I thought this simple haloalkane was supposed to be 3,5-dichloro-2-methylhexane, but my professor said it was 2,4-dichloro-5-methylhexane, (alphabetical ordering?)

How come, shouldn't I simply assign the smallest possible numbers to alll of the supstituents?

Edited May 10, 201411 yr by aelek

You should, which is why your professor's answer is correct.

But the sum in 2,4-dichloro-5-methylhexane is 11, whereas in 3,5-dichloro-2-methylhexane it is 10...?

I haven't heard of adding the numbers up as a method of figuring this sort if thing out before. 2 and 4 are less than 3 and 5, hence you use 2,4 rather than 3,5. You are supposed to give preference to the chlorine substituents, meaning that the carbons they are bonded to need the lowest possible numbers over the carbon bonded to the methyl group.

Reminds me, how many of my classmates did the same mistake, adding of locants. What is required is the minimum locant/numbering as hypervalentiodine tells.

I think I see the point now, thank you guys. Just one more question, I give preference to the chlorine substituents based on the alphabetical order, not because of the atomic mass?

Edited May 11, 201411 yr by aelek