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Some references say that alkanes elute faster than alkenes, whereas in my textbook the higher volatility of alkenes means they elute faster than alkanes. This is for gas chromatography.

Alkenes are more polar than alkanes due to more electron-withdrawing $\text{sp}^2$-hybrid orbitals (more s-character draws electrons closer to nucleus).

Can someone please clarify - bit confused.

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Chromatography in general is a separation method in which the partition equilibrium of a compound between two phases is permanently disturbed since one of the phases is changing/exchanged (mobile phase).

Apparently, the mobile phase in gas chromatography is the carrier gas. The stationary phase isn't just a hollow tube but a capillary covered with various differently functionalized materials, that may be rather unpolar (squalane), more polar (polyethylene glycol) or polar and shape-selective (cyclodextrines). Other very common materials are alkyl-substituted polysiloxanes, such as polydimethylsiloxane.

Using the latter stationary phase, Alan Katritzky and coworkers published QSPR: the correlation and quantitative prediction of chemical and physical properties from structure, Chem. Soc. Rev., 1995, 24, 279-287 (DOI, PDF). Here, the retention times for dodecane and 1-dodecene were given as 4.59 and 4.45 min, respectively.

Ladislav Sojak published a freely available 35-page review Separation and identification of isomeric hydrocarbons by capillary gas chromatography and hyphenated spectrometric techniques in Petroleum & Coal, 2004, 4(3), 1-35 (PDF). You might want to have a look at this article on the effect of having non-terminal $\ce{C=C}$ double bonds and cis-trans isomers of longer alkenes, which therefore have significantly different shapes.

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  • $\begingroup$ ah interesting... so it really depends on the molecule itself then... no general rule of one before the other. $\endgroup$ – confused Apr 10 '14 at 8:19
  • $\begingroup$ @confused It's apparently a complex matter of interaction of the compounds with the stationary phase. Have a look at Fig. 9 in the article by Sojak. There, the order of retention times for trans-6-pentadecene and cis-5-pentadecene is reversed when a column with a thicker coating of the same stationary phase is used. $\endgroup$ – Klaus-Dieter Warzecha Apr 10 '14 at 9:00

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