Achiral molecules mirror image

Now let us look at two molecules, 2-chloropropane and 2-chlorobutane, and their mirror images. Figure 5.2 shows that 2-chloropropane is achiral. Its mirror image is superimposable on the molecule itself. Therefore 2-chloropropane has only one possible structure. 

Certain structural features can sometimes help us determine by inspection whether a mol ecule IS chiral or achiral For example a molecule that has a plane of symmetry or a cen ter of symmetry is superimposable on its mirror image and is achiral... 

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... 

Section 7 1 A molecule is chiral if it cannot be superimposed on its mirror image Nonsuperimposable mirror images are enantiomers of one another Mol ecules m which mirror images are superimposable are achiral... 

Section 7 3 A molecule that has a plane of symmetry or a center of symmetry is achi ral as 4 Methylcyclohexanol (Table 7 2) has a plane of symmetry that bisects the molecule into two mirror image halves and is achiral The same can be said for trans 4 methylcyclohexanol... 

Multiple Chiral Centers. The number of stereoisomers increases rapidly with an increase in the number of chiral centers in a molecule. A molecule possessing two chiral atoms should have four optical isomers, that is, four structures consisting of two pairs of enantiomers. However, if a compound has two chiral centers but both centers have the same four substituents attached, the total number of isomers is three rather than four. One isomer of such a compound is not chiral because it is identical with its mirror image it has an internal mirror plane. This is an example of a diaster-eomer. The achiral structure is denoted as a meso compound. Diastereomers have different physical and chemical properties from the optically active enantiomers. Recognition of a plane of symmetry is usually the easiest way to detect a meso compound. The stereoisomers of tartaric acid are examples of compounds with multiple chiral centers (see Fig. 1.14), and one of its isomers is a meso compound. 

Since the presence of a plane of symmetry in a molecule ensures that it will be achiral, one a q)ro h to classification of stereoisomers as chiral or achiral is to examine the molecule for symmetry elements. There are other elements of symmetry in addition to planes of symmetry that ensure that a molecule will be superimposable on its mirror image. The trans,cis,cis and tmns,trans,cis stereoisomers of l,3-dibromo-rranj-2,4-dimethylcyclobutaijte are illustrative. This molecule does not possess a plane of symmetry, but the mirror images are superimposable, as illustrated below. This molecule possesses a center of symmetry. A center of symmetry is a point from which any line drawn through the molecule encouniters an identical environment in either direction fiom the center of ixnimetry. 

Nonsriperimposable mirror images are enantiomers of one another. Molecules in which minor images are superimposable are achiral. 

A molecule that has a plane of symmetry in any of its possible conformations must be identical to its mirror image and hence must be nonchiraJ, or achiral. Thus, propanoic acid, CH3CH2CO2H, has a plane of symmetry when lined up as shown in Figure 9.4 and is achiral, while lactic acid, CH3CH(0H)C02H, has no plane of symmetry in any conformation and is chiral. 

Figure 9.4 The achiral propanoic acid molecule versus the chiral lactic acid molecule. Propanoic acid has a plane of symmetry that makes one side of the molecule a mirror image of the other side. Lactic acid has no such symmetry plane.

Achiral (Section 9.2) Having a lack of handedness. A molecule is achiral if it has a plane of symmetry and is thus superimposable on its mirror image. 

A chiral complex is one that is not identical to its mirror image. Thus, all optical isomers are chiral. The cis isomers of [CoCl2(en)2 + are chiral, and a chiral complex and its mirror image form a pair of enantiomers. The trans isomer is superimposable on its mirror image complexes with this property are called achiral. Enantiomers differ in one physical property chiral molecules display... 

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 nature, but also in chemical syntheses, many molecules occur in two spatially different structures which behave like mirror images. This can also be seen in two dimensions one of the letters A (Fig. 9.5) can be superimposed upon the second A if it is moved to the right (or left). This is, however, not possible with the letters G . Thus, A is described as not chiral, or achiral, and G as chiral. 

Objects (and molecules) that are superposable on their mirror images are achiral. 

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.

The inactivity in the molecule is due to the fact that it is perfectly symmetric as shown by the dotted line, the upper half exactly coinciding with the lower half. Therefore, molecular asymmetry and not the presence of asymmetric carbon atoms is responsible for optical activity. Since the term asymmetric has been found to be inadequate, now the term chirality has been introduced. The word chiral (the Greek word cheir means hand pronounced kiral) signifies, the property of Handedness . An object that in not superimposable upon its mirror image is chiral and this mirror-image relationship is the same as left hand has with the right. If an object and its mirror image can be made to coincide in space, they are said to be achiral. 

An acceptable division of chiral molecules into right and left means a division of the A-space into two regions (say R and L), such that (i) every chiral molecule is in either R or L, and not on the boundary between them (ii) if a given chiral molecule is in R, then its mirror image is in L, and vice versa and (iii) achiral molecules are in neither R or L, but on the boundary between them.1)... 

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