Wike kusuma wardani

A. Title of Experiment : Infrared Spectroscopy

B. The Aim of Experiment : Identify the functional groups contained in a

organic compound

C. Experiment Objectives

Infrared spectroscopy (IR spectroscopy) is the spectroscopy that deals

with the infrared region of the electromagnetic spectrum, that is light with a

longer wavelength and lower frequency than visible light. It covers a range of

techniques, mostly based on absorption spectroscopy. As with all

spectroscopic techniques, it can be used to identify and study chemicals. A

common laboratory instrument that uses this technique is a Fourier transform

infrared (FTIR) spectrometer.

The infrared portion of the electromagnetic spectrum is usually

divided into three regions; the near-, mid- and far- infrared, named for their

relation to the visible spectrum. The higher energy near-IR, approximately

14000–4000 cm−1 (0.8–2.5μm wavelength) can excite overtone or harmonic

vibrations. The mid-infrared, approximately 4000–400 cm−1 (2.5–25μm) may

be used to study the fundamental vibrations and associated rotational-

vibrational structure. The far-infrared, approximately 400–10 cm−1 (25–

1000μm), lying adjacent to the microwave region, has low energy and may be

used for rotational spectroscopy. The names and classifications of these

subregions are conventions, and are only loosely based on the relative

molecular or electromagnetic properties.

Infrared spectroscopy exploits the fact that molecules absorb specific

frequencies that are characteristic of their structure. These absorptions are

resonant frequencies, i.e. the frequency of the absorbed radiation matches the

frequency of the bond or group that vibrates. The energies are determined by

the shape of the molecular potential energy surfaces, the masses of the atoms,

and the associated vibronic coupling.

In particular, in the Born–Oppenheimer and harmonic

approximations, i.e. when the molecular Hamiltonian corresponding to the

electronic ground state can be approximated by a harmonic oscillator in the

neighborhood of the equilibrium molecular geometry, the resonant frequencies

are determined by the normal modes corresponding to the molecular electronic

ground state potential energy surface. Nevertheless, the resonant frequencies

can be in a first approach related to the strength of the bond, and the mass of

the atoms at either end of it. Thus, the frequency of the vibrations can be

associated with a particular bond type.

The infrared spectrum of a sample is recorded by passing a beam of

infrared light through the sample. Examination of the transmitted light reveals

how much energy was absorbed at each wavelength. This can be done with a

monochromatic beam, which changes in wavelength over time, or by using a

Fourier transform instrument to measure all wavelengths at once. From this, a

transmittance or absorbance spectrum can be produced, showing at which IR

wavelengths the sample absorbs. Analysis of these absorption characteristics

reveals details about the molecular structure of the sample. When the

frequency of the IR is the same as the vibrational frequency of a bond,

absorption occurs.

This technique works almost exclusively on samples with covalent

bonds. Simple spectra are obtained from samples with few IR active bonds

and high levels of purity. More complex molecular structures lead to more

absorption bands and more complex spectra. The technique has been used for

the characterization of very complex mixtures.

Sample preparation

Gaseous samples require a sample cell with a long pathlength

(typically 5–10 cm), to compensate for the diluteness.

Liquid samples can be sandwiched between two plates of a salt

(commonly sodium chloride, or common salt, although a number of other salts

such as potassium bromide or calcium fluoride are also used). The plates are

transparent to the infrared light and do not introduce any lines onto the

spectra.

Solid samples can be prepared in a variety of ways. One common

method is to crush the sample with an oily mulling agent (usually Nujol) in a

marble or agate mortar, with a pestle. A thin film of the mull is smeared onto

salt plates and measured. The second method is to grind a quantity of the

sample with a specially purified salt (usually potassium bromide) finely (to

remove scattering effects from large crystals). This powder mixture is then

pressed in a mechanical press to form a translucent pellet through which the

beam of the spectrometer can pass. A third technique is the "cast film"

technique, which is used mainly for polymeric materials. The sample is first

dissolved in a suitable, non hygroscopic solvent. A drop of this solution is

deposited on surface of KBr or NaCl cell. The solution is then evaporated to

dryness and the film formed on the cell is analysed directly. Care is important

to ensure that the film is not too thick otherwise light cannot pass through.

This technique is suitable for qualitative analysis. The final method is to use

microtomy to cut a thin (20–100 µm) film from a solid sample. This is one of

the most important ways of analysing failed plastic products for example

because the integrity of the solid is preserved.

It is important to note that spectra obtained from different sample

preparation methods will look slightly different from each other due to

differences in the samples' physical states.

