Chem 2221L Section 1 November 27, 2017
Title Lab #8: GC Analysis of a Reaction Mixture (Free Radical Substitution)
Introduction Radical reactions, or the reactions of molecules with unpaired electrons, proceed through three steps that are consistent in every radical reaction. First is initiation, when heat or ultraviolet light causes a homolytic bond cleavage, creating two radicals. Propagation, the second step, produces one radical out of another radical. Finally, termination occurs when two radicals bond to form a molecule with an even number of electrons. Although these reactions account for a small percentage of the total number of reactions studied, radical reactions play an important role in industry preparing polymers and simple halogenated solvents. In this experiment, we performed a radical reaction between 2,3-dimethylbutane and sulfuryl chloride using a t-butyl peroxybenzoate initiator. The initiator is the source of the radicals, since it possesses O-O single bonds that tend to homolytically split without excessive light or heat. The radicals are propagated to the chlorine atoms and the alkane, which produces either 1-chloro-2,3-dimethylbutane or 2- chloro-2,3-dimethylbutane. The products were analyzed via gas chromatography.
Procedure The procedure for experiment #8 in the Chemistry 2221L Class pak was followed with the following modifications. : 0.1 ml sulfuryl chloride was added to the flask by the TA : Heating mantle was set on half power for the whole hour.
Results
See attached GC plot
Peak number
Area
Percent of total
Compound name
4
457181
61.4% 2,3-dimethylbutane
8
152008
20.4% 2-chloro-2,3-dimethylbutane
11
134905
18.1% 1-chloro-2,3-dimethylbutane
Sample calculations Percent of total (peak 4) = peak area/total area = 457181/(457181+152008+134905) = 61.4%
Ratios of two products (peaks 8 and 11) 152008/134905 = 1.13 2-chloro:1-chloro (53% 2-chloro, 47% 1-chloro) 2,3-dimethylbutane has 12 primary hydrogens and 2 tertiary hydrogens
Selectivity = (53/2):(47/12) = 26.5 : 3.9 = 6.79:1 selectivity for chlorination of tertiary hydrogens vs. primary hydrogens
Conclusions The two products of the reaction are found on the GC plot at about 1.23 and 1.42 minutes. On gas chromatography plots, the peaks are given in order of ascending boiling points of the molecules they represent. 2-chloro-2,3-dimethylbutane and 1-chloro-2,3-dimethylbutane have boiling points of 112 C and 122 C, respectively. This means that 2-chloro is the first peak and 1-chloro is the second. On the GC plot there is a larger peak that appears earlier than the two products. This is not one of the products; it is the starting material, 2,3-dimethylbutane. The reaction is not done to completion because there is a limiting reagent: sulfuryl chloride. There is only so many chloride anions produced by .1 mL of sulfuryl chloride, so only about 40% of the alkane is converted to the alkyl halide products. The calculated selectivity of 6.79: 1tertiary to primary is close to the theoretical selectivity ratio of 6:1, suggesting that the reaction was successful. After the reaction was completed, the flask contained an unexpected dark colored solid. The reason for this is unknown; it could have been that the temperature at which the reaction occurred was too high