Organic Chemistry

508 CHAPTER 13 Mass Spectrometry and Infrared Spectroscopy

PROBLEM 21

Which will show an stretch at a higher wavenumber, ethanol dissolved in carbon

disulfide or an undiluted sample of ethanol?

13.11 Absorption Bands

Stretching Vibrations

The strength of a bond depends on the hybridization of the carbon: The greater the

scharacter of the carbon, the stronger the bond it forms (Section 1.14). Therefore, a

bond is stronger when the carbon is sphybridized than when it is hybridized,

which in turn is stronger than when the carbon is hybridized. More energy is needed

to stretch a stronger bond, and this is reflected in the stretch absorption bands,

which occur at if the carbon is sphybridized, at if the carbon

is hybridized, and at if the carbon is hybridized (Table 13.5).

A useful step in the analysis of a spectrum entails looking at the absorption bands in

the vicinity of Figures 13.21, 13.22, and 13.23 show the IR spectra for

methylcyclohexane, cyclohexene, and ethylbenzene, respectively. The only absorption

band in the vicinity of in Figure 13.21 is slightly to the right of that value.

This tells us that the compound has hydrogens bonded to carbons, but none bonded

to or sp^2 spcarbons. Each of the spectra in Figures 13.22 and 13.23 shows absorption

sp^3

3000 cm-^1

3000 cm-^1.

sp^2 ' 2900 cm-^1 sp^3

' 3300 cm-^1 ' 3100 cm-^1

C¬H

sp^3

C¬H sp^2

C¬H

C¬H

O¬H

Tutorial:

IR spectra

Table 13.5 IR Absorptions of Carbon–Hydrogen Bonds

1450 – 1420

1385 – 1365

C C trans 980 – 960

cis 730 – 675

trisubstituted 840 – 800

terminal alkene 990 and 910

HR

R

C

R

C

R

R

C

R

R

H

C

R

H

H

C

R

C

H

H

H

C

H

H

C

R

H

terminal alkene 890

Carbon–Hydrogen

Stretching Vibrations wavenumber (cm−^1 )

∼ 3300

3100 – 3020

2960 – 2850

∼2820 and∼^2720

CH CH 2 CH

CH

Carbon–Hydrogen

Bending Vibrations

Carbon–Hydrogen

Stretching Vibrations Wavenumber (cm−^1 )

Wavenumber (cm−^1 )

Carbon–Hydrogen

Bending Vibrations

3

CCH

O

CCH

CCH

RCH

3