Double bonds rotation restriction

In principle cis 2 butene and trans 2 butene may be mterconverted by rotation about the C 2=C 3 double bond However unlike rotation about the C 2—C 3 single bond in butane which is quite fast mterconversion of the stereoisomeric 2 butenes does not occur under normal circumstances It is sometimes said that rotation about a carbon-carbon double bond is restricted but this is an understatement Conventional lab oratory sources of heat do not provide enough energy for rotation about the double bond m alkenes As shown m Figure 5 2 rotation about a double bond requires the p orbitals of C 2 and C 3 to be twisted from their stable parallel alignment—m effect the tt com ponent of the double bond must be broken at the transition state... 

Geometrical Isomerism. Rotation about a carbon-carbon double bond is restricted because of interaction between the p orbitals which make up the pi bond. Isomerism due to such restricted rotation about a bond is known as geometric isomerism. Parallel overlap of the p orbitals of each carbon atom of the double bond forms the molecular orbital of the pi bond. The relatively large barrier to rotation about the pi bond is estimated to be nearly 63 kcal mol (263 kJ mol-i). 

Maleic andfumaric acids were known in the nineteenth century to have the same chemical composition and the same functional groups and yet they were different compounds—why remained a mystery. That is, until 1874 when van t Hoff proposed that free rotation about double bonds was restricted. This meant that, whenever each carbon atom of a double bond had two different... 

Restricted rotation The rotation around the C - C double bond is restricted. Rotation can only occur if the n bond breaks and then re-forms, a process that is unfavorable (Section 8.2B). 

The sp hybridized carbon atoms of alkenes (olefins) and the atoms or groups attached to these carbons all lie in the same plane, and rotation around the double bond is restricted. As a result, stereoisomerism is possible when each carbon atom of the double bond is asymmetrically substituted. Because geometric isomers are non-superimposable, non-mirror images they... 

Rotation around the double bond is restricted, and substituted alkenes can therefore exist as cis-trans stereoisomers. The geometry of a double bond can be specified by application of the Cahn-Ingold-Prelog sequence... 

Figure 21-9 Two isomers of 1,2-dichloroethene are possible because rotation about the double bond is restricted. This is an example of geometric isomerism. A ball-and-stick model and a space-filling model are shown for each isomer, (a) The cis isomer, (b) The tram isomer.

Rotation around the double bond is restricted because the tt bond would have to be broken to allow rotation. Thus, the double bond is rigid. 

Maleic and fumaric acids were known in the nineteenth century to have the same chemical composition and the same functional groups, and yet they were different compounds—why remained a mystery. That is, until 1874 when van t Hoff proposed that free rotation about double bonds was restricted. This meant that, whenever each carbon atom of a double bond had two different substituents, isomers would be possible. He proposed the terms cis (latin meaning on this side ) and trans (Latin meaning across or on the other side ) for the two isomers. The problem was which isomer was which On heating, maleic acid readily loses water to become maleic anhydride so this isomer must have both acid groups on the same side of the double bond. 

SECTION 24.3 The names of alkenes and alkynes are based on the longest continuous chain of carbon atoms that contains the multiple bond, and the location of the multiple bond is specified by a numerical prefix. Alkenes exhibit not only structural isomerism but geometric (cis-tmiis) isomerism as well. In geometric isomers the bonds are the same, but the molecules have different geometries. Geometric isomerism is possible in alkenes because rotation about the C=C double bond is restricted. 

In alkenes, the double-bonded carbons and the four groups attached to these carbons lie in the same plane. Because rotation about the double bond is restricted, alkenes may exist as geometric, or cis-trans, isomers. This type of stereoisomerism is possible when each double-bonded carbon is attached to two different groups. 

In principle, c/5-2-butene and ra/2 -2-butene may be interconverted by rotation about the C-2—C-3 double bond. However, unlike rotation about single bonds, which is quite fast, rotation about double bonds is restricted. Interconversion of the cis and trans isomers of 2-butene has an activation energy which is 10-15 times greater than that for rotation about the single bond of an alkane and does not occur under normal circumstances. 

The expected length of a single C-N bond is 1.45 A, as in the C -N bond, and that of a C = N double bond is 1.25 A. The actual length of the C -N peptide bond is 1.33 A, showing that it has partial double bond characteristics (40% double bond). Rotation can occur, in principle, around all three bonds [j/, q>, and w, where j/ = (p = w= %Q°. This means that for a protein of 100 residues there are 2 x 10 possible conformations, far more possible conformations than there would be protein molecules, even in a large sample. However, we know that a folded protein has a relatively stable conformation. This is due to many factors, one being the partial double bond characteristics of the C -N peptide bond that limits it to a trans conformation (with the exception of proline), the atoms of the side chains restrict bond rotation due to excluded volume effects that dictate... 

The orbital model explains the facts about double bonds listed in Table 3.1. Rotation about a double bond is restricted because, for rotation to occur, we would have to break the pi bond, as seen in Figure 3.6. For ethylene, it takes about 62 kcal/mol (259 kj/mol) to break the pi bond, much more thermal energy than is available at room temperature. With the pi bond intact, the sp orbitals on each carbon lie in a single plane. The 120° angle between those orbitals minimizes repulsion between the electrons in them. Finally, the carbon-carbon double bond is shorter than the carbon-carbon single bond because the two shared electron pairs draw the nuclei closer together than a single pair does. 

Because rotation at carbon-carbon double bonds is restricted, cis-trans isomerism (geometric isomerism) is possible in appropriately substituted alkenes. For example, 1,2-dichloroethene exists in two different forms ... 

Because rotation about a double bond is restricted, an alkene can exist as cis-trans isomers. The cis isomer has its hydrogens on the same side of the double bond the trans isomer has its hydrogens on opposite sides of the double bond. 

Figure B2.4.1. Proton NMR spectra of the -dimethyl groups in 3-dimethylamino-7-methyl-l,2,4-benzotriazine, as a fiinction of temperature. Because of partial double-bond character, there is restricted rotation about the bond between the dunethylammo group and the ring. As the temperature is raised, the rate of rotation around the bond increases and the NMR signals of the two methyl groups broaden and coalesce.

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