Free Radical Chlorination

Transcription

1 Free Radical Chlorination Although saturated hydrocarbons are inert to most acidic and basic reagents, they can be halogenated in the presence of a free radical initiator. The process is a chain reaction, as shown below for chlorination of ethane with 2, using irradiation to initiate the process. Step 1 2 Initiation Step 2 Step 3 + CH 3 CH 2 H + CH 3 CH 2 H CH 3 CH CH 3 CH 2 Propogation Following Step 3 are a series of steps known as termination steps; these are steps in which two radicals react with one another to generate a non-radical product. These steps, while not shown here, can be found in any good organic text. Another method of chlorination involves the use of sulfuryl chloride (S 2 2 ) and benzoyl peroxide. At relatively low temperatures (70-80 C), the - bond in benzoyl peroxide breaks to form two benzoate radicals; the initiation and propagation steps are shown below (Ph = C 6 H 5 ). 2 Ph S Ph + S 2 + Propagation of the radical chain reaction (Step 3, above) occurs with the alkyl radical reacting with sulfuryl chloride rather than 2 :

2 CH 3 CH 2 S CH 3 CH 2 + S 2 + When a molecule contains more than one type of hydrogen atom, as in 2-methylbutane (or isopentane), a mixture of alkyl halides can result with the observed composition of the mixture being that predicted statistically on the basis of random attack of on the four different groups of hydrogens: a H 3 C a CH 3 CH d b CH 3 CH 2 c Six primary (1 ) group a H s; Three primary (1 ) group b H s; Two secondary (2 ) group c H s; ne tertiary (3 ) group d H s; Specifically, the observed amounts of the two possible primary monochlorides are much smaller than those predicted for a purely random statistical attack of radical on the primary hydrogens present while the amounts of secondary and tertiary chlorides are greater than predicted. This is due to the fact that the radical shows varied selectivity for the different types of H s; experiments have shown that the relative reactivity of 3, 2, and 1 hydrogen atoms toward is 5.0:3.5:1.0, which is the reverse of that predicted if the attack were directed by statistics alone. This reactivity difference is determined by the relative stabilities of the resulting alkyl radicals, since 3 radicals > 2 radicals >1 radicals. The free radical chlorination of 2-methylbutane gives the following results: Compound Type of H abstracted/# of H s Statistical Prediction bserved % Predicted % based on reactivity 1 /6 50% 34% 29% 1 /3 25% 16% 14% 2 /2 16.7% 28% 33% 3 /1 8.3% 22% 24%

3 In this experiment we will attempt to test how general these results are by free radical chlorination of 2,2,4-trimethylpentane. The products obtained will be analyzed and quantitated by gas chromatography. Chlorination of 2, 2, 4-Trimethylpentane (Microscale) SAFETY NTE: Sulfuryl chloride vapors are irritating to the eyes and nose, and the liquid causes burns if spilled on the skin. To reduce the hazard somewhat, the sulfuryl chloride should be dispensed in carbon tetrachloride solution. Carbon tetrachloride is classified as a potential carcinogen. Should any of this solution get on your skin, wipe it off immediately and wash thoroughly with water. Benzoyl peroxide is unstable to heat and friction; transfer by pouring from a paper carton onto a piece of weighing paper, and avoid contact with metal of any kind (do NT use a metal spatula). PRCEDURE Heat a 100-mL beaker of water to about 75 C on a hot plate. While the water is heating, obtain 1.0 ml of a previously prepared sulfuryl chloride/carbon tetrachloride solution (made by dissolving 32 g S 2 2 /100 ml solution) [NTE: this solution will be can be dispensed using a buret] in a clean and dry l0-ml round-bottom flask. To this solution, add 0.5 ml of 2,2,4-trimethylpentane (d = 0.69 g/ml) by pipet; add ca. 25 mg of benzoyl peroxide (estimate amount by volume, comparing with a weighed comparison sample your instructor should have available for you). Attach a water reflux condenser and place a drying tube containing KH pellets on top of the condenser to exclude moisture and absorb the H that is formed (use a glass wool plug at each end of the drying tube to keep the KH from falling out). amp the apparatus in the water bath so that about one half of the flask is submerged, and heat the bath at such a rate that the temperature remains between degrees for 30 minutes (be sure water is running thru the condenser). Remove the flask and condenser from the bath, cool, and add another 25 mg of benzoyl peroxide. Reheat for another 20 minutes. Allow the solution to cool, disassemble the apparatus, and transfer the solution with a disposable pipet to a 25-mL Erlenmeyer flask. Add 6 ml of water and agitate the flask vigorously. Add 2 ml of 10% sodium bicarbonate solution to the flask, and again agitate the flask vigorously (CAUTIN: C 2 evolution). In a microfunnel prepared from a Pasteur pipette with the tip shortened (see Zubrick), gently wedge a very small tuft of glass wool in the bottom and add approximately 0.2 g of anhydrous potassium or sodium carbonate (enough to make a 1- to 1.5-cm column in the pipet). Use a pipet to remove the lower C 4 layer from the flask and filter through the carbonate into a vial. Give this sample to your instructor for GC analysis. Your instructor will provide the order of elution of the compounds from GC column to you. Determine the areas of the chlorotrimethylpentane peaks. Assuming that the areas are proportional to the weights of the various compounds, calculate the total percent of chlorination and the percent distribution among the chlorinated products. If some peaks are not completely resolved, the amounts can be estimated with reasonable accuracy by using an overlapping triangulation method (to be discussed in class). Your report should include the theoretical percentages expected based on the number and reactivity of the various hydrogens present in 2,2,4-trimethylpentane and a discussion of how your actual observed results match to these theoretical numbers.

4 Pre-lab exercise (in lieu of Quiz 8) Fill in the following chart for 2,2,4-trimethylpentane (show calculations in or below the chart to support your answers): Compound Formed from Monochlorination (draw structure) Type of H abstracted/# of H s Statistical Prediction Predicted % based on reactivity Hint: The number of boxes on the chart is correct.

5 PSTLAB QUESTINS 1. Using the relative reactivity of 1 ; 2, and 3 hydrogens for free radical chlorination (1.0:3.5:5.0), predict the percent composition of the monochlorination product mixture from: a. propane b. butane c. 2-methylpropane 2. How does your observed isomer distribution compare with that calculated for monochlorination of 2,2,4-trimethylpentane? Comment on the results. A set of molecular models will be useful here. 3. Attached are the proton NMR spectra of the three monochloro dimethylpentanes obtained in the chlorination of 2,4-dimethylpentane. Identify the compounds by their spectra and explain your choices. 4. Attached are the proton NMR spectra of the monochloropentanes obtained in the experiment you just carried out. Identify the compounds by their spectra and explain your choices.