Chem 2204 Sec 006 Dr. Alfonzo Jordan
Title: Experiment 5B: Reduction of Ketone Using a Metal Hydride
Objective: During this week’s procedure, we will be using a metal hydride reagent, sodium borohydride, in a methanol solvent to carry out the reduction of the ketone, 4-tertbutylcyclohexanone. This will occur through the transfer of a hydride ion from the boron on sodium borohydride to the carbon atom of the carbonyl group of 4-tertbutylcyclohexane. The other hydrogen will be obtained from methanone solution. While the diasterioisomers (cis- and trans-4-tertbutylcyclohexanol) will both be present, trans will be absorbed as the major product.
The procedure was followed as was described in Mayo, pages 158-163, with the modifications listed in the posted Blackboard document. Additional modifications include:
Data & Results
Amount mmol Mp (°C)
TY: 110 mg
0.106 g 4-tert-butylcyclohexanone
Methanol was a clear liquid=causes homogenous mixture
Addition of sodium borohydride is clear
Causes bubbling with addition
Solution is clear
Screw cap has some crystal residue after stirring
1st TLC analysis was not successful due to spot mixing
2nd TLC plate was done
Addition of HCl to crude solution causes bubbling and cloudy, bubbly mixture
During filtering a little product came out
Heat leads to evaporation
Very little product left
MP= 60-62.5 °C
1. Theoretical yields of products
4-tert-butylcyclohexanone: 0.70 mmol * 1 mol/1000 mmol * 156.27 g/1 mol = 0.109 g
~ 0.110 g is theoretical yield of 4-tert-butylcyclohexanol
Trans: 80%: x/110=80/100: x = 88 g trans-4-tert-butylcyclohexanol
Cis: 20%: x/110=20/100: x = 22 g cis-4-tert-butylcyclohexanol
2. TLC Data
a. Ketone Standard
Spot measured: 37 mm
b. All Spots Observed
Co-spot spot measured: 1) blue: 31 mm, 2) red: 38 mm
Rf: 1) blue: 0.39, 2) red: 0.47
Crude Reaction Mixture spot measured: 30 mm
3. Tabular Format
Crude Reaction Mixture
2) 2) Blue
4. Yield of Crude Product
8.483 g – 8.438 = 0.045 g = 45 mg
5. Percent Yield of Crude Product
(45 mg/110 mg) * 100 = 41%
Questions: Answer the following questions. Be as complete as possible, and always explain your reasoning.
1. Your TLC data should have provided experimental evidence that the reduction of 4-tert-butylcyclohexanone with sodium borohydride is a stereoselective reaction.
a) Define the term stereoselective reaction.
A stereoselective reaction is one in which only one specific stereoisomer, out of a mixture of stereoisomers, reacts. It is also defined as a reaction which can produce multiple stereoisomers, but only produces one.
b) Explain the TLC data in this context.
The TLC data depicts that out of the two possible stereoisomers which could have been produced from this reaction, only one was preferred and, therefore, chosen – that being the trans-4-tert-butylcyclohexanol. The intensity of the blue spot after the TLC analysis demonstrates that the trans product was the chosen one.
2. Refer to the FT-IR spectrum, obtained in lab, of your crude reaction product. Include a printout of your FT-IR spectrum with your lab report.
a) In general, on what regions of the IR spectrum would you focus to assess whether or not the desired product 4-tert-butylcyclohexanol was produced from 4-tert-butylcyclohexanone?
List a range for each region, in cm1, and relate each given region to key functional groups in the starting material, product, or both. For each of your answers, also