If you knew that you had either cyclohexane or cyclohexene in the lab but weren't sure which, how could you determine which it was?

Thanks from Sheryl

Since I'm not an organic chemist I would think of an organic test LAST. Density, refractive index, and boiling points of both are too close to differentiate. However, the melting point of cyclohexane is +6 something and cyclohexene is -103 something. +6 should be easily attainable in the lab. That's what I would do. Check out those numbers in your tables. Everyone will tell you not to rely on physical properties but pure samples can tell you a lot. If not that I would try IR and/or NMR. The double bond should be visible in IR. And NMR should show a CH group which the cyclohexane doesn't have. The books tell me n-bromosuccinimide (NBS) will brominate cyclohexene MUCH faster than it will cyclohexane but nothing about how much faster or if the rate is sufficiently different to differentiate between the two.

To determine whether you have cyclohexane or cyclohexene in the lab, you can follow these steps:

1. Melting Point: The first step would be to measure the melting point of the compound. Cyclohexane has a melting point around +6°C, while cyclohexene has a significantly lower melting point of around -103°C. By comparing the measured melting point with the known values, you can get an initial indication.

2. Infrared (IR) Spectroscopy: IR spectroscopy can be used to identify the presence of functional groups. Cyclohexene contains a double bond, which should be visible in the IR spectrum. Look for characteristic peaks in the range of 1600-1660 cm-1 (C=C stretch).

3. Nuclear Magnetic Resonance (NMR) Spectroscopy: NMR spectroscopy can provide detailed information about the molecular structure. Cyclohexane does not have any double bonds, so its NMR spectrum would not display any peaks related to double bonds. On the other hand, cyclohexene should show a peak in the region of 5.5-6.5 ppm, corresponding to the CH group adjacent to the double bond.

4. Functional Group Tests: If necessary, you can perform chemical tests to further confirm the presence of a double bond. For instance, you can use a reagent like bromine water or n-bromosuccinimide (NBS) to test the reaction rate. Cyclohexene will react much faster with these reagents compared to cyclohexane.

By utilizing a combination of these methods, you can confidently determine whether you have cyclohexane or cyclohexene in your lab.