3.10: Additional Problems

Exercise \(\PageIndex{1}\)

For each of the following hand-drawn cartoons of IR spectra, identify the bond(s) that correspond to the indicated peaks.

i.

ii.

iii.

iv.

v.

vi.

vii.

viii.

ix.

x.

xi.

xii.

xiii.

Answer

i. a) O-H b) sp³ C-H c) C-O

ii. a) sp² C-H b) sp³ C-H c) C=C-H (oop bend)

iii. a) N-H (two of them) b) C=O c) C-N

iv. a) sp³ C-H b) H-C-H (CH₂ bend)

v. a) sp³ C-H b) C=O c) H-C-H (CH₂ bend)

vi. a) sp C-H b) sp³ C-H c) C=C

vii. a) N-H b) sp³ C-H

viii. a) C=O b) C-O

ix. a) N-H (two of them) b) sp³ C-H

x. a) sp³ C-H b) aldehyde C-H c) C=O

xi. a) H-C-H (CH₂ bend) b) C-O

xii. a) O-H (very broad in CO₂H) b) sp³ C-H c) C=O d) C-O

xiii. a) sp² C-H b) sp³ C-H c) aromatic overtones d) C=C

Exercise \(\PageIndex{2}\)

For each of the following hand-drawn cartoons of IR spectra, identify the functional group suggested by the spectrum.

a)

b)

c)

d)

e)

f)

g)

h)

i)

j)

k)

Answer

a) The rounded OH peak near 3300 cm^-1 and the strong C-O peak near 1100 cm^-1 suggest an alcohol.

b) The sharp CH peaks below 3000 cm^-1, the weak CH₂ bending modes near 1500 cm^-1 and the absence of any other features suggest an alkane.

c) The strong C=O peak near 1700 cm^-1 and the strong C-O peak near 1100 cm^-1 suggest an ester.

d) The strong C=O peak near 1700 cm^-1 and the absence of additional features other than those associated with saturated hydrocarbons suggest a ketone.

e) The CH peak above 3000 cm^-1 and the strong oop bending modes below 1000 cm^-1 suggest an alkene. The presence of two oop bending peaks may point to a terminal alkene (C=CH₂).

f) The sharp N-H peaks near 3200 cm^-1 and the strong C=O peak near 1600 cm^-1 suggest an amide. the presence of two N-H peaks rather than one points to a primary amide (O=C-NH₂).

g) The small, triangular "sharktooth" peak near 3200 cm^-1 suggests an amine. With just one N-H peak, this is probably a primary amine (R-NH₂).

h) The strong C-O peak near 1100 cm^-1 suggests an ether.

i) The C-H peaks above 3000 cm^-1 and the oop bending modes below 1000 cm^-1 certainly suggest double bonds. The progression of tooth-like "aromatic overtones" between 1600 and 2000 cm^-1 strongly indicates a substituted benzene.

j) The small, triangular "sharktooth" peaks near 3200 cm^-1 suggest an amine. With two N-H peaks, this is probably a secondary amine (R₂NH).

l) The broad, deep OH peak between 3300 cm^-1 and 2600 cm^-1 and the C=O peak near 1700 cm^-1 suggests a carboxylic acid. The O-H peak of a carboxylic acid is often missed; it is moved to lower frequency by hydrogen bonding.

Exercise \(\PageIndex{3}\)

a) Identify at least three important peaks/bonds in each of the following IR spectra.

b) Identify the functional group present in each of these samples. See the Functional Group Section

for help.

c) Draw a possible structure for each of these compounds (there may be many, many correct answers).

problem i)

problem ii)

problem iii)

problem iv)

problem v)

Answer

i)

a)

3200 cm^-1 (sharp): N-H

2900 cm^-1 (sharp): C_sp3-H

1600 cm^-1 (broad): probably NH₂ bending

b) A primary amine.

c)

ii)

a)

3400 cm^-1 (strong, broad): O-H

2900 cm^-1 (sharp): C_sp3-H

1050 cm^-1 (strong): probably C-O

b) Alcohol.

c)

iii)

a)

2900 cm^-1 (sharp): C_sp3-H

1700 cm^-1 (strong): C=O

1200 and 1000 cm^-1 (strong & medium): C-O

b) Ester

c)

iv)

a)

3050 cm^-1 (sharp): C_sp2-H

1700 cm^-1 (strong): C=O

1200 and 1000 cm^-1 (strong & medium): C-O

b) Ester; probably contains an aromatic as well

c)

v)

a)

2750 and 2650 cm^-1 (medium, sharp): C-H of an aldehyde

1700 cm^-1 (strong): C=O

Below 1000 cm^-1 (strong & medium): oop bends

b) Aldehyde; probably contains an aromatic as well

Exercise \(\PageIndex{4}\)

Sketch an approximate IR spectrum for each of the following compounds:

Exercise \(\PageIndex{5}\)

The oop bends are sometimes useful in distinguishing substitution pattrens around a benzene ring. Using the spectra of o-, m-, and p-xylene, formulate some guidelines about what the oop bends look like when substituents are one, two or three carbons away on a benzene ring.

Figure \(\PageIndex{1}\): IR spectrum of m-xylene.

Source: SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology of Japan, 14 July 2008)

Figure \(\PageIndex{2}\): IR spectrum of o-xylene.

Source: SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology of Japan, 14 July 2008)

Figure \(\PageIndex{3}\): IR spectrum of p-xylene.

Source: SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology of Japan, 14 July 2008