Question

In which of the following conversions phosphorus pentachloride is used as a reagent
A. \[{H_2}C = C{H_2} \to C{H_3}C{H_2}Cl\]
B. \[{H_3}C - O - C{H_3} \to C{H_3}Cl\]
C. \[C{H_3}C{H_2}OH \to C{H_3}C{H_2}Cl\]
D. \[HC \equiv CH \to C{H_2} = CHCl\]

Answer 1

Hint: A reagent is an organic compound that is used to obtain desired product. Unlike a catalyst, a reagent is used in the reactions and gets converted into some other side products at the end of the reaction. Phosphorus pentachloride is used as a reagent widely in organic synthesis.

Complete step-by-step answer:Phosphorus has a very high affinity for oxygen. It combines with oxygen to form a double bond. \[P = O\] is a non-conventional double bond. This leads to a large amount of energy release. Hence, it is very stable. Thus, $PC{l_5}$ converts to \[POC{l_3}\] very easily as it is also entropically favoured.

In option B and C, the reactants have oxygen in form ether and alcohol. Hence, $PC{l_5}$ reacts with alcohol and ether and convert them to alkyl chlorides.
Option C is a very famous reaction for conversion of alcohol to chloroalkane. Alcohols also react with $PC{l_3}$ to form chloroalkane and${H_3}P{O_3}$.

Option ‘B’ and ‘C ’ is correct

Note: Phosphorus pentachloride exists but not nitrogen pentachloride. This is because phosphorus has a low-lying vacant \[d\] orbital which is used by phosphorus to expand its covalency from \[3\] to\[5\]. But only the presence of a vacant orbital is not sufficient. Chloride atoms are highly electronegative in nature; this causes contraction of \[d\] orbitals so the energy gap is reduced so the electrons unpair themselves and jump into \[d\] subshell. Phosphorus pentachloride can be converted into Phosphorus trichloride and chlorine gas and helps in many organic reactions to add Cl- group.