1
$\begingroup$

I want to do some experiments on enantiomers. As it involves some calculations of potential energy surfaces, I would like to do this experiment on a simple enantiomer (For example $\ce{CHBrClF}$). But this is in liquid form I think. I need an enantiomer sample in the powder form. Can anyone suggest a simple sample? Either one I can buy or can easily make in a chemical lab with the help of a chemist.

$\endgroup$
  • $\begingroup$ Chirality was discovered by observing that tartaric acid crystallised from wine had crystals which are also chiral. It is widely available: how about that? $\endgroup$ – matt_black Apr 4 '17 at 14:47
3
$\begingroup$

Enantiomorphs are non-superposable mirror images (all signs of one coordinate axis reversed). What is the maximum enantiomeric divergence possible? If you have a quantitatively negligible chiral divergence, there is nearly no quantitative difference to be realized in calculation or measurement. BTW, the maximum energy difference between enantiomers cannot be more than a part-per-trillion difference/average. If it were larger, there would be easily detectable enthalpy of fusion anomalies for calorimetry secondary standard benzil.

Chirality is geometry. Geometric chirality is easily observed (no ${S_n}$ symmetries[A]) and easily calculated[B]. alpha-Quartz and gamma-glycine single crystals have $\text{CHI} = 1 \text{, COR} = 1 \text{, DSI} = 0$, maximum mathematical chirality[C], but it cannot be measured. Solution optical rotations ignore atomic mass distribution[D]. Silver thiogallate, $\ce{AgGaS2}$ in achiral space group ${I}$-${42d}$, rotates 522°/mm along [100] at 497.4 nm[E]. ${P3_121}$ quartz and ${P3_221}$ berlinite are both levorotatory[F]. Achiral $\ce{PhCOCH3}$ impurity changes resolved $\ce{PhCH(OH)CH3}$ optical rotation[G]. CIP notation flips with composition at constant geometry (look at natural protein cysteine versus alanine, both L-amino acids but opposite CIP notations).

How do you obtain $100\%$ optically pure maximally divergent enantiomers to your specs? No problem! Benzil, $\ce{(C6H5CO)2}$, is achiral when molten, gas phase, or dissolved. Solid state crystal lattice forces twist and stack benzil molecules into homochiral helices, space groups ${P3_121}$ (right-handed)or ${P3_221}$ (left-handed)[H].

benzil crystal structure

You obtain (racemic) benzil, modestly resolved (1 $\mathrm{S}$)-(-)-alpha pinene and (1 $\mathrm{R}$)-(+)-alpha pinene (both enantiomers), and mixed xylenes. $20$ ml scintillation vial, foil-lined cap. $100$ mg of benzil or so, $10$ ml resolved, alpha-pinene, heat to $100$ C. Drip in mixed xylenes until the molten benzil is just soluble, add another drop or two, very slowly cool undisturbed. Only one space group crystallizes out.

Enantiomorphic space groups[I] are mathematically perfectly divergent geometric chiralities. Here is the calculation of benzil crystal lattice's atomic mass distribution from Petitjean's software, CHI = $1$ (rapidly asymptotic) COR = $1$ DSI = $0$

enter image description here

If you want a discrete molecule as opposed to a crystal lattice, resolved camphor comes in around $\text{CHI} = 0.5$. Resolved ${D_3}$-trishomocubane is a much better geometric example and is easily synthesized in large quantities starting with benzoquionone and cyclopentadiene (Diels-Alder). If you have a methyl group sticking up from its ${D_3}$-axis, calculated $\text{CHI} = 0.996$.

Look at that methyl-bearing carbon! It has no ${S_n}$ symmetries, for R must reflect or invert as S. This chiral center's sense of chirality cannot be named. Try doing it with CIP notation.

enter image description here

Uncle Al has been very naughty with geometric chirality, "[6.6]Chiralane: a remarkably symmetric chiral molecule," Symmetry: Culture and Science 19(4) 307-316 (2008). Note that the core carbon atom has four rigorously identical substituents yet it is a maximally geometrically chiral center. Chemical chiral nomenclature is fundamentally defective, for said chiral center (and the four to which it is attached) cannot be assigned a direction of handedness by any means, even in theory.

[A] http://osf1.gmu.edu/~bbishop1/CHEM%20814-579%20Stereochemistry%20Lecture%20slides.pdf
pp. 11-13.
[B] J. Math. Phys. 40, 4587 (1999)
http://petitjeanmichel.free.fr/itoweb.petitjean.freeware.html#QCM
[C] http://www.mazepath.com/uncleal/qzdense2.png for alpha-quartz
http://www.mazepath.com/uncleal/glydense2.png for gamma-glycine
[D] http://www.mazepath.com/uncleal/norbors.gif
http://www.mazepath.com/uncleal/norone.png
[E] Appl. Cryst. 33, 126 (2000)
[F] J. Appl. Crystallogr. 19, 108 (1986)
[G] J. Org Chem. 38(10), 1870 (1973)
[H] Acta Cryst. B43 398 (1987)
[I] http://elib.mi.sanu.ac.rs/files/journals/publ/69/7.pdf Section 2: eleven pairs of enantiomorphic space groups;
http://www.math.ru.nl/~souvi/papers/acta03.pdf Section 3ff.

$\endgroup$
2
$\begingroup$

If you value ease of acquisition more than structural simplicity, then chiral protein aminoacids are very cheap, easily obtained crystalline white powders, L-alanine being the simplest among them. Its structure is quite small, but its zwitterionic character could possibly introduce additional complexity depending on how you intend to use it. L-Malic acid is a slightly larger option that's also cheap and has no zwitterionic character. There are probably other examples of relatively small, chiral, biologically important compounds which are isolated as solids in large scale and have good enantiomeric purity.

For maximum structural simplicity (such as a molecule with a single carbon atom that is also chiral), you will need some rather polar substituents in the structure to make sure that such a small molecule is a solid at room temperature, or some very heavy atoms such as iodine and bromine.

Edit: After a bit more searching, lactic acid may be the best combination of simplicity and low cost for an enantiomeric molecule which is a solid at ambient conditions. An enantiomerically pure sample is a white crystalline powder that melts at around 50°C.

$\endgroup$
  • $\begingroup$ en.wikipedia.org/wiki/Mosher's_acid Note that only one atom of 16 non-hydrogens is chiral. For lactic acid, one heavy atom of 6 is chiral. For D_3-trishomocubane, 8/11 skeletal atoms are chiral for having no S_n axes. The molecule is absolutely rigid. Its 8 chiral centers are wonderfully equivalent by symmetry: apex (2) and tertiary (6). One can argue the methylenes are also chiral, since their other two substituents cannot be split by a mirror plane. $\endgroup$ – Uncle Al Apr 7 '14 at 16:26
0
$\begingroup$

Sucrose, common table sugar, is a pure enantiomer that exists as a powder. Many drugs are manufactured as a single enantiomer, Lipitor would be an example. Any chemical supply company will sell numerous pure enantiomers; d- and l-carvone would be a common example.

$\endgroup$
  • $\begingroup$ Sucrose and Lipitor are very complex molecules!!! I am looking for a simple molecules like CHClBrF. $\endgroup$ – albedo Apr 6 '14 at 13:15
  • $\begingroup$ Is an amino acid like d-alanine too complex for your purpose? $\endgroup$ – ron Apr 6 '14 at 13:40

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.