37
votes
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Why is 1-ethylidene-4-methylcyclohexane chiral?
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 ...
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35
votes
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What is the difference between D and L configuration, and + and −?
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 ...
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33
votes
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Is cis-1,2-dimethylcyclohexane a meso compound?
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 ...
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28
votes
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Can heteroatoms with lone pairs be chiral centres?
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 ...
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27
votes
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What are the CIP rules for cyclic substituents?
I presume you want to know the E/Z geometry of the double bond. You need to determine on each end of the double bond which ring has priority. Intuitively one may argue that cyclohexane is larger than ...
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26
votes
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Does trans-decalin really have a plane of symmetry?
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 ...
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26
votes
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How is exact racemization possible?
Racemization isn't "exact," but rather very very close to equality. It is just simple probability.
Think of flipping a coin, p=probability for heads, and q=probability of tails. Now for a fair flip ...
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24
votes
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Is phosphorus a stereogenic centre in this phosphate ester?
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:
$\hspace{7.5cm}$
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23
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Are monosubstituted cyclopropanes achiral or chiral?
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 ...
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23
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How can a molecule with a double bond on one side have a plane of symmetry, and thus, be achiral?
I think this is a case of picture being worth a thousand words. Arguably the easiest way to visualize the plane of symmetry for nonbornene (mirror plane m) is to redraw the molecule in 2D:
...
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21
votes
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Chirality of substituted adamantanes
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 ...
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21
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Atropisomerism of naphthyl alcohol
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 ...
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19
votes
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Are there chiral compounds that don't rotate plane-polarized light?
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 ...
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18
votes
Are our hands really chiral?
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 ...
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18
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Why do Enantiomers have different chemical properties with optically active reagents?
I think a simplified version of the enzyme example is helpful here. Consider this image of binding a molecule to a receptor:
Using a model of enantiomers where the attached groups are just ...
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17
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Are our hands really chiral?
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 ...
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14
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Is there a mathematical generalization for the number of isomers (structural and configurational) of a given compound?
Generally speaking, graph-theoretical enumeration aims at counting chemical compounds as graphs (2D structures).
In other words, it is concerned with constitutional
(or structural) isomers.
The ...
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14
votes
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Why is the inversion barrier larger in PH3 than it is in NH3?
Ammonia is the classic system for $\ce{sp^3}$ hybridisation save methane. The lone pair (and each of the $\unicode[Times]{x3C3}$-bonds) has almost $25~\%$ s-character which corresponds nicely to $\ce{...
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14
votes
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Number of stereoisomers of a polyene
A picture is worth a thousand words so I've redrawn your compound and have numbered the areas to be considered.
Double bond "1" is not capable of displaying cis-trans
stereoisomers since there are ...
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14
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Why is 1-ethylidene-4-methylcyclohexane chiral?
This question has been answered in elegant fashion by @orthocresol. I'd like to expand the discussion somewhat by detailing how digraphs and Cahn-Ingold-Prelog-(Helmchen) rules account for the ZR and ...
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14
votes
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R/S configuration of bridging carbon in bicyclic system
Several days ago I offered a solution to the R/S assignment of the bridgehead carbons of (1R,2S,4R)-2-bromo-2-methylbicyclo[2.2.2]octane 1. During the middle of the night I realized that the method ...
- 16.4k
14
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What are the criteria for a molecule to be chiral?
Mirror images
The very first definition of a chiral molecule is one where it is not superimposable on its mirror image. Therefore, one of the most straightforward ways to determine chirality is to ...
14
votes
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Why is diequatorial trans-1,2-dimethylcyclohexane more stable than diaxial trans-1,2-dimethylcyclohexane?
The key concept is 1,3-diaxial interaction. See the figure 1 below using methylcyclohexane as an example.
Figure 1: Equatorial and axial conformations of methylcyclohexane with 1,3-diaxial strain ...
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14
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Why are the products of Grignard reaction on an alpha-chiral ketone diastereomers rather than a racemate?
First of all, welcome to ChemSE on your first post. Allow me to expand on the Comment of Karsten Theis and provide you with an illustration. If a compound is racemic and bears a single stereocenter, ...
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13
votes
Why is the inversion barrier larger in PH3 than it is in NH3?
Disclaimer: This answer neglects quantum tunnelling effects, which are significant in such compounds. The gist of it is that because nitrogen is smaller and lighter than phosphorus, the rate of ...
- 71.1k
13
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Difference between Epimers and Diastereomers
This scheme I just drew up specifically for you should answer your question.
Diastereomers are stereoisomers that are not enantiomers of each other. That includes conformers (geometric isomers that ...
- 67.3k
13
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Is cis-1,2-dimethylcyclohexane a meso compound?
I'd like to add a few points definition wise from the IUPAC goldbook.
(Because recently the original website seems to be broken I am using links via the internet archive.)
meso
A prefix to ...
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