we will expect 6 double bonds.
Number of double bonds = (42 - 30)/2 = 6
One mole of double bond will react with one mole of bromine. With 6 double bonds, it means 6 moles of bromine.