Branched-chain alkanes isomers

Branched-chain alkanes do not exhibit the same smooth gradation of physical properties as do the continuous-chain alkanes. Usually there is too great a variation in molecular structure for regularities to be apparent. Nevertheless, in any one set of isomeric hydrocarbons, volatility increases with increased branching. This can be seen from the data in Table 4-2, which lists the physical properties of the five hexane isomers. The most striking feature of the data is the 19° difference between the boiling points of hexane and 2,2-dimethylbutane. 

Exercise 22-23 Show explicitly how an alkyl side chain of alkylbenzenesulfonates could be formed with a quaternary carbon, if the C12 alkane used at the start of the synthesis contained any branched-chain C12 isomers. 

For example, butane has a boiling point of -0.5°C, while 2-methylpropane s boiling point is -11.7°C. Only hydrocarbon chains with at least four carbon atoms are capable of forming a branch. Because there must be a way to distinguish isomers, the nomenclature must take these into account. The following are general rules for naming branched-chain alkanes ... 

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.)... 

In the branched-chain alkanes (general formula C H2 +2), the carbon atoms are no longer arranged in a linear sequence, but instead can be bonded to three or four other carbon atoms. This possibility leads to a rich elaboration of structure in which two molecules with the same formula can have different structures, called geometrical isomers, and therefore quite distinct properties. 

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. 

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. 

Figure 4.73 Branched-chain alkanes have lower boiling points than straight-chain 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 ... 

The chain and branched chain saturated hydrocarbons make up a family called the alkanes. Some saturated hydrocarbons with five carbon atoms are shown in Figure 18-11. The first example, containing no branches, is called normal-pentane or, briefly, n-pentane. The second example has a single branch at the end of the chain. Such a structural type is commonly identified by the prefix iso- . Hence this isomer is called /50-pentane. The third example in Figure 18-11 also contains five carbon atoms but it contains the distinctive feature of a cyclic carbon structure. Such a compound is identified by the prefix cyclo in its name—in the case shown, cyclopentane. 

Alkanes with long, unbranched chains tend to have higher melting points, boiling points, and enthalpies of vaporization than those of their branched isomers. The difference arises because, compared with unbranched molecules, the atoms of neighboring branched molecules cannot get as close together (Fig. 18.5). As a result, molecules with branched chains have weaker intermolecular forces than their unbranched isomers. 

Several reaction pathways for the cracking reaction are discussed in the literature. The commonly accepted mechanisms involve carbocations as intermediates. Reactions probably occur in catalytic cracking are visualized in Figure 4.14 [17,18], In a first step, carbocations are formed by interaction with acid sites in the zeolite. Carbenium ions may form by interaction of a paraffin molecule with a Lewis acid site abstracting a hydride ion from the alkane molecule (1), while carbo-nium ions form by direct protonation of paraffin molecules on Bronsted acid sites (2). A carbonium ion then either may eliminate a H2 molecule (3) or it cracks, releases a short-chain alkane and remains as a carbenium ion (4). The carbenium ion then gets either deprotonated and released as an olefin (5,9) or it isomerizes via a hydride (6) or methyl shift (7) to form more stable isomers. A hydride transfer from a second alkane molecule may then result in a branched alkane chain (8). The... 

In isomerizations, zeolites have special merit in their ability to admit straight-chain but not branched-chain molecules into the pores. Thus, normal alkanes up to n-Ci4H30 can penetrate the pores of zeolite 5A to reach the cavities where the C—C or C—H bonds may be catalytically broken the fragments, on reemerging from the pores, can recombine as isomerized molecules. The reverse process is not possible, since the isomers, having... 

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.

Compounds such as those in Figure 14.4 are known as isomers. Isomers are substances which have the same molecular formula but different structural formulae. The different structures of the compounds shown in Figure 14.4 have different melting and boiling points. Molecule b contains a branched chain and has a lower melting point than molecule a, which has no branched chain. All the alkane molecules with four or more carbon atoms possess isomers. Perhaps now you can see why there are so many different organic compounds ... 

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