Free Radical Chlorination

Reading: McM p. 155 "Let's look..." to end of sect 5.3. Sect 10.4

Background: Free radical halogenation is a substitution reaction in which halogen atoms (X) replace hydrogen atoms in alkanes: R-H + X₂ -->R-X + H-X. This free radical reaction requires light; its mechanism is detailed in McM p. 155-156.

In this laboratory, 1-chlorobutane and 2-chlorobutane will be monochlorinated to form dichlorinated butane products:
C₄H₉Cl + Cl₂ --> C₄H₈Cl₂ + HCl.
The Cl₂ is generated by the reaction of sodium hypochlorite and hydrochloric acid:
NaOCl + HCl --> Cl₂ + NaCl + H₂O
Thus the overall reaction is
C₄H₉Cl + NaOCl + HCl --> C₄H₈Cl₂ + NaCl + H₂O.

Each reactant, 1-clorobutane or 2-chlorobutane will produce four C₄H₈Cl₂ products. With 1-chlorobutane as reactant, the products are:

Cl₂CHCH₂CH₂CH₃ + ClCH₂CHClCH₂CH₃ + ClCH₂CH₂CHClCH₃ + ClCH₂CH₂CH₂CH₂Cl

With 2-chlorobutane as reactant, the products are:

ClCH₂CHClCH₂CH₃ + CH₃CCl₂CH₂CH₃ + CH₃CHClCHClCH₃ + CH₃CHClCH₂CH₂Cl

Under each compound above, record the name and the boiling point. These boiling points are found in the Handbook. They will will be needed to identify the gas chromatographic peaks in the analysis below.

Procedure:
Place 1 ml of 1-chlorobutane or 2-chlorobutane in a 5 mL conical vial. Add 1 ml of 5% (by weight) aqueous NaOCl (Clorox bleach) and note which layer, the upper or lower, the aqueous phase is in. In the hood, add 1/2 ml of 3 M aqueous HCl, immediately cap and shake the vial to allow the green Cl₂ dissolved gas to move into the upper layer containing the 1- or 2-chlorobutane. Place the vial behind the unfrosted light bulb; be careful not to look directly at this strong light source. Every 1-2 minutes, shake the vial again. After the green color has essentially disappeared from the vial (or 5 minutes, whichever is later) remove the vial from the light and at your bench, cool the vial to room temperature in an ice bath. Uncap the vial and slowly add, with swirling, 100 mg of Na₂CO₃. When the foaming ceases, use a pipette to withdraw a drop of the aqueous layer and test it to see that its pH is above 7. Which is the organic layer - the upper or lower? To find out, note that the densities of 1- or 2-chlorobutane are less than 1; also note the volumes of each layer and recall how much of organic and aqueous solution was measured initially. Withdraw and discard the entire aqueous layer. The remaining organic product is then transferred to a drying-filter pipette (note 1) clamped vertically over a preweighed 10 ml beaker or Erlenmeyer flask. Collect the liquid that passes through the pipette and reweigh the flask and report the amount of dichlorinated and unreacted compounds recovered. The sample is then analyzed by GC. The first large peak represents the unreacted C₄H₉Cl and the peaks for the C₄H₈Cl₂ products follow in increasing order of their boiling points. Identify and mark the product peaks by comparing your retention times with those of your predecessor's and successor's chormoatograms. They will have started with the other of the two possible reactants. To make this comparison, tabulate the retention times (in minutes or millimeters) of your products along with those of your predecessor and successor on the GC.

Use a full page to insert your chromatogram and include the usual parameters and measurements. Identify the peaks.

Data Analysis:
1. Set up a reaction table using the overall chemical equation, above, (see the file: "Common Practices.."). Calculate the theoretical yield in grams or milligrams of all the dichlorinated products taken together based on the limiting reagent.
2. Why doesn't the theoretical yield calculated agree with the mass of the organic mixture you recovered?
3. Two products of the chlorination of 1-chlorobutane are the same as two products of the chlorination of 2-chlorobutane. What are they? Are the retention times for your peaks the independent of which reactant was used?
4. From the chromatogram, calculate the product distribution taking the total of four product peaks as 100%.
5. On pp. 361 of McM is a discussion of the relative reactivity of primary vs. secondary vs. tertiary hydrogen atoms towards abstraction leading to substitution by halogen atom. In 1-chlorobutane, the presence of chlorine already on the molecule will influence this abstraction step on carbon atoms near it. But a calculation of the comparative amounts of chlorination on the number three and four carbons will be relatively free of this effect. From your chromatogram (or from a student who obtained it) calculate the per hydrogen rate ratio by first taking the percents of 1,3-dichlorobutane vs. 1,4-dichlorobutane, then dividing each percent by the number of hydrogen atoms initially available to react. This results in a relative rate ratio, per hydrogen atom, for free radical chlorination under the conditions of your reaction. Reduce this rate ratio in the same way as on pp. 361-362 (note 2).

Notes:
1. The drying-filter pipette is prepared by first inserting a very small ball of cotton or glass wool into the constriction of a Pasteur pipette, then filling the wider end with 2-3 cm of Na₂SO₄. This pipette serves two purposes. The Na₂SO₄ removes water from (dries) the organic liquid by reacting with water to form a hydrate:     H₂O + Na₂SO₄ --> Na₂SO₄.10H₂O.   The organic compounds are also filtered through the cotton into the receiving flask. If the liquid does not drain, it can be forced into the vial by applying gentle pressure to the pipet with a rubber bulb.
2. In McM p. 361 …"From these and similar reactions…" is given the line of reasoning that you will use. For example, suppose that from 1-chlorobutane, the 1,3-dichlorobutane area was 45% and the 1,4-dichlorobutane's area was 26%. The 45% was produced by abstracting either one of two secondary H atoms, so a per H relative rate at this carbon atom would be 22.5. The 26% is the result of abstraction of any of three primary H atoms, so the relative rate per H atom at the primary carbon is 8.7. Reducing the 22.5 to 8.7 relative rates to a simple ratio results in a 2.6 to 1 relative reactivity for a secondary to a primary hydrogen in your case. On p. 361 of McM, a 3.5 to 1 relative rate ratio is given, but Dr. McMurry's chlorination experiment probably was carried out under entirely different conditions.

References:
"Mayo et al." Mayo, D.W., Pike, R.M., Butcher, S.S. and Trumper, P.K. Microscale Techniques for the Organic Laboratory; Wiley: New York, 1991.
Moore, J.A, and Dalrymple, D.L., Experimental Methods in Organic Chemistry, 2nd Ed.; Saunders: Philadelphia, 1976 p. 130.
The "Handbook" refers to any of the recent editions of: Weast, R.D. Handbook of Chemistry and Physics; The Chemical Rubber Co.: Cleveland, 1960-present.
"McM": McMurry, J. Organic Chemistry, 5th ed., Brooks/Cole Publishing Company, Pacific Grove, CA. 1999.

Rev. January, 2001