infrared Characterization of Linkage Isomers

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Name: - Elizabeth Thomas

Monday 19 March 2012

Demonstrator Name: -

Lab Day: - Monday

Title: - infrared Characterization of Linkage Isomers

Week 3

Objective

The aim of the lab is to synthesis the two linkage isomers pentaamine nitro chlorocobalt(ııı) chloride and pentaamine nitrito chlorocobalt(ııı) two infrared spectrums of each other chlorocobalt(ııı) chloride and to compare the two isomer. Theory

Infrared spectroscopy is a spectroscopic method that uses infrared radiation to study and identify molecules. Energies associated with vibrations of a molecule are observed in the infrared region. A molecule will absorb a certain frequency of IR radiation according to its structure. Gener ally in this spectroscopic technique it is the ret urn of the molecule to the ground state that is observed. The molecule is supplied with a source of infrared, normally in the form of a laser, the mo lecule then absorbs this energy and the spectrum co llected is as a result of the of the molecule emitting this absorbed energy to return to its ground state. For a molecule to be infrared infrared active there must be a net change in dipole moment, not all vibrations that occur in a molecule are infrared active, as t hey may not result in a net c hange in dipole moment. A ligand is an ion or a molecule that is able to form coordinate c oordinate bonds with a central metal. Some ligand molecules though, are able to form bonds with two of the atoms t hat make up the molecule and hence differ in how the ligand is connected to the central metal ion. These isomers are called linkage isomers. Pentaaminechlorocobalt(ııı) chloride is able to form linkage isomers as t he nitrite ligand may either bond to the central metal ion through the nitrogen atom or the oxygen atom. Infrared spectroscopy is used to determine the structures of the two isomers as Co-NO2 and Co-ONO have two different vibrational patterns. Procedure

Carried out as in the lab manual

Results Part A

Mass of pentaaminechlorocobalt(ııı) chloride used Mass of container and salt /g

36.651

Mass of empty container /g Mass of salt used /g

35.603 1.048

Mass of sodium nitrate used Mass of watch glass and salt /g Mass of empty watch glass/g Mass of salt transferred /g

65.266 64.263 1.003

Mass of nitrito isomer obtained Mass of vial and salt/g Mass of empty vial/g Mass of isomer/g

10.177 9.511 0.666

Part B

Mass of pentaaminechlorocobalt(ııı) chloride used Mass of container and salt /g Mass of empty container /g Mass of salt used /g

107.977 106.974 1.003

Mass of sodium nitrate used Mass of watch glass and salt /g Mass of empty watch glass/g Mass of salt transferred /g

30.381 28.980 1.401

Mass of nitro isomer obtained Mass of vial and salt/g Mass of empty vial/g Mass of isomer/g

9.082 8.531 0.551

1. Vibrations in a molecule occur when a molecule absorbs energy in the corresponding vibrational frequency. For a molecular mode to be vibrational active in the infrared mode there must be a net change in dipole of the molecule. I f there is no net change in dipole the molecule is said to be infrared inactive. 2. The background spectrum is used to remove t he interference that is caused by the molecules in the air that fill the spectrometer. If the spectrum is run without the background being removed, the spectrum will not only include the spectrum of the isomers but also of t he molecules in the air that are infrared active, thereby giving an inaccurate spectrum. 3. The peaks observed in the background spectrum are as follows -1

-1

Peaks at 1652.4cm and 1556.9cm result from H2O bend -1

-1

-1

Peaks at 3646.1cm , 3741.4cm and 3850.3cm result from the OH stretch. -1

-1

Peaks at 2929.8cm and 2359.9cm result from the asymmetric CO2 stretch -1

4. The group in the complex that gives r ise to the stretching at 3000 and 3600cm is the primary amine groups. The amine group in the complex is the NH3 primary amine group. 5. The peaks that differ in the two spectrums are -1

PEAKS/cm 2354.5 1570.1 840.7 820.9

NITRO ISOMER Present Absent Present Present

NITRITO ISOMER Absent Present Absent Absent

The peak at 2121.7 located on t he nitrito isomer is likely to be due to the conversion of nitrito to nitro. 6. The conclusion that can be made is that afte r a day the nitrito product has been converted to the nitro product. This testifies to the instability of the nitrito isomer if after one day it is converted to the nitro isomer.

Discussion Generally the nitro isomer has a higher yield than the nitrito isomer. The nitrito and the nitro isomer are both kinetically favorable, that is under the right conditions both reacts will overcome the necessary activation energy and the reaction will go to completion. The stability of the products though are due to

the thermodynamic stability of the product. The nitro product is more thermodynamically stable than the nitrito isomer which is unstable. In the formation of both the nitro and the nitrito products both reaction go through the ONO isomer, t he nitro product though continues to react until the NO2 isomer is formed. The equilibrium though lies to the side of the nitro product and therefore, the nitro product will be more stable and a higher yield should be formed if the two reactions are carried out under the same conditions, this is because the production of the nitro isomer is more favourable than the nitrito isomer. In the experiment carried out though the nitrito had a higher yield than the nitro product, this might be due to overheating of the mixture in the formation of nitrito product or carrying out the reaction below a pH of 7, all this factors lead to the production of the nitro product that may account for the higher yields. Aqueous ammonia is used instead of sodium hydroxide because it is easier to displace the ammonia ligand with the nitrite ligand. Whereas it will be rather difficult to displace the OH ligand, hence ammonia is used instead sodium hydroxide. The nitrito complex is unstable and therefore on overheating it will convert to the nitro isomer.