Chemistry 29a M. Walker

Nucleophilic substitution is a common class of reaction whereby one group is replaced by another, with the latter being a nucleophile, e.g.:

R-X + Y^- --> R-Y + X^-

In today's experiment (where R = n-butyl), we want to convert -OH to -Br. However, OH- is a strong base (i.e. relatively unstable) and therefore a poor leaving group. We make the OH leave by first protonating the butanol to form R-OH₂⁺, which now has water as a (good) leaving group when Br- attacks. The reaction occurs at a primary carbon, and in this case the preferred mechanism for nucleophilic substitution is S_N2. In an S_N2 process, the reaction is concerted, i.e. it occurs without any intermediates, in one step.

Infrared (IR) spectroscopy is an important analytical method, used both to identify and to classify substances, as well as for purity analyses. Read the chapter on IR before writing up your lab, and read the handout on IR before coming to lab.

• Set up the sand bath and start heat when you come into lab. Set up a simple distillation apparatus, using a long neck flask.
• Add the reactants to the flask along with a boiling chip; take special care with sulfuric acid.
• Reflux the contents of the flask for at least 30 min. Ensure that the "condensate ring" does not reach the arm of the distillation head. Cool the neck of the flask with damp paper towel if necessary.
• After reflux, distil off the product, then wash and dry as described in the book.
• WE WILL NOT BE PERFORMING THE FRACTIONAL DISTILLATION
• When you have your dried "crude" product, weigh this, determine % yield and take an IR spectrum of your product.

In your report, analyze the major peaks in the IR spectrum. Show how you know that you have isolated the 1-bromobutane, and the 1-butanol and water are absent.

Assigned problems from Chap 16: 4, 6, 7, 8.

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