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Questions tagged [stereochemistry]

Applicable to questions about the spatial arrangement of atoms in a molecule and how they affect its physical or chemical properties.

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What is Bent's rule?

I'm all bent out of shape trying to figure out what Bent's rule means. I have several formulations of it, and the most common formulation is also the hardest to understand. Atomic s character ...
Dissenter's user avatar
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21 votes
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7k views

Molecular chirality and optical rotation

Why does having molecular chirality result in optical rotation? The dissymetry or chirality of molecules translates to the rotation of plane polarized light, the magnitude and direction depending on ...
stochastic13's user avatar
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Utility of Bent's Rule - What can Bent's rule explain that other qualitative considerations cannot?

Does Bent's rule have any utility? I got a vehement earful for "NO." Points raised by my professor: Coulombic considerations can be used to rationalize bond angles, strengths, and lengths without ...
Dissenter's user avatar
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23 votes
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Why is the inversion barrier larger in PH3 than it is in NH3?

The inversion barrier in $\ce{NH3}$ is approximately $5~\mathrm{kcal~mol^{-1}}$ and that of $\ce{PH3}$ is $35~\mathrm{kcal~mol^{-1}}$. This has well-known stereochemical consequences in that amines ...
wuschi's user avatar
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Why is 1-ethylidene-4-methylcyclohexane chiral?

As far as I can tell, there doesn't seem to be a chiral carbon in the compound, 1-ethylidene-4-methylcyclohexane. The C-4 of the cyclohexane ring has two groups with exactly the same connectivity, and ...
orthocresol's user avatar
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20 votes
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What are the CIP rules for cyclic substituents?

Here's what I read on Wikipedia (section "Cycles"): To handle a molecule containing one or more cycles, one must first expand it into a tree (called a hierarchical digraph by the authors) by ...
stoic-santiago's user avatar
0 votes
1 answer
533 views

Twisting stereoisomers with rings to determine R/S [closed]

I know that if the smallest atomic number group is not in the back, we can twist the molecule so that it is in the back and then determine if it is R/S (Left image). But if the chiral center is ...
Jon's user avatar
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Can heteroatoms with lone pairs be chiral centres?

If a compound has a carbon atom with four different groups covalently bonded to it, it is called asymmetric and enantiomers of the compound can exist. But imagine if one has a different central atom, ...
caconyrn's user avatar
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26 votes
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Is cis-1,2-dimethylcyclohexane a meso compound?

I was watching a video lecture today on hydrocarbons and came across this. The instructor says that, as there is a plane of symmetry, cis-1,2-dimethylcyclohexane is a meso-compound: However, the ...
user avatar
23 votes
3 answers
16k views

Why are allenes chiral?

How is 1-bromo-3-chloropropa-1,2-diene ($\ce{BrHC=C=CHCl}$) chiral? It doesn't have any carbon that is connected to four different groups.
Rajath Radhakrishnan's user avatar
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IUPAC name for 1,2,3-trichlorocyclopropane?

What is the IUPAC name for the two isomers of 1,2,3-trichlorocyclopropane? Similarly what is the name for isomers of 1,2,3,4-tetrachlorocyclobutane, 1,2,3,4,5-pentachlorocyclopentane, etc? 1,2,3-...
ℵ_ϵ's user avatar
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Dihedral angle of gaseous and crystalline HOOH

Why does hydrogen peroxide exhibit a dihedral angle of $111.5^\circ$ in the gaseous state? And a dihedral angle of $90.2^\circ$ in the crystalline phase? I know that in general, there is likely to be ...
Dissenter's user avatar
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5 votes
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Is (2R,3S)-butane-2,3-diol chiral?

Is the attached compound a chiral or achiral overall? My lecturer said that this is an achiral compound because it has a superposable mirror image but I don't get that. So I tried to figure out the ...
Theresa's user avatar
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28 votes
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How to assign E/Z configuration according to the Cahn-Ingold-Prelog rules when subsituents differ only by stereochemistry

I stumbled-upon this compound: I'm wondering how to determine the configuration of the double bond. Is it E or Z? How do I decide on the priorities when the substituents can only differ in the ...
martin-is-my-name's user avatar
27 votes
2 answers
182k views

What is the difference between D and L configuration, and + and −?

What is the difference between D and L configuration, and + and −? My textbook says they are two different things. It also says that the correct way to name glucose is D(+)-glucose. Could someone ...
Mahathi Vempati's user avatar
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Why don't trigonal S and P compounds undergo inversion at room temperature?

Most molecules containing nitrogen atoms in trigonal pyramid configuration undergo a relatively fast process of inversion at room temperature. On the other hand, the free energy barrier for phosphines,...
F'x's user avatar
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Number of diastereomer pairs of 1,3-dichloro-1,2,3-triphenylpropane

How can I determine the number of possible pairs of diastereomers here? My first guess was two: RR with RS and SS with RS. But what about the potential chirality center middle carbon? There are two ...
yolo123's user avatar
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Why is trans-cyclooctene chiral?

