15. Analytical Techniques:IR, NMR, Mass Spect

How does IR work?

A C-D (carbon–deuterium) bond is electronically much like a C-H bond, and it has a similar stiffness, measured by the spring constant, k. The deuterium atom has twice the mass (m) of a hydrogen atom, however. (a) The infrared absorption frequency is approximately proportional to the square root of k/m, when one of the bonded atoms is much heavier than the other, and m is the lighter of the two atoms (H or D in this case). Use this relationship to calculate the IR absorption frequency of a typical C-D bond. Use 3000 cm-1 as a typical C-H absorption frequency. (b) A chemist dissolves a sample in deuterochloroform (CDCl3) and then decides to take the IR spectrum and simply evaporates most of the CDCl3. What functional group will appear to be present in this IR spectrum as a result of the CDCl3 impurity?

Which of the bonds shown in red are expected to have IR-active stretching frequencies? (a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

(j)

Convert the following infrared wavelengths to cm-1. (a) 6.24 𝜇m, typical for an aromatic C=C (b) 3.38 𝜇m, typical for a saturated C-H bond (c) 5.85 𝜇m, typical for a ketone carbonyl

Which of the following compounds has a vibration that is infrared inactive? 1-butyne, 2-butyne, H2

Assuming that the force constant is approximately the same for C-C, C-N, and C-O bonds, predict the relative positions of their stretching vibrations in an IR spectrum.

Why is the C-O absorption band of 1-hexanol at a smaller wavenumber (1060 cm-1) than the C-O absorption band of pentanoic acid (1220 cm-1)?

Which occurs at a larger wavenumber: a. the C-O stretch of phenol or the C-O stretch of cyclohexanol? b. the C=O stretch of a ketone or the C=O stretch of an amide? c. the C-N stretch of cyclohexylamine or the C-N stretch of aniline?

a. Which occurs at a larger wavenumber: 3. a C-N stretch or a C=N stretch? 4. a C=O stretch or a C-O stretch?

Rank the following compounds from highest wavenumber to lowest wavenumber for their C=O absorption bands: b.

Calculate the reduced mass for the following bonds. (a) C―H

Calculate the reduced mass for the following bonds. (c) C―Cl

For each pair, choose the molecule that you expect to have the highest wavenumber for its C=O stretch. (d)

(••) Based on Hooke's law, choose the bond in each pair that you expect to vibrate at a higher wavenumber. (e) C=N vs C≡N

(••) Based on Hooke's law, choose the bond in each pair that you expect to vibrate at a higher wavenumber. (f) C―S vs C=O

(••) Choose the bond in each pair that you expect to have the more intense stretching band (b) C=O vs. C=N

LOOKING AHEAD We explain in Chapter 23 that substituents can transmit electronic information through the benzene ring when appropriately positioned. Assessments 14.55–14.59 allow us to use IR spectroscopy to discover this concept and apply it to an example (Assessment 14.60) that we study there. (•••) Justify the carbonyl stretching frequencies indicated for benzaldehyde and 4-methoxybenzaldehyde.

Choose the bond in each pair that you expect to vibrate at the higher wavenumber. (a) C―N vs. C = N

Choose the bond in each pair that you expect to vibrate at the higher wavenumber. (b) C―H vs. C―O

LOOKING AHEAD We explain in Chapter 23 that substituents can transmit electronic information through the benzene ring when appropriately positioned. Assessments 14.55–14.59 allow us to use IR spectroscopy to discover this concept and apply it to an example (Assessment 14.60) that we study there. (••••) On the basis of your answers to Assessments 14.55–14.58, rank the following acetophenone derivatives on the basis of the carbonyl stretching frequency (1 = highest ; 4 = lowest ). (a)

LOOKING AHEAD We explain in Chapter 23 that substituents can transmit electronic information through the benzene ring when appropriately positioned. Assessments 14.55–14.59 allow us to use IR spectroscopy to discover this concept and apply it to an example (Assessment 14.60) that we study there. (•••) Justify the carbonyl stretching frequencies indicated for 3-chlorobenzaldehyde and 3-fluorobenzaldehyde.

There are three carbon–oxygen bonds in methyl acetate. a. What are their relative bond lengths? b. What are the relative infrared (IR) stretching frequencies of these bonds?