Phosphine ($PH_3$)
Preparation, Drago's Rule, Basicity, and Holmes' Signal.
Phosphine ($PH_3$) is the phosphorus analog of ammonia ($NH_3$). It is a highly toxic, colorless gas famously known for its distinct odor of rotten fish or garlic. While its formula mirrors ammonia, its physical and chemical properties are vastly different, making it a favorite subject for comparison questions in competitive exams.
1. Preparation of Phosphine
A. Laboratory Preparation (Disproportionation)
In the laboratory, phosphine is prepared by heating White Phosphorus ($P_4$) with concentrated Sodium Hydroxide ($NaOH$) solution in an inert atmosphere of $CO_2$.
This is a classic disproportionation reaction. Phosphorus (oxidation state 0) is simultaneously oxidized to Sodium hypophosphite (+1) and reduced to Phosphine (-3).
(Phosphine + Sodium hypophosphite)
Purification: To purify it, the impure gas is passed through Hydroiodic acid ($HI$) to form solid Phosphonium iodide ($PH_4I$). This solid is then treated with a base ($KOH$) to release pure, non-flammable $PH_3$.
B. Hydrolysis of Metal Phosphides
Phosphine can be quickly generated by the action of water or dilute mineral acids on metallic phosphides, like Calcium phosphide ($Ca_3P_2$).
$Ca_3P_2 + 6HCl \rightarrow 3CaCl_2 + \mathbf{2PH_3} \uparrow$
2. Structure: The Drago's Rule Anomaly
Ammonia ($NH_3$) has $sp^3$ hybridization with a bond angle of $107.8^\circ$. You might expect Phosphine to be similar. However, the bond angle in $PH_3$ is a startlingly sharp $93.5^\circ$.
Why is the angle $\sim 90^\circ$?
This is explained perfectly by Drago's Rule. The rule states that hybridization will NOT take place if:
- The central atom belongs to the 3rd period or below (Phosphorus is Period 3).
- The central atom has at least one lone pair.
- The electronegativity of the surrounding atoms is less than or equal to 2.5 (Hydrogen is 2.1).
Conclusion: There is NO hybridization in Phosphine.
Instead of forming hybrid orbitals, Phosphorus uses its pure, unhybridized $3p$ orbitals ($p_x, p_y, p_z$) to form bonds with Hydrogen's 1s orbitals. Because pure p-orbitals are exactly orthogonal ($90^\circ$) to each other, the resulting bond angle in $PH_3$ is extremely close to $90^\circ$ ($93.5^\circ$ due to slight repulsions).
Consequently, the lone pair of electrons in $PH_3$ resides almost entirely in a pure $3s$ orbital (which is spherical and held closely to the nucleus).
Figure 1: Because there is no hybridization, Phosphorus uses its pure $p_x, p_y$, and $p_z$ orbitals for bonding, forcing the $H-P-H$ angle to be approximately 90 degrees.
3. Chemical Properties: Basicity
Phosphine, like ammonia, acts as a Lewis base because it can donate its lone pair of electrons to react with acids (forming phosphonium salts like $PH_4I$).
Why is $PH_3$ a much weaker base than $NH_3$?
This is a guaranteed exam question, and Drago's Rule provides the ultimate answer.
- In $NH_3$ (hybridized), the lone pair resides in an $sp^3$ hybrid orbital. This orbital is directional and points outward, making the electron pair easily available for donation.
- In $PH_3$ (unhybridized), the lone pair resides in a pure $3s$ orbital. The s-orbital is spherical, non-directional, and penetrates deeply towards the nucleus. The nucleus holds onto this lone pair very tightly, making it reluctant to be donated. Thus, $PH_3$ is a very weak base.
4. Application: Holmes' Signal & Smoke Screens
The most famous application of Phosphine relies entirely on the impurity we discussed earlier: Diphosphane ($P_2H_4$). Pure phosphine is not spontaneously flammable, but when generated dynamically from phosphides, the trace $P_2H_4$ causes the gas mixture to violently ignite upon contact with air.
A container filled with a mixture of Calcium Phosphide ($Ca_3P_2$) and Calcium Carbide ($CaC_2$) is pierced and thrown into the sea.
- The sea water enters the canister and hydrolyzes the $Ca_3P_2$ to produce Phosphine ($PH_3$) gas (contaminated with $P_2H_4$).
- Simultaneously, the $CaC_2$ hydrolyzes to produce highly flammable Acetylene ($C_2H_2$) gas.
- The $P_2H_4$ ignites spontaneously in the air, instantly setting fire to both the Phosphine and the Acetylene.
- The burning phosphine produces a dense, bright white cloud of Phosphorus pentoxide ($P_4O_{10}$) smoke, which acts as a highly visible distress signal for ships!
This exact same chemistry ($Ca_3P_2$ + water) is used in military applications to rapidly deploy thick smoke screens to hide troop movements.
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