CIS–TRANS ISOMERISM

CIS–TRANS ISOMERISM Compounds in which rotation is restricted may exhibit cis–trans isomerism. These compounds do not rotate the plane of polarized light (unless they also happen to be chiral), and the properties of the isomers are not identical. The two most important types are isomerism resulting from double bonds and that resulting from rings.

Cis–Trans Isomerism Resulting from Double Bonds

Cis–Trans Isomerism Resulting from Double Bonds Cahn–Ingold–Prelog system for ranking group (Z) (for the German word zusammen meaning together) is (E) (for entgegen meaning opposite)

Cis–Trans Isomerism Resulting from Double Bonds This type of isomerism is also possible with other double bonds, such as C=N,N=N, or even C=S,220 although in these cases only two or three groups are connected to the double-bond atoms. In the case of imines, oximes, and other C=N compounds, if W= Y, 66 may be called syn and 67 anti, although (E) and (Z) are often used here too. In azo compounds, there is no ambiguity. Compound 68 is always syn or (Z) regardless of the nature of W and Y.

Cis–Trans Isomerism Resulting from Double Bonds

Cis–Trans Isomerism and Physical Properties Since they are diastereomers, cis–trans isomers always differ in properties; the differences may range from very slight to considerable. Since trans isomer generally have more symmetry than cis isomers, they in most cases have higher melting points and lowersolubilities in inert solvents. The cis isomer usually has a higher heat of combustion, which indicates a lower thermochemical stability. Other noticeably different properties are densities, acid strengths, boiling points, and various types of spectra, but the differences are too involved to be discussed here. It is also important to note that trans-alkenes are often more stable than cis alkenes due to diminished steric hindrance ,but this is not always the case.

Cis–Trans Isomerism and Physical Properties

Cis–Trans Isomerism of Monocyclic Compounds The presence of a ring, like that of a double bond, prevents rotation. Cis and trans isomers are possible whenever there are two carbons on a ring, each of which is substituted by two different groups. The two carbons need not be adjacent. In some cases, the two stereoisomers can interconvert. In cis- and trans-disubstituted cyclopropanones, for example, there is reversible interconversion that favors the more stable trans isomer. This fluxional isomerization occurs via ring opening to an unseen oxyallyl valence bond isomer.

Cis–Trans Isomerism of Monocyclic Compounds The reference group is indicated by the symbol r. Three stereoisomers named according to this system are c-3,c-5-dimethylcyclohexan-r-1-ol (74), t-3,t-5-dimethylcyclohexan-r- 1-ol (75), c-3,t-5-dimethylcyclohexan-r-1-ol (76), and r-2,c-4-dimethyl-t-6-ethyl-1,3-dioxane (77).

Cis–Trans Isomerism of Fused and Bridged Ring Systems

Isomerism of Fused and Bridged Ring Systems When the two bridges that do not contain the substituent are of unequal length, the rule generally followed is that the prefix endo- is used when the substituent is closer to the longer of the two unsubstituted bridges; the prefix exo- is used when the substituent is closer to the shorter bridge If one of the two bridges contains a functional group, the endo isomer is the one in which the substituent is closer to the functional group

Out–In Isomerism Another type of stereoisomerism, called out–in isomerism (or in–out), is found in salts of tricyclic diamines with nitrogen at the bridgeheads. In medium-sized bicyclic ring systems, in–out isomerisim is possible, and the bridgehead nitrogen atoms adopt whichever arrangement is more stable. (81) favors the out–out isomer, (82) the in,in 1 (83) has nearly planar nitrogen atoms, and 1,9-diazabicyclo[7.3.1]tridecane (84) is in,out.