Today, we will talk about a terminology that you are going to use very often in your Organic Chemistry 1 class. That is the definition of primary, secondary, tertiary, and quaternary carbon atoms in organic chemistry. This is also referred to as the carbon degree of substitution.

 

 

In short, these definitions are assigned to carbon atoms based on the number of other carbon atoms they are connected to:

Primary carbons are connected to one carbon only.
Secondary carbons are connected to two carbon atoms.
Tertiary carbons are connected to three carbon atoms.
And if four carbons are connected to a carbon, then it is a quaternary carbon.

You will encounter this notation when starting to learn the functional groups and continue using it in the nomenclature of organic compounds and later in the semester.

Let’s start by looking at alkanes. The first molecule in this series is methane, and because there is only one carbon, it is not classified by any of the definitions we just discussed.

 

 

The carbons in ethane can, however, be classified. And for this, you need to count the number of carbons that are directly connected to the carbon of interest. We can mark this carbon in red and circle the ones connected to it in blue.

 

 

Notice that it is a symmetrical molecule, and regardless of which carbon you choose, it is going to be a primary carbon, since there is going to be only one carbon connected to it.

Starting from propane, we can locate a primary and a secondary carbon. The CH₃ carbons are primary as they are only connected to the CH₂ carbon. However, the middle carbon (CH₂) is connected to two carbon atoms; therefore, it is a secondary carbon.

 

 

The following isomer of butane contains a tertiary carbon. The central carbon is connected to three carbon atoms of the CH₃ groups:

 

 

Finally, if four carbon atoms are connected to a carbon, then we have a quaternary carbon:

 

 

Before moving on to other functional groups, let’s see how the degree of substitution (1^o, 2^o, 3^o) is also used to distinguish certain fragments in naming organic molecules. For example, butane has four isomers and therefore, four isomeric alkyl groups. Depending on which carbon of the butyl is connected to the parent chain, or another group, we have an n-butyl, sec-butyl, and tert-butyl.

 

 

And these are simply the abbreviations for the carbon’s degree of unsaturation: secondary-sec, tertiary-tert.

This designation of carbon atoms is also applied to carbocations, alkyl halides, alcohols, amines, and amides.

 

Primary, Secondary, and Tertiary Carbocations

The carbocations range from methyl carbocation to tertiary carbocation. The principle for classifying the carbon atoms is the same. You need to count how many carbon atoms are connected to the positively charged carbon.

 

 

There is no such thing as a quaternary carbocation since the four connected carbons satisfy the octet and the central carbon cannot have a formal charge.

 

Primary, Secondary, and Tertiary Alkyl Halides

This classification will be especially important in the nucleophilic substitution and elimination reactions. To distinguish between a primary, secondary, or tertiary alkyl halide, locate the carbon that is connected to the halogen and count how many carbon atoms are connected to it:

 

The nomenclature of alkyl halides is covered in this post. 

 

Primary, Secondary, and Tertiary Alcohols

Depending on the number of carbon atoms connected to the one with the hydroxyl group, the alcohols are also classified as primary, secondary, and tertiary:

 

The nomenclature of alcohols is covered in this post. 

 

Primary, Secondary, and Tertiary Amines

There is a little difference in the way amines are classified! Unlike the previous cases, the amines are classified based on the number of carbons connected to the nitrogen:

 

Another difference with the amines is that the nitrogen can have four groups connected by using the lone pair and getting a positive formal charge. These are called quaternary ammonium salts.

The nomenclature of amines is covered in this post. 

 

Primary, Secondary, and Tertiary Amides

Similar to this, amides are also classified as primary, secondary, and tertiary based on the number of carbons connected to the nitrogen.

 

The nomenclature of amides is covered in this post. 

 

Check Also

• Naming Alkanes by IUPAC Nomenclature Rules Practice Problems
• Naming Bicyclic Compounds
• Naming Bicyclic Compounds-Practice Problems
• How to Name a Compound with Multiple Functional Groups
• Constitutional or Structural Isomers with Practice Problems
• Degrees of Unsaturation or Index of Hydrogen Deficiency
• The Wedge and Dash Representation
• Sawhorse Projections
• Newman Projections with Practice Problems
• Staggered and Eclipsed Conformations
• Conformational Isomers of Propane
• Newman Projection and Conformational Analysis of Butane
• Newman Projection of Chair Conformation
• Gauche Conformation
• Gauche Conformation, Steric, Torsional Strain Energy Practice Problems
• Ring Strain
• Steric vs Torsional Strain
• Conformational Analysis
• Drawing the Chair Conformation of Cyclohexane
• Ring Flip: Drawing Both Chair Conformations with Practice Problems
• 1,3-Diaxial Interactions and A value for Cyclohexanes
• Ring-Flip: Comparing the Stability of Chair Conformations with Practice Problems
• Cis and Trans Decalin
• IUPAC Nomenclature Practice Problems
• IUPAC Nomenclature Summary Quiz
• Alkanes and Cycloalkanes Practice Quiz