Organic Chemistry I University of Illinois at Chicago...

University of Illinois at ChicagoUICCHEM 232

Organic Chemistry I

Lecture 13Organic Chemistry 1

Professor Duncan Wardrop

February 23, 2010

1


Organic Chemistry I

Chapter 13

Spectroscopy &Spectrometry

2


Organic Chemistry I

Sections: 13.1-13.2

Introduction toAnalytical Methods

3

UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide

Lecture 13: February 23

Spectroscopy vs. Spectrometry

4

Spectroscopystudy of the interaction of electromagnetic radiation with matter; typically involves the absorption of electromagnetic radiation

Spectrometryevaluation of molecular identity and/or properties that does not involve interaction with electromagnetic radiation

4



Spectroscopic Methods

5

Method Measurement/Application

InfraredSpectroscopy

• vibrational states: stretching and bending frequencies of covalent bonds that contain a dipole moment

• functional group determination

Ultraviolet-Visible (UV-vis)

Spectroscopy

• electronic states: energy associated with promotion of an electron in a ground state to an exited state

• chromophore determination

MassSpectrometry

• molecular weight: of parent molecule and fragments produced by bombardment with “free” electrons

• fragment and isotope determination

Nuclear Magnetic Resonance

Spectroscopy

• nuclear spin states: energy associated with spin states of nuclei in the presence of a magnetic !eld

• determine structural groups and connectivity

5



Absorption/Transmission Spectroscopy: Simpli!ed Principles

6

• sample absorbs different frequencies of light corresponding to molecular vibrations (IR) or electronic transitions (UV-vis)

• detector determines what frequencies of light passed through (transmittance) and what frequencies of light were absorbed (absorbance)

6



Electromagnetic Spectrum

7

shorter wavelength (λ)higher frequency (ν)

higher energy (E)

longer wavelength (λ)lower frequency (ν)

lower energy (E)

Electromagnetic Radiation

• propagated at the speed of light (3 x108 m/s)

• has properties of particles and waves

• energy is directly proportional to frequency

• energy is indirectly proportional to wavelength

E = hν c = νλ

7



Quantized Energy States

8

Types of States

EnergyRange (λ)

Spectroscopic Method

nuclear spin radiofrequency1-10 m

NMR

rotational microwave10-100 cm

Microwave

vibrational infrared0.78-1000 μm

IR

electronic ultraviolet800-200 nm

UV-visIncreasing Energy

8


Organic Chemistry I

Sections: 13.20-13.22

Infrared Spectroscopy

9



Principles of Infrared Spectroscopy

10

Stretching

Bending

IR: Measures the vibrational energy associated with stretching or bending bonds that contain a dipole moment (µ).

δ−δ− δ−δ+ δ+ δ+

δ+

δ−

δ+

δ−

δ+

δ−

10



Stretching & Bending Vibrations

11

11



Dipole Moment

12

In order to measure the stretching or bending frequency of a covalent bond, it must have a dipole moment (μ).

moreelectronegative

atom

lesselectronegative

atomdipole arrow

δ δ

covalent 2electron bond

(partially negatively charged) (partially positively charged)

12



Hooke’s Law: Bonds are Like Springs

13

Vibrational Energy Depends both on bond strength (spring force constant) and the mass of atoms (objects) attached

(m1 + m2)

(m1 * m2)f *√ν~ = k

ν = vibrational “frequency” in wavenumbers (cm-1)~

k = constant (1/2πc)

f = force constant; strength of bond (spring)

m1, m2 = masses (not molecular weights) of attached atoms

Trends:

↑ bond strength = ↑ frequency

↑ mass = ↓ frequency

13



Spring Analogy

14

smaller mass =higher frequency =

higher energy

stronger spring (bond) =higher frequency =

higher energy

14



Wavenumber (ῡ) and Infrared Scale

15

3400 3000 2200 1000

N-H C NC CCO2

(2380)

C O

C NC C

fingerprintregion

wavenumber (cm-1)2600

O-HC(sp)-H

C(sp2)-HC(sp3)-H

14001800

C O

higher wavenumber (ῡ) =higher frequency (υ) =lower wavelength (λ) =higher energy (E)

lower wavenumber (ῡ) =lower frequency (υ) =

longer wavelength (λ) =lower energy (E)

wavenumber = reciprocal of the wavelength measured in centimeters (cm); directly proportional to frequency

ῡ (cm-1) = 1λ (cm)

15



Infrared Spectrum

16

5001000150020002500300035004000Wavenumbers

20

40

60

80

100

Transmittance: amount of light that passes through sample; not absorbed by molecular vibrations

% T

rans

mis

sion

Frequency: typically measured in wavenumbers; higher wavenumber = higher frequency = higher energy vibration

Bands: frequency of vibration absorbed by molecules; can be broad or narrow; number of bands does not equal number of bonds

16



Characteristic Stretches - Alkanes

17

CH2

CC

CC

CH3

H H

H HH H

HH1

2

3

hexane

H

5001000150020002500300035004000Wavenumbers

20

40

60

80

100

2 31

• 2 = sp3 C-H bond stretching motion; general absorb around 2850-2950 cm-1

• 1 = C-H rocking motion when C atom is part of a methyl group (-CH3); 1370-1350 cm-1

