- Properties of anhydrides
- Chemical reactions
- Hydrolysis
- Esterification
- Amidation
- How are anhydrides formed?
- Cyclic anhydrides
- Nomenclature
- Applications
- Organic anhydrides
- Examples
- Succinic anhydride
- Glutaric anhydride
- References
The anhydrides are chemical compounds originating from the union of two molecules by releasing water. Thus, it could be seen as a dehydration of the initial substances; although it is not exactly true.
In organic and inorganic chemistry they are mentioned, and in both branches their understanding differs to an appreciable degree. For example, in inorganic chemistry the basic and acid oxides are considered as the anhydrides of their hydroxides and acids respectively, since the former react with water to form the latter.
General structure of anhydrides. Source: DrEmmettBrownie, from Wikimedia Commons
Here, confusion can arise between the terms 'anhydrous' and 'anhydride'. Generally, anhydrous refers to a compound to which it has been dehydrated without changes in its chemical nature (there is no reaction); while with an anhydride, there is a chemical change, reflected in the molecular structure.
If the hydroxides and acids are compared with their corresponding oxides (or anhydrides), it will be seen that there was a reaction. In contrast, some oxides or salts can be hydrated, lose water, and remain the same compounds; but, without water, that is, anhydrous.
In organic chemistry, on the other hand, what is meant by anhydride is the initial definition. For example, one of the best known anhydrides are those derived from carboxylic acids (upper image). These consist of the union of two acyl groups (-RCO) through an oxygen atom.
In its general structure, R 1 is indicated for one acyl group, and R 2 for the second acyl group. Because R 1 and R 2 are different, they come from different carboxylic acids and it is then an asymmetric acid anhydride. When both R substituents (whether or not they are aromatic) are the same, in this case we speak of a symmetric acid anhydride.
When two carboxylic acids bind to form the anhydride, water may or may not form, as well as other compounds. Everything will depend on the structure of these acids.
Properties of anhydrides
The properties of anhydrides will depend on which ones you are referring to. Most of them have in common that they react with water. However, for the so-called basic anhydrides in inorganic, actually several of them are even insoluble in water (MgO), so this statement will focus for the anhydrides of carboxylic acids.
The melting and boiling points lie in the molecular structure and in the intermolecular interactions for (RCO) 2 O, this being the general chemical formula of these organic compounds.
If the molecular mass of (RCO) 2 O is low, it is probably a colorless liquid at room temperature and pressure. For example, acetic anhydride (or ethanoic anhydride), (CH 3 CO) 2 O, is a liquid and the one of greatest industrial importance, its production being very vast.
The reaction between acetic anhydride and water is represented by the following chemical equation:
(CH 3 CO) 2 O + H 2 O => 2CH 3 COOH
Note that when the water molecule is added, two molecules of acetic acid are released. The reverse reaction, however, cannot occur for acetic acid:
2CH 3 COOH => (CH 3 CO) 2 O + H 2 O (Does not occur)
It is necessary to resort to another synthetic route. Dicarboxylic acids, on the other hand, can do so by heating; but it will be explained in the next section.
Chemical reactions
Hydrolysis
One of the simplest reactions of anhydrides is their hydrolysis, which has just been shown for acetic anhydride. In addition to this example, there is that of sulfuric acid anhydride:
H 2 S 2 O 7 + H 2 O <=> 2H 2 SO 4
Here you have an inorganic acid anhydride. Note that for H 2 S 2 O 7 (also called disulfuric acid), the reaction is reversible, so heating concentrated H 2 SO 4 results in the formation of its anhydride. If, on the other hand, it is a dilute solution of H 2 SO 4, SO 3, sulfuric anhydride, is released.
Esterification
Acid anhydrides react with alcohols, with pyridine in the middle, to give an ester and a carboxylic acid. For example, consider the reaction between acetic anhydride and ethanol:
(CH 3 CO) 2 O + CH 3 CH 2 OH => CH 3 CO 2 CH 2 CH 3 + CH 3 COOH
Thus forming the ethyl ethanoate ester, CH 3 CO 2 CH 2 CH 3, and ethanoic acid (acetic acid).
