The pKa’s of methanol (CH3OH) and methanethiol (CH3SH) are 16 and 11, respectively.
Which is more basic, KOCH3 or KSCH3?
The Correct Answer and Explanation is:
To determine which compound is more basic between KOCH₃ (potassium methoxide) and KSCH₃ (potassium methanethiolate), we need to consider the pKa values of their conjugate acids: CH₃OH and CH₃SH, respectively.
Correct Answer: KOCH₃ is more basic than KSCH₃.
Explanation:
In acid-base chemistry, basicity is closely tied to the stability of a conjugate acid and the strength of the conjugate base. A stronger base has a weaker conjugate acid, meaning the conjugate acid has a higher pKa value.
Let’s compare the pKa values of the conjugate acids:
- CH₃OH (methanol) has a pKa of ~16
- CH₃SH (methanethiol) has a pKa of ~11
Since methanol has a higher pKa, it is a weaker acid than methanethiol. That means its conjugate base, CH₃O⁻ (methoxide), is stronger (more basic) than CH₃S⁻ (methanethiolate), the conjugate base of methanethiol.
This is explained by the relative electronegativity and size of oxygen and sulfur:
- Oxygen (in CH₃O⁻) is more electronegative than sulfur and better at stabilizing a negative charge through electronegativity, but not as well via size/diffuse orbitals.
- Sulfur (in CH₃S⁻) is less electronegative, but its larger atomic radius spreads out the negative charge better, making CH₃S⁻ more stable, and therefore less reactive, i.e., a weaker base.
So even though sulfur can better stabilize a negative charge, that very stability makes its conjugate base less basic—because basicity is about reactivity and willingness to accept protons.
Thus, between KOCH₃ and KSCH₃:
- KOCH₃ has the stronger base (CH₃O⁻)
- KSCH₃ has the weaker base (CH₃S⁻)
Conclusion:
KOCH₃ (potassium methoxide) is more basic than KSCH₃ (potassium methanethiolate) because its conjugate acid has a higher pKa, meaning the base is stronger.