Conformers isomers Stereoisomers

Conformers or conformational isomers. Stereoisomers that are interconverted by rapid rotation about a single bond. 

The first two classes of selectivity distinguish between constitutional isomers the last one between stereoisomers (configurational isomers and, eventually, conformational isomers). 

Stereoisomers, on the other hand, are compounds with the same molecular formula, and the same sequence of covalently bonded atoms, but with a different spatial orientation. Two major classes of stereoisomers are recognized, conformational isomers and configurational isomers. 

Stereoisomers are isomers having the same molecular formula and the same connectivity, but different spatial arrangements. There are three classes of stereoisomer cis-trans isomers, conformational isomers and enantiomers. 

There are two major types of stereoisomer conformational isomers and configurational isomers. Configurational isomers include optical isomers, geometrical isomers, enantiomers and diastereomers. 

Stereoisomers that are not enantiomers are called diastereoisomers. Three classes may be distinguished configurational, geometrical, and conformational isomers. Configurational diastereomers include molecules with more than one chiral center. Thus 2,3-dichlorobutane can exist in three configurationally... 

The staggered and eclipsed forms of ethane are conformational stereoisomers (conformational isomers, conformers) because they have the same molecular formulas and sequences of bonded elements but different spatial arrangements due to rotations around single bonds. (Actually there are an infinite number of conformational isomers (also called conformations) because there are an infinite number of degrees of rotation around the bond, but normally one only needs to be concerned with energy minima and maxima.)... 

The interconversion of isomers in the case of optical or geometric pairs, if structurally feasible, may only take place by the breaking of a- or 7c-bonds. However, there is a further area of stereoisomerism wherein the isomers are interconvertible by rotation about a single (conformational isomers or conformers. 

Stereoisomers have the same order of atom attachments but different arrangements of the atoms in space. Cis-trans isomerism is one kind of stereoisomerism. For example, two substituents on a cycloalkane can be on either the same (c/s) or opposite (trans) sides of the mean ring plane. Stereoisomers can be divided into two groups, conformational isomers (interconvertible by bond rotation) and configurational isomers (not interconvertible by bond rotation). Cis-trans isomers belong to the latter class. 

All pairs have the same bond patterns and are stereoisomers. Since they are not interconvertible by a-bond rotations, they are configurational isomers, conformational isomers conformational isomers... 

The structures are conformational isomers. Both are achiral. They are diastereomers (stereoisomers but not mirror images). 

Rotamers are conformational isomers that differ by rotation about a single a bond. The rotational barrier is the activation energy required to convert one rotamer to another. Atropisomers are rotamers in which the barrier to rotation about a single a bond is so high, usually due to steric hindrance, that the separate rotamers (stereoisomers) can be isolated <2004T4335>. 

Conformational isomerism, as already defined (Section 3.b), is a property of stereoisomers separated by a low barrier of energy. The separation of isomers at room temperature requires half-lives of several hours, which correspond approximately to a free energy of activation of AG > 20 kcal/mol [56]. An operational and convenient definition of conformational isomerism is thus to consider as conformers those stereoisomers which are not physically separable under ordinary conditions, in other words, which are separated by an energy barrier lower than 20 kcal/mol. Such a definition is further useful in that it sets no conditions as to the chemical process by which conformer interconversion occurs while bond rotation is the most frequently encountered interconversion process, inversion processes are also important. 

Stereoisomers in general, and conformational isomers in particular, are characterized not only by the energy barrier separating them, but also by their free energy difference AG°, which is related to the conformational equilibrium constant K (or conformational ratio) by the equation ... 

A reversible process is one in which a molecule (or set of molecules) changes back and forth between two (or more) different structures (e.g., different conformations, different stereoisomers, even different structural isomers), forming an equilibrium mixture of both (or all) the structures. Recall that although the concentration of each component of an equilibrium mixture does not change with time, equilibrium is nonetheless dynamic because interconversion between the components continues at rates that preserve the composition of the mixture. 

Of the nine stereoisomers of inositol, the scyllo-isomer has no axial hydroxyl, the myo-isomer has one, the epi-, chiro-, and neo-isomers have two, and the alio-, cis-, and mneo-isomers have three hydroxyl groups (Figure 1). Of these, six isomers scyllo-, myo-, epi-, neo-, cis-, and mr/co-i somers) have one or more planes of symmetry in the molecule (meso compounds) and are therefore not chiral. D-chiro- and L-e/u>o-i somers do not have a plane of symmetry and are chiral molecules moreover they are enantiomers of each other. The alio-isomer is unique - the conformational isomer of (10) (Figure 2) is (11) which is also its enantiomer Since interconversion between conformational isomers is rapid, a//o-inositol exists as a 50/50 mixture of the two enantiomers at room temperature. Therefore, although alio-inositol is chiral, the compound is optically inactive at room temperature because it is a racemic mixture a chiral reagent, such as an enzyme, would be expected to preferentially react with one enantiomer and not the other. 

In Sec. 3.5, we saw that there are several different staggered conformations of /i-butane, each of which lies at the bottom of an energy valley—at an energy minimum—separated from the others by energy hills (see Fig. 3.4, p. 79). Different conformations corresponding to energy minima are called conformational isomers, or conformerS. Since conformational isomers differ from each other only in the way their atoms are oriented in space, they, too, are stereoisomers. Like stereoisomers of any kind, a pair of conformers can either be mirror images of each other or not. 

In Secs. 4.20 and 5.6, we learned that stereoisomers can be classified not only as to whether or not they are mirror images, but also—and quite independently of the other classification—as to how they are interconverted. Altogether, we have (a) configurational isomers, interconverted by inversion (turning-inside-out) at a chiral center (b) geometric isomers, interconverted—in principle—by rotation about a double bond and (c) conformational isomers, interconverted by rotations about single bonds. 

For convenience, we laid down (Sec. 4.20) the following ground rule for discussions and problems in this book unless specifically indicated otherwise, the terms stereoisomers," ""enantiomers," and ""diastereomers" will refer only to configurational isomers, including geometric isomers, and will exclude conformational isomers. The latter will be referred to as conformational isomers," con-formers, conformational enantiomers, and conformational diastereomers. ... 

The strain energy values that SpartanBuild calculates have another use besides structure refinement. They also can be used to compare the energies of models that share the same molecular formula—that is, stereoisomers or conformational isomers. Allowed comparisons are shown below. Strain energy differences between these pairs of molecules correspond closely to differences in heat of formation and to differences in free energy. SpartanBuild reports strain energies in kcal/mol (1 kcal/mol = 4.184 kj/mol) in the lower left-hand corner of the SpartanBuild window. 

Note that trans- and cw-decalin are stereoisomers and not conformational isomers cw-Decalin is less stable than rran -decalin by 2.7 kcal/mol. 

Conformational isomers (conformers) Stereoisomers at potential energy minima (local or global) having identical constitution and configuration, which differ only in torsion angles. 

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