Molecule achiral

The enantioselective introduction of chiral centres into an achiral molecule can nowadays be achieved most easily using chiral reductants or oxidants. 

Achiral molecules which can be converted to chiral molecules by the chemical change of one atom — substitution on an sp -atom or addition on an sp -atom — are called prochiral molecules (Y. Izumi, 1977). The atom involved is a prochiral centre. Pairs of atorns or groups... 

A plane of symmetry bisects a molecule so that one half of the molecule is the mirror image of the other half The achiral molecule chlorodifluoromethane for exam pie has the plane of symmetry shown m Figure 7 3... 

Addition to double bonds is not the only kind of reaction that converts an achiral molecule to a chiral one Other possibilities include substitution reactions such as the formation of 2 chlorobutane by free radical chlorination of butane Here again the prod uct IS chiral but racemic... 

Achiral molecules that contain chirality centers are called meso forms Meso forms typically contain (but are not limited to) two equivalently substituted chirality centers They are optically inactive... 

Merrifield method See solid phase peptide synthesis Meso stereoisomer (Section 7 11) An achiral molecule that has chirality centers The most common kind of meso com pound IS a molecule with two chirality centers and a plane of symmetry... 

Prochiral (Section 7 9) The capacity of an achiral molecule to become chiral by replacement of an existing atom or group by a different one... 

Ethanol is an achiral molecule. The plane defined by atoms C-1, C-2, and O is a plane of symmetry. Ary carbon atom with two identical ligands contains a plane of symmetry feat... 

Since chirality is a property of a molecule as a whole, the specific juxtaposition of two or more stereogenic centers in a molecule may result in an achiral molecule. For example, there are three stereoisomers of tartaric acid (2,3-dihydroxybutanedioic acid). Two of these are chiral and optically active but the third is not. 

In this exfflTipIe, addition to the double bond of an alkene converted an achiral molecule to a chiral one. The general term for a structural feature, the alteration of which introduces a chirality center in a molecule, is prochiral. A chirality center is introduced when the double bond of propene reacts with a peroxy acid. The double bond is a prochiral structural unit, and we speak of the top and bottom faces of the double bond as prochiral faces. Because attack at one prochual face gives the enantiomer of the compound formed by attack at the other face, we classify the relationship between the two faces as enantiotopic. 

Only three, not four, stereoisomeric 2,3-butanediols aie possible. These three aie shown in Eigure 7.10. The (2R,3R) and (2.S,3.S) fonns aie enantiomers of each other and have equal and opposite optical rotations. A third combination of chirality centers, (2R,3S), however, gives an achiral structure that is superimposable on its (2S,3R) minor image. Because it is achiral, this third stereoisomer is optically inactive. We call achiral molecules that have chirality centers meso forms. The meso form in Eigure 7.10 is known as iwe50-2,3-butanediol. 

An area in which catalytic olefin metathesis could have a significant impact on future natural product-directed work would be the desymmetrization of achiral molecules through asymmetric RCM (ARCM) or asymmetric ROM... 

In general, it may be said that enantiomers have identical properties in a symmetrical environment, but their properties may differ in an unsymmetrical environment. Besides the important differences previously noted, enantiomers may react at different rates with achiral molecules if an optically active catalyst is present they may have different solubilities in an optically active solvent., they may have different indexes of refraction or absorption spectra when examined with circularly polarized light, and so on. In most cases these differences are too small to be useful and are often too small to be measured. 

It is now possible to see why, as mentioned on page 126, enantiomers react at different rates with other chiral molecules but at the same rate with achiral molecules. In the latter case, the activated complex formed from the (/ ) enantiomer and the other molecule is the mirror image of the activated complex formed from the (S) enantiomer and the other molecule. Since the two activated complexes are... 

In a special case of this type of asymmetric synthesis, a compound (47) with achiral molecules, but whose crystals are chiral, was converted by UV light to a single enantiomer of a chiral product (48). ... 

The photochemical behaviour of 7 OEt is the first example in which the reaction of achiral molecules in an achiral crystal packing does not occur at random but stereospecifically, resulting in a syndiotactic structure. As no external chiral catalyst exists in the reaction, the above result is a unique type of topochemical induction , which is initiated by chance in the formation of the first cyclobutane ring, but followed by syndiotactic cyclobutane formation due to steric repulsions in the crystal cavity. That is, the syndiotactic structure is evolved under moderate control of the reacting crystal lattice. 

It is not easy to control the steric course of photoreactions in solution. Since molelcules are ordered regularly in a crystal, it is rather easy to control the reaction by carrying out the photoreaction in a crystal. However, molecules are not always arranged at an appropriate position for efficient and stereoselective reaction in their crystals. In these cases inclusion chemistry is a useful technique, as it can be employed to position molecules appropriately in the host-guest structure. Chiral host compounds are especially useful in placing prochiral and achiral molecules in suitable positions to yield the desired product upon photoirradiation. Some controls of the steric course of intramolecular and intermolelcular photoreactions in inclusion complexes with a host compound are described. 

When a molecule is adsorbed on a surface, the symmetry of the combined adsorbate-substrate system is very likely to be reduced compared to that of the isolated gas-phase species or the bare adsorption site. This raises the possibility that, if mirror planes present in the isolated achiral molecule and those at the relevant... 

Chirality at surfaces can be manifested in a number of forms including the intrinsic chirality of the surface structure and even the induction of chirality via the adsorption of achiral molecules onto achiral surfaces. The ability of STM to probe surfaces on a local scale with atomic/molecular resolution has revolutionized the understanding of these phenomena. Surfaces that are globally chiral either due to their intrinsic structure or due to the adsorption of chiral molecules have been shown by STM to establish control over the adsorption behavior of prochiral species. This could have profound consequences for the understanding of the origin of homochirality in life on Earth and in the development of new generations of heterogeneous chiral catalysts that may, finally, make a substantial impact on the pharmaceutical industry. 

Achiral molecules are characterised by the presence of symmetry elements of the second kind, for example, planes of symmetry, inversion centres or rotation-reflexion axes. 

Figure 5.13 A beam of plane-polarized light encountering a molecule of 2-propanol (an achiral molecule) in orientation (a) and then a second molecule in the mirror-image orientation (b) The beam emerges from these two encounters with no net rotation of its plane of polarization.

When achiral molecules react to produce a compound with a single tetrahedral stereocenter, the product will be obtained as a racemic form. 

So far we have considered the formation of tubules in systems of fixed molecular chirality. It is also possible that tubules might form out of membranes that undergo a chiral symmetry-breaking transition, in which they spontaneously break reflection symmetry and select a handedness, even if they are composed of achiral molecules. This symmetry breaking has been seen in bent-core liquid crystals which spontaneously form a liquid conglomerate composed of macroscopic chiral domains of either handedness.194 This topic is extensively discussed in Walba s chapter elsewhere in this volume. Some indications of this effect have also been seen in experiments on self-assembled aggregates.195,196... 

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