Straight-chain alkane isomers

Isomers are substances having the same molecular formula and molecular weight, but differing in physical and chemical properties. Since branched and straight-chain alkanes with the same molecular formula can exist as distinct structures having different geometrical arrangement of the atoms, they are termed structural isomers. One example is C H,j (butane) which has two isomers ... 

Figure 7.8 Shape-selective reforming. The straight-chain alkane (left) can enter the zeolite pore and penetrate to the catalytic site, whereas the branched-chain isomer cannot.

Less information is available about the cracking of alkanes. Three sources [212,232,233] confirm that, in the series of straight-chain alkanes, the rate increases with the molecular weight. The data could be correlated by the Taft equation when the molecule was divided into the reaction centre and such substituents as R—CH2—CH3 [125]. The reactivity of hexane isomers was studied at 550°C on an A1203—Si02— Zr02... 

Alkanes having a particular molecular formula can exist as different constitutional isomers. For example, the alkane having the molecular formula C4H10can exist as two constitutional isomers-the straight chain alkane (butane) or the branched alkane (2-methylpropane Following fig.). These are different compounds with different physical and chemical properties. 

The naming rules for straight-chain alkanes can, with a few additions, help you recognize and name other organic compounds. You now know that the name of a straight-chain alkane is composed of a root (such as meth-) plus a suffix (-ane). Earlier in the chapter, you saw the isomers of C6Hi4. Figure 13.13 shows one of them, called 2-methylpentane. 

Its structure is different from the structure of a straight-chain alkane. Like many hydrocarbons, this isomer of CeHi4 has a branch-like structure. Alkanes such as 2-methylpentane are called branched-chain alkanes. (The branch is sometimes called a side-chain.)... 

The natiue of the higlier-molecular-weight aliphatic hydrocarbons from the Murchison has been controversial. Kvenvolden et al. [33] detected a wide variety of coeluting isomers dominated by polycyclic aliphatic structures. Oro el al. [41] reported Cg to Cis metliyl and dimetliyl alkanes, alkenes, and cycloalkanes. Studier et al. [42] detected straight-chain alkanes with some isoprenoidal hydrocarbons. It is now widely believed that straight-chain alkanes were the dominant components of the ahphatic fraction in the solar nebula. However, Cronin and Pizzarello [43] analyzed organic materials from the Murchison under less environmental contamination and concluded lliat tire n-alkanes, methyl alkanes, and isoprenoid alkanes reported in the Murchison were terrestrial contaminants. Tliese authors identified Cis to C30 cyclic alkanes as the major indigenous alipliatic components. 

Free radicals, molecules with unpaired electrons, play a large role in most reactions of alkanes, such as cracking and reformation where long-chain alkanes are converted into shorter-chain alkanes and straight-chain alkanes into branched-chain isomers. 

Straight-chain alkanes are sometimes indicated by the prefix n- (for normal) to distinguish them from branched-chain alkanes having the same number of carbon atoms. Although this is not strictly necessary, the usage is still common in cases where there is an important difference in properties between the straight-chain and branched-chain isomers e.g. n-hexane is a neurotoxin while its branched-chain isomers are not. 

A spedal feature of zeolites which makes them such superb catalysts in some cases is their shape selectivity. The shape selectivity may arise in three ways reactant selectivity, product selectivity and, of lesser importance, transition state selectivity. Reactant selectivity arises from the ability of only certain molecules to be absorbed into the zeolite cavities and thus reach the active acid sites. An important commercial process that exploits this type of reactant selectivity is catalytic dewaxing. Compared to the branched isomers, the straight chain alkanes have low octane numbers and contribute to wax formation in diesel fuel. Product selectivity is derived from the fact that only certain products are of the correct dimension to escape from the zeolite once they have been formed. Transition state selectivity relies upon the fact that certain intermediates, which are formed during a chemical reaction at the active site, will not fit in... 

There are three alkanes with molecular formula C5H12. Pentane is the straight-chain alkane. Isopentane, as its name indicates, has an iso stmctural unit and five carbon atoms. The third isomer is called neopentane. The structural unit with a carbon surrounded by four other carbons is called neo. ... 

Certain catalysts can produce branched-chain alkanes from straight-chain alkanes. This process, called isomer ization, is carried out on a iarge scale commercially. 

Figure 11.2 shows the stmctures of the first four alkanes ( = 1 to n = 4). Nat-mal gas is a mixture of methane, ethane, and a small amount of propane. We discussed the bonding scheme of methane in Chapter 10. The carbon atoms in all the alkanes can be assumed to be -hybridized. The structures of ethane and propane are straightforward, for there is oidy one way to join the carbon atoms in these molecules. Butane, however, has two possible bonding schemes resirlting in different com-poimds called n-butane ( stands for normal) and isobirtane. n-Butane is a straight-chain alkane because the carbon atoms are joined in a continuoirs chain. In a branched-chain alkane like isobutane, one or more carbon atoms are bonded to a nonterminal carbon atom. Isomers that differ in the order in which atoms are connected are called structural isomers. 

A second example may be equally important. It is clear from the information in Table 3.3 (which is illustrative rather than exclusive) that, although the effect is small, less heat is obtained from combustion of branched alkanes than from straight-chain alkanes of the same molecular formula (compare, e.g., the isomers of C5H12 or those of CeH ). Since all of the C5H12 isomers, for example,yield the same amount of CO2 and H2O (Equation 3.2) when combustion occurs, the amount of heat liberated must be a reflection of their relative stabilities ... 

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