Molecules with Chiral Planes

Any molecule with a plane of symmetry or a center of symmetry is achiral but their absence is not sufficient for a molecule to be chiral A molecule lacking a center of symmetry or a plane of symmetry is likely to be chiral but the supenmposability test should be applied to be certain... 

To see whether a chirality center is present, look for a carbon atom bonded to four different groups. To see whether the molecule is chiral, look for the presence or absence of a symmetry plane. Not all molecules with chirality centers are chiral overall—meso compounds are an exception. 

Mirror planes of symmetry are particularly easy to identify from the Fischer projection because this projection is normally the most symmetric conformation. In the first preceding example (propan-2-ol) and in the following example [(2S,3f )-2,3-dibromobutane], the symmetry planes are indicated in red these molecules with symmetry planes cannot be chiral. 

Similarly to the molecular engineering of calamitic molecules to produce ferroelectric smectic C phases [129], disk-like molecules with chiral peripheral chains tilted with respect to the columnar axis were predicted to lead to ferroelectric columnar mesophases [130]. Indeed, as it is the case with all flat disk-shaped mesogenic molecules, the tilt is mainly associated with the flat rigid aromatic cores of the molecules, the side-chains being in a disordered state around the columnar core. Thus, the nearest part of the chains from the cores makes an angle with the plane of the tilted aromatic part of the molecules. If the chiral centre and the dipole moment are located close to the core, then each column possesses a non-zero time averaged dipole moment, and therefore a spontaneous polarization. For reasons of symmetry, this polarization must be, on average, perpendicular to both the columnar axis and to the tilt direction in other words, the polarization is parallel to the axis about which the disk-shaped molecules rotate when they tilt as shown in Fig. 29. 

Enantiomers in which the chiral center is tetracoordinate carbon represent the largest class of chiral molecules, and the student is by now familiar with the fact that while 2-butanol is chiral, ethanol is not. Molecules that are not chiral are said to be achiral. The tetrahedral orientation of ligands to 5/ -carbon requires that when any two of the ligands are identical, the molecule is achiral conversely, when four nonidentical ligands are present, the molecule must be chiral. It is seen that with two identical substituents, the molecule has a plane of symmetry. A molecule with a plane... 

A plane of symmetry (sometimes called a mirror plane) is a plane that passes through a molecule (or object) in such a way that what is on one side of the plane is the exact reflection of what is on the other side. Any molecule with a plane of symmetry is achiral. Chiral molecules do not have a plane of symmetry Seeking a plane of symmetry is usually one quick way to tell whether a molecule is chiral or achiral. 

To summarize, a molecule with an axis of symmetry possesses rotational symmetry, and a molecule with a plane of symmetry possesses reflectional symmetry. With an understanding of these two types of symmetry, we can now explore the relationship between symmetry and chirality. In particular, chirality is not dependent in any way on rotational symmetry. That is, the presence or absence of an axis of symmetry is completely ttrelevant when determining whether a compound is chiral or achiral. We saw that tnt f-l,2-dimethylcyclohexane possesses rotational symmetry nevertheless, the compound is still chiral and exists as a pair of enantiomers ... 

The use of symmetry in determining which molecules can be chiral can be discussed based on the fact that enantiomers are related to one another by reflection. Any object has only one mirror image it does not matter where the mirror is positioned to reflect the object. This immediately tells us that a molecule with a plane of symmetry cannot be chiral, because we generate an identical molecule when it is reflected in the symmetry plane and so the molecule must be indistinguishable from its mirror image. In fact, if there is any symmetry operation that links a molecule with its mirror image, then the molecule cannot be chiral. 

Now let s consider the consequences of formation of the cyclic bromonium ion derived from rns j -2-butene followed by nucleophilic attack by bromide ion (Figure 8.22b). The bromonium ion results from attack on the top. Bromide ion attacks equally well at the right and left carbon atoms, giving the 2S,3R and 2R,3S structures, respectively. This pattern corresponds to two equivalently substituted chiral carbon atoms in a molecule with a plane of symmetry thus, this isomer corresponds to a single meso compound. 

Label A-E in Model 8 with the terms cis, trans, and/or meso, as appropriate. Two terms may apply. (Review) meso = molecule with chiral centers and an internal mirror plane such that it is not chiral. 

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... 

A molecule that contains just one chiral carbon atom (defined as a carbon atom connected to four different groups also called an asymmetric or stereogenic carbon atom) is always chiral, and hence optically active. As seen in Figure 4.1, such a molecule cannot have a plane of symmetry, whatever the identity of W, X, Y, and Z, as long as they are all different. However, the presence of a chiral carbon is neither a necessary nor a sufficient condition for optical activity, since optical activity may be present in molecules with no chiral atom and since some molecules with two or more chiral carbon atoms are superimposable on their mirror images, and hence inactive. Examples of such compounds will be discussed subsequently. 

With these restrictions Fischer projections may be used instead of models to test whether a molecule containing asymmetric carbons is superimposable on its mirror image. However, there are no such conventions for molecules whose chirality arises from anything other than chiral atoms when such molecules are examined on paper, three-dimensional pictures must be used. With models or three-dimensional pictures there are no restrictions about the plane of the paper. 

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