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Chapter 15

CHM 2211 Chapter Notes - Chapter 15: Ketone, Unpaired Electron, Alkyne

Course Code
CHM 2211
Mohammed Daoudi

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Spectroscopy: used for structure determination
A. The Nature of Light
i. Wavelength (lambda): describes the distance between adjacent peaks of an
oscillating field
ii. Frequency(v): describes the number of wavelengths that pass a particular point
in space per unit time
iii. Wavelength and frequency are inversely proportional (v=c/lambda (c= constant
speed of light)
iv. Photons: packets of energy; directly proportional to its frequency (E=hv
(h=planck’s constant=6.66 x ^-34 Jxs)
v. Electromagnetic spectrum: range of all frequencies
A. Vibrational Excitation
i. IR radiation causes vibrational excitation of the bonds in a molecule; different
types due to the different ways bonds store vibrational energy
1. Stretching and beinding vibrations
B. Identification of Functional Groups with IR Spectroscopy
i. Energy gap for a CH bond is much larger than the energy gap for a CO bond
1. CH will absorb a higher energy in photons
C. The General Shape of an IR Absorbance Spectrum
i. Absorption spectrum: a plot that is created when an IR spectrometer measures
the percent transmittance as a function of frequency
1. All signals point down
2. Location of the signal on the spectrum can be specified by corresponding
wavelengths or frequency of radation that was absorbed
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ii. Wave number: frequency of light/speed of light (a constant); measured in
inverse centimeters
iii. Every signal in an IR spectrum has three characteristics: wavenumber, intensity,
and shape
A. Hooke’s Law
i. Wavenumber of absorption is associated with bond stretching is dependent on
bond strength and the masses of the atoms sharing the bond
ii. Hooke’s law:enables us to approximate the frequency of vibration for a bond
between two atoms of mas m1 and m2 (see book for equation)
1. From the equation we see that smaller atoms give bonds that vibrate at
higher frequencies, creating a higher wavenumber
a. Example: CH (3000 cm^-1) > CD (2200 cm^-1) > CO (1100cm^-
1) > CCl (700 cm ^-1)
2. Stronger bonds will vibrate at higher frequencies, creating a higher
wavenumber of absorption
a. Example: C (triple bond) N (2200 cm ^-1) > C (double bond) N (1600
cm ^-1) > CN (1100 cm ^-1)
iii. View figure 15.6
iv. There are two main regions of the IR spectra
1. Diagnostic region: fewer peaks and provides the clearest information;
contains all signals from double, triple, and xH bonds
2. Fingerprint region: contains signals from vibrational excitation of most
single bonds; more difficult to analyze
3. View figure 15.7
B. Effect of Hybridization States on Wavenumber of Absorption
i. Wavenumber of carbon is dependent on hypbridization states
1. Example: CH carbon
a. Sp^3 (2900 cm^-1) < sp^2 (3100 cm^-1) < sp (3300 cm ^-1)
ii. To compare the spectra of an alkane, alkene, and alkyne:
1. Draw a line at 3000 cm ^-1
2. View figure 15.10
C. Effect of Resonance on Wavenumber of Absorption
i. An unsaturated, conjugated ketone produces a signal with a lower wavenumber
compared to a carbonyl group of a saturated ketone
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