1°, 2°, 3°, 4°

by James

in Alcohols, Aldehydes, Alkanes, Alkenes, Alkyl Halides

Chemistry is a language unto itself, with its share of slang and linguistic shortcuts. Organic chemists, with carbon chemistry as their subject, have developed all kinds of shorthand phrases to describe structures and phenomena that might otherwise take a sentence of two to explain. Here’s today’s example: the terminology of carbon-containing functional groups: primary, secondary, tertiary, quaternary.

  • Primary carbons, are carbons attached to one other carbon. (Hydrogens – although usually 3 in number in this case – are ignored in this terminology, as we shall see).
  • Secondary carbons are attached to two other carbons.
  • Tertiary carbons are attached to three other carbons.
  • Finally, quaternary carbons are attached to four other carbons.

You can’t go higher than that. To have five substitutents, you’d need 10 electrons around carbon, a clear violation of the octet rule. When people do write 5 covalent bonds around carbon, it’s a mistake. (In the trade, these are often called Texas carbons – 1) because it resembles a star, 2) because everything is bigger in Texas, and 3) because the only man who can put five bonds on carbon is also known as Walker, Texas Ranger.)

1-1234

We use the same terminology for carbocations. A primary carbocation is attached to one other carbon, a secondary to two, and a tertiary to three. You can’t have a quaternary carbocation without violating the octet rule either (you’d need an extra empty p orbital for that, bringing the total to 5).

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Alcohols also follow the primary/secondary/tertiary nomenclature. The rule for alcohols is that they are named according to the number of carbons attached to the carbon bearing the hydroxyl group: in other words, whether the hydroxyl bound to a primary, secondary, or tertiary carbon. You can’t have a quaternary alcohol – again, that would involve breaking the octet rule. [A bit of non-essential nomenclature: the carbon attached to the OH is sometimes referred to as the “carbinol” carbon].

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A related category of compounds are the alkyl halides, which we encounter in SN1/SN2/E1/E2 reactions (among many others). The naming for alkyl halides is similar to that for alcohols: they are named according to the number of carbons attached to the halogen, where halogen is fluorine, chlorine, bromine, or iodine.

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Next, we come to amines, which are a little bit different. They are named according to the number of carbons attached to nitrogen.  Primary, secondary, and tertiary amines are nitrogens bound to one, two and three carbons, respectively. Since the nitrogen has a lone pair, it is still possible to form another bond to carbon. These are called quaternary amines, although they bear a positive charge on nitrogen and are not at all basic. They are often referred to as quaternary ammonium salts. You’ll see the -ium ending quite a bit – it designates a positively charged species.

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Finally, amides also fall into this category. A primary amide is bound to one carbon – the carbonyl carbon. Successive substitutions of hydrogen for carbon turn the amide into secondary and tertiary amides. [You might ask – can you have quaternary amides? Well, yes. Except nobody calls them that – they’re quite unstable, and go by another name. Why? Because. Why is the plural of goose, “geese”, but the plural of moose is “moose”? There is no satisfying answer to nomenclature questions]

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{ 12 comments… read them below or add one }

Brian H.

Hi James,

Just quickly looked at your website here. It looks good. I like to see other organic chemists getting busy with blogs, exploiting Web 2.0. I started off with a blog and decided I would build myself a website. It is my overglorified CV but I often post articles from industry. Check it out at pharmnbiofuel.com.

Brian

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Natalee

Thank you for your site! It’s saving my Hiney!!

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Chris

Accutally you CAN put 5 substituents at one carbon – and the guy how can do this is called George Olah!

http://www.nobelprize.org/nobel_prizes/chemistry/laureates/1994/

SCNR Chris

Reply

Daxx

Finally! Someone was able to explain the primary, secondary, and tertiary amine thing. I’m not an O-chem guy at all, but I totally got this page. Thanks for putting it together. It finally makes sense.

D

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Ivy

Hi there, may i know if we can describe the carbons in an aromatic compound as primary, secondary, tertiary or quaternary carbon? Or they are only applied to aliphatic carbons? Hope you can help. Thank you so much!

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Rich Aversa

For alkenes, the convention is to use “monosubstituted”, “disubstituted”, etc. Same for arenes.

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Bakhodur

Can secondary carbon be attached to two carbons with one single bond, and one double bond?
Here is the problem which is on:
http://ir.kmu.edu.tw/bitstream/310902000/36554/2/%E9%86%AB%E8%97%A5%E6%9A%A8%E6%87%89%E7%94%A8%E5%8C%96%E5%AD%B8%E7%B3%BB%E7%A2%A9%E5%A3%AB%E7%8F%AD%E8%80%83%E5%8F%A4.pdf

The structure below is a potent analgesic agent (pain reliever) (J. Med. Chem., 2009, 5685). How many secondary carbons are in this structure?

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James

I believe the definitions, “primary, secondary, tertiary” only hold for alkyl groups.
http://goldbook.iupac.org/A00228.html

So there would only be 6 secondary carbons and the alkenes do not count.

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OJ

Hello James,
Is there a technical term to call a carbon with one hydroxyl group attached? Can we call them “hydroxyl carbon” or “hydroxylic carbon”?
Thanks,
OJ

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James

“Carbinol” is the preferred nomenclature

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Jesal

Was searching answer for my chemistry homework…..didnt got on book….remebered google has answer to each question….came here….understood everything in matter of seconds…blessed you with all my heart a good life…….THANK YOU James…👌👌👌😀😀😀

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Jesal

So, Hello and can you describe the same things for hydrogen. Like 1° 2° 3°
Hydrogen. If Yes plz let me know.Thanx.

Reply

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