Isomeric Alkanes The Butanes

Methane is the lowest boiling alkane, followed by ethane, then propane. 

This will generally be true as we proceed to look at other alkanes as the number of carbon atoms increases, so does the boiling point. All the alkanes with four carbons or less are gases at room temperature and atmospheric pressure. With the highest boiling point of the three, propane is the easiest one to liquefy. We are all familiar with propane tanks. These are steel containers in which a propane-rich mixture of hydrocarbons called liquefied petroleum gas (LPG) is maintained in a liquid state under high pressure as a convenient elean-buming fuel. 

Methane is the only alkane of moleeular formula CH4, ethane the only one that is C2Hg, and propane the only one that is C3H8. Beginning with C4H10, however, constitutional isomers (Seetion 1.8) are possible two alkanes have this particular molecular formula. In one, called -butane, four earbons are joined in a eontinuous chain. The n in n-butane stands for normal and means that the carbon chain is unbranched. The seeond isomer has a branehed earbon chain and is called isobutane. 

As noted earlier (Seetion 1.16), CH3 is called a methyl group. In addition to having methyl groups at both ends, n-butane eontains two CH2, or methylene groups. Isobutane contains three methyl groups bonded to a CH unit. The CH unit is ealled a methine group. 

FIGURE 2.1 Structures of methane, ethane, and propane showing bond distances and bond angles. 

Boiling points cited in this text are at 1 atm (760 mm of mercury) unless otherwise stated. 

Generally speaking, the three models offer complementary information. Organic chemists use all three, emphasizing whichever one best suits a particular feature of stracture or reactivity. Until recently, the Lewis and orbital hybridization models were used far more than the molecular orbital model. But that is changing. 

The Lewis rules are relatively straightforward, easiest to master, and the most familiar. You will find that your ability to write Lewis formulas inaeases rapidly with experience. Get as much practice as you can early in the course. Success in organic chemistry depends on writing correct Lewis formulas. 

Orbital hybridization descriptions, because they too are based on the shared electron-pair bond, enhance the information content of Lewis formulas by distinguishing among various types of atoms, electrons, and bonds. As you become more familiar with a variety of structural types, you will find that the term sp -hybridized carbon triggers associations in your mind that are different from those of some other term, such as sp -hybridized carbon, for example. 

Each half-filled sp orbital overlaps with a half-filled hydrogen Is orbital along a line between them giving a tetrahedral arrangement of four a bonds. Only the major lobe of each sp orbital is shown. Each orbital contains a smaller back lobe, which has been omitted for clarity. 

The orbital hybridization model accounts for carbon having four bonds rather than two, the bonds are stronger than they would be in the absence of hybridization, and they are arranged in a tetrahedral fashion around carbon. 

Describe the bonding in propane according to the orbital hybridization model. 

We will return to the orbital hybridization model to discuss bonding in other aliphatic hydrocarbons—alkenes and alkynes—later in the chapter. At this point, however, weTl explore alkanes as a class in more detail. 

---

Isomeric Alkanes The Butanes 65 Higher n-Alkanes 66 The C5H,2 Isomers 66 lUPAC Nomenclature of Unbranched Alkanes 68... 

Two constitutionally isomeric alkanes have the molecular formula C4H10 One has an unbranched chain (CH3CH2CH2CH3) and is called n butane, the other has a branched chain [(CH3)3CH] and is called isobutane Both n butane and isobutane are common names... 

When the reactions of alkane molecules larger than the butanes or neopentane are studied, and in particular when the molecule is large enough to form a Cs or a Ce ring, the complexity of the reaction pathway is considerably increased and an important feature is the occurrence, in addition to isomerization product, of important amounts of cyclic reaction products, particularly methylcyclopentane, formed by dehydrocycliza-tion this suggests the existence of adsorbed cyclic species. The question is whether the reaction paths for dehydrocyclization and isomerization are related. There is convincing evidence that they are. Skeletal interconversions involving n-hexane, 2- and 3-methylpentane may be represented. 

Low-molecular-weight hydrocarbons (C4 and C5 alkanes) usually undergo isomerization through a simple bond shift. The transformation of [l-l3C]-butane, for instance, yields isobutane via skeletal isomerization and the isotopomer [2-13C]-butane 155... 

According to an early report, sulfated zirconia promoted with 1.5% Fe and 0.5% Mn increased the rate of isomerization of n-butane to isobutane by several orders of magnitude at modest temperature (28°C).299 This reactivity is surprising, since the isomerization of n-butane in strong liquid acids takes place at a rate much lower than that of higher alkanes, which is due to the involvement of the primary carbocationic intermediate. In addition, other solid acids, such as zeolites, did not show activity under such mild conditions. Evidence by isotope labeling studies with double-labeled n-butane unequivocally shows, however, that the isomerization of... 

The skeletal isomerization of straight-chain paraffins is important for the enhancement of the octane numbers of light petroleum fractions. The isomerization of H-butane to isobutane has attracted much attention because isobutane is a feedstock for alkylation with olefins and MTBE synthesis. It is widely believed that the low-temperature transformation of n-alkanes can be catalyzed only by superacidic sites, and this reaction has often been used to test for the presence of these sites. 

Anderson and his co-workers examined the reactions of small alkanes mainly on platinum and palladium 45-48). Isobutane was isomerized to n-butane on platinum and on palladium, neopentane isomerized to isopentane on platinum, whereas other metals (including palladium) caused hydrogenolysis predominantly or exclusively. It was proposed that the slow step in the isomerization was the formation of a bridged intermediate (C) from an aory-triadsorbed species (A, B) (Fig. 11).1 Hiickel MO calculations based on this proposal suggested,... 

Very recent work (60b) has confirmed that Ir films do not isomerize neopentane most of the transition metals as well as palladium (60c) rearrange isobutane to k-butane but are also inactive for the former conversion. This clearly indicates that isomerization of neopentane on Pt is mechanistically rather special and, in view of the known propensity of Pt to promote ay exchange with deuterium of paraffins (5,49), refocuses attention on the ay species diadsorbed on one metal atom as the precursor for bond shift in simple alkanes. The following mechanism for neopentane isomerization on Pt is feasible, where the shifting... 

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. 

Moreover, the different yields of CH3T (and of C2H5T) from the protonation of butane and isobutane and the fact that no isomerization of the tritiated C4 alkanes takes place, were taken as evidence against... 

But nearly every alkane can have a number of isomeric structures, and there must be an unambiguous name for each of these isomers. The butanes and pentanes are distinguished by the use of prefixes w-butane and isobutan penta, isopentane and neopentane. But there are 5 hexane T lTieptanes, and 75 decanes it would be diflicnli to devise, and even more difficult to remember, a different prefix for each of these isomers. It is obvious that some systematic method of naming is needed. 

Sommer and coworkers have made important observations with respect to the activation of alkanes over sulfated zirconia, a new type of solid superacid. Whereas isotope exchange of small alkanes occurs with the involvement of the corresponding pentacoordinate ions, a classical carbenium ion-type mechanism was found to be operative for larger homologs (propane, isobutane). The exception is the isomerization of n-butane over sulfated zirconia promoted by Pt and alumina, where the initiation step for isomerization was suggested to be the protolysis of the C-H bond. ... 

---