How does trans-cyclooctene exhibit chirality if there are no stereocenters? Related follow-on questions: Are all higher cycloalkenes chiral? Do more double bonds cause a bigger number of ...
EJC's user avatar
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Are a meso compound and its mirror image isomers?

How are meso isomers even isomers? By definition, meso isomers are compounds that are superposable on their mirror images, but doesn't that mean that the mirror image is just the same compound? Can ...
Mahathi Vempati's user avatar
15 votes
4 answers
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Mismatch of plane of symmetry vs Gold Book's definition for a pseudochiral carbon

Here is pentane-2,3,4-triol: At first glance, without deciding the configurations at each chiral carbon, we can clearly see that this molecule has a plane of symmetry, perpendicular to the plane of ...
Gaurang Tandon's user avatar
11 votes
1 answer
2k views

Are there chiral compounds that don't rotate plane-polarized light?

I know that meso compounds have chiral centers but don't rotate plane-polarized light, and I know that there can be non-traditionally chiral compounds (e.g. ones with large substituents that prohibit ...
Carl Schildkraut's user avatar
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Chirality of heteroatoms [duplicate]

Why is a phosphorus atom with three different substituents chiral, but nitrogen isn't? Nitrogen inverts fast, while phosphorus obviously much slower. How is that explained?
EJC's user avatar
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Do molecules with axial chirality have "stereogenic units"?

This question is a follow-up to a recent question on discodermolide stereochemistry that I answered here. In the course of reading IUPAC's definition of "stereogenic center", I wondered if axially ...
Curt F.'s user avatar
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10 votes
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Why does nitrous acid exhibit (unexpectedly) different bond angles?

Data Point 1 - quantum chemical calculations http://nzc.iap.ac.cn/uploadpdf/Wang_et_al._CSB_2007a.pdf Key points in paper: There is some sort of interaction between the hydrogen and the terminal ...
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Conformations of Cyclohexane

Is there any such thing as a twist chair conformation? Also, the boat and the chair conformations are achiral, while the twist boat conformation of cyclohexane is chiral and dissymetric. What about ...
Charles's user avatar
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Does trans 1,3-dichlorocyclobutane have zero dipole moment?

Does this compound have zero dipole moment? I think it has a non-zero dipole moment. If I assume the ring to be planar, then the dipole moments of each $\ce{C-Cl}$ bond cancel out. (One is upward and ...
Aditya Dev's user avatar
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9 votes
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Why do only chiral molecules rotate the plane of polarized light and how do they rotate it?

I have learnt that only chiral or unsymmetrical molecules can rotate the plane of linearly polarized light. But, why is it so? And how can molecules rotate the plane or what does it actually mean by ...
Rajath Radhakrishnan's user avatar
8 votes
1 answer
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Does 1,4-dimethylcyclohexane have a chiral center?

My textbook says that 1,4-dimethylcyclohexane (cis or trans) does not have a chiral centre, since two groups on the tertiary carbon (that are the part of the ring) are identical, and thus the compound ...
user avatar
6 votes
3 answers
3k views

Is there a mathematical generalization for the number of isomers (structural and configurational) of a given compound?

The original question was: How do I find the number of structural and configurational isomers of a given compound? is there any formula? For a specific example, how would I find the number of ...
krazkat's user avatar
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3 votes
2 answers
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Selectivity in organic reactions

I am finding great difficulty in understanding the differences between stereospecific, stereoselective, regiospecific and regioselective nature of reactions. Can you explain it in simple terms without ...
cuebits's user avatar
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3 votes
2 answers
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"Asymmetric molecules are necessarily polar"

Is a molecule with no symmetry necessarily polar? Can a polar molecule still have some elements of symmetry (i.e. mirror image is the same as itself)? I think so ... because we can have achiral ...
Dissenter's user avatar
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44 votes
2 answers
108k views

How to derive these general formulae for number of stereoisomers of a compound with a possible plane of symmetry?

These formulae are used if the molecule has a possible plane of symmetry. One such example would be: Here the carbons marked with an asterisk are stereogenic centres (the asterisk is not used to mark ...
FreezingFire's user avatar
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29 votes
3 answers
67k views

How do you separate enantiomers?

