Question

Organic Chemistry: Intro to Gas Chromatography 6 known compounds are: isopropyl acetate, ethyll acetate, methanol, ethanol, 2-butanone, 2-pentanone 1. Discuss the retention times of the six known compounds with regards to their molecular weights. Describe any pattern that emerges. 2. Discuss the retention times of the six known compounds with regard to their boiling points. Describe any pattern that emerges. 3. Identify the compounds that are present in your unknown mixture. Support your identification. For the unknown, I got three different retention times: .885, 1.355, 1.877 isopropyl acetate: 2.530 ethyl acetate: 1.810 methanol:.885 ethanol:1.040 2-butanone:1.760 2-pentanone: 2.745

Answer 1

1. In gas chromatography, lower molecular weight compounds have lower retention time ( lower molecular weight compounds are eluted first) and higher molecular weight compounds have higher retention time.

Molecular weight of isopropyl acetate is:102 g/mol

Molecular weight of ethyl acetate is:88 g/ mol

Molecular weight of methanol is: 32 g/mol

Molecular weight of ethanol: 46 g/ mol

Molecular weight of 2- butanone: 72 g/ mol

Molecular weight of 2- pentanone: 86 g/ mol

So, methanol should have the lowest retention time and isopropyl acetate should have the highest retention time. The increasing order of retention time is: methanol< ethanol< 2- butanone< 2-pentanone< ethyl acetate< isopropyl acetate

The theoretical predictions based on molecular weight match with the practical values of retention time of methanol, ethanol, 2- butanone. But the practical values donot merge with theoretical predictions for the rest three.

2. In gas chromatography, lower boiling liquids have lower retention time and higher boiling liquids have higher retention time.

Boiling points of methanol. Ethanol, 2- butanone, 2- pentanone, ethyl acetate and isopropyl acetate are : 64.7 oC, 78.4 oC, 79.64 oC, 101 oC, 77.1 oC, 89 oC

So, according to boiling points, the increasing order of retention times are: methanol<ethanol<ethyl acetate<2-butanone<iso propyl acetate<2- pentanone

This theoretical predictions match with practical values.

3. Unknown mixture contains methanol ( retention time 0.885) , ethyl acetate( retention time 1.877). The third compound with retention time 1.355 does not match with any known compound from the given six compounds.