Iman of reactions via lowering the activation

 

Iman Izzati Bte Mohamed Musharraf

 

Nanyang Technological
University; School of Physical and Mathematical Science, Division of Chemistry
and Biological Chemistry. 

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ABSTRACT: Metal-based oxidizing agents are commonly used
in organic synthesis to obtain ranges of oxidations. However, this method
brings disadvantages, mainly the over-oxidation to carboxylic acids. The
objective of this experiment is to use a catalytic system with CuBr, bpy, NMI,
TEMPO and atmospheric oxygen as the stoichiometric oxidant, with the catalyst
lowering the activation energy of the process; the primary alcohol can be
converted to the corresponding aldehyde.

Metal-based oxidizing agents, such as
pyridinium chlorochromate (PCC), KMnO4, CrO3 in conc. H2SO4
and OsO4, are commonly used in organic synthesis to obtain
ranges of oxidations. However, disadvantages arise from these reagents, such as
the yield of by-products, stoichiometric amounts used and promotion of
over-oxidation to carboxylic acids. These disadvantages are usually challenging
to scale up stoichiometric metal-based oxidation reactions for synthesis. Thus,
a catalyst is used in a sub-stoichiometric amount to increase rate of reactions
via lowering the activation energy of the process. The catalyst itself does not
change the reactants or products’ energy and itself not being chemically
consumed or changed.

 

Steps

Colour
Change

CuBr
added to Alcohol B

Green

Bpy
and TEMPO added

Light
Green

4
drops of NMI upon stirring

Yellow-green

Rapid
stirring for ~30 minutes

Blue-green

Extraction
with pentane

Organic
layer : Colourless       Aqueous layer:
Blue

Addition
of MgSO4

Pale
pink liquid

Evaporation
of pentane

Orange
oil

Table 1. Summary of colour
changes of the compound

The
experiment was modified slightly, as a few more NMI drops were added at
intervals of 10 minutes while stirring, as there was no significant colour
change.

 

Mass
of product obtained: 0.2541g                 
         Appearance of product: An
orange oil                                 
Mass of starting material Alcohol B: 0.3566g                  Yield of aldehyde product:
0.2541/0.3566 = 71.26g               
Percentage yield of product: 71.26 x 100% = 71.3%         The yield of the product was not very
high and this may be due to some product being lost during the extraction,
where little amount of the organic layer might be removed together with the
aqueous layer, resulting in lesser organic product in the end. The transferring
of the products was also not completely efficient, leading to some products
being left in some apparatuses that also results to the lower yield of product
obtained.                                                  
Confirmation of molecular formula and structure of B:  Based on the EI-MS spectrum, there is a ratio
of peak intensities of 3:1 at 139 and 141, indicating presence of Cl. Using
rule of 13, (142.64-35.5)/13 = C8H11 followed by
subtracting CH4 (MW=16) due to having an oxygen, results in the
molecular formula of C7H7O, with an unsaturated index of
4, indicating a benzene ring.                                        1H
NMR of reactant

Dshift/ppm

Signal

No. of H

Coupling Constant, J/Hz

Corresponding H

7.34

m

2

6.18

HD

7.34

m

2

2.61

HC

4.67

d

2

5.85

HB

1.71

t

1

5.88

HA

Table 2. 1H NMR
spectrum data of alcohol B with the use of 300 MHz NMR machine and solvent CDCl3

 

13C NMR of reactant

 

D shift/ppm

Signal

No. of C

Corresponding C

139.24

s

1

CB

133.34

s

1

CE

128.65

s

1

CD

128.24

s

1

CC

64.51

s

1

CA

Table 3. 13C NMR spectrum data of alcohol B with the use
of 75 MHz NMR machine and solvent CDCl3.     

 

With reference to table
2, the 1H NMR spectrum of alcohol B, HA is the most
upfield as it belongs to the –OH group with is electron donating, where the
signal is in the range of 0 to 5 ppm. HC and HD are
aromatic protons due to the peaks in the range of 7 to 8 ppm. HD is
closer in proximity to the electron-withdrawing Cl group and hence is slightly
more downfield. HD is thus les shielded than HC and the
multiplet formed at 7.34, with the coupling constant of J=6.18 Hz and 2.61 Hz
respectively.

 

With reference to table
3, the 13C NMR spectrum, CB, Cc, CD
and CE are aromatic protons, having their NMR signals in the region
of 110 to 170 ppm. CB is the most downfield carbon as it is at the
ipso position with the –CH2OH substituent, via the resonance effect,
which the singlet at 139.24 ppm is observed. CE is more downfield
than CC and CD as it is bonded to the
electron-withdrawing Cl group directly, thus being slightly more deshielded
relatively, where the singlet at 133.34 ppm is observed. CC can be
concluded to be in the ortho position while CD is in the meta
position in the aromatic ring. Cc’s slight upfield shift is due to
resonance structures, hence a singlet at 128.65ppm and 128.24ppm can be
observed for CD and CC respectively. Lastly, CA
is the most upfield as it is an alkyl carbon and is close in proximity to the
electron-donating OH group, giving a signal within the alkyl region.