Organic Chemistry Lab 1: Thin Layer Chromatography

CHM 1321 Section A02

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TA: Guoxian Zang

Submission Date: January 23rd,
2018

 

 

 

 

 

 

Protocol

The procedure/protocol is described in the lab manual
on pages 13-19 (CHM
1321 Introductory Organic Chemistry Laboratory Manual 2017, Dr. William Ogilvie
and Dr. Tony Durst, 2018). No modifications were made.

Observations

–      
Silica gel
is white, dry, odourless

–      
Dichloromethane
is a transparent, clear, colourless liquid with a strong odour

–      
The assigned
sample #9 is a solid, crystallized, white substance (no odour)

–      
The ethyl
acetate (2:8 mixture) is transparent, colourless, and had a strong odour

–      
Both biphenyl
and benzophenone are transparent, colourless liquids

–      
After placing
the substances on the silica gel, it got darker and then the liquid spots disappeared

–      
Pure ethyl
acetate is transparent and has a very strong odour

–      
Hexanes look
similar to all others (transparent, clear, colourless) and also has an odour

–      
XX sample
given is faintly yellow, transparent

–      
Sample A
given (ortho-bromonitrobenzene) is faintly yellow and transparent (similar to
sample given)

–      
Sample B given
(meta-bromonitrobenzene) is transparent and colourless

–      
Sample C
given (para-bromonitrobenzene) is transparent and colourless

Data/Results

 

 

 

Table 1: Part A with 2:8 Ethyl
Acetate as Eluant

 
Eluant

Lanes

Rf Values

Reference

Co-Spot

Sample #9

Reference

Sample

2:8 Ethyl
Acetate

Biphenyl

Biphenyl and
Sample #9

Sample #9

0.84

0.7

2:8 Ethyl
Acetate

Benzophenone

Benzophenone
and Sample #9

Sample #9

0.7

0.7

 

Table 2: Part B with Pure Ethyl
Acetate Eluant

 
Eluant

Lanes

Rf Values

Reference

Co-Spot

Sample #9

Reference

Sample

Ethyl
Acetate

Biphenyl

Biphenyl and
Sample #9

Sample #9

0.9

0.9

Ethyl
Acetate

Benzophenone

Benzophenone
and Sample #9

Sample #9

0.89

0.89

 

Table 3: Part B with Hexane Eluant

 
Eluant

Lanes

Rf Values

Reference

Co-Spot

Sample #9

Reference

Sample

Hexane

Biphenyl

Biphenyl and
Sample #9

Sample #9

0.3

0

Hexane

Benzophenone

Benzophenone
and Sample #9

Sample #9

0

0

 

Table 4: Part C with 1:9 Mixture of
Ethyl Acetate Eluant

 
Eluant

Lanes

Rf Values

Reference

Co-Spot

Sample XX

Reference

Sample XX

1:9 Ethyl
Acetate

A
(O-Bromonitrobenzene)

A and XX

XX

0.33

0.33

0.57

1:9 Ethyl
Acetate

B
(M-Bromonitrobenzene)

B and XX

XX

0.48

0.6

0.37

1:9 Ethyl
Acetate

C
(P-Bromonitrobenzene)

C and XX

XX

0.52

0.52

0.31

 

TLC Drawings/Figures

Figure 1:
Part A Plate 1

Figure 2:
Part A Plate 2

Figure 3: Part
B Plate 1

Figure 4:
Part B Plate 2

Figure 5:
Part B Plate 3

Figure 6:
Part B Plate 4

Figure 7:
Part C Plate 1

Figure 8:
Part C Plate 2

Figure 9:
Part C Plate 3

Figure 10:
Part C TCL Plates Under UV Light Stained with O-Bromonitrobenzene,
P-Bromonitrobenzene, and M-Bromonitrobenzene

 

 

Calculations for percentage of absorption

Sample Rf
Value Calculation

Calculation
of reference (biphenyl) in part A, plate 1

Rf= Compound
travelling distance/solvent travelling distance

= d1/d2

= 4.2cm/5cm

= 0.84

Therefore,
Rf value for biphenyl in part A, plate 1 is 0.84.

Determining
Component Ratio of Sample XX

Absorbance
of Para-Bromonitrobenzene= 34768.037

Absorbance of
Ortho-Bromonitrobenzene= 27683.576

Percent Absorbance
(Para)  

=Absorbance
of Para/ (total absorbance) x100

= 34768.037/
(34768.037+27683.576) X100

=55.672%

Percent
Absorbance (Ortho)

= 100-absorbance
from para

=100-55.672

=44.328%

The percent
of the ortho-isomer within the ortho-para mixture of bromonitrobenzene is
44.328% while the percent of para-isomer is 55.672%.

Determination
of Para-Isomer Within Mixture

Substitute
the percent of the ortho-isomer (44.328%) into the calibration curve in order
to determine mole percent (of para-isomer). The mole percent of para-isomer
within the ortho-para mixture is represented by “x”.

y= 1.0114x-2.0208

44.328= 1.0114x-2.0208

44.328+2.0208+
1.0114x

x=45.826%

The mole
percent of para-isomer in the ortho-para mixture of bromonitrobenzene is
45.826%. Now this number is subtracted from 100% to find the mole percent of
ortho-isomer in the mixture.

100-x

100-57.04

= 54.174%

To conclude,
the ortho:para ratio is a mixture of approximately 44:54 (22:27 when
simplified).

Discussion

In part A of the experiment, an unknown sample’s (#9) polarity
was compared to two references (biphenyl and benzophenone) on a silica gel TLC
plate in order to determine its composition. Biphenyl is a nonpolar molecule
due to equal distribution of charges in the symmetrical structure.
Benzophenone, on the other hand, has an unequal distribution of charges and a
nonsymmetrical structure, making it polar. When compared with benzophenone, the
spots on the plate did not separate, however, when compared with biphenyl, they
did. The lack of separation on the benzophenone containing plate suggests that
both the sample and the reference had the same levels of both adsorption and
polarity. Due to the neutrality of the solvent system (2:8 ethyl acetate:hexanes),
both the sample and the reference were able to cross the plate based on their
affinity to the gel, meaning there was minimal interactions between the solvent
system and the benzophenone/sample. The TLC plates suggest that the unknown
sample was benzophenone at this point. Furthermore, the lower the Rf value is,
the stronger the affinity for the silica gel. The Rf values for the plate with
the biphenyl and the sample were 0.84 and 0.7, respectively, meaning they were
not the same. However, the Rf values for the benzophenone and sample were both
0.7. It can be concluded that the unknown sample is, in fact, benzophenone. In
addition, the lower Rf value suggests that the unknown compound travelled a
shorter distance than the biphenyl, ultimately resulting in the separation of
compounds. It can then be concluded that the sample was more polar than
biphenyl.

In part B of the experiment, more TLC plates were
created in order to test the effects that two different solvent systems had on
the development of the plate. Ethyl acetate is more polar than hexanes are,
meaning that it strongly interacted with the silica gel.