Raman Spectra of Alcoholic Molecules

 

Abstract: in this post we describe the application of the Raman spectrometer to the analysis of alcoholic molecules: methyl alcohol, ethyl alcohol and isopropyl alcohol.

Introduction

In previous articles we have shown the DIY Raman spectrometer for analyzing liquid samples. We also illustrated the application of Raman spectroscopy to the study of the water molecule and of organic and inorganic molecules. In this post we show the results obtained in the study of alcoholic molecules, starting with the simplest, methyl alcohol, continuing with ethyl alcohol and propyl alcohol. In Fig. 1 we compare the Raman spectra of these three chemicals.

Fig 1 – Comparison between the Raman spectra of Methyl, Ethyl and Isopropyl alcohol

Methyl alcohol

Methanol, also known as methyl alcohol is the simplest of the alcohols. Its chemical formula is CH3OH (or, for short, MeOH or MetOH). The carbon atom is at the center of a roughly regular tetrahedron whose vertices are occupied by the oxygen atom and by the three hydrogen atoms directly linked to it. At room temperature, it appears as a colorless liquid with a characteristic odor. It is very volatile and extremely flammable. It is completely soluble in many organic solvents, such as chloroform, and in water. In Fig. 2 we report the Raman spectrum of methanol. The interpretation of the spectrum is quite simple: around 2800-3000 cm-1 there are the maximums linked to the symmetrical and asymmetrical stretching modes of the methyl group CH3, at 1500 cm-1 there is the bending mode always of the methyl group , while at lower frequencies, around 1000 cm-1 there is the C-O bond stretching mode. The stretching mode of the O-H bond is also present at about 3400 cm-1.

Fig 2 – Raman spectrum of Methyl alcohol

Ethyl alcohol

Ethanol (or ethyl alcohol) is a linear alkyl chain alcohol whose condensed formula is CH3CH2OH. It is also called, by definition, simply alcohol, being the basis of all alcoholic beverages. In chemistry, it can also be found abbreviated as EtOH. At room temperature it appears as a colorless liquid, with a characteristic (ethereal) and pungent odor, with a slightly sweet, burning taste (pseudo-caloric effect) and with a dehydrating tactile effect. It tends to be volatile and extremely flammable. The flame it produces during combustion is pale blue. In Fig. 3 we report the Raman spectrum of ethanol. We distinguish around 2800-3000 cm-1 the maximums linked to the stretching modes of the CH3 and CH2 groups, at 1500 cm-1 there is the bending mode, while at lower frequencies there are the stretching modes of the C-O bond and of the C-C bond.

Fig 3 – Raman spectrum of Ethyl alcohol

Isopropyl alcohol

Isopropyl alcohol, or 2-propanol, is an alcohol with the formula CH3CH(OH)CH3, colorless and moderately volatile, with a strong characteristic odor that is not unpleasant if pure. Isopropyl alcohol is miscible in water, ethanol, ether and chloroform. In Fig. 4 we report the Raman spectrum of the isopropyl alcohol. Also for this alcohol we distinguish all the maxima linked to the CH3 and CH2groups and to the C-O and C-C bonds.

Fig 4 – Raman spectrum of Isopropyl alcohol

Conclusions

The comparative analysis of the Raman spectra of the three alcoholic molecules made it possible to test the quality of the instrument and the applicability of the spectroscopic technique for determining the type of alcohol present in the sample. The comparison with calibrated aqueous solutions of known molarity would also easily allow the quantification of the alcohol content present.

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