Numbering In cycohexane (2013 final, Q6A)

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LeilaBushweller1F
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Numbering In cycohexane (2013 final, Q6A)

Postby LeilaBushweller1F » Sat Mar 14, 2015 11:28 am

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
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Re: Numbering In cycohexane (2013 final, Q6A)

Postby Niharika Reddy 1D » Sat Mar 14, 2015 11:59 am

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.

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Re: Numbering In cycohexane (2013 final, Q6A)

Postby Chem_Mod » Sat Mar 14, 2015 12:05 pm

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.


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