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Re: help naming an organic compound

Submitted by alkaline262 on Thu, 12/31/2009 - 13:09

heres the link above for the picture, how do they arrive at all parts of that name (1) - (methyl) - (ethyl)

if you combine methyl(CH3-) and ethyl(CH3CH2) you get the textbook propane CH3CH2CH3, but the only way this would work is if you assume the radical is a hydrogen, and if you did couldn't you just call it propane to begin with? >_< i am really confused here.

An alkyl group has the general formula CnH2n+1 and is named after an alkane which fits the general formula CnH2n+2

Alkane         Formula           Alkyl group    Alkyl formula
Methane        CH4                  methyl          CH3-
Ethane         C2H4                  ethyl          C2H5-
Propane        C3H6                propyl          C3H7-


Alkyl groups exist as part of a compound and not as separate molecules. They need another atom or group of atoms to replace the 'lost' hydrogen atom.

eg ethylbromide         C2H5-Br (also called bromoethane)
propylchloride         C3H7-Cl (also called chloropropane)

Now with chloropropane it is possible to show two different structures depending on the position of the Cl atom which can either be on C1 or on C2. This would give us 1-chloropropane or propyl-1-chloride, and 2-chloropropane or propyl-2-chloride.

In the days before we numbered carbon atoms to indicate the position of attached group we used other names such as 'isopropyl chloride (same as 2-chloropropane). The isopropyl group is CH3 - CH - CH3

CH3 - CH - CH3

is isopropylchloride. There is no such substance as isopropyl.

              CH3    C2H5  CH3
              l        l        l
  CH3 -  C  -  C    -  C  -  CH2 - CH3
              l        l        l
              CH3    H        H

The structure above has a continuous carbon chain of 6 carbon atoms so as an alkane is based on hexane.
It has three CH3 groups attached and these are methyl groups. It also has one C2H5 or ethyl group attached.
So it could be called ethyl-trimethyl-hexane.
More accurately, it could be called 3-ethyl-2,2,4-trimethylhexane as the six carbon chain is numbered from the left (to give smaller numbers than if numbered from the right).

kingchemist wrote:

CH3 - CH - CH3

is isopropylchloride. There is no such substance as isopropyl.

I would name this guy as 2-Chloropropane. For the most part, I avoid using industrial nomenclature just because IUPAC nomenclature is so systematic. The isopropyl group itself is a trivial name, just like "vinyl" and "allyl".  People back in the day just decided to name organic compounds without any logical approach. Most of these names give no indication of the chemical structures, unless you memorize the structure that belongs with the name. This is why I prefer IUPAC-no memorization required. But if you take a look at the name isopropyl, you could probably tell that it is an isomer of the propyl group, hence the name isopropyl. And there is only one isomer of propane, so the isopropyl group makes sense. But a lot of them don't, just like allyl and vinyl.

I would get in the habit of naming it the way Kyle mentioned. IUPAC nomenclature will serve you well in academia - it's universally recognized.