Free Radical Substitution

Topics: Chemical reaction, Molecule, Ion Pages: 6 (873 words) Published: April 1, 2015


The objective of this experiment is to conduct a free radical reaction and measure the ratio of products through a GC analysis. Free radical reactions are important to understand because they allow chemists to design synthesis of complex molecules from very simple organic molecules. The reagents used were 2,3-dimethylbutane as the carbon chain, t-butyl peroxybenzoate as the initiator, and sulfuryl chloride as the source of chloride radicals. The predicted products are 1-chloro-2,3-dimethylbutane and 2-chloro-2,3-dimethylbutane based on the primary or tertiary hydrogen being replaced with chlorine. The predicted ratio of these products is 45% tertiary and 54% primary. The GC analysis of the products showed that the solution contained 64% tertiary and 36% primary.

Introduction
The objectives of this lab are to understand free radical reactions and characterize the selectivity and ratio of the reaction. In order to achieve these goals the product will be analyzed in a gas chromatograph to show the selectivity of the reaction. A free radical reaction has three steps: initiation, propagation, and termination. In the initiation stage the free radicals are formed through hemolytic cleavage and the addition of heat. The image below shows the initiation using the reagents from the experiment. T-butyl peroxybenzoate is used as the initiator and sulfuryl chloride is used as the source of chloride atoms.

The propagation stage is where the free radical breaks the bonds in the molecule forming a free radical on the carbon chain. The free radical on the carbon chain then attacks sulfuryl chloride to take a chloride atom and form a product. The left over sulfuryl chloride then splits in two in order to form more chloride free radicals. This group of reactions continue to take place until in a loop until termination stops them. And finally in termination, the unreacted carbon chain free radicals and chloride free radicals come together to end the chain. Also, the chloride free radicals come together to form Cl2.

Radical reactions are important to study because they allow us to design the synthesis of complex substances by starting from simple organic molecules. An example of an important free radical reaction is the chlorine-atom chain reaction that destroys the ozone in the upper atmosphere. It is predicted that two products will be formed: 1-chloro-2,3-dimethylbutane, and 2-chloro-2,3-dimethylbutane. The selectivity of a chlorine radical taking away a hydrogen from another molecule for tertiary, secondary, and primary hydrogens is 5:3.8:1. And the ratio of primary to tertiary product is: primary, 12H*1 = 12; tertiary, 2H*5 = 10. Thus primary should be favored slightly. The predicted amount of each is 45 % tertiary and 54 % primary.

Results and Discussion
Table 1: Reagent Data
Compound
MW
(g/mol)
Density
(g/mL)
Volume
(mL)
b.p. (ͦ C)
2-3 dimethylbutane
86.18
0.662
1.6
58.7
t-butylperoxybenzoate
194.23

1.1
3 drops
282.4
Sulfuryl Chloride
134.97
1.67
0.05

1-chloro-2,3-dimethylbutane
120.62
0.9
-----
117.7
2-chloro-2,3-dimethylbutane
120.62
0.9
-----
112.0

The table above shows the data for reagents used in the experiment Table 2: GC Data
Peak #
Ret. Time (s)
Area
4
2.977
123473737
5
3.901
70401307

Compounds that move farther interact (short retention time) more with the inert gas and thus have a lower boiling point. In comparison, compounds that have a longer retention time interact more with the stationary phase and thus have higher boiling points. 2-chloro-2,3-dimethylbutane was found to have a lower boiling point, so on the GC data attached it should correspond with peak # 4. The data shows that the tertiary molecule (2-chloro-2,3-dimethylbutane) was favored in the reaction as there is a higher peak and greater area for #4 in comparison to 5. The measured ratio of tertiary to primary molecules is calclulated via the area under the...
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