Comparing to a reference

Schematics of two-beam absorption spectrometer. A beam of infrared

light is produced, passed through an interferometer (not shown), and then split

into two separate beams. One is passed through the sample, the other passed

through a reference. The beams are both reflected back towards a detector,

however first they pass through a splitter, which quickly alternates which of

the two beams enters the detector. The two signals are then compared and a

printout is obtained. This "two-beam" setup gives accurate spectra even if the

intensity of the light source drifts over time.

To take the infrared spectrum of a sample, it is necessary to measure

both the sample and a "reference" (or "control"). This is because each

measurement is affected by not only the light-absorption properties of the

sample, but also the properties of the instrument (for example, what light

source is used, what detector is used, etc.). The reference measurement makes

it possible to eliminate the instrument influence. Mathematically, the sample

transmission spectrum is divided by the reference transmission spectrum.

The appropriate "reference" depends on the measurement and its

goal. The simplest reference measurement is to simply remove the sample

(replacing it by air). However, sometimes a different reference is more useful.

For example, if the sample is a dilute solute dissolved in water in a beaker,

then a good reference measurement might be to measure pure water in the

same beaker. Then the reference measurement would cancel out not only all

the instrumental properties (like what light source is used), but also the light-

absorbing and light-reflecting properties of the water and beaker, and the final

result would just show the properties of the solute (at least approximately).

A common way to compare to a reference is sequentially: First

measure the reference, then replace the reference by the sample, then measure

the sample. This technique is not perfectly reliable: If the infrared lamp is a bit

brighter during the reference measurement, then a bit dimmer during the

sample measurement, the measurement will be distorted. More elaborate

methods, such as a "two-beam" setup, can correct for these types of effects to

give very accurate results.

The following steps can be used as basic guidelines to interpret an

infrared spectrum.

1. Begin to pay attention to absorption (bands) from the left and do the initial

observations in the region 4000-1500 cm-1. For the moment ignore the

detailed observations in the region around 3000 cm-1 is the absorption of

vibration of the linier CH however the passing mark every band that

exists.

2. When possible the marking ribbon vibration in step one followed by a

demonstration of the remaining area between 1500-400 cm-1.

3. Note the area slightly above 3000cm-1 to investigate the presence or

absence of H atoms attached to carbon is not saturated. Where the

absorption in the region try to estimate the type of unsaturated carbon

observed. What is a double bond or triple. For H atoms attached to the C =

C then it will be seen the tape on the area 3095-3010 cm-1 and the ribbon

in the region 1600-1650 cm-1 for absorption C = C. whereas if the H atom

attached to the C=C would Visible band at 3300 cm-1 and bands at 2260 -

2100 cm-1 for absorption C-C.

4. Investigate the absorption bands for the fingerprint region of 1500-40 cm -1.

In this area will be found a cluster of CO ether, halogen CX where X = F,

Cl, Br, I. Another important thing in this area is that it can be used to

determine the type of double bond such as cis or trans or the number of

substituents on the benzene ring.

Characteristic IR Absorption Frequencies of Organic Functional Groups

Functional Group

Type of Vibration

Characteristic Absorptions (cm-1)

Intensity

Alcohol

O-H(stretch, H-

bonded)3200-3600 strong, broad

O-H (stretch, free) 3500-3700 strong, sharp

C-O (stretch) 1050-1150 Strong

Alkane

C-H Stretch 2850-3000 Strong

-C-H Bending 1350-1480 Variable

Alkene

=C-H Stretch 3010-3100 Medium

=C-H Bending 675-1000 Strong

C=C Stretch 1620-1680 Variable

Alkyl Halide

C-F Stretch 1000-1400 Strong

C-Cl Stretch 600-800 Strong

C-Br Stretch 500-600 Strong

C-I Stretch 500 Strong

Alkyne

C-H Stretch 3300 strong,sharp

stretch 2100-2260variable, not present in symmetrical alkynes

Amine

N-H Stretch 3300-3500medium (primary amines have two bands; secondary have one

band, often very weak)

C-N Stretch 1080-1360 medium-weak

N-H Bending 1600 Medium

Aromatic

C-H Stretch 3000-3100 Medium

C=C Stretch 1400-1600 medium-weak, multiple bands

Analysis of C-H out-of-plane bending can often distinguish substitution patterns

Carbonyl Detailed Information on Carbonyl IR

C=O Stretch 1670-1820 Strong

(conjugation moves absorptions to lower wave numbers)

Ether

C-O Stretch1000-1300 (1070-

1150)Strong

Nitrile

CN Stretch 2210-2260 Medium

Nitro

N-O Stretch1515-1560 & 1345-

1385strong, two bands

IR Absorption Frequencies of Functional Groups Containing a Carbonyl (C=O)

