(a) Interpretation: It is to be explained why the pKa values of all three isomers in which a methyl group is attached to the ring are higher than the pKa of phenol itself. Concept introduction: When comparing uncharged acids, the stronger acid is the one whose negatively charged conjugate base is more stable. Electron donating substituents attached to the ring decrease the acidity as compared to non-substituted rings. Alkyl group is an electron donating group, thus, it will destabilize the negatively charged conjugate base to a greater extent than non-substituted rings.

Question

(a 4 shows scanning electron microscope (SEM) images of extruded actions of packing bed for two capillary columns of different diameters, al 750 (bottom image) and b) 30-μm-i.d. Both columns are packed with the same stationary phase, spherical particles with 1-um diameter. A) When the columns were prepared, the figure shows that the column with the larger diameter has more packing irregularities. Explain this observation. B) Predict what affect this should have on band broadening and discuss your prediction using the van Deemter terms. C) Does this figure support your explanations in application question 33? Explain why or why not and make any changes in your answers in light of this figure. Figure 4 SEM images of sections of packed columns for a) 750 and b) 30-um-i.d. capillary columns.³

Answer 1

Question

Chapter 6, Problem 6.77P

Interpretation Introduction

(a)

Interpretation:

It is to be explained why the pKa values of all three isomers in which a methyl group is attached to the ring are higher than the pKa of phenol itself.

Concept introduction:

When comparing uncharged acids, the stronger acid is the one whose negatively charged conjugate base is more stable. Electron donating substituents attached to the ring decrease the acidity as compared to non-substituted rings. Alkyl group is an electron donating group, thus, it will destabilize the negatively charged conjugate base to a greater extent than non-substituted rings.

Interpretation Introduction

(b)

Interpretation:

Why the meta isomer has the lowest pKa value of the three isomers is to be explained.

Concept introduction:

When comparing uncharged acids, the stronger acid is the one whose negatively charged conjugate base is more stable. Electron donating substituents attached to the ring decrease the acidity as compared to non-substituted rings. Methyl groups is an electron donating group, thus, it will destabilize the negatively charged conjugate base to a greater extent than non-substituted rings. Greater the resonance stabilization of the conjugate base, stronger is the acid. In the resonance contributors, the structure in which an atom bearing a negative charge becomes destabilized as the number of electron-donating groups attached to it increases. This structure will not contribute significantly to the resonance hybrid.

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