Question

Differentiating between reactants and products using gas chromatography, infrared spectroscopy, and mass spectrometry:

I am having trouble figuring out how you would tell the difference between the reactant and products using each of the techniques below. Please explain and answer completely.

How would you differentiate between 2-methyl-cyclohexan-1-ol and the three alkene products by each of the following types of data: (a) Gas chromatography (GC) (b) Infrared spectroscopy (IR) (c) Mass spectrometry (MS) IN HO-P-OH OH phosphoric acid .o. Heat 1-methylcyclohex-1-ane 3-methylcyclohex-1-ene cis/trans 2-methyl-cyclohexan-1-ol methylenecyclohexane Molecular formula C-H140 Molecular formula for each isomer is C H12

Answer 1

(a) GC analysis: Boiling point of given compounds is in the order of :Cis and trans 2-methyl cyclo 1-hexanol > 1-methyl cyclohex-1-ene > 3-methyl cyclohex-1-ene > methyelene cyclohexane. Clearly, the polarity of reactant is higher than that of its products. Hence, it will interact more with the stationary phase and its retention time will be more than that of products. Products come out of the column first followed by the reactant. Maximum retention time obtained is for the reactant.

(b) IR analysis is based on functional group present. The reactant molecule shows a broad O-H stretching peak (due to intermolecular interactions) at around 3200-3300 cm-1. However, the products show no such characteristic peak above 3000 cm-1. It shows a peak at around 1600 cm-1 for C=C stretching.

(c) Mass spectrometry is based on destruction of the compound to give stable products. Reactant has a cyclic ring with no double bond and it gets easily destorted being unstable and the most intense peak below 60 m/z would be obtained corresponding to straight alkene which is more stable than cyclic ring. However, in case of products, double bond gives the ring stability and the most intense peak obtained is at around 80 m/z value which is a cyclic alkene after removal of -CH3 groups.