Practically, what happens is the substitution of the hydrogen of the hydroxyl group, by an acyl group:
R 1 -OH => R 1 -OCOR 2
In the case of (CH 3 CO) 2 O, its acyl group is -COCH 3. Therefore, the OH group is said to be undergoing acylation. However, acylation and esterification are not interchangeable concepts; acylation can occur directly on an aromatic ring, known as the Friedel-Crafts acylation.
Thus, alcohols in the presence of acid anhydrides are esterified by acylation.
On the other hand, only one of the two acyl groups reacts with the alcohol, the other stays with the hydrogen, forming a carboxylic acid; that for the case of (CH 3 CO) 2 O, it is ethanoic acid.
Amidation
Acid anhydrides react with ammonia or amines (primary and secondary) to give rise to amides. The reaction is very similar to the esterification just described, but the ROH is replaced by an amine; for example, a secondary amine, R 2 NH.
Again, the reaction between (CH 3 CO) 2 O and diethylamine, Et 2 NH is considered:
(CH 3 CO) 2 O + 2Et 2 NH => CH 3 CONEt 2 + CH 3 COO - + NH 2 Et 2
And diethylacetamide, CH 3 CONEt 2, and a carboxylated ammonium salt, CH 3 COO - + NH 2 Et 2 are formed.
Although the equation may seem a bit difficult to understand, it is enough to observe how the group –COCH 3 replaces the H of an Et 2 NH to form the amide:
Et 2 NH => Et 2 NCOCH 3
Rather than amidation, the reaction is still acylation. Everything is summed up in that word; this time, the amine undergoes acylation and not the alcohol.
How are anhydrides formed?
Inorganic anhydrides are formed by reacting the element with oxygen. Thus, if the element is metallic, a metallic oxide or basic anhydride is formed; and if it is non-metallic, a non-metallic oxide or acid anhydride is formed.
For organic anhydrides, the reaction is different. Two carboxylic acids cannot join directly to release water and form acid anhydride; the participation of a compound that has not been mentioned yet is necessary: acyl chloride, RCOCl.
The carboxylic acid reacts with the acyl chloride, producing the respective anhydride and hydrogen chloride:
R 1 COCl + R 2 COOH => (R 1 CO) O (COR 2) + HCl
CH 3 COCl + CH 3 COOH => (CH 3 CO) 2 O + HCl
One CH 3 comes from the acetyl group, CH 3 CO–, and the other is already present in acetic acid. The choice of a specific acyl chloride, as well as the carboxylic acid, can lead to the synthesis of a symmetric or asymmetric acid anhydride.
Cyclic anhydrides
Unlike the other carboxylic acids that require an acyl chloride, dicarboxylic acids can condense into their corresponding anhydride. To do this, they need to be heated to promote the release of H 2 O. For example, the formation of phthalic anhydride from phthalic acid is shown.
Formation of phthalic anhydride. Source: Jü, from Wikimedia Commons
Note how the pentagonal ring is completed, and the oxygen that joins both C = O groups is part of it; this is a cyclic anhydride. Also, it can be seen that phthalic anhydride is a symmetric anhydride, since both R 1 and R 2 are identical: an aromatic ring.
Not all dicarboxylic acids are capable of forming their anhydride, since when their COOH groups are widely separated, they are forced to complete larger and larger rings. The largest ring that can be formed is a hexagonal one, larger than that the reaction does not take place.
Nomenclature
How are anhydrides named? Leaving aside the inorganic ones, pertinent to oxide issues, the names of the organic anhydrides explained so far depend on the identity of R 1 and R 2; that is, of its acyl groups.
If the two R's are the same, simply replace the word 'acid' with 'anhydride' in the respective name of the carboxylic acid. And if, on the contrary, the two Rs are different, they are named in alphabetical order. Therefore, to know what to call it, you must first see if it is a symmetric or asymmetric acid anhydride.
(CH 3 CO) 2 O is symmetric, since R 1 = R 2 = CH 3. It derives from acetic or ethanoic acid, so its name is, following the previous explanation: acetic or ethanoic anhydride. The same is true of the phthalic anhydride just mentioned.
Suppose we have the following anhydride:
CH 3 CO (O) COCH 2 CH 2 CH 2 CH 2 CH 2 CH 3
The acetyl group on the left comes from acetic acid, and the one on the right comes from heptanoic acid. To name this anhydride you must name its R groups in alphabetical order. So, its name is: heptanoic acetic anhydride.
