Synthesis of a Bromohydrin
Complete the TPC below. Copy and paste it in your lab notebook.
Table of Physical Constants (TPC)1
physical constants were obtained from the CRC Handbook of Chemistry
Physics, 52nd ed., unless otherwise noted.
In the second step of the mechanism, water acts as a nucleophile and adds to the carbon which is best able to support positive charge in accordance with Markovnikov's rule, which can be predicted by looking at the Bromonium Ion below:
Of the two intermediates depicted above, the intermediate on the left is lower in energy relative to the intermediate on the right because the partial positive charge is more stable on a tertiary carbon. Therefore, the rate of nucleophilic attack of water at this carbon should be faster relative to addition of water to the secondary carbon of the intermediate on the right.
The expected product of Markovnikov addition is 2-Bromo-1-Hydroxy-1-Methylcyclohexane (also named 2-Bromo-1-Methylcyclohexanol), as seen in Figure 2.
To a small 13x100 mm test tube, add 350 mg of N-bromosuccinimide*, 1.0 mL distilled water, and 0.75 mL tetrahydrofuran. To this heterogeneous mixture, transfer 0.25 mL 1-methylcyclohexene via a disposable pipet. Note which layer is the organic material and which one is aqueous-based.
Facilitate gentle swirling of the reaction mixture by means of a vortex mixer; be sure the setting on the vortex mixer is low or set to "shake." Allow the reaction mixture to stir at room temperature until no solid NBS is observed in the colorless solution (about ten minutes). Record all observations in your notebook. If the mixture is still yellow after 10 minutes, add another 1-2 drops of 1-methylcyclohexene.
Dilute the resulting mixture with 2 mL water and stir for an additional 2 minutes. Allow the organic and aqueous layers to separate.
Add 1 mL CH2Cl2 to the test tube and stir, then allow the layers to separate. Transfer the organic layer containing the bromohydrin to a clean test tube via a disposable glass pipet. Extract the aqueous layer with an additional 1 mL of CH2Cl2 and combine organic layers.
Filter the organic solution through phase separation paper and anhydrous magnesium sulfate into a clean vial. Transfer this solution to a numbered GC-MS vial and dilute to 1.5 mL with CH2Cl2, if necessary. Give the vial to the Lab Assistant for analysis.
*N-bromosuccinimide (NBS) is corrosive; avoid contact with skin and wear gloves when handling the solid or solutions of NBS
2Modified slightly from the previously published report: Porter, D.J.; Stewart, Andrea T.; Wigal, Carl T. J. Chem. Ed. 1995, 72, 1039-40.
OUT YOUR PERSONAL TOTE!