What is the approximate pK_a value of a terminal alkyne?

• A. ~ 50
• B. ~ 20
• C. ~ 25
• D. ~ 38

Answer: (C)

Verified step by step guidance

1. Understand the concept of pKa: pKa is a measure of the acidity of a compound. It is the negative logarithm of the acid dissociation constant (Ka). Lower pKa values indicate stronger acids.
2. Identify the functional group: A terminal alkyne has the structure R-C≡C-H, where the hydrogen is attached to the carbon in the triple bond.
3. Consider the acidity of the hydrogen in a terminal alkyne: The hydrogen atom in a terminal alkyne is relatively acidic compared to other hydrocarbons due to the sp hybridization of the carbon atom, which holds the electrons more tightly.
4. Compare with other hydrocarbons: Terminal alkynes are more acidic than alkanes and alkenes but less acidic than alcohols and carboxylic acids. This places their pKa value in the range of 20-25.
5. Select the approximate pKa value: Based on the acidity of terminal alkynes, the pKa value is approximately 25, which is consistent with the typical acidity of terminal alkynes.