1.how do you know whats the most acidic or basic compound ? 2.how do you know whats the most stable resonance ?
Hey Irene! In this question we have three type of compounds:
a. 'Water' that is a weak acid (pKa = 15)
b. 'Ammonia' that is a weak base (pKa = 36)
c. and Hydrocarbons
In order to compare the acidity between ethane, ethene and ethyne we can look for their hybridization. Here we have ethane that is Sp3 hybridized (S character = 25%), ethene is Sp2 (S character = 33%) and ethyne is Sp hybridized (S character = 50%). For stronger acids we know that its conjugate base should be more stable. In case of ethyne, we see 50% s-character stabilize the negative charge of conjugate base more because of the reason that s-orbital is more closer to the nucleus. Therefore ethyne has a pKa of 25. The general trend of acidity in hydrocarbons depending upon their hybridization follows as below:
Sp > Sp2 > Sp3
Based on this information we can arrange these compounds with decreasing acidity as below:
H2O > C2H2 (ethyne) > NH3 > C2H4 (ethene) > C2H6 (ethane)
So, The correct option is "D"
Hope this makes sense :-)