• 3 = scissor motion of -CH3 hydrogen atoms; 1470-1450 cm-1

• 1300-900 cm-1 = !ngerprint region for organic molecules; typically complex and unhelpful

17



Characteristic Stretches - Alkenes

18

H3CC

CC

CC

H H

H H

HH

4

1-hexene

H

H

H5

5001000150020002500300035004000Wavenumbers

20

40

60

80

100

5 4• 5: notice sp2 C-H (~3100

cm-1) at higher frequency than sp3 C-H (~2950 cm-1)

• more s-character = stronger bond = higher frequency

• 4: also, C=C bond at higher frequency than C-C bond; ~1600 cm-1

18



Characteristic Stretches - Alkynes

19

CC

H

H3C 7

6

1-hexyne

6

• 7: notice sp C-H (~3300 cm-1) at higher frequency than sp2 C-H (~3100 cm-1), which was higher than sp3 C-H (~2950 cm-1)

• 6: C≡C stretch is very weak because carbons have almost identical electronegativities = small dipole moment

7

19



Characteristic Stretches - Alcohols

20

5001000150020002500300035004000Wavenumbers

20

40

60

80

100

5

9

4

HO

CC

C

H H

4H

H

H5

prop-2-en-1-ol(allyl alcohol)

9

• 9: hydroxyl groups (-OH) exhibit strong broad bands; ~3300 cm-1

• broad peak is a result of hydrogen bonding; width depends on solution concentration

• lower concentration = less hydrogen bonding = more narrow -OH band

20



Characteristic Stretches - NItriles

21

OC

NH

8

9

3-hydroxy-propionitrile

5001000150020002500300035004000Wavenumbers

20

40

60

80

100

8

9

• 8: nitriles ~2200 cm-1

• nitriles (C≡N) absorb a greater magnitude of energy than alkynes (C≡C) because they have a larger dipole moment

• larger dipole moment = more intense peak

• size of the dipole does NOT affect frequency of vibration

21



Example: Ester, Amine, Benzene

22

5001000150020002500300035004000Wavenumbers

20

40

60

80

100

11

104

C

C

CO

O

NH

H

10

11

4

2-amino-benzoic acid butyl ester

• 10: strong carbonyl (C=O) band ~1700 cm-1

• 11: amines; secondary amines (-NH) give one band; primary amines (-NH2) gives two bands

• 4: several alkene bands ~1600 cm-1 for benzene ring C=C double bonds

22



Characteristic Stretches - Carboxylic Acids

23

• 10: strong carbonyl (C=O) band ~1700 cm-1

• 9: hydroxyl band (-OH) can be less intense and sharper in carboxylic acids

• 4: weak alkene band (C=C) since small dipole moment

CC

CO

H

O

H

H5 4

10

9

cyclohex-2-enecarboxylic acid

5001000150020002500300035004000Wavenumbers

20

40

60

80

1009

10

4

23



Characteristic Stretches - Aldehydes

24

5001000150020002500300035004000Wavenumbers

20

40

60

80

100

10

4

12

CC

CH

O 10

12

H

H4

hept-2-enal

• 12: usually two bands for C-H of aldehydes; may overlap with sp3 C-H bands

24



Self Test Questions

25

Which molecule is represented by the IR below?

A. aB. bC.cD.d

OH

H3C CH3

OH2C O CH3

H2C

CH3

OHH

O

cyclobutanol 2-butanone ethyl vinyl ether 2-methyl-2-propen-1-ol 2-methylpropanal

a. b. c. d. e.

25



Self Test Questions

26


A. aB. bC.cD.d

OH

H3C CH3

OH2C O CH3

H2C

CH3

OHH

O


a. b. c. d.

26



Self Test Questions

27


A. aB. bC.cD.d

OH

H3C CH3

OH2C O CH3

H2C

CH3

OHH

O


a. b. c. d. e.

27



Self Test Questions

28


A. aB. bC.cD.d

OH

H3C CH3

OH2C O CH3

H2C

CH3

OHH

O


a. b. c. d. e.

28



Self Test Questions

29


A. aB. bC.cD.d

OH

H3C CH3

OH2C O CH3

H2C

CH3

OHH

O


a. b. c. d. e.

29



Self Test Questions

30


A. aB. bC.cD.d

OH

H3C CH3

OH2C O CH3

H2C

CH3

OHH

O


a. b. c. d. e.

30



Example

31

A. aB. bC.cD.d

a. b. c. d. e.

Greater s character =

stronger, shorter bonds = higher

frequency

OH

H3C CH3

OH2C O CH3

H2C

CH3

OHH

O


31


Organic Chemistry I

Section: 13.23This topic will be covered in Chapter 10.

Ultraviolet-VisibleSpectroscopy

32


Organic Chemistry I

Chapter 13: Sections 13.23,13.24, 13.25

Next Lecture. . .

Quiz This Week. . .Chapter 5 & 6

Problem Set 1 has been posted

Quiz This Week. . .Chapter 5 & 6

33

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