Primary, Secondary, Tertiary, Quaternary In Organic Chemistry

How To Recognize Primary, Secondary, Tertiary (and sometimes Quaternary) Hydrocarbons, Carbocations, Alkyl Halides, Alcohols, Amines, and Amides.

• Primary (1°), secondary (2°), tertiary (3°) and quaternary (4°) alkyl carbons are defined according to the number of carbons directly attached to that carbon.
• Similar nomenclature can be used for carbocations. Primary (1°), secondary (2°), and tertiary (3°) carbocations are defined according to the number of carbons directly attached to the carbon bearing the positive charge.
• Primary (1°), secondary (2°), and tertiary (3°) alcohols are defined according to the number of carbons directly attached to the carbon bearing the hydroxyl group.
• Primary (1°), secondary (2°), and tertiary (3°) alkyl halides are defined similarly to alcohols.
• Amines and amides are defined according to the number of carbons directly attached to the nitrogen atom.
• This classification system only applies to sp³ hybridized carbons (alkyl).

Table of Contents

1. Primary, Secondary, Tertiary and Quaternary Alkyl Hydrocarbons
2. Primary, Secondary, and Tertiary Carbocations
3. Primary, Secondary and Tertiary Alcohols
4. Primary, Secondary, and Tertiary Alkyl Halides
5. The Special Case of Methane and Methyl
6. Primary, Secondary, and Tertiary Amines (and Quaternary Ammonium Salts)
7. Primary, Secondary and Tertiary Amides
8. Notes
9. Quiz Yourself!
10. (Advanced) References and Further Reading

1. Primary, Secondary, Tertiary, and Quaternary Alkyl Hydrocarbons

There are four possible bonding patterns for alkyl carbons in hydrocarbons.

• Primary carbons (1°), are carbons attached to one other carbon and three hydrogens. Also known as a methyl (CH₃)
• Secondary carbons (2°) are attached to two other carbons and two hydrogens. Also known as methylene (CH₂) carbons.
• Tertiary carbons (3°) are attached to three other carbons and one hydrogen. Also known as methine (R₃CH) carbons.
• Finally, quaternary carbons (4°) are attached to four other carbons.

You can’t go higher than that!  To have five substituents, 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. [Note 1]

Ready for a test? See if you can properly identify all the labelled carbons on testosterone.

Click to Flip

2. Primary, Secondary, and Tertiary Carbocations

Carbocations can also be classified as primary, secondary, or tertiary according to the number of carbons directly attached to the positively charged carbon.

Quaternary carbocations don’t exist. The problem is that the extra p-orbital on carbon would bring the number of orbitals on carbon to 5, violating the octet rule.

See if you can properly classify the following carbocations as primary, secondary, or tertiary.

Click to Flip

It’s important to note that the terms primary, secondary, tertiary and quaternary only apply for alkyl carbons and carbocations. When the carbon participates in pi-bonding, different names are applied.

3. Primary, Secondary, and Tertiary Alcohols

Primary, secondary, and tertiary alcohols are named according to the number of carbons directly attached to the C-OH carbon. (This carbon is sometimes known as the carbinol carbon). 

There is no such thing as a quaternary alcohol because that would require having 5 bonds to carbon.

See if you can classify the following alcohols as primary, secondary, or tertiary.

Click to Flip

4. Primary, Secondary, and Tertiary Alkyl Halides

Alkyl halides have an sp³ hybridized carbon directly attached to a halogen.

Like alcohols, they are named according to the number of carbons directly attached to the carbon containing the halogen.

See if you can properly classify the following alkyl halides as primary, secondary, or tertiary.

Click to Flip

Just to give you fair warning, being able to classify alkyl halides as primary, secondary or tertiary is a key skill. When you encounter nucleophilic substitution and elimination reactions, you’ll see that the reactivity of alkyl halides is significantly affected by their substitution pattern.

5. A Special Case: Methane And Methyl Groups

Up to this point we’ve left out the special case of methane, the only hydrocarbon which lacks any carbon-carbon bonds.

Methane, the methyl carbocation, methanol, and methyl halides are distinct from the “primary, secondary, tertiary” naming convention.

6. Primary, Secondary, and Tertiary Amines (and Quaternary Ammonium Salts)

Next, we come to amines, which are named a little bit differently than the previous examples. [Note 2]

Amines are named according to the number of carbons attached to the nitrogen.

Primary, secondary, and tertiary amines are nitrogens bound to one, two and three carbons, respectively.

It is possible for the nitrogen to be bound to a fourth carbon. This species is known as an alkylammonium salt. It is not technically an amine since it lacks a lone pair on nitrogen and cannot act as a base.

Remember that the positive formal charge on nitrogen doesn’t imply that there is an empty p orbital there! Always assume a full octet on positively charged nitrogen and oxygen.

See if you can tell the difference between these primary, secondary and tertiary amines.

Click to Flip

7. Primary, Secondary, and Tertiary Amides

The naming of amides is similar to that of amines.

A primary amide has the nitrogen bonded to one carbon – the carbonyl carbon.

Secondary and tertiary amides have the nitrogen bonded to two and three carbons respectively. (There is an amide counterpart to a quaternary ammonium salt, but it goes by a different name and you might go through introductory organic chemistry without ever seeing an example. – Note 3)

Quiz yourself on naming amides, here.

Click to Flip

Notes

Note 1.  These are often called Texas carbons because it resembles a star and because, y’know, “everything is bigger in Texas”.

Note 2. Note that methylamine is a primary amine, but methyl alcohol is not a primary alcohol.

Note 3. These are called acylammonium salts.

Quiz Yourself!

Quiz yourself here.

Click to Flip

(Advanced) References and Further Reading

Some IUPAC definitions on nomenclature:

1. Alcohols. From the IUPAC Gold Book.
   DOI: 10.1351/goldbook.A00204
2. Amines. From the IUPAC Gold Book.
   DOI: 10.1351/goldbook.A00274
3. Amides. From the IUPAC Gold Book.
   DOI: 10.1351/goldbook.A00266
   Note that IUPAC’s definition of primary, secondary, and tertiary amides is not the definition of primary, secondary, and tertiary amides that is in colloquial use.