Organic Chemistry Basics - Result Question 169

####20. Which of the following will have least hindered rotation about carbon-carbon bond?

(a) Ethane

(b) Ethylene

(c) Acetylene

(d) Hexachloroethane

(1987, 1M)

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Answer:

Correct Answer: 20. (b)

Solution:

  1. Allene is the name given to propdiene, $H _2 C=C=CH _2$

Hybridisation of an atom is determined by determining the number of hybrid orbitals at that atom which is equal to the number of sigma $(\sigma)$ bonds plus number of lone pairs at the concerned atom.

$\operatorname{Pi}(\pi)$ bonds are not formed by hybrid orbitals, therefore, not counted for hybridisation.

Here, the terminal carbons have only three sigma bonds associated with them, therefore, hybridisation of terminal carbons is $s p^{3}$. The central carbon has only two sigma bonds associated, hence hybridisation at central carbon is $s p$.



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