Chemistry 29a M. Walker

This is actually the first lab where you actually make something, in this case 4 dichlorobutane isomers, starting from 1-chlorobutane* . This means you must do a "mole table" in your prelab (see p20 of the book for an example, though don’t go overboard with solubilities on everything). Also, you must calculate your yield using moles, don’t just divide one mass by the other! These two things apply to all "synthesis" labs.

This reaction is the same as the reaction we are covering in lecture in chapter 4 of Wade. We have four possible free radicals formed as intermediates, with the radical being at position 1, 2, 3 or 4 of the chain (leading to the 1,1-, the 1,2-, the 1,3- or the 1,4 isomer respectively of the dichlorobutane). The stability of each intermediate radical will determine how much of each isomer of dichlorobutane you will get. Things you will need to consider will be: is the radical more stable at a secondary carbon or a primary carbon, and also what would you think might be the effect of the chlorine? Discuss these in your report in the light of your results.

We will be examining a new technique this week, gas chromatography (GC). This is discussed in the book in chapter 11; you should read the section on GC before you write up your report. GC involves injecting a small amount of a liquid into a narrow tube containing a hot stream of inert gas. A higher boiling liquid will usually be carried the tube along more slowly than a lower boiling one. If a mixture of two such liquids is injected, the GC machine will separate them, and a detector at the end of the tube will detect the lower boiling material first followed by the higher boiling material. The area of each peak generated by the detector (calculated as on the attached sheet) is proportional to the amount of each component present, so you can work out the relative proportions of each component in your mixture. The time taken for each component to reach the detector should be the same for every injection (assuming that temperatures and flow rates remain the same), and this is called the retention time. You can calculate the retention time from measuring the distance the chart paper goes after you inject.

There will be no separate assigned problems this week, and the final 20 points will be assigned to part of these calculations. You should calculate the following:

1. The retention times for 1-chlorobutane and the four dichlorobutanes; identify which isomer is which by their boiling points. If a tiny peak is present before the biggest one, this is just a little air from the injection and it should be ignored.
2. The area of each peak, correlated with the identity of each peak from (a).
3. The % of each component. Ignore the 1-chlorobutane (or methylcyclopentane) in this, just look at the four products.
4. Calculate the relative reaction rate per hydrogen at each position. This is done by dividing the % from (c) by the no. of hydrogens at that position .

Then you should use your results as a basis for your discussion and conclusion.

• Take care with your gas trap- do not get the cotton too thick or soaking wet so it blocks the exit of gas, otherwise your apparatus will pressurize and cause a major release of gas, however the cotton must be a little moist to work.
• When you are separating two layers, always remove the lower layer first (even if you want to discard the upper layer), otherwise you will have a poor separation.
• Usually the dichlorobutane product forms the lower layer, but sometimes it forms the top layer. You should test your mixture with a little water to make sure of this. If the leayers don’t separate well, add a few drops of water, then if necessary a few drops of chlorobutane. [Note: if you perform the methylcyclopentane experiment, your product is likely to form the top layer throughout].
• Take care with the syringes- they are fragile and expensive. Make sure you hold the plunger while injecting, otherwise the pressure inside the GC may cause it to be blown out.

* Two students will be asked to make chloromethylcyclopentane from methylcyclopentane.

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