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Also known as double displacement, double decomposition or metathesis reactions, are those in which two chemical substances react with each other exchanging neutral groups or ions of the same charge to give rise to two new compounds in which the same types of bonds are present. .
Double replacement reactions can be viewed as a pair swapping. The two original pairs represent the two starting reagents, and after the reaction two new pairs are obtained.
During a metathesis or double replacement reaction, one of the groups or ions in one molecule displaces a similar group in the other, while one of the groups in the second displaces a similar group in the first. This is the reason why they are called double displacement reactions.
How to recognize double replacement reactions?
Metathesis reactions are easy to recognize. First, they always involve two reactants and two products. Secondly, they always follow the following scheme:
Both A and B, C and D can correspond to individual atoms or to groups of atoms. As can be seen in this schematic, group A is displacing or replacing group C in the CD molecule, while, at the same time, group C replaces group A (or group D replaces group D, depending on the point). of sight).
The bond between groups A and B and between C and D can be both ionic and covalent. The only thing that matters is that the link formed in the two products is of the same type.
Types of double replacement reactions
Depending on the types of reagents and the types of products that are formed, different types of metathesis or double replacement reactions can be distinguished:
salt metathesis
In salt metathesis reactions both reactants and both products are salts (ionic compounds). The general scheme shown above, in the case of salt metathesis looks like this:
In these cases, the two reactants are usually salts soluble in water or another solvent, while one of the two products is an insoluble salt that precipitates.
Examples of double salt replacement reactions
Most precipitation reactions are double replacement reactions. Some examples are:
In this reaction, silver(I) and potassium ions replace each other to form the insoluble silver chloride salt and the soluble silver nitrate salt that remains in solution.
Here, sodium and barium cations are exchanged to give insoluble barium sulfate and sodium chloride that remains in solution.
Acid-Base Neutralization Reactions
Another very common class of double displacement reactions are acid-base neutralization reactions. These reactions begin with the reaction between a base (for example, a metal hydroxide) and an acid (which can be a hydroacid, oxyacid, or an organic acid) and give salt and water as products:
Examples of acid-base neutralization reactions
This is a reaction between a strong acid (nitric acid) and a strong base (potassium hydroxide). However, it could also involve weak acids and bases, such as:
metal exchange reactions
This type of double replacement reactions occurs with Organometallic compounds. Unlike previous examples involving the breaking and forming of ionic bonds, here coordinate covalent bonds are broken and formed between a metal (acting as a Lewis acid) and an organic group that donates the bonding pair of electrons (the which acts as a Lewis base).
Examples of metal exchange reactions
Organic Double Displacement Reactions
The substitution reactions that organic compounds such as alkyl halides and alcohols (to name a few) undergo are also double replacement or double displacement reactions. In them, the incoming group replaces the outgoing group, while the outgoing group joins the original counterpart of the incoming group.
Hydrolysis reactions of some organic compounds are also examples of double displacement reactions .
Examples of organic double displacement reactions
An example of a nucleophilic substitution reaction in which bromine is replaced by iodine and vice versa is given below.
Another example is the hydrolysis of esters. In this case
References
Carey, F. (1999). Organic Chemistry (3rd ed.). Madrid, Spain: McGraw-Hill Companies.
Chang, R., & Goldsby, KA (2012). Chemistry, 11th Edition (11th ed.). New York City, New York: McGraw-Hill Education.
