## Final 2013 Q6 - Numbering

104781135
Posts: 13
Joined: Wed Sep 21, 2016 2:56 pm

### Final 2013 Q6 - Numbering

Hi everyone!
Can anyone explain why the methyls of the cyclohexane in Question 6A of the 2013 Final are numbered 1,5,5,6 and not 1,2,3,3?

ntruong2H
Posts: 50
Joined: Sat Jul 09, 2016 3:00 am
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### Re: Final 2013 Q6 - Numbering

104781135 wrote:Hi everyone!
Can anyone explain why the methyls of the cyclohexane in Question 6A of the 2013 Final are numbered 1,5,5,6 and not 1,2,3,3?

It's because you have a double bond. When you have a double or a triple bond, you must count going across across the bond. You started at the double bond, but then went to the right, and that is how you got 1,2,3,3. However, you must count going across the double bond, so start at the same carbon, and go to the left. That is how you get 1,5,5,6. Double bonds get priority before single bonds. Also, if you started at the other end of the double bond and went across it and started numbering, you would get 2,3,4,4. Which is also incorrect, as 2 > 1, and you want the lowest number possible. Hope this helps!

Mackenzie Sober 2E
Posts: 10
Joined: Fri Jul 08, 2016 3:00 am

### Re: Final 2013 Q6 - Numbering

ntruong2H wrote:
104781135 wrote:Hi everyone!
Can anyone explain why the methyls of the cyclohexane in Question 6A of the 2013 Final are numbered 1,5,5,6 and not 1,2,3,3?

It's because you have a double bond. When you have a double or a triple bond, you must count going across across the bond. You started at the double bond, but then went to the right, and that is how you got 1,2,3,3. However, you must count going across the double bond, so start at the same carbon, and go to the left. That is how you get 1,5,5,6. Double bonds get priority before single bonds. Also, if you started at the other end of the double bond and went across it and started numbering, you would get 2,3,4,4. Which is also incorrect, as 2 > 1, and you want the lowest number possible. Hope this helps!

Just for clarification, you want to start the numbering with the lowest possible number? i.e. 1<2 in this case. Not necessarily that the sum of all the numbers is lower?

Posts: 17
Joined: Sat Jul 09, 2016 3:00 am

### Re: Final 2013 Q6 - Numbering

Mackenzie Sober 2E wrote:
ntruong2H wrote:
104781135 wrote:Hi everyone!
Can anyone explain why the methyls of the cyclohexane in Question 6A of the 2013 Final are numbered 1,5,5,6 and not 1,2,3,3?

It's because you have a double bond. When you have a double or a triple bond, you must count going across across the bond. You started at the double bond, but then went to the right, and that is how you got 1,2,3,3. However, you must count going across the double bond, so start at the same carbon, and go to the left. That is how you get 1,5,5,6. Double bonds get priority before single bonds. Also, if you started at the other end of the double bond and went across it and started numbering, you would get 2,3,4,4. Which is also incorrect, as 2 > 1, and you want the lowest number possible. Hope this helps!

Just for clarification, you want to start the numbering with the lowest possible number? i.e. 1<2 in this case. Not necessarily that the sum of all the numbers is lower?

Yes exactly. For example you would use 1,6 instead of 2,3 even though 1,6 adds up to 7 and 2,3 adds up to 5.