# Which of the following is the most basic compound?

Which of the following is the most basic compound?

1. $$\ce{HS-}$$
2. $$\ce{H3Si-}$$
3. $$\ce{H2P-}$$
4. $$\ce{Br-}$$

I can't figure out which of these is the strongest base. Would it be $$\ce{H3Si-}$$, since $$\ce{Si}$$ is the least electronegative of all these central elements?

• The size of the atom carrying the charge is generally more important than the electronegativity. – Michael Lautman May 22 '19 at 15:29

I agree with Doctor Insult's argument that "the stronger the base, weaker is the conjugate acid." In that sense, higher the $$\mathrm{p}K_\mathrm{a}$$ of acid, weaker is the acid, and hence, stronger is the conjugate base. The $$\mathrm{p}K_\mathrm{a1}$$ values of $$\ce{HBr}$$, $$\ce{H2S}$$, $$\ce{H3P}$$, and $$\ce{H4Si}$$ are -8.7, 6.89, 27, and ~35, respectively (Ref.1). Therefore, the order of the strength of their conjugate bases should be: $$\ce{H3Si- \gt H2P- \gt HS- \gt Br-}$$ Hence, strongest base is $$\ce{H3Si-}$$.

Note: Ironically, this is the same decreasing order of electronegativity of center atoms $$\ce{Si, P, S, Br}$$: $$1.90 \lt 2.19 \lt 2.58 \lt 2.96$$, respectively.

References:

1. William L. Jolly, In Modern Inorganic Chemistry; 1st Edn.; McGraw-Hill, Inc.: New York, NY, 1984, p 177.

Basic strength of a base is inversely proportional to the electronic negative charge density on the charged atom, because it will more readily to try to stabilize it charge by abstracting a proton. Size varies as Br>Si>S>P, so $$\ce{H2P-}$$ will be the strongest base. Even from the $${K_a}$$ data, I concluded that $$\ce{H2P-}$$ will be the strongest base. Look up the ionisation constants of their conjugate acids. The stronger the base, weaker is the conjugate acid. Hope that helps.

• In this case, the charged atoms are all similar in size, so the electronegativity is the dominant effect. – Michael Lautman May 22 '19 at 16:59