Achiral objects

Achiral (Section 7 1) Opposite of chiral An achiral object is supenmposable on its mirror image Acid According to the Arrhenius definition (Section 1 12) a substance that ionizes in water to produce protons Accord mg to the Br0nsted-Lowry definition (Section 1 13) a sub stance that donates a proton to some other substance According to the Lewis definition (Section 1 17) an electron pair acceptor... 

Achiral (Section 7.1) Opposite of chiral. An achiral object is superimposable on its mirror image. 

Fig 10.4 Some common examples of chiral and achiral objects... 

By comparison, achiral objects, such as a simple nail, ball, basket, white pocketless T-shirt, are all superimposible on their mirror image. 

Give two examples of chiral objects and two examples of achiral objects. 

Achiral objects can be assembled into chiral solid-state structures, and this is frequently the case for urea 1 when it encloses guests. Other compounds adopt a chiral conformation in solution and therefore may ultimately produce either chiral or achiral host structures. On the other hand, thiourea 2 forms an inclusion lattice that is achiral. This arrangement is nonetheless very effective in enclosing guest molecules. 

The achiral objects (teacup, football, tennis racket, and pencil) can be used with equal ease by right- or left-handed persons. Their mirror images are superimposable on the objects themselves. On the other hand, a golf club must be either left- or right-handed and is chiral a shoe will fit a left or a right foot a corkscrew may have a right- or left-handed spiral. These objects, as well as a portrait, have mirror images that are not identical with the objects themselves, and thus they are chiral. 

In Fig. 3.1, we have represented a comparison of chiral objects based on their sizes. If we define an object whose mirror image is nonsuperposable with it as chiral, almost all objects of our life are chiral a perfect macroscopic achiral object does not exist. But usually, we use the term chiral in the sense that its mirror image... 

Since chirality is a geometrical property, all serious discussions on this topic require a mathematical treatment that is much out of this review. Note, however, that if you cut by the middle of a Klein bottle (an achiral object having a plane of symmetry), you obtain two Mobius strips both chiral and mutually enantiomorph (Fig. 3.5). This pure mathematical result is closely related to the situation of meso compounds described above [11]. 

It has been recognized147 that chirality measures can be subdivided into two types those that gauge the extent to which a chiroid differs from an achiral reference object (measures of the first kind) and those that gauge the extent to which two enantiomorphs differ from one another (measures of the second kind). In chirality measures of the first kind, the question to be answered is How dissimilar are the chiroid and its achiral reference object In chirality measures of the second kind, the question is How dissimilar are the two enantiomorphs of a chiroid In both cases the underlying concept is that of a distance, measured either between a chiral and an achiral object or between two enantiomorphous chiroids. That is, the degree of chirality of a chiroid X is defined in relation to another, chiral or achiral, reference object Xref The less these two objects match, the more chiral is X. 

A chiral object cannot be superimposed on its mirror image whilst an achiral object can be superimposed on its mirror image. The two forms of a chiral molecule are known as enantiomers. A single chiral isomer is said to be enantiomerically or optically pure. Whereas an equimolar mixture of enantiomers is said to be racemic. 

Among the many chemical processes in which chirality/achirality relationships may be important is the fragmentation of some molecules and the reverse process of the association of molecular fragments. Such fragmentation and association can be considered generally and not just for molecules. The usual cases are those in which an achiral object is bisected into achiral or heterochiral halves. On the... 

For an everyday analogy, consider what happens when you are handed an achiral object like a pen and a chiral object like a right-handed glove. Your left and right hands are enantiomers, but they can both hold the achiral pen in the same way. With the glove, however, only your right hand can fit inside it, not your left. 

By applying the SNSM similarity measure to mirror images, the quantity is a measure of achirality, whereas the dissimilarity measure d A,A ), denoted as Xs J A), is a measure of chirality, where the interrelation (137) between Xs,J A) and implies that this measure can take values from the unit interval. The measure Xs A), first proposed as an example of dissimilarity measures of the second kind, is zero for achiral objects and takes positive values for all chiral objects. Objects perceived as having prominent chirality tend to have large Xs A) values. The SNSM measures have also been applied to more general molecular shape problems. More recently, Klein showed that by a logarithmic transformation of the scaling factors s g, a metric can be constructed to provide a proper distance-like measure of dissimilarity of shapes. 

The quantity Xsab(A) is zero for achiral objects, and it is a positive value for chiral objects. In general, XsabC ) tends to have larger values for objects perceived as having more prominent chirality. This measure differs from measures based on maximum overlap between mirror images [46,48-53,57,58,242]. 

Following the approach of the original description [240], we shall first consider chirality. For a chiral object T, the largest achiral object that fits within T, as well as the smallest achiral object that contains T are of special importance. We shall compare the volumes v of these objects, and use these comparisons to assess the degree of the deviation of the object T from achirality. 

The above is a simple idea however, as the following example shows, some caution is in order. Consider two solid balls of the same radius, where one ball has a spiral line issued from its surface. The first object is achiral whereas the second one is chiral, and the first object is the largest achiral object that fits within the second one. Clearly, the two objects have the same volume, hence comparing volumes is not appropriate for assessing the degree of chirality (i.e., the degree of "achirality deficiency") of the second object. This problem is caused by the presence of the infinitely thin spiral line of zero volume. In order to avoid such pathological cases, we shall consider only objects T that are "nowhere infinitely thin" and have finite, nonzero volume [240]. 

An object that is chiral is an object that can not be superimposed on its mirror image. Chiral objects don t have a plane of symmetry. An achiral object has a plane of symmetry or a rotation-reflection axis, i.e. reflection gives a rotated version. 

So far we have looked largely at substrates which are flat achiral objects that have chiral centres introduced by means of a reagent. Desymmetrisations are slightly different. Molecules which are desymmetrised tend to be of a meso type. That is, they are achiral because they have a mirror plane and the sides of the molecule contain left and right handed portions 219. This is in contrast to the C2 axis present in many catalysts such as the TADDOLate 218. Desymmetrisations are powerful because there may be several chiral centres embedded in an achiral molecule which suddenly become much more useful in the newly formed chiral molecule. There are a large variety of symmetrical substrates that have been enantioselectively desymmetrised48 and we look at a few of the more important classes. 

Related to the distinction between chiral and achiral objects, symmetry elements can be conveniently classified into two categories simple axes of symmetry (Cn), and alternating (or mirror) axes of symmetry (S ) (15). 

A chiral object has a nonsuperimposable mirror image. In other words, its mirror image is not the same as itself. A hand is chiral because if you look at your left hand in a mirror, you do not see your left hand you see your right hand (Figure 5.1). In contrast, a chair is not chiral—it looks the same in the mirror. Objects that are not chiral are said to be achiral. An achiral object has a superimposable mirror image. Some other achiral objects would be a table, a fork, and a glass. 

Measures of the first kind those that gauge the extent to which a chiroid object differs from an achiral object... 

By structure, we don t just mean chemical structure the same rules apply to everyday objects. Some examples from among more familiar objects in the world around us should help make these ideas clear. Look around you and find a chiral object—a pair of scissors, a screw (but not the screwdriver), a car, and anything with writing on it, like this page. Look again for achiral objects with planes of symmetry—a plain mug, saucepan, chair, most simple manufactured objects without writing on them. The most significant chiral object near you is the hand you write with. 

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