Applications
Inorganic anhydrides have a myriad of applications, from the synthesis and formulation of materials, ceramics, catalysts, cements, electrodes, fertilizers, etc., to as a coating of the earth's crust with its thousands of iron and aluminum minerals, and dioxide of carbon exhaled by living organisms.
They represent the starting source, the point where many compounds used in inorganic syntheses are derived. One of the most important anhydrides is carbon dioxide, CO 2. It is, along with water, essential for photosynthesis. And at an industrial level, SO 3 is essential since the demanded sulfuric acid is obtained from it.
Perhaps, the anhydride with the most applications and for having (while there is life) is one from phosphoric acid: adenosine triphosphate, better known as ATP, present in DNA and the "energy currency" of metabolism.
Organic anhydrides
Acidic anhydrides react through acylation, either to an alcohol, forming an ester, an amine, giving rise to an amide, or an aromatic ring.
There are millions of each of these compounds, and hundreds of thousands of carboxylic acid options to make an anhydride; therefore, the synthetic possibilities grow dramatically.
Thus, one of the main applications is to incorporate an acyl group into a compound, substituting one of the atoms or groups of its structure.
Each separate anhydride has its own applications, but generally speaking they all react in a similar way. For this reason, these types of compounds are used to modify polymeric structures, creating new polymers; i.e. copolymers, resins, coatings, etc.
For example, acetic anhydride is used to acetylate all the OH groups of cellulose (lower image). With this, each H of the OH is replaced by an acetyl group, COCH 3.
Cellulose. Source: NEUROtiker, from Wikimedia Commons
In this way, the cellulose acetate polymer is obtained. The same reaction can be outlined with other polymeric structures with NH 2 groups, also susceptible to acylation.
These acylation reactions are also useful for the synthesis of drugs, such as aspirin (acetylsalicylic acid).
Examples
Some other examples of organic anhydrides are shown to finish. Although no mention will be made of them, oxygen atoms can be replaced by sulfur, giving sulfur, or even phosphorous anhydrides.
-C 6 H 5 CO (O) COC 6 H 5: benzoic anhydride. The group C 6 H 5 represents a benzene ring. Its hydrolysis produces two benzoic acids.
-HCO (O) COH: formic anhydride. Its hydrolysis produces two formic acids.
- C 6 H 5 CO (O) COCH 2 CH 3: benzoic propanoic anhydride. Its hydrolysis produces benzoic and propanoic acids.
-C 6 H 11 CO (O) COC 6 H 11: cyclohexanecarboxylic anhydride. Unlike aromatic rings, these are saturated, without double bonds.
-CH 3 CH 2 CH 2 CO (O) COCH 2 CH 3: propanoic butanoic anhydride.
Succinic anhydride
Succinic anhydride. Source: Ninjatacoshell, from Wikimedia Commons
Here you have another cyclic, derived from succinic acid, a dicarboxylic acid. Note how the three oxygen atoms reveal the chemical nature of this type of compound.
Maleic anhydride is very similar to succinic anhydride, with the difference that there is a double bond between the carbons that form the base of the pentagon.
Glutaric anhydride
Glutaric anhydride. Source: Choij, from Wikimedia Commons
And finally, glutaric acid anhydride is shown. This structurally differs from all the others by consisting of a hexagonal ring. Again, the three oxygen atoms stand out in the structure.
Other anhydrides, more complex, can always be evidenced by the three oxygen atoms very close to each other.
References
- The Editors of Encyclopaedia Britannica. (2019). Anhydride. Enclyclopaedia Britannica. Recovered from: britannica.com
- Helmenstine, Anne Marie, Ph.D. (January 8, 2019). Acid Anhydride Definition in Chemistry. Recovered from: thoughtco.com
- Chemistry LibreTexts. (sf). Anhydrides. Recovered from: chem.libretexts.org
- Graham Solomons TW, Craig B. Fryhle. (2011). Organic Chemistry. Amines. (10 th edition.). Wiley Plus.
- Carey F. (2008). Organic Chemistry. (Sixth edition). Mc Graw Hill.
- Whitten, Davis, Peck & Stanley. (2008). Chemistry. (8th ed.). CENGAGE Learning.
- Morrison and Boyd. (1987). Organic chemistry. (Fifth edition). Addison-Wesley Iberoamericana.
- Wikipedia. (2019). Organic acid anhydride. Recovered from: en.wikipedia.org