Question

Which of the following alkynes is most acidic?
A. $C{H_3}C \equiv CH$
B. $C{H_3}C \equiv CC{H_3}$
C. $C{H_3}C{H_2}C \equiv CH$
D. $CH \equiv CH$

Answer 1

The compound which loses hydrogen ion (${H^ + }$) easily is more acidic than the other compound. So from the above the most acidic alkyne is the one which loses ${H^ + }$ ion easily.

Step by step answer: First of all you should know that if the hydrogen is removed easily from the organic compound then the compound will be more acidic secondly there are many factors which affect acidity of the compound. If the carbon is sp hybridized and is having hydrogen on it then it will be more acidic. And also if the no. of carbon groups attached to it which increases +I effect will decrease the acidic nature of that compound.
Now, from the above discussion let us go to each compound one by one,
Firstly, $C{H_3}C \equiv CC{H_3}$ that is But-2-yne has no hydrogen attached to sp hybridized carbon so it is least acidic among the given compounds.
Now, $C{H_3}C \equiv CH$and $C{H_3}C{H_2}C \equiv CH$ that is But-1-yne and Propyne respectively have groups having +I effect attached to triple bond and as the groups having +I effect increases, the acidic character of compound decreases. Thus $CH \equiv CH$ that is ethyne is the most acidic of all four compounds.

Hence the correct option is D.

Note: There are so many factors on which acidity of a compound depends, and the net effect of all these factors tells you that which is most acidic in nature. If the +I effect of the groups is more then the compound will be less acidic. As +I effect is inversely proportional to the acidic character of the compound.