Name: Mohit Mulani Experiment Due Date:11/20/13

Section instructor: Dr. Thomas Kwok Section: 223

Dehydrobromination of Meso­stilbene Dibromide, GC/MS Analysis

Abstract

This experiment carries out the conversion of meso­stilbene dibromide to 1,2 diphenylethyne via

concerted E2 reactions. Vacuum filtration and recrystallization are then used to extract and purify the

desired product which is put through the Bayer and Br2/Ch2Cl2 tests for unsaturation. It is also further

analyzed through Gas Chromatography/Mass Spectrometry. MP is 57­59°C, GC times are 11.10 for

1,2 diphenylethyne and 12.07 for transition compound­(1­bromoethene­1,2­diyl)dibenzene. M/z values

for product are 178.2 and 179.2; for transition compound they are 258.2 and 260.2,

Introduction

(antiperiplanar conformation)

This compound is rotated 120 degrees so H and Br are antiperiplanar and E2 reaction can occur. The

transition state haloalkene is synperiplanar because it has a double bond which cannot be rotated. This

state is more stable than an anti transition state and therefore requires more energy for the second E2

mechanism. This is why we need a high temperature and the addition of triethylene glycol helps add

energy for bond rotation and elimination by increasing the boiling point of the solution. In the first E2

mechanism, the Bromine attaches to the K+ from KOH and the hydrogen “donates” its electron to the

single bond. In the second E2 mechanism, the same thing happens but the two groups (H and Br leaving

group) are in syn periplanar orientation. The temperature at these reactions is 190­200°C.

After the reaction, to separate out the desired 1,2 diphenylethyne from KBr, we add water and place

the room temperature cooled solution in an ice bath. The KBr, being polar is soluble in water enabling

us to perform vacuum filtration. This is done with a Hirsch funnel and a vacuum flask with a side tube

that is attached to running water to create a vacuum. After the crude product (insoluble) is collected, we

have to further purify it. To do this we recrystallize in very small amounts of 95% ethanol. This involves

the addition of ethanol to the crude mixture and heating in a hot water bath until most everything

dissolves. Then the crude mixture is cooled in an ice bath and vacuum filtration is performed again. This

final insoluble product is purified 1,2 diphenylethyne­ an assumption that is tested and confirmed by

analysis.

Gas chromatography and mass spectrometry are also used to analyse our product. The gas

chromatography divides substances into 2 phases (a gaseous mobile phase and liquid stationary phase)

which utilizes relative vapour pressure and polarity differences. The data collected from this is called

Retention time and is the time between when the compound is entered into the injection port and its

elution. A mass spectrometer ionizes the compounds and gives us spectral peaks on a graph, abundance

vs (m/z). The parent molecular ion which is formed by the ionization is usually at the highest m/z point

and its abundance can be seen on the graph. Isotopes in several cases can be accounted for in these

graphs by minor (or major depending on the proportion of isotopes to each other) peaks around the

main ion.

SECTION 2 : RESULTS

Reagents/Products Equivalents MW(g/mol)/Density(g/cm3)

Moles Theoreticalwt/volume

Actual Mass or Volume(Purified,g)

1 340.05 g/mol 0.00089 0.3037g 0.3037g

­ 259.15 g/mol 0.0012 0.3037g ­

0.035 178.24 g/mol 0.000031

0.3037g 0.0055g

KOH 3.8856.1056 g/mol

0.00346 0.1945 g 0.1945 g

meso stilbene dibromide is the limiting reagent.

MP of product is seen to be 57­59°C.

OBSERVATIONS:

Action Observed Change

Put the mixture of meso stilbene dibromide, KOH and triethylene glycol on a heating block

Mixture is brownish colour

On heating at 190­200 °C. Mixture starts boiling vigorously and solid precipitates

Add 4ml water and put in ice bath Some of the mixture dissolves, substantial amount of precipitate remains undissolved

Add ethanol and heat filtered out ppt Most ppt dissolves, some remains

Cool in ice bath White crystals begin forming

Add 5mg product to tube and add 1 drop bromine in dichloromethane. Add some more drops

After the first drop, the solution turns colourless. After the second and third drops, a pale colour forms.

