Phosphorus Trichloride ($PCl_3$)
Preparation, Pyramidal Structure, Hydrolysis, and Organic Applications.
Phosphorus forms two major series of halides: $PX_3$ and $PX_5$. Among them, Phosphorus Trichloride ($PCl_3$) is a colorless oily liquid that acts as a vital bridge between inorganic and organic chemistry. It is famously known for its violent reactivity with moisture and its immense utility as a chlorinating agent.
1. Preparation of $PCl_3$
A. Direct Chlorination of White Phosphorus
$PCl_3$ is obtained by passing dry chlorine gas over heated white phosphorus. It is important that the chlorine is not in excessive abundance, otherwise, $PCl_5$ will form.
B. Action of Thionyl Chloride ($SOCl_2$)
This is a highly tested laboratory method. When white phosphorus is treated with Thionyl chloride ($SOCl_2$), it yields $PCl_3$ along with two gaseous/volatile byproducts.
Examiners frequently ask to identify the byproducts of this specific reaction. Memorize that it produces Sulfur dioxide ($SO_2$) and Disulfur dichloride ($S_2Cl_2$).
2. Structure and Geometry
The central phosphorus atom in $PCl_3$ has 5 valence electrons. It shares 3 electrons with three chlorine atoms, leaving one lone pair.
- Hybridization: $sp^3$ (3 bond pairs + 1 lone pair = 4 electron domains).
- Shape: Because of lone pair-bond pair repulsion, the perfectly tetrahedral geometry is distorted into a Trigonal Pyramidal shape.
- Bond Angle: The $Cl-P-Cl$ bond angle is $100.4^\circ$ (compressed from the ideal $109.5^\circ$).
- Lewis Base: Because of the non-bonding lone pair, $PCl_3$ can act as a Lewis base (electron pair donor).
Figure 1: The lone pair exerts repulsion on the three P-Cl bond pairs, compressing the angle to $100.4^\circ$.
3. Chemical Properties & Key Reactions
$PCl_3$ is highly reactive. Its chemistry is dominated by its violent reaction with water and its ability to replace hydroxyl ($-OH$) groups with chlorine atoms.
A. Violent Hydrolysis (Fuming in Air)
$PCl_3$ reacts violently with water to form Phosphorous acid ($H_3PO_3$) and Hydrogen Chloride ($HCl$) gas.
Exam Insight: If a bottle of $PCl_3$ is opened, it fumes in moist air. This is because it instantly reacts with the moisture in the atmosphere to release microscopic droplets of highly acidic $HCl$ gas.
B. The Ultimate Chlorinating Agent
In organic chemistry, $PCl_3$ is a standard reagent used to replace hydroxyl ($-OH$) groups with Chlorine ($-Cl$) atoms. It reacts smoothly with alcohols and carboxylic acids, and the inorganic byproduct is always Phosphorous acid ($H_3PO_3$).
(Ethanol $\rightarrow$ Ethyl Chloride)
(Acetic acid $\rightarrow$ Acetyl chloride)
C. Other Important Reactions
- Oxidation by Oxygen: When heated in oxygen, it gets oxidized to Phosphorus oxychloride.
$2PCl_3 + O_2 \rightarrow 2POCl_3$
- Addition of Chlorine: It reacts with excess chlorine to form Phosphorus pentachloride ($PCl_5$).
$PCl_3 + Cl_2 \rightarrow PCl_5$
Mastery Check: Phosphorus Trichloride
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