53

Intriguing question. First, the best yield would be achieved by selectively producing one enantiomer instead of the other. In this case, White wants D-methamphetamine (powerful psychoactive drug), not L-methamphetamine (Vicks Vapor Inhaler). Reaction processes designed to do this are known as "asymmetric synthesis" reactions, because they favor production ...


47

That's a good, concise statement of Bent's rule. Of course we could have just as correctly said that p character tends to concentrate in orbitals directed at electronegative elements. We'll use this latter phrasing when we examine methyl fluoride below. But first, let's expand on the definition a bit so that it is clear to all. Bent's rule speaks to the ...


28

There are several ways that enantiomers can be separated, but none of them are particularly simple. The first way to separate them is chiral chromatography. In chiral chromatography, silica gel is bonded to chiral molecules to form what is called a chiral stationary phase. The enantiomers will then separate as they run down the column because one of the ...


27

$\alpha$-D-glucose and $\beta$-D-glucose are stereoisomers - they differ in the 3-dimensional configuration of atoms/groups at one or more positions. $\alpha$-D-glucose $\beta$-D-glucose Note that the structures are almost identical, except that in the $\alpha$ form, the $\ce{OH}$ group on the far right is down, and, in the $\beta$ form, the $\ce{OH}...


26

Asked and answered, but I think one thing that's missing is that allenes are not planar like alkenes or alkynes are. You can refer to this question for an explanation. The dihedral angle between the two halogens is 90 degrees (ideally). Here's an animation hopefully providing a better view of the 3D structure: This results in the two mirror images being ...


26

It's not easy to see from a diagram, because it distorts bonds and angles. I recommend building it with a balls-and-sticks model set. You can also use a molecular viewer to model it; there are a couple of open-source (or at least free) ones out there. I have calculated the molecule on the DF-BP86/def2-SVP level of theory. The point group of the molecule is ...


24

since for every orientation of the molecule, we can reverse the orientation such that the light appears to be falling on the molecule from a direction other than the one for our original molecule. This is false. Let's take 2-butanol. For this stereoisomer, light is turning clockwise when viewed from the right side (I'm not sure of this, but we can assume). ...


24

The D-L system corresponds to the configuration of the molecule: spatial arrangement of its atoms around the chirality center. While (+) and (-) notation corresponds to the optical activity of the substance, whether it rotates the plane of polarized light clockwise (+) or counterclockwise (-). D-L system tells us about the relative configuration of the ...


24

Generally, amine nitrogens will not behave like a normal asymmetric carbon. Simple amines are roughly $\mathrm{sp^3}$ hybridiized and the molecules you use as examples do have 4 (we include the lone pair of electrons as a substituent) different substituents around the central nitrogen atom. So in principle me might consider it asymmetric or chiral. But ...


23

Very interesting question! The key word you are looking for is planar chirality. In trans-cyclooctene, the polymethylene bridge can either go "in front of" or go "behind" the plane of the double bond, assuming you fix the double bond and the two hydrogens in place. As pointed out by @jerepierre, they are considered different molecules due to a high-energy ...


23

In compound A, the negative and double bonded oxygens bound to the phosphorus are equivalent: $\hspace{5.1cm}$ In compound B, they are not equivalent: $\hspace{7.5cm}$


22

Meth doesn't have to be optically pure to be "pure". A mixture of d,l-methamphetamine is still pure, but I get where you're going with this. He has a few options: Chiral resolution - he could make the racemic meth and they resolve it by selectively crystallizing out the desired enantiomer. Chiral acids like tartaric acid can be used to do this. He could ...


19

Achiral cyclic compounds like 1,2,3-trichlorocyclopropane may contain pseudoasymmetric centers. Pseudoasymmetric centers have distinguishable ligands (“a”, “b”, “c”, “d”), two of which are nonsuperposable mirror images of each other (enantiomorphic). The lower case stereodescriptors “r” and “s” are used to designate the absolute configuration of ...


19

The strict criterion for a compound to display chirality is that it must not be superimposable upon its mirror image. Let's ignore the chair conformation of the ring for a while, and assume it adopts a planar conformation. You could draw a side-on view of the ring like this: Its mirror image would look like this. This is an example of axial chirality (...


19

cis-1,2-Dimethylcyclohexane is achiral, not because there is a plane of symmetry, but because it consists of two enantiomeric conformations which interconvert rapidly via ring flipping at normal temperatures. This is exactly the same case as amine inversion. "Chiral nitrogens" such as that in $\ce{NHMeEt}$ do not lead to chirality or optical activity ...


18

Chirality is a property of objects in which they lack certain symmetry operations, specifically improper rotations, including the mirror plane and inversion operations. For example, 3-dimensional chiral objects lack mirror symmetry. According to Wikipedia: The feature that is most often the cause of chirality in molecules is the presence of an asymmetric ...


