Final answer:
Among the given compounds, ethane (CH₃CH₃) has the highest pKa, indicating it is the least acidic compound because it has a very weak ability to donate a proton.
Step-by-step explanation:
The question asks which of the following compounds has the highest pKa: CH₃OH (methanol), CH₃NH₂ (methylamine), HCCH (acetylene), CH₃CH₃ (ethane), and CH₂=CH₂ (ethylene). The pKa is an indicator of acid strength; the higher the pKa, the weaker the acid. Considering the acid strength increases with the electronegativity of the atom carrying the acidic proton within the same row of the periodic table, and based on general knowledge, ethane (CH₃CH₃) has the highest pKa value among the given compounds, due to its negligible acidity.