There are some stereochemical reactions that result in the presence of enantiomers. When moving forward with a practical organic synthesis, how does one usually separate them in order to continue with ...
Cardtrick's user avatar
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16 votes
1 answer
412 views

Atropisomerism of naphthyl alcohol

In March's Advanced Organic Chemistry (6th ed.), it's stated that the above two are atropisomers. I don't get why is it so. How is the topmost $\ce{C}$ locked in its place? Surely, there's steric ...
user avatar
15 votes
2 answers
568 views

Molecular knots absolute configuration

How is the absolute configuration of chiral molecular knots determined? What rules should be applied? For centrochiral compounds, the CIP rules are applied, for axially chiral is described here and ...
EJC's user avatar
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14 votes
2 answers
3k views

How is the Felkin-Anh model of stereoinduction correctly explained with MO theory?

One method to predict stereoinduction in asymmetric reactions is the Felkin-Anh model (derived from the more basic Cram model) that applies to nucleophile attacks on α-chiral aldehydes. In a ...
Jan's user avatar
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11 votes
1 answer
794 views

Which alkene has a higher Cahn-Ingold-Prelog priority in (2Z,5E)-hepta-2,5-dien-4-ol?

How do I determine whether the central carbon of this compound is (R) or (S)? On both the sides of carbon, the groups are same. They differ only in the relative position of hydrogen atoms. On what ...
Tejas's user avatar
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11 votes
2 answers
2k views

Are all higher cycloalkenes chiral?

Are all higher trans-cycloalkenes chiral as trans-cyclooctene is? This question supplements my previous question: Why is trans-cyclooctene chiral? See also the follow-on question: Do more double ...
EJC's user avatar
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8 votes
3 answers
33k views

DL vs dl notation [closed]

As the L-isomer of glucose, it is the enantiomer of the more common D-glucose. Source: Wikipedia As far as I know enantiomers or Optical isomers are non superimposable mirror image structures with ...
Tyto alba's user avatar
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6 votes
2 answers
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What's the H-C-H bond angle in ethene?

The carbon is $\mathrm{sp^2}$ hybridised and is therefore planar and should also, theoretically be $120^\circ$. However, VSEPR theory suggests that the π bond would "need more space" due to ...
RobChem's user avatar
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5 votes
2 answers
3k views

When is rotation around double bonds to form cis-trans isomers allowed?

Textbook Question: The rhodopsin molecules in the retina contain a derivative of vitamin A called cis-retinal (Figure 6). On exposure to light, cis-retinal becomes an isomer called trans-retinal. ...
K-Feldspar's user avatar
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4 votes
2 answers
6k views

Dependence of the angle of rotation on the wavelength of plane polarized light

Wavelength difference is a big deal, I know. It can solely change the whole interaction between the chiral molecule & the light. But I am not sure what's the mechanism by which light of different ...
Mockingbird's user avatar
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3 votes
1 answer
870 views

Stereoisomers of octahedral co-ordination compounds of the form [Ma3b3]

Does $\ce{[Co(H2O)3Br3]}$ show geometrical and optical isomerism? According to me $\ce{Co}$ should be $\mathrm{sp^3d^2}$ hybridised giving an octahedral geometry. It is of the form $\ce{[Ma3b3]}$ type....
Utkarsh Barsaiyan's user avatar
2 votes
2 answers
6k views

What is the optical nature of the tris(ethylenediamine)cobalt(III) cation?

The molecule I've drawn has neither a plane of symmetry nor a center of symmetry, even though some books refer to this as optically inactive. Can you please elaborate why is this so, or those books ...
Arvind Tiwari's user avatar
1 vote
1 answer
6k views

Why are substituted biphenyls optically active?

Why are some substituted biphenyl compounds optically active and what kind of substituted biphenyls are optically active . I read this in a book but no satisfactory reasons were given and no examples ...
Nitro phenol's user avatar
1 vote
1 answer
3k views

Why is trans-1,2-dimethylcyclohexane an enantiomer?

Can't the two be superimposed over each other after ring inversion.
Avyansh Katiyar's user avatar
1 vote
1 answer
4k views

Enantiomers of 1,2-dichlorocyclohexane

The other day in class we were discussing the enantiomers of 1,2-dichlorocyclohexane and my teacher told us that (R,R) and (S,S) are different isomers while (R,S) and (S,R) are the same. I understand ...
Beast's user avatar
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26 votes
2 answers
12k views

What does lowercase r-s notation mean?

I came across a naming convention which I haven't seen before. I let ChemDraw name the following compound for me and got a name containing lowercase "r" and "s" configurations. Can someone tell me ...
Philipp's user avatar
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21 votes
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What are known examples of drugs that racemize/stereoconvert in vivo, and how are they converted?

It is known that although only the (S)-enantiomer of the infamous sedative thalidomide possesses teratogenic properties, it is not very useful to administer the pure (R)-enantiomer since it is ...
user avatar
19 votes
3 answers
3k views

Can geometrical isomers be enantiomers?

I have just been pondering upon the question above. I have found this example and I am sure they are enantiomers, but do they also count as geometrical isomers?
Harsh Katara's user avatar