Bond rotation, alkanes alkenes

Cis/Trans Isomerism There is normally free rotation around the carbon-carbon single bonds in alkanes. The alkene functional group has two carbon-carbon bonds. The introduction of the second bond freezes rotation around the... 

In addition to the locations of the double bonds, another difference of alkenes is the molecule s inability to rotate at the double bond. With alkanes, when substituent groups attach to a carbon, the molecule can rotate around the C-C bonds in response to electron-electron repulsions. Because the double bond in the alkene is composed of both sigma and pi bonds, the molecule can t rotate around the double bond (see Chapter 6). What this means for alkenes is that the molecule can have different structural orientations around the double bond. These different orientations allow a new kind of isomerism, known as geometrical isomerism. When the non-hydrogen parts of the molecule are on the same side of the molecule, the term cis- is placed in front of the name. When the non-hydrogen parts are placed on opposite sides of the molecule, the term trans- is placed in front of the name. In the previous section, you saw that the alkane butane has only two isomers. Because of geometrical isomerism, butene has four isomers, shown in Figure 19.12. 

Molecules react because they move. Atoms have (limited) movement within molecules— you saw in Chapter 3 how the stretching and bending of bonds can be detected by infrared spectroscopy, and we explained In Chapter 4 how the a bonds of alkanes (but not the n bonds of alkenes) rotate freely. On top of that, in a liquid or a gas whole molecules move around continuously. They bump into each other, into the walls of the container, maybe into solvent... 

An important difference between alkanes and alkenes is the degree of flexibility of the carbon—carbon bonds in the molecules. Rotation around single carbon—carbon bonds in alkanes occurs readily at room temperature, but the carbon—carbon double bond in alkenes is strong enough to prevent free rotation about the bond. Consider ethene (GgH ). Its six atoms lie in the same plane, with bond angles of approximately 120°. 

As these two examples demonstrate, the names of alkanes end with -ene and nnmbers are nsed to indicate the position of the double bond. The parent chain is nnmbered so that the lowest nnmber possible is given to one of the carbon atoms in the donble bond, regardless of any other substitnents present in the compound (for example, alkyl gronps or halides). The numbers in the names of alkenes refer to the lowest numbered carbon atom in the chain that is part of the C=C bond of the alkene. The name butene means that there are four carbon atoms in the longest chain. Because of restricted rotation abont the carbon-carbon double bond, alkenes can form geometric isomers (Section 4.4). In this case, the name of an alkene must also specify whether the isomer is cis or trans ... 

E. ROTATION ABOUT SIGMA (a) BONDS IN ACYCLIC ALKANES, ALKENES, ALKYNES, AND ALKYL-SUBSTITUTED ARENES... 

The C—C bond allows rotation of bonded groups, so the atoms in an alkane continually change their relative positions in contrast, the tt bond of the alkene C=C bond restricts rotation, which fixes the relative positions of the atoms bonded to it. 

We know that free rotation around carbon—carbon single bonds is fast at room temperature (Section 4.11). Therefore, alkanes can exist in many conformations. Free rotation does not occur around the carbon—carbon double bond of an alkene at room temperature because of its 7t bond, which forms by side-by-side overlap of two 2p orbitals. About 240 kj mole is required to break a it bond (Figure 5.2). This quantity is the difference between the bond dissociation energies of a carbon—carbon double bond and a carbon—carbon single bond. 

The C=C group and all four atoms attached to it lie in the same plane and are locked into that arrangement by the resistance to twisting of the TT-bond (Fig. 18.7). Because alkene molecules cannot roll up into a ball as compactly as alkanes or rotate into favorable positions, they cannot pack together as closely as alkanes so alkenes have lower melting points than alkanes of similar molar mass. 

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. 

The presence of the k bond confers properties on an alkene that mark it out as different from an alkane. In particular, the n bond, by the nature of its sideways overlap of the constituent p orbitals, is weaker than a a bond. Moreover, the electrons of the n bond are relatively exposed, above and below the plane of the alkene. These electrons are the source of reactivity of the alkene toward electrophiles, as in, say, electrophilic addition of bromine (Chapter 4). The n bond in ethene (and other alkenes) is, however, sufficiently strong that it prevents rotation around the carbon-carbon a bond, which is a well-documented property of the carbon-carbon bond in ethane (Section 1.6). The bonding between sp2... 

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. 

For larger alkenes, hyperconjugation with an alkyl group a to an olefinic carbon atom eliminates the need for rotation, so the radical cations of almost all alkenes other than ethene are planar. One-electron oxidation of alkanes leads to a-radical cations. Such ionization removes an electron from an orbital associated with o bonding among carbon atoms (Scheme 2.44). 

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 ... 

The effect of introducing sp -hybridized atoms into open-chain molecules has been discussed previously, and it has been noted that the torsional barriers in 1-alkenes and in aldehydes and ketones are smaller than those in alkanes. Similar properties carry over to incorporation of sp centers in six-membered rings. Whereas the free energy of activation for ring inversion in cyclohexane is 10.3 kcal/mol, the barrier is reduced to 7.7 kcal/mol in methylenecyclohexane, and to 4.9 kcal/mol in cyclohexanone." The decrease in activation energy is related to the lower torsional barriers for rotation about sp -sp bonds, and to the decreased steric requirements of a carbonyl or methylene group. 

These 7t bonds, composed of overlapping 2p orbitals, have substantial impact upon the shape (stereochemistry) of the molecule. Alkenes can exist in cis (Z) or trans ( ) forms—they have sides. Unlike the alkanes, which contain only O bonds which have very low barriers to rotation, there is a substantial barrier... 

The two areas of orbital overlap in the n bond (above and below the plane of the molecule) cause ethene to be a rigid molecule there is no rotation around the carbon—carbon bond. This is in contrast to ethane, and other non-cyclic alkanes, and leads to the existence of cis-trans (geometric) isomerism in alkenes (Chapter 20). 

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