Isomer, rotational, butane

For example, the isomer of butan-2-ol that rotates the plane of polarized light clockwise is named ( + )-butan-2-ol or tZ-butan-2-ol. Its enantiomer, ( —)-butan-2-ol or Z-butan-2-ol, rotates the plane counterclockwise by exactly the same amount. 

Although the —CH2— group could be inserted in other places, the free rotation about single C—C bonds in hydrocarbons allows the resulting molecules to be twisted into one or the other of these two isomers. Both compounds are gases, but butane (24) condenses at —1CC, whereas methylpropane (25) condenses at — 12°C. Two molecules that differ only by rotation about one or more bonds may look different on paper, but they are not isomers of each other they are different conformations of the same molecule. Example 18.3 illustrates how to tell if two molecules are different isomers or different conformations of the same isomer. 

A mixture containing two enantiomers in equal proportions will have zero optical rotation, as the rotation due to one isomer will be cancelled by the rotation due to the other isomer. Such a mixture is known as racemic mixture or racemic modification. A racemic mixture is represented by prefixing dl or (+) before the name, for example ( ) butan-2-ol. The process of conversion of enantiomer into a racemic mixture is known as racemisation. 

A very common approximation goes by the name of rotational isomeric state model (RIS). With such a model it is assumed that the molecular population is placed exclusively in a few energy minima, always according to the Boltzmann distribution. The conformations corresponding to the minima are called conformers, or rotational isomers, and are indicated by one or more letters (G, T) for butane three conformers are considered (G, T, G ), for pentane five (TT, G T, TG, G G, G G ). A more detailed examination of pentane reveals the existence of two further wells (fl, = 65 , 2 = 260° and 0, = 100°, 02 = 295°), sometimes indicated by G G and G G , respectively, close to the forbidden G G" conformation, but with a much tower energy (157, 158). 

The enthalpy difference befween the AA and the GA conformers in the [C4CiIm][BFJ is much smaller than the corresponding enthalpy difference befween the conformers of a free butane chain. This indicates that the 1-butyl-3-methylimidazolium cations most likely form local liquid structures specific fo each rofafional isomers [50]. Coexistence of these local structures, incorporating different rotational isomers, seems to hinder crystallization. This is probably the reason for fhe low melting points of such ILs. 

Butane (C4H10) can exist in two different isomeric forms, e.g. n-butane and isobutane (2-methylpropane). Open chain alkanes have free rotation about their C—C bonds, hut cycloalkanes cannot undergo free rotation, so substituted cycloalkanes can give rise to cis and trans isomers (see Section 3.2.2). 

In Bo-butane [Fig. 9-46(c) and (d) one isomer (c) has the Cl on the central carbon and there is only one other isomer (d) because the three terminal carbons are identical. A point worth noting is that all of the C to C bonds are single bonds this means that, since there is free rotation about a single bond, all of the hydrogens bonded to a particular carbon are identical. Because of this factor, replacing one H on a carbon with a Cl is the same as replacing any other. 

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. 

Can any manipulation of the model, by twisting or turning of the model or by rotation of any of the bonds, give you the butane system If these two, butane and 2-methylpropane (isobutane), are isomers, then how may we recognize that any two structures are isomers (6g) ... 

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. 

Does this mean that (/ >butan-2-ol is the dextrorotatory isomer because it is named (.R), and (S)-butan-2-ol is levorotatory because it is named (5) Not at all The rotation of a compound, (+) or (-), is something that we measure in the polarimeter, depending on how the molecule interacts with light. The (R) and (S) nomenclature is our own artificial way of describing how the atoms are arranged in space. 

Suppose we had a mixture of equal amounts of ( + )-butan-2-ol and (-)-butan-2-ol. The (+) isomer would rotate polarized light clockwise with a specific rotation of +13.5°, and the (-) isomer would rotate the polarized light counterclockwise by exactly the same amount. We would observe a rotation of zero, just as though butan-2-ol were achiral. A solution of equal amounts of two enantiomers, so that the mixture is optically inactive, is called a racemic mixture. Sometimes a racemic mixture is called a racemate, a ( ) pair, or a (d,l) pair. A racemic mixture is symbolized by placing ( ) or ((1,1) in front of the name of the compound. For example, racemic butan-2-ol would be symbolized by ( )-butan-2-ol or (d,/)-butan-2-ol. ... 

Considerations of minimum overlap of radii of nonbonded substituents on the polymer chain are useful in understanding the preferred conformations of macromolecules in crystallites. The simplest example for our purposes is the polyethylene (1-3) chain in which the energy barriers to rotation can be expected to be similar to those in /i-butane. Figure 4-2 shows sawhorse projections of the conformational isomers of two adjacent carbon atoms in the polyethylene chain and the corresponding rotational energy barriers (not to scale). The angle of rotation is that between the polymer chain substitutents and is taken here to be zero when the two chain segments are as far as possible from each other. 

Single bonds between C, H, O, N, S, P, and halogen are free to rotate. (Exceptions to this generalization will be discussed as the need arises.) This results in different conformations or conformational isomers for the same molecule. The following are two of the different conformations of the compound butane (C4H,o) ... 

The special change that occurs with the longer molecules (n > 4) is connected with their flexibility, i.e., the possible rotation about the interior bonds of the molecule. It becomes first possible in butane with its three conformational isomers, as illustrated in Fig. 1.37. Figure 5.123 shows that for the paraffins with even n from hexane (n = 6) to dodecane (n = 12), there is an increase in AS 20 J K mol for each pair of additional bonds around which rotation is possible. Similar observations can be made for the paraffins with odd numbers of carbon atoms. The difference in absolute level of AS between the odd and even series arises from differences in crystal packing as was analyzed with thermodynamic parameters in Fig. 4.52 for decane. The few paraffins with exceptionally low AS are, at present, not fully understood. Their low AS may be caused by experimental error caused by incomplete crystallization, mobility in the crystal through mesophase formation, or by differences in the packing in the crystal or stmcture in the melt. 

Two conformations of butane (also called rotational isomers)... 

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