36 votes

Five-pointed crown alkane C25H40: does it exist?

These polyspiro compounds such as the one you gave are known as [m,n]rotanes, where m is the size of the central ring and n is the size of the decorating rings. So, your compound would be called [5,5]-...
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19 votes
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How can the gauche-effect be explained?

The gauche effect is commonly explained with LCAO-based bond orbitals. LCAO is short for linear combination of atomic orbitals and implies that we can take two atomic orbitals $\phi_1, \phi_2$ and ...
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19 votes

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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17 votes

What is the difference between steric strain and torsional strain?

TL;DR Torsional strain can be thought as the repulsion due to electrostatice forces between electrons in adjacent MOs. Meanwhile steric strain (also known as van der Waals strain) can be thought as ...
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When can a molecule be considered freely rotating at room temperature?

I'm not a transition-state physical chemist, but I think a good approach to this problem is transition-state theory, specifically the Eyring equation: $$k = \kappa \frac{k_b T}{h} e^{\frac{-\Delta G^{...
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16 votes

Why are conformational isomers considered as isomers?

Gaurang's answer deals well with the theory part I just wanted to give a visual aid. (Taken from Atropisomerism of naphthyl alcohol) Let's call the rotating substituent on the naphtahlene a fan. The ...
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Multiplet shape in proton NMR of morpholines

It looks as if the NMR of morpholine is an AA′XX′ spectrum (the chemical shift difference is 0.80 ppm, or 320 Hz on your spectrometer, two orders of magnitude larger than the coupling constant). ...
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14 votes

Why are conformational isomers considered as isomers?

I was taught that geometric (cis-trans) isomers are considered isomers because there is a high energy barrier to breaking the double bond... There's a problem here. Geometrical isomers are isomers ...
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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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12 votes
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Conformations of Cyclohexane

Short answer, no. There is not a "twist-chair." To convince yourself, it probably helps to make a physical model with a chemistry model kit. If you try to twist the chair, you can't do it without ...
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12 votes
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Why is the eclipsed conformer more stable than the bisected conformer in 1-butene?

In alkanes staggered conformers (all anti) are local minima, while eclipsed conformers represent transition states. The first is "stable", while the latter is not. The rotational barrier has been ...
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12 votes

Five-pointed crown alkane C25H40: does it exist?

Fascinating molecule. I am not offering anything substantially, new or original here; merely here to confirm what has already been said. I couldn't find any evidence that suggests this particular ...
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Does trans 1,3-dichlorocyclobutane have zero dipole moment?

Cyclobutane and its substituted derivatives readily undergo a ring flip (or ring inversion) as pictured below. The barrier to ring flipping is very low, around 1.5 kcal/mole, so at room temperature ...
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11 votes

Why is the cis isomer of 1,3-dimethylcyclobutane more stable than the trans isomer?

The four carbon atoms in cyclobutane are not coplanar, as that would lead to a large degree of unfavourable eclipsing interactions. So, it is not a flat square; it adopts a puckered conformation. One ...
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11 votes
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Flexibility of long-chain aliphatic compounds

PubChem 3D has limits on the number of 'rotatable bonds' I recently answered a related question about PubChem 3D: Beyond the limits of MMFF94 and MMFF94s themselves, PubChem3D had several limits ...
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Trying to understand SMILES(?) patterns

First off this is SMARTS not SMILES. Think of SMARTS as like a regular expression language for molecules (e.g., SMILES). So let's break down some of those characters: ...
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10 votes

Which is most stable conformation of optically inactive butane-2,3-diol?

The argument made by Ben can be supported by computational chemistry. I calculated three conformers of the compound. Conformation B1 includes an intramolecular hydrogen bond, while in conformation B2 ...
10 votes

Define, (actually define) the "stability" and "energy" of a compound

As a chemist, I would agree that textbooks are not clear about the term stability and energy. It is not your fault and undergraduate organic chemistry books make the situation worse. I cannot recall ...
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10 votes
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Stable conformer of protonated ethane-1,2-diamine

I'm not 100% convinced that both amines will protonate at the same time, since protonation of a charged molecule is different than the neutral. At the moment, I don't have the time to calculate the $\...
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9 votes
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Hydrogen bonding in staggered vs eclipsed conformations

Disclaimer: I am not a computational chemist, I am completely new to this stuff. Just a humble student of @pentavalentcarbon However, my thought is that if you have interacting OH groups, the ...
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9 votes
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Why is a conformer of hydrogen peroxide rotationally inactive?

"Rotationally inactive" is probably about the rotational spectrum of the molecule I'm assuming the (unsaid) context of the question is that it is about rotational spectroscopy as this is ...
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8 votes
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Number of gauche interactions in an alkane

When we sight down a carbon-carbon single bond and draw a Newman projection of a particular conformation, any two groups on the adjacent (vicinal) carbons that have a dihedral angle of 60° between ...
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8 votes

Which conformations of 2,3-dimethylbutane are more stable?

The correct arrangement (from least stable to most stable) is probably: $4 > 2 > 3 \gtrapprox 1$ based on my calculations for the four gas phase rotamers you can see in the figure below. Your ...
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Is there a way to determine the orientation of a benzene-ring ethyl group in synthesis reactions?

Unless the temperature is really low, you will not be able to distinguish the two conformation isomers (conformers). Rotation about single C-C bonds is rarely hindered. It can be, by using bulky ...
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8 votes

Define, (actually define) the "stability" and "energy" of a compound

Finding minimums: a general view When minimizing energy you are searching (using numerical methods) for the minimum of Potential Energy). You are at the minimum when the derivative(Jacobian) is equal ...
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8 votes
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All-Gauche Alkane: Is It Possible?

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 ...
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7 votes
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How can rotamers of alpha-keto esters be named to distinguish between different geometries?

The IUPAC recommends s-cis and s-trans for the rotamers of conjugated dienes. The nomenclature derives from having a "cis-like" or "trans-like" geometry about a sigma bond. I feel like something ...
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7 votes

Effect of periodic acid on cyclohexane derivatives

The literature reveals that cis-1,2-cyclohexanediol 1 cleaves about 20-times faster than trans-1,2-cyclohexanediol 2 although the equilibrium between the diol and the cyclic periodate intermediate ...
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7 votes

Conformation of six-membered ring constituting a bridged compound

According to structure determined by electron diffraction, norbornane is present in bridged boat conformation. http://pubs.acs.org/doi/abs/10.1021/ja01014a032, see Figure. Norbornane and its ...
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What are the implications of having different conformations?

That's an interesting question. To start off, what Mith said in the comments is absolutely right. Let's say that two conformations of a molecule have different smells (which is in fact not an absurd ...
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