How do you name #"H"_3"C"{"C"("CH"_3)_2}_2("CH"_2)_2"CH"_3# and #"H"_3"C""CH"_2("CH""CH"_2"CH"_3)("CH"_2)_2"CH"("CH"_3)_2#?

You'll have to be able to draw these out to name them. It is impossible to name by inspection of their chemical formulas.

1. I've highlighted the longest hydrocarbon chain, which you would have had to identify. #(a)# is therefore a kind of hexane, while #(b)# is a kind of heptane (hex = six, hept = seven carbons in the main chain).

2. Now you simply count from each end to determine the lowest possible set of carbon indices, and account for duplicate functional groups using prefixes. Alphabetize them afterwards.

NOTE: the smallest index should be the first point of difference between two sets of numberings.

There are four methyl groups in #(a)#, while there is one methyl and one ethyl group in #(b)#.

Counting from the left:

#(a)# is then denoted #ul"2,2,3,3-tetramethylhexane"#.

#(b)# is then denoted #"3-ethyl-6-methylheptane"#.

Counting from the right:

#(a)# is then denoted #"4,4,5,5-tetramethylhexane"#.

#(b)# is then denoted #ul"5-ethyl-2-methylheptane"#.

The first name for #(a)# is correct since its indices are minimized.

The second name for #(b)# is correct since the first encountered functional group with the lowest index is methyl, but since ethyl comes before it alphabetically, we rearrange these while keeping the indices as they are.

#(b)# is more difficult, so remember that example.