Tag Info

28

The correct definition of chirality is given in the IUPAC gold book as follows: chirality The geometric property of a rigid object (or spatial arrangement of points or atoms) of being non-superposable on its mirror image; such an object has no symmetry elements of the second kind (a mirror plane, σ = S1, a centre of inversion, i = S2, a rotation-...

27

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

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 ...

26

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: We can see that if we reflect the molecule (in the plane of the ring + double bond),...

25

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). ...

25

I think Martin has provided an excellent answer, and I would like to supplement it with a few additional details and examples that might prove insightful. So as I already mentioned in the comments, the definition of chirality is rooted in symmetry. A chiral compound can contain no improper axis of rotation ($S_n$), which includes planes of symmetry ($S_1 = \... 24 Very interesting question! The key word you are looking for is planar chirality. In trans-cyclooctene, the polymethylene bridge can either go "in front of" or go "behind" the plane of the double bond, assuming you fix the double bond and the two hydrogens in place. As pointed out by @jerepierre, they are considered different molecules due to a high-energy ... 20 Chirality is a property of objects in which they lack certain symmetry operations, specifically improper rotations, including the mirror plane and inversion operations. For example, 3-dimensional chiral objects lack mirror symmetry. According to Wikipedia: The feature that is most often the cause of chirality in molecules is the presence of an asymmetric ... 18 Yes, this compound is chiral. The polycyclic backbone is called adamantane. It has$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 ... 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 ... 17 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-... 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. ... 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 ... 15 Background One can draw conformations of n-butane with the carbon-carbon bonds oriented in certain directions and the methyl hydrogens pointing in certain directions that are chiral. However, since rotation about single bonds is typically fast at room temperature, these chiral conformers of n-butane would only be resolvable at extremely low temperatures. ... 15 No, the biphenyl above is not chiral. 2,2′,6,6′-tetrasubstituted biphenyls are nonplanar at room temperature in order to avoid steric clashes between the substituents. This can in general give rise to chirality depending on the substituents, such as in this example. However, this chirality is only true under certain conditions, namely: The substituent ... 15 No, for three reasons: Firstly, you need a relatively large molecule and/or a high viscosity and/or a strong shear/extensional flow, before the molecule even notices that it´s in a nonuniform environment. The molecule, if it has an elongated form, will then align perpendicular to the normal vector of that flow field. That only occurs for sufficiently ... 14 Is there a simple way to get the circular dichroism of a molecule from it's structure? Short Answer: Yes, for many molecules if you know the molecule's structure, then you can predict the shape of the optical rotary dispersion (ORD) or circular dichroism (CD) curve. Conversely, and perhaps more importantly, if you know the shape of the ORD or CD curve, ... 14 Yes, the molecule should exhibit chirality, but this can likely only be observed below room temperature. In the optimised1 molecular configuration the molecule cannot be superimposed on its mirror image. This is because of the out-of-plane rotation of the nitro groups. The thermodynamic most favourable structure is ofC_2$symmetry and has no mirror plane (... 14 Mirror images The very first definition of a chiral molecule is one where it is not superimposable on its mirror image. Therefore, one of the most straightforward ways to determine chirality is to construct the molecule and its mirror image (perhaps via a model), and then to see whether they are superimposable. Symmetry elements A fully equivalent ... 13 Circularly polarized light is like a helix that twists through space. The two components are mirror images of each other. Now, every molecule interacts with both the left-handed twisting light and the right handed twisting light. The interactions differ. Every molecule, in different orientations, interact differently with the left-handed and the right-handed ... 12 Ammonia is the classic system for$\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$\ce{sp^3}$. However, the whole system can also swing around, changing its hybridisation to$\ce{sp^2}$and back; a process during which the lone pair is temporarily ... 12 In the second row of the periodic table, elements have relatively small differences between the size their$\mathrm s$- and$\mathrm p$-orbitals. Therefore, the orbitals of$\ce{NR3}$can go from$\mathrm {sp}^3$to$\mathrm {sp}^2$with relatively little energy increase, so an amine can become planar and then reorient with the inverted stereochemistry. The ... 12 You are actually right, that stereocenter does have R stereochemistry. The four substituents to the chiral center in question are: 1 -$\ce{CO(O)}$=$\ce{C(O)3}$2 -$\ce{C(O)(C)(H)}$3 -$\ce{C(N)(C)(H)}$4 -$\ce{H}$These are ranked and numbered according to the Cahn-Ingold-Prelog priority rules. Adding the numbering to the structure: Orienting the ... 11 Look for carbons with four different groups attached to identify potential chiral centers. Draw your molecule with wedges and dashes and then draw a mirror image of the molecule. If the molecule in the mirror image is the same molecule, it is achiral. If they are different molecules, then it is chiral. Here you're particularly interested in the C in the CH.... 11 Disclaimer: This answer neglects quantum tunnelling effects, which are significant in such compounds. The gist of it is that because nitrogen is smaller and lighter than phosphorus, the rate of tunnelling and hence inversion of chirality is much faster. This issue is discussed to some extent in this question. I've attached a Walsh diagram for the$D_\mathrm{...

11

The simplified point of view in a way that students should be expected to answer the question It is not the axis of symmetry (which is a $C_2$ axis in this case) that is important but rather that the molecule additionally features planes of symmetry ($\sigma_\mathrm{v}$). There are two perpendicular planes of symmetry in your example compound which both ...

11

From a 2007 paper:: Here we show that instrumental advances in Raman optical activity, combined with quantum chemical computations, make it possible to determine the absolute configuration of (R)-[$\ce{^2H1}$, $\ce{^2H2}$, $\ce{^2H3}$]-neopentane. This saturated hydrocarbon represents the archetype of all molecules that are chiral as a result of a ...

10

Ah, now I think I understand what you're asking. You're saying that we have a central carbon to which is attached a hydrogen and 3 groups with the same structure. Each of these 3 groups contains 2 chiral carbons. One group is R,R one is R,S and one is S,S. So yes, the chirality of the central carbon (it is chiral, it is not meso or prochiral - there is no ...

Only top voted, non community-wiki answers of a minimum length are eligible