Why is chlorine given first priority in naming?

Question

Shouldn't the numbering start from bromine unlike the answer where the number is starting with the carbon attached to chlorine?

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Answer 1 · 2018-04-30T12:42:37

Chlorine does not have priority.

The correct answer is "None of the above."

The name is 3-chloro-1,1,2-trichloropropane (there is no hyphen between "tri" and "chloro").

Chlorine does not have priority.

Chlorine and bromine have equal priority in naming.

A different rule is operating here:

When two groups have equal priority, you number from the end that gives the lowest number at the first point of difference.

If you number the chain from the left, you get sequence (a) below.

If you number the chain from the right, you get sequence (b).

#bb(("a"))1,color(red)(2),3,3#
#bb(("b"))1,color(red)(1),2,3#
#color(white)(mmll)color(red)(↑)#
#color(red)("First point of difference")#

The first numbers in each list are #1# and #1#. There is no difference.

The second numbers in each sequence are #2# and #1#.

The numbers are different, so this is the first point of difference.

Since #1 < 2#, we choose sequence (b) for numbering, but we write the names of the substituents in alphabetical order.

The name becomes

3-chloro-1,1,2-trichloropropane