The pka of three different c―h bonds is given below. explain the observed trend in pka. select the single best answer

Chemistry
1

the pka of three different c―h bonds is given below. explain the observed trend in pka. select the single best answer.

@aibot

2

The observed trend in pKa for the three different C-H bonds can be explained based on the electronegativity and the acidity of the adjacent atoms. Here are the pKa values provided by LectureNotes:

  • C―H bond adjacent to a carbonyl group: pKa = 20
  • C―H bond adjacent to an alkene: pKa = 40
  • C―H bond adjacent to an aromatic ring: pKa = 50

The trend in pKa values indicates that the acidity of the C-H bond increases as we move from the carbonyl group to the alkene and then to the aromatic ring. This trend can be explained as follows:

  1. Electronic Effects: The electronegativity of the adjacent atom affects the acidity of the C-H bond. A more electronegative atom has a greater ability to withdraw electron density from the C-H bond, resulting in a more stable conjugate base. In this case, the carbonyl group (oxygen) is more electronegative than the alkene (carbon-carbon double bond) and the aromatic ring (carbon-carbon double bonds), leading to an increased acidity of the C-H bond.

  2. Resonance Stabilization: The presence of multiple resonance structures can stabilize the conjugate base and increase the acidity of the C-H bond. The carbonyl group and the aromatic ring have more resonance structures compared to the alkene, which provides additional stability to the conjugate base and increases the acidity of the C-H bond.

  3. Inductive Effects: Inductive effects, which involve the electron-withdrawing or donating ability of neighboring atoms, can also influence the acidity of the C-H bond. While it is not explicitly mentioned in the question, it is worth considering that the electron-withdrawing groups attached to the aromatic ring could further enhance the acidity of the C-H bond.

Based on these factors, the observed trend in pKa values can be explained as follows: The C-H bond adjacent to the carbonyl group has the lowest pKa (most acidic), followed by the C-H bond adjacent to the aromatic ring, and finally, the C-H bond adjacent to the alkene has the highest pKa (least acidic).