Chapter 16, Problem 16.85P

Interpretation Introduction

(a)

Interpretation:

Which $\text{α-carbon}$ from cyclohexanone or cyclobutanone has a greater effective electronegativity is to be determined.

Concept introduction:

In a ${}^{\text{1}}\text{H}$ NMR spectrum, the position of the signal due to a particular hydrogen along the chemical shift axis is determined by the electron environment around it. The presence of an electronegative atom within three bonds increases the chemical shift $\text{δ}$. This does not necessarily mean a nearby atom like oxygen, nitrogen or a halogen. The effective electronegativity of the carbon bonded to the proton will also affect the position of the signal. The higher the effective electronegativity of the carbon bonded to the proton, the higher the chemical shift.

Interpretation Introduction

(b)

Interpretation:

The higher effective electronegativity of the $\text{α-carbon}$ in cylcobutanone is to be explained on the basis of the s and p character of the carbon hybridized orbitals.

Concept introduction:

The effective electronegativity of a carbon depends on hybridization. As the s character of the hybrid orbital increases (the number of contributing p orbital decreases), the effective electronegativity increases. This is the result of the reduced distance between the bond electron pair and the nucleus as the s character increases.