The hydrogen at C-1 of 1-butyne is much more acidic than the one at C-1 in 1-butene. Explain.

Verified step by step guidance

1. Identify the functional groups in 1-butyne and 1-butene. 1-butyne has a triple bond (alkyne) at C-1, while 1-butene has a double bond (alkene) at C-1.
2. Recall that the acidity of a hydrogen atom is influenced by the hybridization of the carbon atom to which it is attached. In 1-butyne, the carbon at C-1 is sp-hybridized, while in 1-butene, the carbon at C-1 is sp2-hybridized.
3. Understand that sp-hybridized carbons have more s-character (50%) compared to sp2-hybridized carbons (33%). The higher s-character in sp-hybridized carbons means that the electrons are held closer to the nucleus, making the hydrogen more acidic.
4. Consider the stability of the conjugate base formed after deprotonation. The conjugate base of 1-butyne (an acetylide ion) is more stable due to the higher electronegativity of the sp-hybridized carbon, which can better accommodate the negative charge.
5. Conclude that the hydrogen at C-1 of 1-butyne is more acidic than the hydrogen at C-1 of 1-butene due to the higher s-character of the sp-hybridized carbon and the greater stability of the conjugate base.

Key Concepts

Acidity and pKa

Acidity in organic chemistry refers to the tendency of a compound to donate a proton (H+). The strength of an acid is often measured by its pKa value; lower pKa values indicate stronger acids. In the case of 1-butyne and 1-butene, the hydrogen at C-1 in 1-butyne is more acidic due to the stability of the resulting anion after deprotonation, which is influenced by the hybridization of the carbon atom.

Hybridization

Hybridization is the concept of mixing atomic orbitals to form new hybrid orbitals that can accommodate bonding. In 1-butyne, the C-1 carbon is sp-hybridized, while in 1-butene, it is sp2-hybridized. The sp-hybridized carbon has a higher s-character (50%) compared to sp2 (33%), which means the electrons are held closer to the nucleus, making the hydrogen more acidic in 1-butyne.

Resonance Stabilization

Resonance stabilization occurs when a molecule can be represented by two or more valid Lewis structures, leading to a more stable overall structure. In the case of the anion formed from deprotonating 1-butyne, resonance can stabilize the negative charge, enhancing the acidity of the hydrogen at C-1. In contrast, the anion from 1-butene lacks similar resonance stabilization, making it less acidic.