Definition Structure Preparation Reactions and Uses of Hypophosphorous Acid and Hypophosphite Salts
Hypophosphorous acid (HPA), which is also called phosphinic acid, is defined as a phosphorus oxyacid and a powerful reducing agent having the molecular formula H3PO2. It is a low-melting and colourless compound, soluble in dioxane, water, and alcohols. Generally, the formula for this acid is written as H3PO2, but a more descriptive presentation can be HOP(O)H2, which highlights its monoprotic character. Salts that are derived from this acid are known as hypophosphites.
Structure of Hypophosphorous Acid
The structure of Hypophosphorous acid is pseudo-tetrahedral, which is a molecular shape:
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Preparation and Availability
First, the hypophosphorous acid was prepared by the French chemist named Pierre Louis Dulong (1785–1838) in 1816.
Industrially, the acid is prepared via a two-step process: Firstly, the alkali and alkaline earth metals' hypophosphite salts result from the white phosphorus reaction with a hot aqueous solution of the appropriate hydroxide, for example, Ca(OH)2.
P4 + 4 OH− + 4 H2O → 4 H2PO-2 + 2 H2
Then, the salt is treated with a non-oxidizing and strong acid to give the free hypophosphorous acid as:
H2PO−2 + H+ → H3PO2
Usually, HPA is supplied as a 50 percent aqueous solution. Also, anhydrous acid can't be obtained by simple evaporation of the water because the acid ready oxidizes to phosphoric acid, phosphorous acid, and also disproportionates to phosphine and phosphorous acid. Pure anhydrous hypophosphorous acid may be formed by the continuous extraction of the aqueous solutions with diethyl ether.
Organic Chemistry
In organic chemistry, H3PO2 may be used for the reduction of arene diazonium salts by converting ArN+2 to Ar–H. When diazotized in the hypophosphorous acid's concentrated solution, an amine substituent can be removed from the arenas.
Owing to its ability to function as an oxygen scavenger and mild reducing agent, sometimes, it is used as an additive in the Fischer esterification reactions, where it prevents the coloured impurities formation.
It can be used to prepare the derivatives of phosphinic acid.
What are Hypophosphite Salts?
Sodium hypophosphite (NaPO2H2, which is also called sodium phosphinate) is defined as the sodium salt of hypophosphorous acid, and often, it is encountered as the monohydrate, NaPO2H2·H2O. At room temperature, it is solid, appearing as odourless white crystals. And, it is soluble in water and can absorb moisture from the air easily.
Eventually, the name has changed from sodium hypophosphite to hypophosphorous acid.
Sodium hypophosphite or the hydrated sodium hypophosphite must be kept in a cool and dry place, isolated from the oxidizing materials. It decomposes into a phosphine that is irritating to disodium phosphate and the respiratory tract.
2 NaH2PO2 → PH3 + Na2HPO4
Uses
Mainly, the sodium hypophosphite may be used for electroless nickel plating (Ni-P). With this particular method, a durable nickel-phosphorus film may coat objects with irregular surfaces, such as in aviation, petroleum fields, and avionics.
Sodium hypophosphite is also capable of reducing the nickel ions in solution to the metallic nickel on metal substrates and plastic substrates as well. The latter needs that the substrate can be activated with fine particles of palladium. The resulting nickel deposit contains around 15 percent of phosphorus.
It may be used as a food additive.
Industrial Uses
Sodium sulfate is essential in textile manufacturing, especially in Japan, where it is such a largest application. Sodium sulfate also helps in "levelling," reducing the negative charges present on fibres so that the dyes may penetrate evenly. It does not corrode the stainless steel vessels used in dyeing, unlike the alternative sodium chloride. This application consumed approximately 100,000 tonnes in Japan and the US in 2006.
Small-scale Applications
Anhydrous sodium sulfate is widely used as an inert drying agent in the laboratory for removing traces of water from the organic solutions.
It is more slower-acting but efficient compared to the similar agent magnesium sulfate. Only it is effective below up to a temperature of 30 °C, but it may be used with various materials since it is chemically fairly inert. Sodium sulfate can be added to the solution until the crystals, together, no longer clump.
Phosphorous vs. Phosphoric acid
Phosphoric acid (H3PO4) should not get confused with phosphorous acid (H3PO3). The first is a completely hydrated and oxidised form of P, while the second is a partially hydrated and oxidised form. Thus, phosphorous acid can be described as a powerful reducing agent, whereas phosphoric acid is not. Polyphosphoric acid is an odourless liquid with high viscosity and hygroscopic properties.
