Why is the structure of $\ce{POCl3}$ not like the left? I know the elements in period 3 or higher don’t have to follow the octet rule, but the left structure doesn’t seem to have any problems:
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1$\begingroup$ chemistry.stackexchange.com/questions/180471/… chemistry.stackexchange.com/questions/139367/… $\endgroup$– MithoronCommented May 21 at 17:58
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3 Answers
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There are two main reasons:
- The bond energy of $\ce{P=O}$ is very high as compared $\ce{P-O}$ and $\ce{O-Cl}$ combined. {The bond energy of $\ce{P=O}$ is driving force in many organic reactions. -See Reference below}
- There is high Lone pair repulsions between O and Cl.
Reference:
The P=O bond, with its bond energy of $\ce{575 kJ mol^{−1}}$, is one of the strongest double bonds in chemistry, and the Wittig reaction is irreversible and is driven forward by the formation of this P=O bond.
{Clayden, J., Greeves, N., Warren, S. Organic chemistry, 2nd ed.; Oxford University Press: New York, 2012.}
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Phosphorus has the lowest ionisation energy, and thus, it should be in the centre. When constructing Lewis structures, the atom with the lowest ionisation energy must be in the middle since it can form the most bonds with other elements, as a lower ionisation energy allows it to form bonds (gain or lose electrons) more readily. The amount of energy required to do so would also be lower.
Remember, however, that there are exceptions to this rule.
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Phosphorus oxychloride $\ce{POCl3}$ has no bond O-Cl. It is related to its synthesis. There are a lot of possible synthesis. Here are two possible oxidation reactions, that show that the Oxygen atom is directly fixed on the free doublet of the phosphorus atom, and not between $\ce{P}$ and $\ce{Cl}$ : $$\ce{2 PCl3 + O2 -> 2 POCl3}$$ $$\ce{3 PCl3 + KClO3 -> 3 POCl3 + KCl}$$
