Without looking at the structures, give molecular formulas for the compounds in Problem 3-8 (a) and (b). Use the names of the groups to determine the number of carbon atoms, then use the (2n+2) rule.
Solution 9P :
Here, we are asked to provide molecular formula and determine the number of carbon atoms based on the name of the group :
(a) 4 - (1,1- dimethylethyl)octane :
The given compound is an alkane and the prefix ‘oct’ in the group name ‘octane’, tells the longest chain in the compound has 8 carbon atoms.
Also, there is (1,1- dimethylethyl) group on the 4th carbon atom. Ethyl group has 2 carbon atoms and Methyl group has 1 carbon atom and the name ‘diethyl’ indicates there are 2 ethyl groups and hence there are 2 carbon atoms and totally there are 4 carbon atoms in the group.
So, the total number of carbon atoms in the compound which includes the number of carbon atoms in the functional group and the main chain is :
= (8+4) = 12.
Now using the (2n+2) rule, we determine the number of hydrogen atoms in the compound :
= 2(12) +2
Hence, the molecular formula of the above compound is C12H26.