Substituent question

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Cecilia Tsai 3C
Posts: 16
Joined: Fri Sep 25, 2015 3:00 am

Substituent question

Postby Cecilia Tsai 3C » Wed Feb 24, 2016 10:18 pm

If the substituent has the same number, what happens?

I know in the course reader it says "the 1st named group gets lower number" but what does that mean? Can someone provide an example please?

Peter2715
Posts: 20
Joined: Fri Sep 25, 2015 3:00 am

Re: Substituent question

Postby Peter2715 » Thu Feb 25, 2016 12:08 am

When looking at alkanes, you first find the longest chain of carbons, this length will give you the root name to use like methane, ethane, propane, etc.

Next, you look at what branches off the path of this main chain, which will be your constituents. The way you determine your constituent number is by counting the number of carbon from which your constituent branches off from. So if a methyl group branches off the Second carbon from a 5 carbon long chain, you would name it 2-methylpentane. (the name pentane came from it being a 5 carbon long main chain)

You want to number your chain from the side that will give you the LEAST number values. For example, if you counted from the other side of the 5 carbon chain, you would have ended up counting the 4th carbon and mistakenly named it 4-methylpentane.

***If you have multiple substituents, such as in the case of 3-ethyl-5-methylpentane (from page 6 in your orange organic chemistry reader), you would still prioritize your counting so that you end up with the lowest possible values. In this case a 3 and 5 are correct. In THIS particular case, however, counting from the opposite side would ALSO yield 3, 5; but since you always want to name substituents in alphabetical order (ethyl then methyl) it is another rule that you assign the lower value to the first named substituent since the numbering is equivalent, meaning it works out to be 3,5 REGARDLESS from which side you start numbering.

To clarify, naming this molecule results in ___ethyl___methylpentane due to alphabetical ordering, so you could put 3,5 or 5,3 depending on which side you counted from; since these are equivalent numbers, it is a rule that you simply put the smaller number on the first named substituent which is ethyl.


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