The pKa of acetonitrile, when considered as a carbon acid by deprotonating its alpha-hydrogens, is 25. This value is provided in standard pKa tables for organic compounds.
Understanding Acetonitrile's Acidity and its Conjugate Acid
To provide a precise answer, it's important to differentiate between the acidity of acetonitrile itself and the acidity of its conjugate acid.
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Acetonitrile as a Carbon Acid: The pKa of 25 for acetonitrile (CH₃CN) refers to its ability to act as a weak acid by donating a proton from its methyl group (α-hydrogen). This reaction forms a carbanion, specifically the acetonitrile anion (CH₂⁻CN):
CH₃CN ⇌ CH₂⁻CN + H⁺ (pKa ≈ 25)This relatively high pKa indicates that acetonitrile is a very weak carbon acid, requiring strong bases for deprotonation.
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Acetonitrile Conjugate Acid (Protonated Acetonitrile): The term "acetonitrile conjugate acid" typically refers to the species formed when acetonitrile (CH₃CN) acts as a base and accepts a proton. This protonation usually occurs at the nitrogen atom, forming the iminium ion CH₃C=NH⁺:
CH₃CN + H⁺ ⇌ CH₃CNH⁺The pKa of this protonated form (CH₃CNH⁺) is a measure of its acidity—its tendency to donate the newly acquired proton back. Nitriles are generally very weak bases, meaning their conjugate acids are relatively strong acids. The pKa of protonated acetonitrile (CH₃CNH⁺) is significantly lower than 25, commonly cited in the range of -10 to -5. This indicates that CH₃CNH⁺ is a strong acid, readily donating its proton.
Therefore, while the pKa of acetonitrile (as a carbon acid) is 25, the pKa of its conjugate acid (protonated acetonitrile, CH₃CNH⁺) is a distinct and much lower value.
