Should we number this molecule right to left or left to right?

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1 Answer
Apr 26, 2018

You number it from left to right.

Explanation:

Whenever you have a tie for the first number in a chain, you number in both directions an choose the one that gives the lowest number at the first point of difference.

If you number from left to right, you get the numbers #"3,3,5"#.

If you number from right to left, you get the numbers #"3,5,5"#.

You compare the two sets of numbers,

The first number in each set is #"3"#, so there is no difference.

The second number in the first set is #"3"#.

The second number in the other set is #"5"#, so there is a difference. We call this the first point of difference.

Thus, at the first point of difference, #bb"3 < 5"#,

The correct name is 3,3,5-trimethylheptane.