- Characteristics of the bases
- Physical properties
- Strength of a base
- pH greater than 7
- Ability to neutralize acids
- Oxide reduction capacity
- They release OH
- Conjugate bases
- They have nitrogen atoms or substituents that attract electron density
- Bases with aromatic rings
- Examples of bases
- NaOH
- CH
- Alkali hydroxides
- Organic bases
- NaHCO
- References
The foundations are all those chemical compounds that can donate electrons or accept protons. In nature or artificially there are both inorganic and organic bases. Therefore its behavior can be predicted for many ionic molecules or solids.
However, what differentiates a base from the rest of the chemical substances is its marked tendency to donate electrons compared to, for example, species poor in electron density. This is possible only if the electronic pair is located. As a consequence of this, bases have electron-rich regions, δ-.
Soaps are weak bases formed by the reaction of fatty acids with sodium hydroxide or potassium hydroxide.
What organoleptic properties allow the bases to be identified? They are generally caustic substances, which cause severe burns through physical contact. At the same time, they have a soapy touch, and dissolve fats easily. Furthermore, its flavors are bitter.
Where are they in daily life? A commercial and routine source of foundations is cleaning products, from detergents to hand soaps. For this reason, the image of bubbles suspended in the air can help to remember the bases, even though behind them there are many physicochemical phenomena involved.
Many bases exhibit totally different properties. For example, some have foul and strong odors, such as organic amines. Others, on the other hand, such as ammonia, are penetrating and irritating. They can also be colorless liquids, or ionic white solids.
However, all bases have one thing in common: they react with acids to produce soluble salts in polar solvents, such as water.
Characteristics of the bases
Soap is a base
Apart from what has already been mentioned, what specific characteristics should all the bases have? How can they accept protons or donate electrons? The answer lies in the electronegativity of the atoms of the molecule or ion; and among all of them, oxygen is the predominant one, especially when it is found as an hydroxyl ion, OH -.
Physical properties
The bases have a sour taste and, except for ammonia, are odorless. Its texture is slippery and has the ability to change the color of litmus paper to blue, methyl orange to yellow, and phenolphthalein to purple.
Strength of a base
Bases are classified into strong bases and weak bases. The strength of a base is associated with its equilibrium constant, hence, in the case of bases, these constants are called basicity constants Kb.
Thus, strong bases have a large basicity constant so they tend to dissociate completely. Examples of these acids are alkalis such as sodium or potassium hydroxide, whose basicity constants are so large that they cannot be measured in water.
On the other hand, a weak base is one whose dissociation constant is low so it is in chemical equilibrium.
Examples of these are ammonia and amines whose acid constants are in the order of 10 -4. Figure 1 shows the different acidity constants for different bases.
Base dissociation constants.
pH greater than 7
The pH scale measures the alkalinity or acidity level of a solution. The scale ranges from zero to 14. A pH less than 7 is acidic. A pH greater than 7 is basic. Midpoint 7 represents a neutral pH. A neutral solution is neither acidic nor alkaline.
The pH scale is obtained as a function of the concentration of H + in the solution and is inversely proportional to this. Bases, by decreasing the concentration of protons, increase the pH of a solution.
Ability to neutralize acids
Arrhenius, in his theory, proposes that acids, being able to generate protons, react with the hydroxyl of the bases to form salt and water in the following way:
HCl + NaOH → NaCl + H 2 O.
This reaction is called neutralization and is the basis of the analytical technique called titration.
Oxide reduction capacity
Given their ability to produce charged species, bases are used as a medium for electron transfer in redox reactions.
Bases also have a tendency to oxidize since they have the ability to donate free electrons.
Bases contain OH- ions. They can act to donate electrons. Aluminum is a metal that reacts with bases.
2Al + 2NaOH + 6H 2 O → 2NaAl (OH) 4 + 3H 2
They do not corrode many metals, because metals tend to lose rather than accept electrons, but bases are highly corrosive to organic substances like those that make up the cell membrane.
These reactions are usually exothermic, which produces severe burns on contact with the skin, so this type of substance must be handled with care. Figure 3 is the safety indicator when a substance is corrosive.
Marking of corrosive substances.
They release OH
To begin with, OH - can be present in many compounds, mainly in metal hydroxides, since in the company of metals it tends to "take" protons to form water. Thus, a base can be any substance that releases this ion in solution through a solubility equilibrium:
M (OH) 2 <=> M 2+ + 2OH -
If the hydroxide is very soluble the equilibrium is totally shifted to the right of the chemical equation and we speak of a strong base. M (OH) 2, on the other hand, is a weak base, since it does not completely release its OH - ions into the water. Once the OH - is produced it can neutralize any acid that is around it:
OH - + HA => A - + H 2 O
And so the OH - deprotona to the acid HA to transform into water. Why? Because the oxygen atom is very electronegative and also has an excess electronic density due to the negative charge.
O has three pairs of free electrons, and can donate any one of them to the partially positively charged H atom, δ +. Also, the great energy stability of the water molecule favors the reaction. In other words: H 2 O is much more stable than HA, and when this is true the neutralization reaction will occur.
