Introduction:
When diethyl ether (C4H10O) is treated with phosphorus pentachloride (PCl5), a substitution reaction known as the ether cleavage reaction occurs. This reaction is commonly referred to as the "decomposition" or "cleavage" of the ether.

Reaction:
The reaction between diethyl ether and phosphorus pentachloride can be represented by the following equation:
C4H10O + PCl5 → 2C2H5Cl + POCl3

Explanation:
The reaction takes place in the presence of a Lewis acid catalyst, which is PCl5 in this case. The reaction occurs in three main steps:

1. Formation of an intermediate complex:
Initially, diethyl ether (C4H10O) and phosphorus pentachloride (PCl5) react to form an intermediate complex. The oxygen atom of the ether acts as a nucleophile and attacks the phosphorus atom of PCl5. This leads to the formation of a positively charged oxygen atom and a negatively charged phosphorus atom.

2. Formation of an oxonium ion:
The intermediate complex then undergoes a rearrangement, resulting in the formation of an oxonium ion. The oxygen atom of the ether coordinates with one of the chlorine atoms of PCl5, while one of the ethyl groups shifts to the oxygen atom. This rearrangement leads to the formation of a positively charged oxygen atom and a negatively charged phosphorus atom.

3. Substitution reaction:
In the final step, the oxonium ion reacts with another molecule of PCl5, resulting in the substitution of the oxygen atom with a chlorine atom. This leads to the formation of two molecules of ethyl chloride (C2H5Cl) and one molecule of phosphoryl chloride (POCl3) as byproducts.

Summary:
In summary, when diethyl ether is treated with phosphorus pentachloride, a substitution reaction occurs, resulting in the cleavage of the ether and the formation of ethyl chloride and phosphoryl chloride. This reaction is commonly used to convert ethers into alkyl halides and is an important transformation in organic chemistry.