## Numbering In cycohexane (2013 final, Q6A)

LeilaBushweller1F
Posts: 12
Joined: Fri Sep 26, 2014 2:02 pm

### Numbering In cycohexane (2013 final, Q6A)

Hello. I am confused on how the substituents were numbered in this problem. I got 2,3,4,4-tetramethylcyclohexene as the name, but the solution has 1,5,5,6-tetramethylcyclohexene as the answer. Why is it numbered this way? I thought you want to minimize the numbering (lowest to the double bond), so that the substituents are the smallest numbers possible?

Niharika Reddy 1D
Posts: 127
Joined: Fri Sep 26, 2014 2:02 pm

### Re: Numbering In cycohexane (2013 final, Q6A)

The double bond does get priority but the correct way to number it also gives a lower number for the first substituent than how you numbered it (1<2) while still having the double bond at carbon 1. Usually, it's better to start numbering on the carbon that has a substituent attached AND has a double bond, which is why the lowest number for the first substituent can be obtained by numbering counterclockwise from the carbon that has a methyl group attached and has the double bond. Although this gives larger numbers for the other substituents, we pick the numbering that gives the first lowest number, and 1 is smaller than 2.

Chem_Mod
Posts: 19769
Joined: Thu Aug 04, 2011 1:53 pm
Has upvoted: 902 times

### Re: Numbering In cycohexane (2013 final, Q6A)

The set (1,5,5,6) is better than (2,3,4,4) because 1 < 2. Compare the point of first difference and then forget about the rest of them. Only if the first numbers have a tie, would you move to compare the 2nd ones, etc.