Question

How many atoms lie in a straight line in the molecule 2 - butyne, $C{H_3}C \equiv CC{H_3}$?
a) 10
b) 8
c) 6
d) 4
e) 2

Answer 1

Hint: The total number of atoms that lie in a straight line in the molecule 2 - butyne, $C{H_3}C \equiv CC{H_3}$ molecule is equal to the number of carbon atoms present in the molecule. The sp hybridised atoms have a bond angle of 180$^0$ in them while the $s{p^3}$ hybridised atoms have a bond angle of 109.4$^0$ between them. The atoms with bond angle 180$^0$ are in a straight linear chain.

Complete step by step answer:
The 2 - butyne is alkyne. It contains a triple bond and is sp hybridised. We know that the triple bond has the angle of 180$^0$ between two atoms which form a triple bond. So, both carbon atoms that form triple bonds have 180$^0$ angle between them. This means they are in a straight line.
The atoms attached to these carbons will also be in a straight line.
But the carbon attached to these are $s{p^3}$ hybridised. These atoms have a bond angle of 109.4$^0$.

Let us once see the structure of a tetrahedral methyl group attached. It will make things more clear.

So, this can be one fragment of the 2 - butyne.

The complete structure can be seen as -

So, now it is clear from this diagram that the side groups present are tetrahedral in geometry while the central carbon atoms are only present in the linear chain.
Thus, calculating we get a total 4 atoms that lie in a straight line in the molecule 2 - butyne, $C{H_3}C \equiv CC{H_3}$.

So, the correct answer is “Option D”.

Note: It must be noted that there are a little deviations in the bond angles due to bond pair bond pair repulsions which occur very little. So, one can not say that these will have exact values of these bond angles but yes, it will be very much closer to the value. Further, drawing the structure of a molecule, we can easily guess the answer.