CONVERSION OF AN ALCOHOL TO AN ALKYL BROMIDE WITH REARRANGEMENT Experiment 11: CONVERSION OF AN ALCOHOL TO AN ALKYL BROMIDE WITH REARRANGEMENT

Objectives To synthesize isomeric alkyl halides from an alcohol using an SN1 dehydration reaction. To determine product substitution based on reaction rate with AgNO3. To characterize reactants and products using IR spectroscopy.

SN1 MECHANISM 2. Forms new O-H bond, oxygen bears a positive charge (oxonium ion). 3. Water is eliminated-forms 2o carbocation. 1. hydroxyl oxygen attacks and removes a proton from hydrobromic acid. 4. Products may form from the 2o carbocation, but it is more likely that the 2o C+ will rearrange to a 3o C+. 5. Reaction of the carbocation with the bromide ion yields the alkyl halide. 5. Reaction of the carbocation with the bromide ion yields the alkyl halide.

EXPERIMENTAL PROCEDURE (Synthesis) Add 2-methylcyclohexanol, 48% HBr and 3 boiling chips to flask. Clamp flask to ring stand. Attach clear hoses to condenser and place condenser on top of flask. Begin water flow through condenser and apply heat. Reflux the solution for 30 minutes. Cool flask in beaker of tap water.

EXPERIMENTAL PROCEDURE (Purification) Transfer solution to separatory funnel. Add concentrated sulfuric acid. Stir solution gently with glass rod. Do NOT agitate. Draw off bottom acidic layer into an Erlenmeyer flask. Transfer the top layer to a preweighed beaker using a glass Pasteur pipet. Reweigh beaker to obtain final mass. Perform silver nitrate test.

Table 11.1 Theoretical Yield (g) Must determine which reactant is your limiting reagent first, 48% HBr or 2-methylcyclohexanol. Calculate the amount of product that can be formed based on the limiting reagent. Actual Yield (g) This mass will be obtained by weighing an empty small beaker, then reweighing after the product has been added. The difference in mass is the actual product yield. % Yield Actual yield (g) X 100 Theoretical yield (g) Product Appearance Physical state and color of product.

EXPERIMENTAL PROCEDURE (Silver Nitrate Test) When an alkyl halide is treated with a solution of alcoholic silver nitrate, a precipitate of silver halide forms. The rate limiting step is the formation of the carbocation.

Reaction Rate (min/sec) 2-bromo-2-methylpropane Table 11.2 Test Solution Reaction Rate (min/sec) 1-bromobutane You only need 3 drops of each of these! Get ONLY what you need! 2-bromobutane 2-bromo-2-methylpropane Product sample

IR SPECTROSCOPY

Table 11.3 Functional Group Base Values (cm-1) 2-methylcyclohexanol Alkyl halide product Frequency (cm-1) OH stretch 3200-3600 C-O stretch 1000-1200 sp3 CH stretch 2850-3000 C-Br stretch 500-700

SAFETY CONCERNS Sulfuric acid is very corrosive and must be used with EXTREME caution! Silver nitrate will stain the skin! Use gloves at all times! Alkyl bromides are lachrymators! Keep covered at all times to avoid inhalation!

WASTE MANAGEMENT Place the acidic solution from extraction in bottle labeled “AQUEOUS ACIDIC WASTE”. Place alkyl bromide product in the bottle labeled “ALKYL BROMIDE WASTE”. Pour the contents of the test tubes used in the silver nitrate test in the container labeled “SILVER NITRATE WASTE”.

For Next Lab… Experiment 12 Pre-lab notebook entry will be due at the beginning of lab!