Experiment 3: Alkanes: Chlorination Jeremy Wolf

July 14, 2008 TA: Stefanie Lenz

Introduction:

The purpose of this lab experiment was to perform a free-radical chain reaction and determine the relative reactivity of C-H bonds towards chlorination. The chain reaction was composed of an initiation, propagation, and termination steps; each step having a unique mechanism. In order to enable the typically unreactive alkane to undergo the chain reaction, chlorine radicals were produced from a liquid compound containing chlorine that could absorb energy and create a radical chlorine atom.

The alkane, 1-chlorobenzene, was used in this reaction to also study the effect of a chlorine atom on the alkane molecule. The chlorine atom had an effect on the radical alkane formation, which in turn, had an effect on the chlorination of the alkane. This relatively high electronegativity of the chlorine atom on the 1-chlorobutane causes the chlorine to pull electron density towards the chlorine atom. This has the effect of increasing the stability of the 1-chlorobutane radicals. This experiment tested the reactivity of those different radicals by measuring the percent of the different dichlorobutane molecules produced from the reaction by gas chromatography.

Main Reaction and Mechanism:

Main Reactions Chart

Stoichiometry

2 mL SO2Cl2 x 1.667 g/mL = 3.334 g SO2Cl2

3.334 g SO2Cl2 x 1 mol/134.97 g = 0.002470 mol SO2Cl2

Assuming a 1:1 ratio, then

0.02470 mol SO2Cl2 = 0.02470 mol SO2 x 64.07 g/mol = 1.583 g SO2

0.02470 mol SO2Cl2 = 0.02470 mol HCl x 36.46 g/mol = 0.9006 g HCl

1.583 g SO2 + 0.9006 g HCl = 2.484 g of gases produced from the reaction, so the flask should have lost a maximum of 2.48 grams of mass when the reaction was complete.

Reaction was 90% complete when 0.90 x 2.484 g = 2.235 grams of mass was lost from the reaction flask.

Table of Reactants

Table of Products

Yield Data

Theoretical Yield of dichlorobutane = moles of limiting reactant (sulfuryl chloride) x molar weight of dichlorobutane

Theoretical Yield = 0.02470 x 127.01 g/mol = 3.137 grams product

Actual Yield = 1.176 grams

Percent Yield = Actual/Theoretical x 100

= 1.176 g/3.137 g x 100 = 37%

Synopsis of and Notes on Experimental Procedure – Results

Figure 1: Experimental Set-up

The reaction was set up according to the apparatus in Figure 1. The initial reaction vessel contained 0.10 grams of ABCN, 5 mL of 1-chlorobutane, 2 mL of sulfuryl chloride, and a small stir bar. The total mass of the flask and reactants was 65.92 grams. The first heating lasted a total of 20 – 25 minutes, but not all of the time was at full reflux. After the first heating, the system lost only 0.54 grams.

Another 0.09 grams of ABCN was added to the reaction vessel and the system was heated to reflux for another 15 minutes of reflux. After the second heating, the system lost 2.81 grams of mass total. The product was then rinsed with a brine solution and a carbonate solution, then separated in a separatory funnel. Once the aqueous layer tested basic to litmus (after the first carbonate rinse) the organic layer was rinsed with another brine solution and separated. Then the solution was dried with anhydrous sodium sulfate. The resulting solution was clear. After the solution was weighed, it was examined by gas chromatography.

Figure 2: Gas Chromatograph of Dichlorobenzene Mixture

Link to Google Document

The gas chromatography data shows the following:

Observed Properties of Product:

The only observed properties of the product were that it was a white solid when it precipitated briefly from solution. No other properties were observed.

Side Reactions:

SO2Cl2 + H2O ➔ H2SO4 + HCl (not balanced)

The first three reactions are termination steps of the chlorination process. If 2 chlorine radicals react together, it stops the chain reaction process. The result is the same if two 1-chlorobutane radicals react or if a chlorine radical reacts with the initiator radical. The final reaction would have occurred if the water from the gas trap siphoned back into the reaction mixture. The first three side reactions were bad because they would end the reaction before all of the reactants available for conversion had a chance to react, which would reduce the percent yield. The last side reaction was bad because it would have caused steam and hot acid to be quickly created and possible explode all over the lab, which would have ruined everyone’s day.

Method of Purification:

Conclusions:

The purpose of this laboratory experiment was achieved. The reactivity of the C-H bonds was shown, even though the yield of this reaction was fairly low, at 37%. According to the gas chromatography data and relative reactivity calculations, the 20 hydrogen on the C3 bond was the most reactive and the C1 carbon was least reactive. The data obtained in the lab experiment supports the explanation given in the lab manual (Dailey, p. 24), which said that the chlorine atom causes an electron-attracting withdrawing effect on the rest of the carbon atoms in the chain. This effect lowered the energy of the secondary hydrogen radicals and increased their stability.

The secondary hydrogens were more reactive than the primary hydrogen. The C3 bond was 3.5 times more reactive than the methyl carbon and the C2 bond was 1.8 times more reactive. This confirms the stability of the electron-attracting withdrawing effect.

The methyl carbon was more reactive than the chlorinated carbon atom in the 1-chlorobutane molecule. This is also supported by electron repulsion theory, since the chlorine atom would cause steric interaction with the addition of another chlorine atom to the molecule at the C1 bond. The electron-attracting withdrawing effect would also destabilize the C1 radical. Both steric interactions and the electron-attracting withdrawing effect would both cause the effect seen in the lab experiment.

The only disappointing aspect of the lab experiment was the low yield of the reaction. The low yield could have been caused by a longer reflux time in the second heating. Since more mass was lost than was expected from the loss of mass due to the production of gases, sulfur dioxide and hydrogen chloride, it is reasonable to assume that some of the additional mass that was lost was due to evaporated product or unreacted 1-chlorobutane. If the lab were to be repeated, care should be taken to follow the directions for the reflux time more carefully.

Answers to Assigned Questions:

Page 261, Exercises #2 and 9

#2 – The amount of sulfuryl chloride used in this reaction is less than the required amount to convert all of the 1-chlorobutane to product for two reasons:

• The lower concentration of sulfuryl chloride produces only dichlorobutane. A higher concentration of sulfuryl chloride would produce trichlorobutane in a significant, or detectable, amount. During the reaction the concentration of 1-chlorobutane is much, much higher than the concentration of dichlorobutane, so the likelihood of chlorine radicals reacting with the 1-chlorobutane is much higher than the chance of the chlorine radicals reacting with dichlorobutane.
• In order to keep the termination steps limited, the concentration of unreacted 1-chlorobutane is consistently higher than the radical chlorine atoms. In this way, it was much more likely for the chlorine radicals to react with reactant than other chlorine atoms, which could terminate the reaction prematurely.

#9 –