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I've learned about anti and gauche conformations throughout my scholastic journey, and I have heard often that straight-chain alkanes always adopt an all-anti conformation in their lowest energy state. I also know that, in a pure sample of any given straight-chain alkane, (eg. hexane) the anti and gauche conformations are in a certain equilibrium with each other, And that this ratio of gauche to anti shifts towards gauche as the available energy of the sample (i.e. temperature/heat/ pressure) approaches infinity. My question is as follows: Has anyone in the scientific community to date successfully forced an entire sample of straight-chain alkanes into the all-gauche conformation without significantly raising the conditions above STP? I have scoured the internet but have not surfaced with a definitive answer, so I now turn to the scientific community.

All contributions are greatly appreciated.

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    $\begingroup$ No, the ratio will approach a finite value, likely sth in the range of 2:1. If two states have only a marginal energy difference compared to $kT$, they will be equally occupied. That´s one of the most basic principles in QM and statistical thermodynamics. $\endgroup$
    – Karl
    Apr 3, 2020 at 9:01
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    $\begingroup$ The best example would be solid isotactic polypropylene, where one in six (or five?) carbon carbon bonds is in gauche. In a linear alkane it is sth like one in twelve or fifteen at RT. $\endgroup$
    – Karl
    Apr 3, 2020 at 9:05
  • $\begingroup$ @Karl but isotactic polypropylene is branched, not linear. Sill a cool tip, tho. Thanks! $\endgroup$
    – mpprogram6771
    Apr 3, 2020 at 14:20

1 Answer 1

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At Last! Success!

I found this research paper, which describes a "Self-assembling nanocapsule", which is held together by plenty of hydrogen bonding, and can assemble around a linear molecule of all-anti tetradecane, $\ce{(C14H20)}$ using the energy afforded by the hydrogen bonding to squeeze/twist the alkane into a helical all-gauche conformation. Have a look:

enter image description here

Cool, right?

Sources:

Scarso A, Trembleau L, Rebek J Jr. Helical folding of alkanes in a self-assembled, cylindrical capsule. J Am Chem Soc. 2004 Oct 20;126(41):13512-8. PubMed PMID: 15479107.

Note:

Other answers are still greatly appreciated!

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    $\begingroup$ Gosh, supramolecular chemistry never ceases to amaze! $\endgroup$
    – Nicolau Saker Neto
    Apr 4, 2020 at 0:49
  • $\begingroup$ @NicolauSakerNeto I know, right? It's so fascinating! $\endgroup$
    – mpprogram6771
    Apr 4, 2020 at 3:53

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