Question

What is the total number of $\sigma $and $\pi $ bonds in pyrophosphoric acid?
(A) $8,2$
(B) $10,2$
(C) $12,2$
(D) $8,4$

Answer 1

Hint: As we know that phosphorus forms a number of oxoacids and the condensation of two molecules of phosphoric acid $({H_3}P{O_4})$ results in the formation of pyrophosphoric acid with a chemical formula $({H_4}{P_2}{O_7})$and sigma and pi bonds are formed by overlapping of atomic orbitals.

Complete Step by step answer: We know that phosphorus on oxidation and condensation forms various oxoacids and the condensation of two molecules of phosphoric acid results in the pyrophosphoric acid formation that has a formula of $({H_4}{P_2}{O_7})$.
We also know that $\sigma $and $\pi $bonds are formed by overlapping atomic orbitals. End to end overlapping of atomic orbitals results in sigma bonds formation and pi bond is formed by sideways overlapping of atomic orbitals.
Now let us first see the structure of pyrophosphoric acid to make it clear:
 

As we can see in the structure that it contains four $P - OH$ bonds, two $P = O$ bonds and one $P - O - P$ bond and we know that sigma bonds are the result of head to head overlapping of orbitals thus it generally forms a single bond. Therefore we can say that all the single bonds present in pyrophosphoric acid are sigma bonds. So, a total of 10 single bonds are there between oxygen and hydrogen and phosphorus and oxygen.
Now talking about the double bonds, we know that they are formed by sideways overlapping of orbitals and their axes are parallel to each other resulting in double bond which generally exist in combination with a sigma bond thus one of the double bond is sigma and the other is pi. Therefore, in pyrophosphoric acid there are a total two pi bonds between phosphorus and oxygen and two more sigma bonds make it a total of twelve sigma bonds.

Therefore the correct answer is (C) i.e. $12,2$.


Note: Always remember that a sigma bond corresponds to a single bond and a double bond corresponds to one sigma and one pi-bond whereas a triple bond generally involves one sigma and two pi-bonds.