6. Based on your gas chromatography results, predict the approximate retention t

Question

6. Based on your gas chromatography results, predict the approximate retention times that probably would be obtained for the folowing compounds,and explain your predictions. (Hint: Compare the boiling points and intermolecular forces for each compound.)

A. 3-Methyl-2-Butanone (CAS Reg. No. 563-80-4)

B. 5-Methyl-2hexanone (CAS Reg. No. 110-12-3)

Results:

IUPAC NAME BOILING POINT(C) RETENTION TIME(MIN)

2-propanone 56.05C 0.525 min

2-Butanone 79.59C 0.825 min

2-pentanone 102.26C 1.505 min

4-methyl-2-pentanone 116.5C 2.290 min

2-Hexanone 127.6C 3.455 min

Explanation / Answer

As given in the data

the boiling point is increasing with increase in molecular weight of the given carbonyl compounds.

Also the boiling point of two isomers (with same molecular weight) is higher for the one with strainght chain structure.

Example : 4-methyl-2-pentanone and 2-Hexanone

Reason : Van der Waals dispersion forces are acting in carbonyl compounds which are proportional to surface area. Larger the molecule more the surface area more the boiling point

So retention time and boiling point for 5-Methyl-2hexanone will be greater than that of 2-hexanone.

And retentiona time and boiling point for 3-Methyl-2-Butanone will be in between butanone and pentanone

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