18

Yes, this compound is chiral. The polycyclic backbone is called adamantane. It has $T_\mathrm{d}$ symmetry, meaning that as far as chirality goes, it behaves like a perfect tetrahedron, somewhat like methane does. It also has the interesting property that if you extend the C-F, C-Cl, ... bonds inwards, they will all meet at the same point. Those red dotted ...


18

The hindered rotation is due to the hydrogens at the naphthyl moiety. The following shows the rotation calculated at the DF-B97D3/def2-SVP level of theory. To better visualise this, I have chosen a mode with large atoms (not actually using the van-der-Waals radii, because that looked very strange; click here for a ball and stick version): We can observe ...


18

Here is a 3-D conformer from PubChem As you can clearly see, a plane of symmetry can be sent along the black line perpendicular to the plane of the screen. Hence, the molecule is achiral. If you take a mirror image, you can ultimately super-impose it again on the parent form Here is an illustrative 3D image(courtesy of andselisk) which clearly shows the ...


17

The name (1​s,4​s)-1-ethyl-4-methylcyclohexane is correct; it is even the preferred IUPAC name (PIN). It refers to the cis isomer of 1-ethyl-4-methylcyclohexane. The preferred IUPAC name for the corresponding trans isomer is (1​r,4​r)-1-ethyl-4-methylcyclohexane. In general nomenclature, according to Subsection P-93.5.1.2 of the current version of ...


17

There is a reason for everything. Does Bent's rule have any utility? YES! It wouldn't be there if there was not. But I will get back to this at the end of this post. Let me go through the points raised by your teacher first: Coulombic considerations can be used to rationalize bond angles, strengths, and lengths without the use of Bent's rule. ...


17

Holding your hands in this way merely proves that your hands are mirror images. If you take any object (chiral or not) and hold it up to a mirror, you can always align common features. Imagine instead placing one of your hands inside the other. You may be able to align the overall thumbs and fingers, but they will be facing opposite directions, thus are not ...


16

I basically agree with Ron's answer, but had to draw all of the possible structures to confirm it. The complication is that carbon-2 is non-stereogenic but it may be chirotopic depending on the configuration of the neighboring atoms. The IUPAC Gold Book calls this a pseudo-asymmetric carbon atom. Carbon-2 is not a stereocenter, because it does not have four ...


16

But if i rotate my left hand by 180 degrees ie now palm of my left hand faces away from me then both the left and right hand are superimposable . Turning your hands this way only makes them superimposable if you make the assumption that they are two-dimensional objects, where the normal vectors coming out of both sides of a given hand are indistinguishable. ...


16

Good question. There's a phenomenon named cryptochirality[1] (meaning “hidden chirality”), when a compound, though chiral, has practically unmeasurable optical rotation activity. It can happen to molecules with chiral center(s) bearing very similar substituents. (So, no tricks with bonded slightly modified enantiomeric pairs are needed.) An example is 5-...


15

Although I can't think of any drug examples other than thalidomide, here's information on thalidomide's mechanism: The chiral carbon of thalidomide can tautomerize in basic conditions into an enol, which is achiral. A reversal back to the ketone results in a mix of (R) and (S) enantiomers. In the body, this tautomerization is generally catalyzed by basic ...


15

Have you read the Wikipedia article to Bent's rule (especially the Justification paragraph). I think it explains the things rather well. In the example of $\ce{H3CF}$ the $\ce{H}$ is more electropositive than $\ce{C}$ and the $\ce{F}$ is more electronegative than $\ce{C}$. So, using the assumption that like in $\ce{CH4}$ the $\ce{C}$ atom is $\mathrm{sp}^3$ ...


15

It is all about minimizing the energy of a molecule. In the case of carbon, the only molecule that adopts a perfect hexagonal geometry in its ground state is benzene (and its derivatives that possess a 6-fold rotational axis). In this case the hexagonal geometry is adopted because all of the carbons are $\ce{sp^2}$ hybridized. The ideal geometry (lowest ...


15

For a molecule to be chiral it must have non-superimposable mirror images. Here is a drawing of the two mirror images for 2-bromobutane. The chiral carbon atom is denoted by an asterisk. In the case of 2-bromobutane there are 4 different substituents attached to it. The molecule is chiral, you can't pick up one of the mirror images and superimpose it on ...


14

The physicochemical properties of enantiomers being the same (except for optical activity), one needs to make use of the chirality of the molecules to separate them. This is done by asymmetrizing the environment of the molecules. I'll explain. Lets say you have a racemic mixture of a chiral carboxylic acid. To effectively differentiate both enantiomers, you ...


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