Phosphoric acids having less than 95 percent of H3PO4 (68 percent of P2O5) contain simple orthophosphoric acid. In higher concentrations, the acid holds a combination of pyro, ortho, tetra tri, and phosphoric acid that is highly concentrated. For this specific reason, acids with a concentration of more than 68%, and P2O5 is better referred to as polyphosphoric acid. Polyphosphoric acid can be insoluble in water, hydrolyzing into orthophosphoric acid by producing heat. They are insoluble in both hydrocarbons and halogenated hydrocarbons.
FAQs on Hypophosphorous Acid and Its Hypophosphite Salts in Chemistry
1. What is hypophosphorous acid?
Hypophosphorous acid is a monobasic oxyacid of phosphorus with the chemical formula H3PO2, also known as phosphinic acid. It contains one ionizable hydrogen atom attached to oxygen, while the other two hydrogens are directly bonded to phosphorus. Structurally, it is represented as HOP(O)H2. It is a strong reducing agent and is commonly used in electroless plating and chemical synthesis.
2. What is the formula and structure of hypophosphorous acid?
The formula of hypophosphorous acid is H3PO2, and its structure is best written as HOP(O)H2. In this structure:
- One hydrogen is bonded to oxygen (–OH group) and is ionizable.
- Two hydrogens are directly bonded to phosphorus (P–H bonds).
- Phosphorus is double-bonded to one oxygen (P=O).
3. Why is hypophosphorous acid monobasic?
Hypophosphorous acid is monobasic because only one hydrogen atom is replaceable or ionizable in aqueous solution. In H3PO2:
- Only the hydrogen attached to oxygen (–OH) can dissociate as H+.
- The two hydrogens attached directly to phosphorus (P–H) are not acidic.
H3PO2(aq) ⇌ H+(aq) + H2PO2−(aq).
4. What is the oxidation state of phosphorus in hypophosphorous acid?
The oxidation state of phosphorus in H3PO2 is +1. It is calculated as follows:
- Let oxidation state of P = x.
- Hydrogen = +1 each (3 × +1 = +3).
- Oxygen = −2 each (2 × −2 = −4).
5. Why is hypophosphorous acid a strong reducing agent?
Hypophosphorous acid is a strong reducing agent because phosphorus is in the low oxidation state of +1 and can be easily oxidized to higher states like +3 or +5. During oxidation, it forms products such as phosphorous acid or phosphoric acid. For example:
H3PO2 + 2AgNO3 + H2O → H3PO3 + 2Ag(s) + 2HNO3
Here, Ag+ is reduced to metallic silver, showing the reducing property.
6. What are hypophosphite salts?
Hypophosphite salts are salts containing the hypophosphite ion H2PO2−, formed when hypophosphorous acid loses one proton. They are produced by neutralization reactions such as:
H3PO2(aq) + NaOH(aq) → NaH2PO2(aq) + H2O(l)
Common examples include sodium hypophosphite (NaH2PO2) and calcium hypophosphite.
7. How is hypophosphorous acid prepared?
Hypophosphorous acid is commonly prepared by the hydrolysis of phosphorus trichloride in the presence of moist air or controlled conditions. A simplified preparation involves:
PCl3 + 3H2O → H3PO3 + 3HCl
Further reduction steps can yield H3PO2. Industrially, it is often obtained by acidifying hypophosphite salts such as sodium hypophosphite with a strong acid like HCl.
8. What happens when hypophosphorous acid is heated?
When heated, hypophosphorous acid undergoes disproportionation to form phosphine and phosphorous acid. The balanced reaction is:
2H3PO2 → PH3 + H3PO3
In this reaction:
- One phosphorus atom is reduced to −3 in PH3.
- The other is oxidized to +3 in H3PO3.
9. What is the difference between hypophosphorous acid and phosphorous acid?
The main difference is that hypophosphorous acid (H3PO2) is monobasic, while phosphorous acid (H3PO3) is dibasic. Key differences include:
- Basicity: H3PO2 (1 ionizable H), H3PO3 (2 ionizable H).
- Oxidation state of P: +1 in H3PO2, +3 in H3PO3.
- Reducing power: H3PO2 is a stronger reducing agent.
10. What are the uses of hypophosphorous acid and hypophosphite salts?
Hypophosphorous acid and hypophosphite salts are mainly used as reducing agents and in electroless plating. Important uses include:
- Electroless nickel plating (using sodium hypophosphite as a reducing agent).
- Preparation of pharmaceuticals and fine chemicals.
- As an antioxidant and stabilizer in polymers.
- Laboratory reduction reactions.