Conjugate bases
And what about OH - and A - ? Both are bases, with the difference that A - is the conjugate base of acid HA. Also, A - is a much weaker base than OH -. From here the following conclusion is reached: a base reacts to generate a weaker one.
Base Strong + Acid Strong => Base Weak + Acid Weak
As can be seen from the general chemical equation, the same is true for acids.
Conjugate base A - can deprotonate a molecule in a reaction known as hydrolysis:
A - + H 2 O <=> HA + OH -
However, unlike OH -, it establishes an equilibrium when neutralized with water. Again this is because A - is a much weaker base, but enough to cause a change in the pH of the solution.
Therefore, all those salts that contain A - are known as basic salts. An example of them is sodium carbonate, Na 2 CO 3, which after dissolving basifies the solution through the hydrolysis reaction:
CO 3 2– + H 2 O <=> HCO 3 - + OH -
They have nitrogen atoms or substituents that attract electron density
A base is not only ionic solids with OH - anions in their crystal lattice, but they can also have other electronegative atoms such as nitrogen. These types of bases belong to organic chemistry, and among the most common are amines.
What is the amine group? R-NH 2. On the nitrogen atom there is an unshared electronic pair, which can, like OH -, deprotonate a water molecule:
R – NH 2 + H 2 O <=> RNH 3 + + OH -
The equilibrium is far to the left, since amine, although basic, is much weaker than OH -. Note that the reaction is similar to that given for the ammonia molecule:
NH 3 + H 2 O <=> NH 4 + + OH -
Only that the amines cannot form the cation, NH 4 +; although RNH 3 + is the ammonium cation with a monosubstitution.
And can it react with other compounds? Yes, with anyone who has sufficiently acidic hydrogen, even if the reaction does not occur completely. That is, only a very strong amine reacts without establishing equilibrium. Likewise, amines can donate their pair of electrons to species other than H (such as alkyl radicals: -CH 3).
Bases with aromatic rings
Amines can also have aromatic rings. If its pair of electrons can be "lost" inside the ring, because the ring attracts electron density, then its basicity will decrease. Why? Because the more localized that pair is within the structure, the faster it will react with the electron-poor species.
For example, NH 3 is basic because its pair of electrons has nowhere to go. The same occurs with amines, whether they are primary (RNH 2), secondary (R 2 NH) or tertiary (R 3 N). These are more basic than ammonia because, in addition to what has just been explained, nitrogen attracts higher electronic densities of the R substituents, thus increasing δ-.
But when there is an aromatic ring, said pair can enter into resonance within it, making it impossible to participate in the formation of bonds with H or other species. Therefore, aromatic amines tend to be less basic, unless the electron pair remains fixed on nitrogen (as with the pyridine molecule).
Examples of bases
NaOH
Sodium hydroxide is one of the most widely used bases worldwide. Its applications are innumerable, but among them we can mention its use to saponify some fats and thus make basic salts of fatty acids (soaps).
CH
Structurally acetone may appear to not accept protons (or donate electrons), yet it does, even though it is a very weak base. This is because the electronegative O atom attracts the electron clouds of the CH 3 groups, accentuating the presence of its two pairs of electrons (: O:).
Alkali hydroxides
Besides NaOH, alkali metal hydroxides are also strong bases (with the slight exception of LiOH). Thus, among other bases there are the following:
-KOH: potassium hydroxide or caustic potash, it is one of the most widely used bases in the laboratory or in industry, due to its great degreasing power.
-RbOH: rubidium hydroxide.
-CsOH: cesium hydroxide.
-FrOH: francium hydroxide, whose basicity is theoretically presumed to be one of the strongest ever known.
Organic bases
-CH 3 CH 2 NH 2: ethylamine.
-LiNH 2: lithium amide. Together with the sodium amide, NaNH 2, they are one of the strongest organic bases. In them the amide anion, NH 2 - is the base that deprotonates water or reacts with acids.
-CH 3 ONa: sodium methoxide. Here the base is the anion CH 3 O -, which can react with acids to give methanol, CH 3 OH.
-The Grignard reagents: they have a metal atom and a halogen, RMX. In this case, the radical R is the base, but not because it takes away an acid hydrogen, but because it gives up its pair of electrons that it shares with the metal atom. For example: ethylmagnesium bromide, CH 3 CH 2 MgBr. They are very useful in organic synthesis.
NaHCO
Baking soda is used to neutralize acidity in mild conditions, for example, inside the mouth as an additive in toothpastes.
References
- Merck KGaA. (2018). Organic Bases. Taken from: sigmaaldrich.com
- Wikipedia. (2018). Bases (chemistry). Taken from: es.wikipedia.org
- Chemistry 1010. Acids and Bases: What They Are and Where They Are Found.. Taken from: cactus.dixie.edu
- Acids, Bases, and the pH Scale. Taken from: 2.nau.edu
- The Bodner Group. Definitions of Acids and Bases and the Role of Water. Taken from: chemed.chem.purdue.edu
- Chemistry LibreTexts. Bases: Properties and Examples. Taken from: chem.libretexts.org
- Shiver & Atkins. (2008). Inorganic chemistry. In Acids and Bases. (fourth edition). Mc Graw Hill.
- Helmenstine, Todd. (August 04, 2018). Names of 10 Bases. Recovered from: thoughtco.com