The Alkanes

FIGURE 7.2 The two —CH3 groups in ethane rotate easily about the bond that joins them. 

FIGURE 7.3 Two of the many possible conformations of the alkane CieH34. The carbon atoms are not shown explicitly, but they lie at the black intersections. Hydrogen atoms are at the white ends. Eliminating the spheres representing atoms in these tube (or Dreiding) models reveals the conformations more clearly. 

A mixture of hydrocarbons such as petroleum does not boil at a single, sharply defined temperature. Instead, as such a mixture is heated, the compounds with lower boiling points (the most volatile) boil off first, and as the temperature increases, more and more of the material vaporizes. The existence of a boiling-point range permits components of a mixture to be separated by distillation (see discussion in Section 11.6). The earliest petroleum distillation was a simple batch process The crude oil was heated in a still, the volatile fractions were removed at the top and condensed to gasoline, and the still was cleaned for another batch. Modern petroleum refineries use much more sophisticated and efficient distillation methods, in which crude oil is added continuously and fractions of different volatility are tapped off at various points up and down the distillation column (Fig. 7.5). To save on energy costs, heat exchangers capture the heat liberated from condensation of the liquid products. 

Distillation allows hydrocarbons to be separated by boiling point, and thus by molecular mass. A mixture of gases emerges from the top of the column, resembling the natural gas that collects in rock cavities above petroleum deposits. These gas mixtures contain ethane, propane, and butane, which can be separated further by redissolving them in a liquid solvent such as hexane. The methane-rich mixture of gases that remains is used for chemical synthesis or is shipped by pipeline to 

FIGURE 7.4 The melting and boiling points of the straight-chain alkanes increase with chain length n. Note the alternation in the melting points Alkanes with n odd tend to have lower melting points because they are more difficult to pack into a crystal lattice. 

---

The largest series is that of the alkanes or paraffins, which are open chain molecules with saturated bonds, and have the general formula C H2n+2-... 

Figure 5.13 Examples from the alkane (paraffin) series...

Under standard conditions of temperature and pressure (STP), the first four members of the alkane series (methane, ethane, propane, and butane) are gases. As length of the carbon increases the density of the compound increases (pentane) to C yHgg are liquids, and from C.,gH3g, the compounds exist as wax-like solids at STP. 

The most common prefixes are written below using the alkane series as an example, and the prefixes are highlighted ... 

Alkanes from CH to C4gFlg2 typically appear in crude oil, and represent up to 20% of the oil by volume. The alkanes are largely chemically inert (hence the name paraffins, meaning little affinity), owing to the fact that the carbon bonds are fully saturated and therefore cannot be broken to form new bonds with other atoms. This probably explains why they remain unchanged over long periods of geological time, despite their exposure to elevated temperatures and pressures. 

Olefins are uncommon in crude oils due to the high chemical activity of these compounds which causes them to become saturated with hydrogen. Similarly, acetylene is virtually absent from crude oil, which tends to contain a large proportion of the saturated hydrocarbons, such as the alkanes. 

There are a total of eighteen different hydrocarbon series, of which the most common constituents of crude oil have been presented - the alkanes, cycloalkanes, and the arenes. The more recent classifications of hydrocarbons are based on a division of the hydrocarbons in three main groups alkanes, naphthanes and aromatics, along with the organic compounds containing the non-hydrocarbon atoms of sulphur, nitrogen and oxygen. 

Alkanethiols and other sulfur-bearing hydrocarbons covalently attach to metal surfaces alkanethiol onto gold is the most widely studied of these systems [27-29,31,32,45]. These SAMs are ordered provided the alkane chain contains nine or more carbons [32]. Binary solutions of two alkanethiols also appear... 

The initial step in alkane hydrogenolysis is the dissoeiative adsorption, or reaetive stieking of the alkane. One might suspeet that this first step may be the key to the stnieture sensitivity of this reaetion over Ni surfaees. Indeed, the reaetive stieking of alkanes has been shown to depend markedly on surfaee stnieture... 

Silicon, unlike carbon, does notiorm a very large number of hydrides. A series of covalently bonded volatile hydrides called silanes analogous to the alkane hydrocarbons is known, with the general formula Si H2 + 2- I uf less than ten members of the series have so far been prepared. Mono- and disilanes are more readily prepared by the reaction of the corresponding silicon chloride with lithium aluminium hydride in ether ... 

The bond matrix expresses 2 C—C bonds plus 8 C—H bonds for propane and 3 C—C bonds plus 10 C—H bonds for n-butane. Eaeh enthalpy of atomization is obtained by subtraeting the enthalpy of formation of the alkane from the sum of atomie atomization enthalpies (C 716 H 218 kJ mol ) for that moleeule. For example, the moleeular atomization enthalpy of propane is 3(716) +8(218) — (—104) = 3996 kJ mol . Enthalpies of formation are available from Pedley et al. (1986) or on-line at www.webbook.nist.gov. 

