Numbering counterclockwise or clockwise

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terdaf
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Numbering counterclockwise or clockwise

Postby terdaf » Tue Mar 14, 2017 8:20 pm

Is the numbering in cycloalkanes (and cycloalkenes for that matter) clockwise or counterclockwise? Ive read that it doesnt matter and to go with whatever of the two provides the lowest numbers (I think there was some exercise where both ways were used in different subquestions) but in page 102 of the course reader for 1,6-dichlorocyclohexene clockwise seems to be used even though counterclockwise would give number 1 to the double bond and the first Cl and number 2 to the second Cl rather than 1,6. So what is the rule?

Pratyush Kandimalla 2O
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Re: Numbering counterclockwise or clockwise

Postby Pratyush Kandimalla 2O » Tue Mar 14, 2017 11:07 pm

The first priority of numbering is given to the alkene group, that is the double bond.
While numbering the carbons, we always go along the double bond.


So in this case, we choose the lowest number for the carbon with the double bond, with the Cl group attached to it.
This makes our double bond and one of the substituents have a low number (ie 1).
Now, to continue numbering, we go along the double bond.
So the carbon which will get the next number (ie 2) is the one which is joined to the FIRST CARBON via the double bond.
And so the numbering continues in a clock wise direction.
Hence the other chloro gets the number 6.

So, name is --> 1,6 Dichloro hexene



Following the same idea,
if we would have had the double bond between the two carbons containing the Chloro groups,
then we would have numbered them in an anticlockwise direction to give --> 1,2 Dichloro hexene
(In this case you could also number clockwise and get the same name due to symmetry)


Hope this helps.

Mihika Sridhar 3H
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Joined: Wed Sep 21, 2016 2:58 pm

Re: Numbering counterclockwise or clockwise

Postby Mihika Sridhar 3H » Wed Mar 15, 2017 2:18 pm

I have a related question - what if there is a functional group as well as a double bond? I know that the carbon attached to the functional group would be the first one but if the double bond isn't attached to that same first carbon, which was does the numbering go? Or does it not matter?

404639305
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Re: Numbering counterclockwise or clockwise

Postby 404639305 » Wed Mar 15, 2017 2:22 pm

Mihika Sridhar 3H wrote:I have a related question - what if there is a functional group as well as a double bond? I know that the carbon attached to the functional group would be the first one but if the double bond isn't attached to that same first carbon, which was does the numbering go? Or does it not matter?


A double bond has higher priority over a functional group so Carbon #1 would have to go on the double bond. Whether it goes counter clockwise or clockwise would depend on where the functional group is (whichever way is closest to the functional group).

Pratyush Kandimalla 2O
Posts: 33
Joined: Wed Sep 21, 2016 3:00 pm
Been upvoted: 2 times

Re: Numbering counterclockwise or clockwise

Postby Pratyush Kandimalla 2O » Wed Mar 15, 2017 11:35 pm

The priority goes --

Functional Groups > Bonds (triple>double) > substituents.

So you would choose a numbering such that the functional group gets the lowest value.


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