PRIMARY CARBON: CHARACTERISTICS, TYPES AND EXAMPLES - CHEMISTRY - 2025

The primary carbon is one that in any compound, regardless of its molecular environment, form link to the least one carbon atom. This bond can be single, double (=), or triple (≡), as long as there are only two carbon atoms linked and in adjacent positions (logically).

The hydrogens present on this carbon are called primary hydrogens. However, the chemical characteristics of the primary, secondary and tertiary hydrogens differ little and are predominantly subject to the molecular environments of carbon. It is for this reason that primary carbon (1 °) is usually treated with more importance than its hydrogens.

Primary carbons in hypothetical molecule. Source: Gabriel Bolívar.

And what does a primary carbon look like? The answer depends, as has been mentioned, on your molecular or chemical environment. For example, the image above shows the primary carbons, enclosed in red circles, in the structure of a hypothetical (although probably real) molecule.

If you look carefully, you will find that three of them are identical; while the other three are totally different. The first three consist of methyl groups, -CH ₃ (to the right of the molecule), and the others are the methylol groups, -CH ₂ OH, nitrile, -CN, and an amide, RCONH ₂ (to the left of the molecule and below it).

Characteristics of primary carbon

Location and links

Six primary carbons were shown above, with no comment other than their locations and what other atoms or groups accompany them. They can be anywhere in the structure, and wherever they are, they mark the "end of the road"; that is, where a section of the skeleton ends. This is why they are sometimes referred to as terminal carbons.

Thus, it is obvious that the -CH ₃ groups are terminal and their carbon is 1 °. Note that this carbon binds to three hydrogens (which have been omitted in the image) and to a single carbon, completing their four respective bonds.

Therefore, they are all characterized by having a CC bond, a bond that can also be double (C = CH ₂) or triple (C≡CH). This remains true even if there are other atoms or groups attached to these carbons; just like the other three remaining 1 ° carbons in the image.

Low steric impediment

It was mentioned that the primary carbons are terminal. By signaling the end of a stretch of the skeleton, there are no other atoms interfering with them spatially. For example, -CH ₃ groups can interact with atoms of other molecules; but their interactions with neighboring atoms of the same molecule are low. The same is true for -CH ₂ OH and -CN.

This is because they are practically exposed to "vacuum". Therefore, they generally present low steric hindrance in relation to the other types of carbon (2nd, 3rd and 4th).

However, there are exceptions, the product of a molecular structure with too many substituents, high flexibility, or a tendency to close in on itself.

Reactivity

One of the consequences of the lower steric hindrance around the 1st carbon is a greater exposure to react with other molecules. The fewer atoms blocking the path of the attacking molecule towards it, the more likely it will react.

But, this is true only from the steric point of view. Actually the most important factor is the electronic one; that is, what is the environment of said 1 ° carbons.

The carbon adjacent to the primary one, transfers part of its electron density to it; and the same can happen in the opposite direction, favoring a certain type of chemical reaction.

Thus, the steric and electronic factors explain why it is usually the most reactive; although, there really is no global reactivity rule for all primary carbons.

Types

Primary carbons lack an intrinsic classification. Instead, they are classified on the basis of the groups of atoms to which they belong or to which they are bonded; These are the functional groups. And since each functional group defines a specific type of organic compound, there are different primary carbons.

For example, the group -CH ₂ OH is derived from the primary alcohol RCH ₂ OH. Primary alcohols therefore consist of 1 ° carbons attached to the hydroxyl group, -OH.

The nitrile group, -CN or -C≡N, on the other hand, can only be attached directly to a carbon atom via the single C-CN bond. In this way, the existence of secondary (R ₂ CN) or much less tertiary (R ₃ CN) nitriles would not be expected.

A similar case occurs with the substituent derived from the amide, -CONH ₂. It can undergo substitutions of the hydrogens of the nitrogen atom; but its carbon can only bind to another carbon, and therefore it will always be considered as primary, C-CONH ₂.

And with respect to the group -CH ₃, it is an alkyl substitute which can only be linked to another carbon, being therefore primary. If the ethyl group, -CH ₂ CH ₃, is considered on the other hand, it will immediately be noted that the CH ₂, methylene group, is a 2 ° carbon because it is linked to two carbons (C-CH ₂ CH ₃).

Examples

Aldehydes and carboxylic acids

Mention has been made of some examples of primary carbons. In addition to them, there is the following pair of groups: -CHO and -COOH, called formyl and carboxyl, respectively. The carbons of these two groups are primary, since they will always form compounds with formulas RCHO (aldehydes) and RCOOH (carboxylic acids).

This pair is closely related to each other due to the oxidation reactions that the formyl group undergoes to transform into carboxyl:

RCHO => RCOOH

Reaction suffered by aldehydes or the -CHO group if it is as a substituent in a molecule.

In linear amines

The classification of amines depends exclusively on the degree of substitution of the -NH ₂ group hydrogens. However, in linear amines, primary carbons can be observed, as in propanamine:

CH ₃ -CH ₂ -CH ₂ -NH ₂

Note that CH ₃ will always be a 1st carbon, but this time the CH ₂ on the right is also 1st as it is bonded to a single carbon and the NH ₂ group.

In alkyl halides

An example very similar to the previous one is given with the alkyl halides (and in many other organic compounds). Suppose bromopropane:

CH ₃ -CH ₂ -CH ₂ -Br

In it the primary carbons remain the same.

By way of conclusion, the 1st carbons transcend the type of organic compound (and even organometallic), because they can be present in any of them and are identified simply because they are linked to a single carbon.

References

1. Graham Solomons TW, Craig B. Fryhle. (2011). Organic Chemistry. Amines. (10 ^th edition.). Wiley Plus.
2. Carey F. (2008). Organic Chemistry. (Sixth edition). Mc Graw Hill.
3. Morrison, RT and Boyd, RN (1987). Organic Chemistry. (5 ^ta Edition). Editorial Addison-Wesley Interamericana.
4. Ashenhurst J. (June 16, 2010). Primary, Secondary, Tertiary, Quaternary In Organic Chemistry. Master Organic Chemistry. Recovered from: masterorganicchemistry.com
5. Wikipedia. (2019). Primary carbon. Recovered from: en.wikipedia.org