The hydrogen atom attached to an alkane molecule vibrates along the bond axis at a frequency of about 3000 cm. What wavelength of electromagnetic radiation is resonant with this vibration What is the frequency in hertz What is the force constant of the C II bond if the alkane is taken to be a stationary mass because of its size and the H atom is assumed to execute simple harmonic motion ... 

Much of the communication between insects involves chemical messengers called pheromones A species of cockroach secretes a substance from its mandibular glands that alerts other cockroaches to its presence and causes them to congregate One of the principal components of this aggregation pheromone is the alkane shown in the bond line formula that follows Give the molecular formula of this substance and represent it by a condensed formula... 

The lUPAC rules assign names to unbranched alkanes as shown m Table 2 2 Methane ethane propane and butane are retained for CH4 CH3CH3 CH3CH2CH3 and CH3CH2CH2CH3 respectively Thereafter the number of carbon atoms m the chain is specified by a Latin or Greek prefix preceding the suffix ane which identifies the com pound as a member of the alkane family Notice that the prefix n is not part of the lUPAC system The lUPAC name for CH3CH2CH2CH3 is butane not n butane... 

As numbered on the structural formula the longest continuous chain contains eight car bons and so the compound is named as a derivative of octane Numbering begins at the end nearest the branch and so the ethyl substituent is located at C 4 and the name of the alkane is 4 ethyloctane... 

The longest continuous chain in the alkane shown is SIX carbons... 

The alkane bears two methyl groups and an ethyl group It IS an ethyldimethylhexane... 

Two different chains contain five carbons in the alkane... 

All the parts of this problem refer to the alkane having the carbon skeleton shown... 

Consider three compounds similar m size and shape the alkane propane the alkyl halide fluoroethane and the alcohol ethanol... 

The rest of this chapter describes a completely different method for preparing alkyl halides one that uses alkanes as reactants It involves substitution of a halogen atom for one of the alkane s hydrogens... 

The alkane is said to undergo fluonnation chlorination bromination or wdinatwn according to whether X2 is F2 CI2 Bi2 or I2 respectively The general term is halo genation Chlorination and hromination are the most widely used... 

In both the following exercises assume that all the methylene groups in the alkane are equally reactive as sites of free radical chlorination... 

The uncatalyzed addition of hydrogen to an alkene although exothermic is very slow The rate of hydrogenation increases dramatically however m the presence of cer tain finely divided metal catalysts Platinum is the hydrogenation catalyst most often used although palladium nickel and rhodium are also effective Metal catalyzed addi tion of hydrogen is normally rapid at room temperature and the alkane is produced m high yield usually as the only product... 

Step 4 The second hydrogen atom is transferred forming the alkane The sites on the catalyst surface at which the reaction occurred are free to accept additional hydrogen and alkene molecules... 

Substitutive lUPAC nomenclature names epoxides as epoxy derivatives of alkanes According to this system ethylene oxide becomes epoxyethane and propylene oxide becomes 1 2 epoxypropane The prefix epoxy always immediately precedes the alkane ending it is not listed m alphabetical order like other substituents... 

The alkane formed by hydrogenation of (S) 4 methyl 1 hexyne is optically active but the one formed by hydrogenation of (S) 3 methyl 1 pentyne is not Explain Would you expect the products of hydrogenation of these two compounds in the presence of Lindlar palladium to be optically active" ... 

Alkanes. The saturated open-chain (acyclic) hydrocarbons (C H2 +2) have names ending in -ane. The first four members have the trivial names methane (CH4), ethane (CH3CH3 or C2H5), propane (C3H8), and butane (C4Hjo). For the remainder of the alkanes, the first portion of the name... 

Univalent radicals derived from saturated unbranched alkanes by removal of hydrogen from a terminal carbon atom are named by adding -yl in place of -ane to the stem name. Thus the alkane... 

Bivalent radicals derived from saturated unbranched alkanes by removal of two hydrogen atoms are named as follows (1) If both free bonds are on the same carbon atom, the ending -ane of the hydrocarbon is replaced with -ylidene. However, for the first member of the alkanes it is methylene... 

Monocyclic Aliphatic Hydrocarbons. Monocyclic aliphatic hydrocarbons (with no side chains) are named by prefixing cyclo- to the name of the corresponding open-chain hydrocarbon having the same number of carbon atoms as the ring. Radicals are formed as with the alkanes, alkenes, and alkynes. Examples ... 

---