Question

Suppose that, on a GC column, methyl hexanoate eluted in 1.87 minutes, methyl heptanoate eluted in 2.35 minutes, and methyl octanoate eluted in 3.0 minutes. What is the structure of an unknown methyl ester, known to belong to the same homologous series, that elutes at 4.7 minutes?

Answer 1

Notice that the elution time for each methyl ester increases by a factor of about 1.25...that is to say that the elution

time of methyl heptanoate (2.35min) can be predicted by looking at the elution time of the methyl ester that

preceeds it in the series-methyl hexanoate (1.87 min) and multiplying by 1.25 (1.87* 1.25 =2.33 or approx 2.35).

Similarly, the elution time for methyl octanoate (3.0 min) can be predicted my multiplying the elution time of methyl

heptanoate by 1.25...(2.35*1.25=2.9 or approx 3.0). Following this pattern, the predicted elution time of methyl

nonanoate is 3.75min (3.0* 1.25= 3.75), and the elution time of methyl decanoate is 4.7 min...(3.75* 1.25=4.68).

Therefore, the next ester in the homologous series is methyl decanoate