Identifying substituents in the molecular formula

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Albert Agabekyan 1A
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Joined: Mon Oct 06, 2014 3:00 am

Identifying substituents in the molecular formula

Postby Albert Agabekyan 1A » Mon Feb 29, 2016 11:17 pm

When given the molecular formula, sometimes the substituents are given in parentheses. However, sometimes that just indicates another atom in the molecule that determines the overall shape. How do you know when the atom in the parentheses is a substituent vs a carbon atom part of the main chain?

Cindy Chen_2I
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Joined: Wed Nov 18, 2015 3:00 am

Re: Identifying substituents in the molecular formula

Postby Cindy Chen_2I » Mon Feb 29, 2016 11:40 pm

Usually if you see (CH2)x, it's in the middle of a chain because the C is attached to 2H and a 2 other Cs on the main chain.
If you see a (RCH3), it's usually the end of a chain because the C is already connected to 3H and a C on the left, so it cannot be attached to anything else.

Albert Agabekyan 1A
Posts: 18
Joined: Mon Oct 06, 2014 3:00 am

Re: Identifying substituents in the molecular formula

Postby Albert Agabekyan 1A » Tue Mar 01, 2016 12:01 am

So for CH3CH=CHCH(CH3)2, their is only one substituent which is the CH3 and that is connected to the second carbon from the right, correct?

Also, the textbook answer is 4-methyl-2-pentene. Does the number of the shape have higher priority over the number of the substituent? Is that why it is counted starting from the left?

The 2 is chosen from the first carbon that is connected to the double bond?

Aya Ghoneum 1H
Posts: 22
Joined: Fri Sep 25, 2015 3:00 am

Re: Identifying substituents in the molecular formula

Postby Aya Ghoneum 1H » Tue Mar 01, 2016 12:22 pm

Hello, double bonds take priority over alkanes, so you would need to start on the side of the molecule that would get you to number the double bond first. It doesn’t matter if the double bond is part of the main chain or just a substituent, you still need to start numbering so that you can get to it as soon as possible (in this case it is part of the main chain). It may be helpful to google a “priority list of functional groups” to determine the list of priority for all functional groups we will come across in class. Here is the link to my previous post about how to name both cyclic organic compounds and chain organic compounds: viewtopic.php?f=83&t=12935

Going back to your initial question, the (CH3)2 means there are 2 –CH3 groups coming off the previous –CH group. Another way to confirm this is by the fact that the previous –CH carbon is only bonded to one hydrogen and the previous carbon, so there must be two other things it is bonded to since all carbons want 4 bonds total.

After knowing the structure of the compound, we can name it. The longest chain goes from the –CH3 group on the left all the way through to one of the –CH3 groups on the right. (I am making my base chain from left to right in this case because this will get us to number the double bond the quickest). Now that we numbered correctly, we can name the compound. The compound’s base chain is 2-pentene, and the one other substituent we need to add is 4-methyl, so the name becomes 4-methyl-2-pentene.

I hope this helps!


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