Butane structure

Molecular formulas merely include the kinds of atoms and the number of each in a molecule (as C4H , for butane). Structural formulas show the arrangement of atoms in a molecule (see Fig. 1-1). When unshared electrons are included, the latter are called Lewis (electron-dot) structures [see Fig. 1-1(/)]. Covalences of the common elements—the numbers of covalent bonds they usually form—are given in Table 1-1 these help us to write Lewis structures. Multicovalent elements such as C, O. and N may have multiple bonds, as shown in Table 1-2. In condensed structural formulas all H s and branched groups are written immediately after the C atom to which they are attached. Thus the condensed formula for isobutane [Fig. l-l(f>)) is CH,CH(CH,)... 

Both of the butane structures have 4 carbon atoms and 10 hydrogens, but the atoms are arranged in a different sequence, which means that they will be different compounds with somewhat different properties -different boiling and melting points, for example. 

The alkene with four carbons, however, is a different story. See the two butane structures in Figure 14.15. The two different structures can be drawru They are different because of the location of the double bond. These two structures are isomers, since they have the same molecular formula (C4H8) but a different structure. Remember when drawing organic structures that all carbons must have exactly four bonds. Notice that a number and a hyphen to the left of the name butene indicates the carbon in the chain on which the double bond first appears (counting from the left). The lowest number possible is always used. For example, if we had counted from the right, 1-butene would have been called 3-butene, which is incorrect. 

Decomposition of the metallocycle butane, structures I and II, can lead to the metathesis product however, it can also reductively eliminate a cyclopropane. The energetics are as follows ... 

Engstrom J R, Goodman D Wand Weinberg W H 1986 Hydrogenolysis of n-butane over the (111) and (110)-(1 2) surfaces of iridium a direct correlation between catalytic selectivity and surface structure J. Am. Chem. Soc. 108 4653... 

Figure 7.13 reprinted with permission from Jorgensen W L, R C Binning Jr and B Bigot. Structures md Properties of Organic Liquids u-Butane and 1,2-Dichloroethane and Their Conformational Equilibria. The Journal of the American Chemical Society 103 4393-4399. 1981 American Chemical Society. 

C Binning Jr and B Bigot 1931. Structures and Properties of Organic Liquids n-Butane and 1,2-... 

At Its most basic level separating the total strain of a structure into its components is a qualita tive exercise For example a computer drawn model of the eclipsed conformation of butane using ideal bond angles and bond distances (Figure 3 8) reveals that two pairs of hydrogens are separated by a distance of only 175 pm a value considerably smaller than the sum of their van der Waals radii (2 X 120 pm = 240 pm) Thus this conformation is destabilized not only by the torsional strain associ ated with its eclipsed bonds but also by van der Waals strain... 

We can relate the conformational preference for an equatorial methyl group m methylcyclohexane to the conformation of a noncyclic hydrocarbon we discussed ear her butane The red bonds m the following structural formulas trace paths through four carbons beginning at an equatorial methyl group The zigzag arrangement described by each path mimics the anti conformation of butane... 

Replacing one of them by some different atom or group gives the enantiomer of the structure obtained by replacing the other therefore the methylene hydrogens at C 2 of butane are enantiotopic The same is true for the hydrogens at C 3... 

Example The reaction coordinate for rotation about the central carbon-carbon bond in -butane has several stationary points. A, C, E, and G are minima and B, D, and Fare maxima. Only the structures at the minima represent stable species and of these, the anti conformation is more stable than the gauche. 

As discussed in Sec. 4, the icomplex function of temperature, pressure, and equilibrium vapor- and hquid-phase compositions. However, for mixtures of compounds of similar molecular structure and size, the K value depends mainly on temperature and pressure. For example, several major graphical ilight-hydrocarbon systems. The easiest to use are the DePriester charts [Chem. Eng. Prog. Symp. Ser 7, 49, 1 (1953)], which cover 12 hydrocarbons (methane, ethylene, ethane, propylene, propane, isobutane, isobutylene, /i-butane, isopentane, /1-pentane, /i-hexane, and /i-heptane). These charts are a simplification of the Kellogg charts [Liquid-Vapor Equilibiia in Mixtures of Light Hydrocarbons, MWK Equilibnum Con.stants, Polyco Data, (1950)] and include additional experimental data. The Kellogg charts, and hence the DePriester charts, are based primarily on the Benedict-Webb-Rubin equation of state [Chem. Eng. Prog., 47,419 (1951) 47, 449 (1951)], which can represent both the liquid and the vapor phases and can predict K values quite accurately when the equation constants are available for the components in question. 

Isomers — different structural arrangements with the same chemical formula (e.g., n-butane and t-butane). 

Beginning with the fourth alkane, butane, we find we can draw a structural formula of a compound with four atoms and ten hydrogen atoms in two ways the first is as the normal butane exists and the second is as follows, with the name isobutane (refer to Table 1 for properties). 

The principal components of the cut are butene-1, butene-2, isobutylene and butadiene-1,3. Methyl, ethyl, and vinyl acetylenes, butane and butadiene-1,2 are present in small quantities. Butadiene is recovered from the C4 fraction by extraction with cuprous ammonium acetate (CAA) solution, or by extractive distillation with aqueous acetonitrile (ACN). The former process is a liquid-liquid separation, and the latter a vapor-liquid separation. Both take advantage of differences in structure and reactivity of the various C4 components to bring about the desired separation. 

Sketch a potential energy diagram for rotation around a carbon-carbon bond in propane. Clearly identify each potential energy maximum and minimum with a structural formula that shows the conformation of propane at that point. Does your diagram more closely resemble that of ethane or of butane Would you expect the activation energy for bond rotation in propane to be more than or less than that of ethane Of butane ... 

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