7 junio, 2024

Primary carbon: what it is, characteristics, types and examples

What is primary carbon?

He primary carbon It is one that in any compound, regardless of its molecular environment, forms a bond with at least one other carbon atom. This bond can be simple, 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 primary, secondary, and tertiary hydrogens differ little and are predominantly subject to the molecular environments of carbon. It is for this reason that the primary carbon (1°) is usually treated with more importance than its hydrogens.

A primary carbon looks depending on its molecular or chemical environment. For example, the image above indicates the primary carbons, circled in red, in the structure of a hypothetical (although probably real) molecule.

If they are carefully observed, three of them will be found to be identical, while the other three are totally different. The first three consist of methyl groups, -CH3 (to the right of the molecule), and the others are the methylol groups, -CH2OH, nitrile, -CN, and an amide, RCONH2 (to the left of the molecule and below she).

Primary carbon characteristics

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. That is why they are sometimes referred to as terminal carbons.
The -CH3 groups are terminal and their carbon is 1°. Notice that this carbon bonds to three hydrogens (which have been omitted in the image) and to a single carbon, completing their four respective bonds.
All are characterized by having a CC bond, a bond that can also be double (C=CH2) or triple (C≡CH). This is true even if there are other atoms or groups attached to those carbons, as is the case with the other three remaining 1° carbons in the image.

low steric hindrance

By marking the end of a stretch of the skeleton, there are no other atoms interfering with them spatially. For example, -CH3 groups can interact with atoms of other molecules, but their interactions with neighboring atoms of the same molecule are low. The same applies to -CH2OH and -CN.
They are practically exposed to the “vacuum”. Therefore, they generally exhibit low steric hindrance relative to the other carbon types (2nd, 3rd, and 4th).
There are exceptions, due to 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 carbon 1 is a greater exposure to react with other molecules. The fewer atoms blocking the path of the attacking molecule towards it, the more likely its reaction will be.
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 carbons 1°.
The carbon adjacent to the primary transfers part of its electronic density to it, and the same can happen in the opposite direction, favoring a certain type of chemical reaction.
The steric and electronic factors explain why it is usually the most reactive, although there is not really a global reactivity rule for all primary carbons.

Guys

The primary carbons lack an intrinsic classification. Instead, they are classified according to the groups of atoms to which they belong or 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 -CH2OH is derived from the primary alcohol RCH2OH. 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 C-CN single bond. Thus, the existence of secondary (R2CN) or much less tertiary (R3CN) nitriles could not be expected.

A similar case occurs with the substituent derived from the amide, -CONH2. It can undergo substitutions of the hydrogens of the nitrogen atom, but its carbon can only bond to another carbon, and, therefore, it will always be considered as primary, C-CONH2.

And with respect to the -CH3 group, it is an alkyl substituent, which can only be linked to another carbon, thus being primary. If, on the other hand, the ethyl group, -CH2CH3, is considered, it will be immediately noted that the CH2, methylene group, is a 2nd carbon because it is bonded to two carbons (C-CH2CH3).

examples

aldehydes and carboxylic acids

Mention has been made of some examples of primary carbons. Additional to them is the following pair of groups: -CHO and -COOH, called formyl and carboxyl, respectively. The carbons in these two groups are primary, as they will always form compounds with the 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 become carboxyl:

RCHO => RCOOH

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

In linear amines

The classification of amines depends exclusively on the degree of substitution of the hydrogens of the -NH2 group. However, in linear amines primary carbons can be observed, as in propanamine:

CH3-CH2-CH2-NH2

Note that CH3 will always be a 1st carbon, but this time the CH2 on the right is also 1st, since it is bonded to only one carbon and the NH2 group.

In alkyl halides

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

CH3-CH2-CH2-Br

In it the primary carbons remain the same.

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

References

Graham Solomons, TW, Craig B. Fryhle. Organic Chemistry. Amines (10th ed.). WileyPlus.
Carey F. Organic Chemistry. (Sixth edition). Mc Graw Hill.
Morrison, RT and Boyd, RN Organic Chemistry. (5th Edition). Editorial Addison-Wesley Interamericana.
Primary carbon. Retrieved from en.wikipedia.org.

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