Question

Explain lowest sum rule. Also explain the longest chain rule.

Answer 1

Hint: While numbering an organic compound, it should be done in such a manner that the first preference is given to functional groups and then to the multiple bonds and finally to the substituents. Also the sum of the numbers given to the branches should be the lowest.

Complete answer:
As we know that IUPAC has given certain rules to uniquely determine the name of every organic compound. Among these, two important rules are the lowest sum rule, longest chain rule.
Let’s see both of these rules separately:-
Longest Chain rule: According to this rule, while selecting the parent carbon chain, selection should be done in such a manner that the longest chain is prioritized first. According to this rule the longest, most substituted carbon chain containing a functional group will be prioritized while selecting the parent carbon chain.
Ex:

In this compound we can have different possible carbon chains like:
(1)-(2)-(3)-(4)-(5)-(6)-(7)-(8)
 (1)-(2)-(b)
 (1)-(2)-(3)-(4)-(5)-(a)
But since we have to select the longest carbon chain, we will select the (1)-(2)-(3)-(4)-(5)-(6)-(7)-(8) carbon chain. Therefore, the name of this compound will be 2, 5 di-methyl octane.
Lowest sum rule: According to this rule carbon bonded to a functional group must have the lowest possible carbon number and if there are no functional groups, then any substituent present must have the lowest possible number. When the functional groups are of the same priority and are connected to a long carbon chain, then this rule is followed.
Ex: $C{{H}_{3}}-C{{H}_{2}}-CH(F)-CH(Br)-C{{H}_{2}}-C{{H}_{2}}-C{{H}_{3}}$
If we number this carbon chain from the left side then carbon with F and carbon with Br would get a number of 3 and 4 respectively, but if we number it from the right-side C with F and C with Br would get a number of 5 and 4 respectively.
Sum of the numbers given to locants (F and Br)
Left side: 3+4=7
Right Side: 5+4=9
Now, according to the Lowest Sum rule since the sum of locants is less while numbering from the left side, we will name the compound according to this numbering. Therefore, the name of this compound will be 4-bromo-3-chloroheptane.

Note:
While giving the IUPAC name of a compound, both the Longest Chain rule and the Lowest Sum rule must be followed. After selecting and numbering the carbon chain according to these rules, we then write the name of the compound taking alphabetical order into consideration.