Add to 5mg product and 2ml 95% ethanol drops of KMnO4 and compare observation to drops of KMnO4 added to control of ethanol.

One drop causes change in colour to purple in both cases, however the purple colour change in the control (ethanol) is much deeper than the purple colour of the product ethanol mix.

Spectroscopic Data (GC/MS analysis)

GC Peak Retention

Time (minutes)Detected Mass (M/Z) Structure

11.08 179.2

with 1 13C

11.08 178.2

with no 13C

12.10 260.2

with 81Br

12.10 258.2

with 79Br

12.10 180.3

with 1 13C

12.10 179.3

with no 13C

Calculations:

Moles=Given Mass/Mol Mass

=0.3037g/340.05 g/mol

=0.00089 moles

SECTION 3

Methods/Procedures

The procedure is taken from Gilbert, J. C., Martin, S. F., Experimental Organic Chemistry, A Miniscale

and Microscale Approach, 403­405, 93­101, 867­868. There were no significant deviations from the

assigned procedure.

Discussion and interpretation of results

The experiment ran how we expected it to. The desired product 1,2 diphenylethyne is clearly formed

along with some of the transition haloalkene ­(1­bromoethene­1,2­diyl)dibenzene.

This much can be concluded from the GC/MS data. Two retention times are seen on the GC graph,

11.10 and 12.07. From the MS graph (they work complementary to each other) we can tell that the

11.10 represents the product and 12.07 represents the transition compound. The relative abundances of

these tell us that more of the transition compound was present in the sample that was injected.

From the MS data, we see the product in the first graph at 11.10 minutes. It has the m/z value of 178.2

and 179.2 representing the M and M+1 peak we can expect when the isotopes 12C and 13C are

present. The percentage probability of the M+1 peak is about 15%; the natural proportion is 99:1 but

as there are 14 carbons in this product, the probability and size of peak is increased. At 12.07 minutes,

we see the transition compound at 258.2 and 260.2. The difference in these peaks is caused by the

presence of 81Br and 79Br. The proportion of these is almost equal and so are the sizes of the peaks. As

the exact ratio is about 0.51:0.49, we see a little higher peak for 79Br. We also see peaks at 180.3 and

179.3. These represent the transition compound with the bromine removed (hence the positive charge).

The different isotopes of carbon are again represented.

Examining the GC/MS data, we come to the conclusion that as the mechanism predicted, we have

formed 1,2 diphenylethyne ny E2 reaction. Comparing with the chemical tests, we can conclude that

none of the original meso compound was left (as the tests checked for unsaturation and the original

compound was saturated). The chemical tests couldn’t have distinguished between the transition

compound and the final product as they were both unsaturated. This where the GC/MS data was more

specific. The GC data by itself too wouldn’t be a reliable way to identify compounds, but when used

together, the GC/MS graphs tell us the final compounds we have created. Thus the GC separates out

the coumpounds and the MS gives us specific data about the mass of the compounds so we can identify

them. By themselves, the individual methods face limitations that can be overcome by using them

together. Even then however, it is prone to errors. Including the chance of false positives and negatives,

any mistakes made while performing separation with Gas chromatography carry over to the Mass

Spectrometer influencing the spectra we get (creating “noise” on the graph).

Errors in this graph are manifested in the form of the transition compound still being present with the

product. This implies that the whole reaction didn’t occur which means the energy wasn’t enough to

favour the entire second E2 mechanism. As is seen from GC/MS analysis and the MP (57­59°C), the

product isn’t completely pure 1,2 diphenylethyne. The presence of imurities could be external or from

the transition compound (1­bromoethene­1,2­diyl)dibenzene.

Conclusions:

At the end of the experiment, we have used GC/MS analysis to conclude the formation of the product

1,2 diphenylethyne and a transition haloalkene. The Bayer test and Bromine in dicholoromethane test tell

us that the product we have is unsaturated (i.e either an alkene or alkyne) which is what can be

confirmed through GC/MS. The meso compound wasn’t completely converted into the product which

explains the low yield and GC/MS spectra.

References

Gilbert, J. C., Martin, S. F., Experimental Organic Chemistry, A Miniscale and Microscale Approach,

5th edition