Question

Bond angle between C-C in alkyne is:
A) ${{109}^{0}}{{28}^{'}}$
B) ${{120}^{0}}$
C) ${{180}^{0}}$
D) ${{60}^{0}}$

Answer 1

The concept of hybridisation and the theories based on it that is mainly molecular orbital theory explains bond angles by the geometry of molecules. The answer to this question lies in this concept.

Complete Solution :
We have come across the chapters dealing with various theories to determine the shape, bond angles, chemical bonds etc., like molecular orbital theory, valence bond theory and many more.
Let us now focus on the bond angle from which we can approach the required answer on the basis of molecular orbital theory (MOT).
- Molecular orbital theory basically involves the molecular orbital diagram constructed on the basis of electronic configuration of atoms in a molecule.
- This theory gives the information about the bond order and thus the bond angle.
But, we know that the hybridisation is calculated based on the valence electrons.

Now, in alkyne molecules there are three bonds in between two carbon atoms among which one is sigma bond and the other two are pi bonds. The structure of alkyne in general is as shown below,
$-C\equiv C-$

- Now, since there are one sigma and two double bonds, the hybridisation of alkynes will be sp.
Therefore, the bond angle assigned for sp hybridisation is ${{180}^{0}}$ that is the molecule is linear in structure.
So, the correct answer is “Option C”.

Note: Note that the bond angles slightly vary from the actual bond angle assigned for the particular hybridisation. This is because of the presence of lone pair of electrons and also type of atoms attached to the central atom resulting in lone pair-lone pair or lone pair-bond pair or bond pair-bond pair repulsion.