Question

Choose which of these compounds has proton(s) that can be pulled off with NaH:

a) hexane,
b) 1-hexene
c) 1-hexyne.

In your answer give the approximate pKa of the most extractable proton in each. Give the mechanism and products of the acid/base reaction.

Answer 1

Among hexane, 1-hexene, and 1-hexyne, 1-hexyne is the only compound with a proton that can be pulled off with NaH, due to its relatively low pKa of approximately 25. NaH acts as a base and 1-hexyne donates the acidic proton, resulting in a hexyne anion and hydrogen gas. So, the correct option is c) 1-hexyne.

Step-by-step explanation:

The compound with protons that can be pulled off with sodium hydride (NaH) among hexane, 1-hexene, and 1-hexyne is 1-hexyne. Sodium hydride is a strong base that can deprotonate relatively acidic hydrogens, such as those on terminal alkynes.

The approximate pKa values of the most extractable proton for each compound are:

• Hexane: pKa ≈ 50
• 1-hexene: pKa ≈ 44
• 1-hexyne: pKa ≈ 25

Therefore, the 1-hexyne has the most acidic proton. In the acid/base reaction mechanism, the hydride ion from NaH acts as a base and abstracts the acidic proton from 1-hexyne, yielding a hexyne anion and hydrogen gas.

The reactions are as follows:

• Hexane: No reaction with NaH due to very low acidity.
• 1-hexene: No reaction with NaH due to low acidity.
• 1-hexyne: 1-hexyne + NaH → hexyne anion + Na+ + H2

Mentioning the correct option in the final part, 1-hexyne is the compound that undergoes deprotonation with NaH.