Functional Group

Type of Vibration

Characteristic Absorptions (cm-1)

Intensity

Carbonyl

C=O Stretch 1670-1820 Strong

(conjugation moves absorptions to lower wave numbers)

Acid

C=O Stretch 1700-1725 Strong

O-H Stretch 2500-3300 strong, very broad

C-O Stretch 1210-1320 Strong

Aldehyde

C=O Stretch 1740-1720 Strong

=C-H Stretch 2820-2850 & 2720-2750 medium, two peaks

Amide

C=O Stretch 1640-1690 Strong

N-H Stretch 3100-3500unsubstituted have

two bands

N-H Bending 1550-1640

Anhydride

C=O Stretch 1800-1830 & 1740-1775 two bands

Ester

C=O Stretch 1735-1750 Strong

C-O Stretch 1000-1300 two bands or more

Ketone

Acyclic Stretch 1705-1725 Strong

Cyclic Stretch

3-membered - 18504-membered - 17805-membered - 17456-membered - 17157-membered - 1705

Strong

,-unsaturated

Stretch 1665-1685 strong

aryl ketone Stretch 1680-1700 Strong

D. Equipment and Materials

1. Equipments

- Infrared spectrofotometer Buck-500 scientific

- A set of KBr pellet maker tool (Hydraulic compressor more than 12

tons, sample place + KBr (at e 4 holder’s disks), vacuum pump,

agate’s mortar)

- Analitical balance

- Watch glass

- Spatula

1. Materials

- The organic sample of B

- The KBr crystall free water [made by storing the KBr crystall (spectra

grade) in exicator or heat in oven at 50oC. if KBr not spectra grade, it

must be heated in oven at 120oC for 24 hours before it was used]

E. Procedures

As many as 1-2 mg sample and 300 mg KBr free water are inserted in

mortar agate, then mixed bray perfectly for 5 minutes or until obtained the

homogenous ground powder. Here after mixed that is inserted in the pellet

maker tool. The pellet is measured its infrared spectrum among wave number

region 4000-600 cm-1.

F. Data of Observations

Data of observation is a spectra as follow:

G. Analysis

The aim of practice is to identify the functional group from unknown

sample. This is sample B. First step to identify the functional groups is make a

pellet, the sample mixed with KBr crystal until homogenous and then press.

The KBr must be free from water because it affects the spectra. It will be

identified as a O-H stretching. It can reduce the accuracy of result. Finally,

pellet is measured its infrared spectrum among wave number region 4000-600

cm-1.

The IR spectrum from sample B uses to determine the functional

group. First, the wave number of peak is 2920.8/cm and 2852.7/cm on the

high intensity that indicate the streching C-H and become strong with wave

number of peak 1467/cm in a fingerprint region. Second, in a wave number of

peak 1736.7/cm indicate streching C=O cetones.

From the uptake was in the area show the results of a helping group

1300-1100/cm C-C-C and bending of C = O. absorption in the region 1230-

1100/cm show alifatis ketones.

In conclusion we predict the organic compound is a kind of cetone

because we only find the C=O and C-H functional group.

H. Conclusion

Uses spectrofotometry IR we cannot determine the structure of organic

compound, we only determine the functional group of compound. The

functional group from spectra are :

Stretching C-H alkyl at 2920.8/cm and 2852.7/cm.

Bending C-H alkyl at 1467/cm.

Stretching C=O cetones at 1736.7/cm.

C-C-C at 1300-1100/cm.

bending of C = O 1230-1100/cm.

I. Tasks

1. Determine the functional group contained in an organic sample!

Answer:

The sample C has the following functional groups:

Stretching C-H alkyl at 2920.8/cm and 2852.7/cm.

Bending C-H alkyl at 1467/cm.

Stretching C=O cetones at 1736.7/cm.

C-C-C at 1300-1100/cm.

bending of C = O 1230-1100/cm

2. Why must KBr crystall be free from water?

Answer:

KBr crystall must be free from water because the molecule of

water will influence the absorption of functional group in IR spectometer,

so that it will make the result of spectrum read is not valid

J. References

Gary, D. Christian. 1994. Analytical Chemistry. John wiley and sons : New York.

Harvey, david. 2000. Modern Analytical Chemistry. MC Graw Hill Company : United States.

Sastroamidjojo, Harjono. 1985. Spektroskopi. Liberty : Yogyakarta.

Smith dkk. 2000. Vogel’s text book of practical organic chemistry.longman house : England.