jb wm4 instrumental analysis

8/9/2019 JB WM4 Instrumental Analysis

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WM4 Instrumental analysis


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The 3 key instrumental techniques

How do we know that salicylic acid contains

OH and COOH groups?

Mass spectroscopy (m.s.).

Infrared (i.r.) spectroscopy.

Nuclear magnetic resonance (n.m.r.)

spectroscopy.


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Making use of infrared spectroscopy

Any unidentified, new substance has its i.r. spectrum

recorded.


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6.4 Infrared spectroscopy

Energy in molecules is quantised.

In i.r., molecules are exposed to radiationbetween 1014Hz 1013Hz (wavelengths 2.5m

-15 m). Remember: c = v

Bonds vibrate and stretch (pull apart then

push together again) as they absorb energy.


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Vibrational changes of CO2

Asymmetric stretch

Symmetric stretch

bending


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Infrared spectra: signals = stretches

An IR spectrum hangs down from a baseline (100%

transmittance = no absorbtion).

The signals (look like icicles) on an IR spectra

correspond to bonds absorbing a packet of energy

and vibrating more.

The OH and CO bonds in salicylic acid absorb

energy at specific wavelengths ()/m and so

wavenumbers (1/ )/ cm-1.


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Generally, particular bonds can be matched

to a particular absorption region.

Bond Location Wavenumber/cm-1 Intensity

C-H Alkanes 2850 -2950 M-S

C=C Alkenes 1620 1680 M

Arenes Sev peaks 1450-1650 Variable

C C Alkynes 2100 2260 M

C=O Aldehydes 1720 1740 S

C-O Alcohols, ethersphenols

1050 - 1300 S

C-F fluoroalkanes 1000 1400 S

O-H Alcohols 3600 3640 S

N-H 1o amines 3300 - 3500 M-S


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Interpreting spectra: 2-ethylbut-1-ene

?


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Interpreting spectra: propanone


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Regions in the IR spectrum where

typical absorptions occur

Absorption range/cm-1 Bonds responsible Examples

4000 2500 Single bonds to hydrogen O-H, C-H, N-H,

2500 2000 Triple bonds C C, N N

2000 1500 Double bonds C=C, C=O

Below 1500

(fingerprint region)

Various

(not used to ID functional groups)

C-O,

C-X (halogen)

Label these regions on the previous two examples; sketch the structures andlink them to the main signals. Do this for the following examples, too.


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Examples of infrared spectra

Butane

Strongabsorption at2970 cm-1characteristic ofC-H stretchingin aliphaticcompounds.

No indication ofany functionalgroups.


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Methylbenzene 2 types of C-H

absorption ~3000cm-1 (above = C-H on benzene;below = C-H onmethyl group).

No indication ofany functionalgroups.

Absorption pattern~700 cm-1 is typicalof a benzene ringwith a substitutedgroup.


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Benzoic acid A sharp

absorption at3580 cm-1 is dueto O-H bond.

A strongabsorption at1760 cm-1 is theC=O group.

Position of C-Habsorptionsuggests anaromaticcompound.


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Summary of IR spectroscopy

An IR spectrum measures the extent to

which electromagnetic radiation istransmitted through a sample of substance.

Frequency ranges absorbed give clues about

functional groups which are present.

IR spectrum of salicylic acid gives evidenceofC=O andOH groups.


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Evidence from nuclear magnetic

resonance (n.m.r.) spectroscopy.

This technique helps to determine structure,as it investigates the different environmentsin which (hydrogen) nuclei are situated. Then.m.r. spectrum for salicylic acid showssignals for the different environments of the 6protons:

One proton in a COOH environment.One proton in a phenolic OH environment. Four protons attached to a benzene ring.


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n.m.r. spectrum for salicylic acid

12 11

Ab

sorp

tion


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The evidence so far.

A combination of i.r. and n.m.r. spectroscopyshows that salicylic acid has anOH groupandCOOH group both attached to abenzene ring; we can now rename itHYDROXYBENZOIC ACID.

However, it could be one of 3 possibleisomers: 2-hydroxybenzoic acid, 3-hydroxybenzoic acid and 4-hydroxybenzoicacid.

Mass spectroscopy can determine whichisomer we have.


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The mass spectrum of salicylic acid

Signals correspond to positively charged ions formed

from the parent compound, and fragment ions.

Parent or

molecular ion

The fragmentation pattern

is characteristic of a particular

compoundthe fragment at

120 can only come from 2-

hydroxybenzoic acidcanyou see why?

mass


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Fragmentation

Positive ions in a mass spectrometer canbreak down into building blocks.

Example: 2-ethoxybutane.

CH3-CH2-CH-O-CH2CH3

CH3 (M=102)

+

CH3-CH2-CH-O-CH2CH3

(M=87)?

+

HO-CH-CH3 (M=45)

Loss andrearrangement of

CH3CH=CHCH3

+

CH3-CH2-CH

(M=87) CH3

?

Loss of

CH3-CH2?

?

?

+

Loss of

CH3

CH3-CH2(M=29)+


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Positively charged fragments form.

Mass difference suggests functional groups.

For each fragmentation,

one product has apositive charge:

M+ A+ + B

A + B+

The most stable ion

usually forms.

MassDifference

Group that issuggested

15 CH3

17 OH

28 C=O or C2H4

29 C2H5

43 COCH3

45 COOH

77 C6H5


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Isotope peaks: heights are in the same

ratio of abundance for particular elements.

Pairs of peaks correspond to

isotopes of35 Cl and 37 Cl in the

ratio of 75%:25% ie. 3:1.

Highlight these.

mass


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Now its over to you!

Do activity WM4: use accurate Mrvalues,

isotope peaks and a database to lead you tothe formula of salicylic acid.

It shows you how chemists use fragmentation

patterns to deduce or confirm a molecular

structure. Do assignments 1 and 2 C.S. p110-111

Do Problems for 6.5 on mass spectrometry,

C I p145-146

jb wm4 instrumental analysis

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