Preparation of Tert-Butyl Chloride

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Tert-butyl chloride (also called 2-chloro-2-methylpropane), b.p. 51 degrees C, C.A.S. number 507-20-0, is a useful reagent that is simple to prepare. Combining Tert-butyl alcohol and hydrochloric acid, the reaction proceeds via an Sn1 mechanism. The task then becomes one of separating out the desired product in a purified form, free of reactants and byproducts.

Basic Reactions

  • The tert-butyl alcohol first interacts with a hydrochloric acid proton (H+) to form a protonated alcohol and a chloride ion. The basic reactions for this are:

    HCl ' (H+) + (Cl-) and

    (CH3)3-C-OH + (H+) ' (CH3)3-C-(OH2+)

    The (OH2+) group slowly eliminates a neutral molecule of water, leaving the plus charge on the central carbon atom. This can be written:

    (CH3)3-C-(OH2+) ' (CH3)3-C+ + H2O

    At this point, the remaining negatively charged chlorine quickly attaches where the water left off, forming the tert-butyl chloride.

    (CH3)3-C+ +(Cl-) ' (CH3)3-C-Cl

Beginner's Preparation of Tert-butyl Chloride

  • Into a small flask in a hood, combine tert-butyl alcohol with enough hydrochloric acid to use up all the alcohol while leaving a slight excess of acid. The tert-butyl alcohol dissolves in the hydrochloric acid because it is water-soluble, but since tert-butyl chloride is not very soluble, it forms a second layer that floats on top of the water layer.

    Separate the two layers using a separatory funnel. The layer which contains the tert-butyl chloride is the upper layer, since the product is lighter than water. Wash this upper layer with just a little dilute sodium bicarbonate solution to remove residual acid from the butyl chloride. Once again separating the upper layer into another vessel, add just a little anhydrous sodium sulfate to it to get rid of remaining moisture. After this step, filter and keep the liquid, and then distill it using a simple distilling flask and condenser apparatus. The resultant distillate is pure tert-butyl chloride.

Discussion

  • Although manufacture of tert-butyl chloride follows the mechanism termed Sn1 (nucleophilic substitution accomplished in one step), in the case of n-butyl chloride, or straight-chained butyl chloride, its formation would be by an Sn2 reaction (two-step). In the case of the formation of secondary butyl chloride, however, both reaction mechanisms, Sn1 and Sn2, apply.

Material Safety Data Sheet

  • The primary hazard of tert-butyl chloride is its flammability. See the appropriate MSDS data sheet for further details.

References

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