The amino group is one that is present in various nitrogenous organic compounds, for example amines, and is represented by the formula -NH 2. Amines are the most representative compounds where we find this group, since when they are aliphatic they have the formula RNH 2; while when they are aromatic, they have the formula ArNH 2.
Amides, RC (O) NH 2, with the carbonyl group C = O, are also another example of compounds containing the amino group. In many other compounds, amino groups are found as mere substituents, since in the rest of the structure there may be oxygenated groups with greater chemical relevance.
Amino group highlighted with blue color. Source: MaChe / Public domain
The amino group is considered a by-product of ammonia, NH 3. As its three NH bonds are replaced by NC bonds, primary, secondary, and tertiary amines emerge, respectively. The same reasoning applies to amides.
Compounds with amino groups are characterized by being basic or alkaline substances. They are also part of a myriad of biomolecules, such as proteins and enzymes, and pharmaceutical products. Of all the functional groups, it is probably the most diverse due to the substitutions or transformations that it is capable of undergoing.
Structure
Structural formula of the amino group. Source: Kes47 via Wikipedia.
In the upper image we have the structural formula of the amino group. In it, its molecular geometry is discovered, which is tetrahedral. The aliphatic side chain R 1, and the two hydrogen atoms H, are positioned at the ends of a tetrahedron, while the lone pair of electrons is at the top. Hence, the wedges move away from or out of the plane of the observer.
From a stereochemical point of view, the NH 2 group is highly mobile; it is dynamic, its R 1 -N bond can rotate or vibrate, and the same happens with its NH bonds. The geometry of this group is not affected by the incorporation of other R 2 or R 3 side chains.
This means that the tetrahedral geometry observed for this primary amine remains the same as for the secondary (R 2 NH) or tertiary (R 3 N) amines. However, it is normal to expect that the angles of the tetrahedron will be distorted, since there will be greater electronic repulsion around the nitrogen atom; that is, R 1, R 2, and R 3 will repel each other.
And not to mention the space occupied by the lone pair of electrons on nitrogen, which can form bonds with the protons in the middle. Hence the basicity of the amino group.
Properties
Basicity
The amino group is characterized by being basic. Therefore, its aqueous solutions must have pH values above 7, with the presence of OH - anions predominant. This is explained by its hydrolysis equilibrium:
RNH 2 + H 2 O ⇌ RNH 3 + + OH -
Being RNH 3 + the resulting conjugated acid. The R side chain helps decrease the density of positive charge that now appears on the nitrogen atom. Thus, the more R groups there are, the less this positive charge will "feel", so the stability of the conjugated acid will increase; which in turn, implies that the amine is more basic.
A similar reasoning can be applied considering that the R chains contribute electronic density to the nitrogen atom, "reinforcing" the negative density of the lone pair of electrons, thereby increasing the basic character of the amine.
It is then said that the basicity of the amino group increases as it is more substituted. Of all the amines, the tertiary ones are the most basic. The same happens with amides and other compounds.
Polarity and intermolecular interactions
Amino groups confer polarity to the molecule to which they are attached due to their electronegative nitrogen atom.
Therefore, compounds that have NH 2 are not only basic, they are also polar. This means that they tend to solubilize in polar solvents like water or alcohols.
Its melting or boiling points are also considerably high, a product of dipole-dipole interactions; specifically, of the hydrogen bridges that are established between two NH 2 of neighboring molecules (RH 2 N-HNHR).
It is expected that the more substituted the amino group is, the less likely it is to form a hydrogen bond. For example, tertiary amines cannot even establish one because they are completely devoid of hydrogens (R 3 N: -: NR 3).
Even when the NH 2 group contributes polarity and strong molecular interactions to the compound, its effect is less compared, for example, to that of the OH or COOH groups.
Acidity
Although the amino group is distinguished by its basicity, it also has a certain acid character: it will react with strong bases or can be neutralized by them. Consider the following neutralization reaction:
RNH 2 + NaOH → RNHNa + H 2 O
In it, the anion RNH - is formed, which electrostatically attracts the sodium cation. Water is a weak base compared to NaOH or KOH, capable of neutralizing NH 2 and causing it to behave like an acid.
Examples
Some examples of compounds containing the NH 2 group, without substitutions, will be listed below; that is, secondary or tertiary amines will not be considered. We then have:
-Methylamine, CH 3 NH 2
-Ethylamine, CH 3 CH 2 NH 2
-Butanamine, CH 3 CH 2 CH 2 CH 2 NH 2
-Isobutylamine, (CH 3) 2 CHNH 2
-Formamide, HCONH 2
-Hydroxylamine, NH 2 OH
-Benzylamine, C 6 H 5 CH 2 NH 2
-Acrylamide, CH 2 = CHCONH 2
-Phenylamine, C 6 H 5 NH 2
-Arginine, with R = - (CH 2) 3 NH-C (NH) NH 2
-Asparagine, with R = -CH 2 CONH 2
-Glutamine, with R = -CH 2 CH 2 CONH 2
-Lysin, with R = - (CH 2) 4 NH 2
The last four examples correspond to amino acids, fundamental pieces with which proteins are built and whose molecular structures have both the NH 2 group and the COOH group.
These four amino acids contain in their side chains R a NH 2 further, so that the formation of the peptide bond (union of two amino acids by their ends NH 2 and COOH) does not disappear NH 2 in the resulting proteins.
Histamine, another example of compounds with the NH2 group. Source: Vaccinationist / Public domain
In addition to amino acids, in the human body we have other compounds that carry the NH 2 group: such is the case of histamine (above), one of many neurotransmitters. Note how highly nitrogenous its molecular structure is.
Structural formula of amphetamine. Source: Boghog / Public domain
Structural formula of serotonin. Source: CYL / Public domain
And finally, we have other examples of substances that play a role in the central nervous system: amphetamine and serotonin. The first is a stimulant used to treat some mental disorders, and the second is a neurotransmitter popularly associated with happiness.
References
- Graham Solomons TW, Craig B. Fryhle. (2011). Organic Chemistr and. (10 th edition.). Wiley Plus.
- Carey F. (2008). Organic Chemistry. (Sixth edition). Mc Graw Hill.
- Morrison and Boyd. (1987). Organic chemistry. (Fifth edition). Addison-Wesley Iberoamericana.
- Wikipedia. (2020). Amine. Recovered from: en.wikipedia.org
- Peter AS Smith & Eric Block. (2020). Amine. Encyclopædia Britannica. Recovered from: britannica.com
- Brian C. Smith. (March 1, 2019). Organic Nitrogen Compounds II: Primary Amines. Recovered from: spectroscopyonline.com
- William Reusch. (May 5, 2013). Chemistry of Amines. Recovered from: 2.chemistry.msu.edu