Double bonds restricted rotation around

There are carbon-carbon double bonds and rotation around them is restricted... 

Interconversion Around a "Partial Double Bond" (Restricted Rotation) At room temperature, a neat sample of dimethylformamide shows two CH3 peaks because the rate of rotation around the hindered partial double bond is slow. At... 

In sulfenamides (R S—NR R ), the cation-radicals keep an nnpaired electron occupying a n orbital. This orbital is localized between the sulfur and nitrogen atoms. As a matter of fact, a slight S=N double bond character exists in the neutral sulfenamides. A consequence of this double bond character is an increase in the energy barrier, which restricts rotation around the S—N bond. Restricted rotation about the S—N bond is known in neutral sulfenamides (Kost and Raban 1990). Notably, the energy barrier to this rotation is greater for the derived cation-radicals when compared to the parent compounds (Bassindale and Iley 1990). 

The restricted rotation around the C=N double bond in oximes (2) gives rise to two possible isomers, 9A and 9B for aldoximes and lOA and lOB for ketoximes. For aldoximes, these are labeled syn (9A) and anti (9B). 

In Table 2 are listed the hydroxylamines, oximes and hydroxamic acids for which we have determined the gas phase structures. We tried to select a representative group in each category. There are two types of oximes, as indicated, aldoximes and ketoximes. Due to restricted rotation around the C=N double bond, these can exist in two isomeric forms (except when R = H for an aldoxime and R = R" for a ketoxime). We have investigated both isomers in nearly every instance. For aldoximes, they are generally labeled syn when the H and OH are on the same side of the double bond and anti when on opposite sides. Note that the ketoximes in Table 2 contain one pair of isomers in which the >C=NOH group is not bonded to two carbons instead one bond is to a chlorine. One of these isomers wiU be of interest in Section B.D in the context of hydrogen bonding vi lone pair—lone pair repulsion. 

In contrast, resonance stabilization is less in an amide because the resonance forms Ai and A2 given below are very different in energy. Nevertheless, because an amide is a resonance hybrid of Ai and A2, it is predicted diat tliere should be some double-bond character in die bond between carbon and nitrogen. This is in fact die case since many amides show restricted rotation around die C-N bond (typical of a 7r bond). Moreover, die nitrogen atom in amides is nearly planar and not very basic, also indicating that the lone pail" is delocalized. 

This is not strictly true. There are some special (and rare) classes of molecules that are chiral because of restricted rotation around single or double bonds that impose a nonplanar and chiral conformation. Substituted allenes are the classic example of axially dissymmetric chiral molecules. 

Because the amide C-N bond has partial double bond character, there is restricted rotation around this bond. Therefore, in addition to the three signals expected for the thiophene protons in the NMR spectrum, there are two broad singulets for the two amide protons since these are both chemically and magnetically non-equivalent. The amide hydrogen atoms are diastereotopic and therefore can be distinguished by the descriptors pro-Z and pro-E. 

The activation energy for a torsional process around a covalent bond depends, among other factors, on the 71-electron density associated with this bond. It is of interest to investigate whether the energy barrier for rotation around a carbon-carbon double bond can be sufficiently reduced to allow the establishment of the dynamic equilibrium between the isomers 26 and 28 in the ground state of the system. The complete equilibrium must also include the transformations 26 29 and 28 30, which are associated with restricted rotation around the C—N bond (Scheme 1). 

N-Substituted 3-aminocrotonic esters (67) tend to adopt a planar or near-planar structure. As a consequence of their push-pull nature these molecules show an increased facility for Z-67-is-67 isomerization around the C=C double bond as well as restricted rotation around the C—N and C—COOR2 single bonds. The IR and XH-NMR spectra of simple 3-(alkylamino)crotonic esters have shown that these substances exist either in the liquid state or in solution as equilibrium mixtures of the Z and E configurations, respectively. The position of the equilibrium is solvent-dependent, and the energy difference between the isomers varies from ca 7.3 kJ mol-1 in non-polar solvents to ca 0.8 kJ mol"1 in dimethyl sulphoxide, the intramolecularly bonded Z-form 68 or 69 being the most stable111-113. 

Note from Fig. 22.7 that the p orbitals on the two carbon atoms in ethylene must be lined up (parallel) to allow formation of the -tr bond. This prevents rotation of the two CH2 groups relative to each other at ordinary temperatures, in contrast to alkanes, where free rotation is possible (see Fig. 22.8). The restricted rotation around doubly bonded carbon atoms means that alkenes exhibit cis-trans isomerism. For example, there are two stereoisomers of 2-butene (Fig. 22.9). Identical substituents on the same side of the double bond are designated cis and those on opposite sides are labeled trans. 

Reoall from Section 1.9B that there is restricted rotation around carbon-carbon double bonds. Maleic acid and fumaric acid are two isomers with vastly different physioal properties and pKg values for loss of both protons. Explain why each of these differenoes occurs. 

There must be restricted rotation around a double bond. 

The amide bond that links different amino acids together in peptides is no different from any other amide bond (Section 24.4). Amide nitrogens are nonbasic because their unshared electron pair is delocalised by interaction with the carbonyl group. This overlap of the nitrogen p orbital with the p orbitals of the carbonyl group imparts a certain amount of double-bond character to the C-N bond and restricts rotation around it. As indicated by the stereo views of alanylserine and serylalanine shown in the previous section, the amide bond is planar and the N-H is oriented 180° to the C=0. 

The presence of a double bond in the hydrocarbon chain gives rise to geometrical isomerism, which is due to restricted rotation around carbon-carbon double bonds and is exemplified by fumaric and maleic acids. 

Cis-trans isomerism (Often called geometric isomerism although this term refers to all stereoisomers) is a form of stereoisomerism and describes the orientation of functional groups at the ends of a bond around which no rotation is possible. Both alkenes and cycloalkanes have restricted rotation around certain bonds. In alkenes, the double bond restricts movement and rotation, as does the looped structure of cycloalkanes. 

Restricted rotation around double bonds is partially responsible for the conformation and hence the activity of many biological molecules that we will study later. 

The C=C Bond and Geometric (cis-trans) Isomerism There are two major structural differences between alkenes and alkanes. First, alkanes have a tetrahedral geometry (bond angles of —109.5°) around each C atom, whereas the double-bonded C atoms in alkenes are. trigonal planar (—120°). Second, the C—C bond allows rotation of bonded groups, so the atoms in an alkane continually change their relative positions. In contrast, the -n bond of the C=C bond restricts rotation, which fixes the relative positions of the atoms bonded to it. 

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