Single bond, rotational isomerism relative

Rotational isomerism relative to a single bond is illustrated by ethane and 1,2-dichloroethane, both depicted in Figure 3-4. First, take the ethane molecule, H3C-CH3. During a complete rotation of one methyl group around the C-C bond relative to the other methyl group,... 

The concept of cis-trans (Z-E) isomerism, originally used for the description of the relative geometry of olefins, has been extended to many other functions which feature a double bond character (pseudo double bonds), such as amides, as well as single bonds with a partial or complete limited rotation due to steric or stereoelec-tronic effects. Cis-trans isomerization (CTI) therefore exists in non-re-bonded or overcrowded molecules that switch from a given stable conformational state to another. This is the case of biaryl compounds which have been utilized in organic chemistry as the basis of molecular switches and rotors [1,2]. Nature has also exploited CTI of single bonds to increasing molecular diversity, in particular with the bulky thyroxin, a thyroid hormone, and the well-known disulfide bond which plays a critical role in the structure of peptides and in the conformation of proteins. 

In addition to geometry, alkenes also differ from open-chain alkanes in that the double bonds prevent the relatively free rotation that is characteristic of carbon atoms bonded by single bonds. As a result, alkenes can exhibit geometric isomerism, the same type of stereoisomerism seen earlier for the cycloalkanes (Section 1.9). There are two geometric isomers of 2-butene ... 

Cis/trans double bond isomerization in enones is also possible if there is a suitable proton available to be removed to give the dienol/dienolate. So if 17.22 is treated with base, a proton is removed from the y-position to give the dienolate anion (Figure 17.13). In this intermediate, the bond between the a- and p-carbon atoms is a single bond and so may rotate freely the position of the equilibrium will depend on the relative stabilities of the two species, in this case determined by steric factors. 

Halophenols constitute the simplest structural model for the analysis of an intramolecular hydrogen bond since the syn-anti isomerization involves only the rotation of the OH proton around the single C—O bond. Furthermore, the two forms coexist in apolar solvents and give two characteristic absorptions in the OH stretching region of the IR spectrum with the notable exception of the fluoro derivative. In a series of papers Okuyama and Ikawa ° "° have re-examined by FTIR the relative stability of the syn-anti isomers by varying the temperature and the pressure. The enthalpies of isomerization are reported in Table 10. 

The stereochemistry of acetylcholine resides in the different arrangements in space of its atoms by virtue of rotation about o bonds (i.e., conformational isomerism). Because of relatively unrestricted rotation about these single covalent bonds, acetylcholine can exist in an infinite number of conformations. Most studies of the conformational isomerism of acetylcholine have focused on torsion angles between the ester oxygen atom and the quaternary nitrogen resulting from rotation about the Ca-C 3 bond. Four of these conformations are illustrated by Newman projections in Fig. 12.10. 

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