Bond rotation, alkanes

Carbon-carbon single bonds in alkanes are formed by a overlap of carbon sjy hybrid orbitals. Rotation is possible around a bonds because of their cylindrical... 

A cycloalkane is a saturated cyclic hydrocarbon with the general formula C H2 . In contrast to open-chain alkanes, where nearly free rotation occurs around C, -C bonds, rotation is greatly reduced in cycloalkanes. Disubstituted cycloalkanes can therefore exist as cis-trans isomers. The cis isomer has both substituents on the same face of the ring the trans isomer has substituents on opposite faces. Cis-trans isomers are just one kind of stereoisomers—isomers... 

Some comments on the liquid state conformations of n-alkanes are made. Calculated are the average probability

and number of gauche bond rotations assumed by internal bonds, and the probability Pg(central) that the central bond in each of the even members (Cg, C3,...,C22) is in a gauche rotational state. 

As we have seen, the anomeric effect confers a double-bond character to each C—0 bond of conformer D the energy barrier for a C —0 bond rotation in acetals must therefore be higher than that observed in simple alkanes. Borgen and Dale (41) may have provided the first evidence for this point by observing that 1,3,7,9-tetraoxacyclododecane (37) has a much higher conformational barrier (11 kcal/mol) than comparable 12-membered rings such as cyclododecane (7.3 kcal/mol (42) or 1,4,7,10-tetraoxacyclododecane (5.5 and 6.8 kcal/mol (43)). It was also shown that the two 1,3-dioxa groupings in 37 exist in a conformation identical to that of dimethoxymethane, i.e. the conformation D. 

There is free rotation around carbon-carbon single bonds. Therefore, alkane chains are quite flexible and can adopt a large number of conformations. For example, two conformations of butane are shown in Figure 11.5. The first structure converts into the second by rotation around the C2-C3 bond. These two structures are not isomers of each other because it is not possible to separate them. 

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. 

In contrast to the rotational freedom around single bonds in open-chain alkanes, there is much less freedom in cycloalkanes. Cyclopropane, for example, must be a rigid, planar molecule (three points define a plane). No bond rotation can take place around a cyclopropane carbon-carbon bond without breaking open the ring (Figure 3.8, p. 100). 

Ethane is the prototype molecule for rotation around carbon-carbon single bonds in alkanes, and has therefore probably been the most frequently studied molecule with respect to hindered rotation. 

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... 

Chemists don t agree on the minimum energy barrier for bond rotation that allows isolation of enantiomeric atropisomers at room temperature, but it is on the order of 100 kJ/mol (24 kcal/mol). Recall that the activation energy for rotation about C—C single bonds in alkanes is about 12 kJ/mol (3 kcal/mol). 

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°. 

Many of the major changes in molecular conformations are due to bond rotations. The torsion interactions account for the rotation around bonds of four adjacent sites or the motion of dihedral angles. The torsional potentials are 27t-periodic and symmetric at 0 and n. For alkanes, the Ryckaert and Belleman [55] torsional potential is often used ... 

Rotation around Carbon-Carbon Bonds in Alkanes Ethane... 

In Chapter 5 (Section 5.1.1.2), 2-octene was named, but it was noted, without explanation, that this molecule has another isomer. Remember that the C=C unit is rigid and there is no rotation about that bond as there is around C-C single bonds in alkanes (see Chapter 8, Section 8.1). Once attached to the C=C, a group is locked on one side of the double bond or the other, leading to different isomers. These particular isomers are discussed in Section 9.4. 

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