Stability of alkanes

Butanes are chosen as the simplest models for the normal and branched isomers. Both branched and normal isomers contain a C-C bond (2 ) interacting with the terminal C-H bonds (2 and 2 ) (Scheme 26a). The cyclic -aj-a2 -a3 a2- interaction (Scheme 26b) occurs in the polarization of the middle C-C a-bond by the interactions with the antiperiplanar C-H a-bonds. The orbital phase is continuous in the branched isomer and discontinuous in the normal isomer (cf Scheme 4). The branched isomer is more stable. The basic rule of the branching effects on the stability of alkanes is ... 

The number (nj) of the cross ct conjugations of the trios of a C-C bond and two antiperiplanar C-H bonds is important for the stabilities of alkanes. The cross conjugation number (nj) of an alkane is defined as that of the conformer where the longest C-C chain has trans a zigzag structure. For example, there are three cross conjugations (n = 3) in isobutene and none in n-butane (n = 0) (Scheme 27). Isobutane is more stable than n-butane [34, 35]. 

Gronert [42] and Schleyer [43] are not aware of our theory [41]. Branched alkanes are stabilized by the C-C bond polarization by two antiperiplanar C-H bonds. The polarization is favored by the orbital phase continuity. We can predict the relative stabilities of alkanes only by counting the number of the vicinal bond trios. Neither the Gronert nor the Schleyer model contains any vicinal interactions. 

ELUmo is a measure of the ability of a compound to accept electrons (i.e., to act as an electrophile or undergo reduction). The above correlations show a decrease in ELUMO as the number of chlorines increases. As ELUM0 decreases, the ability of a compound to behave as an electrophile increases however, properties that increase stability increase LUMO energy and decrease reactivity. For example, between two-carbon alkanes and alkenes, r = -0.5104 and r = -0.9948, respectively. These data agree with Richard and Hunter (1996) in regard to the stability of alkanes over alkenes. 

Extensive thermodynamic data are available for the major classes of hydrocarbons. Table 3.1 gives data for the C4-C6 and Cj alkanes and the C4-C6 alkenes, and several general relationships become apparent. One is that chain branching increases the stability of alkanes. This relationship is clear, for example, in the data for the Cg alkanes, with a total enthalpy difference of nearly 4kcal/mol between the straight-chain hexane and the tetra-substituted 2,2-dimethylbutane. There is a similar range of 4.5kcal/mol between the least stable (octane) and most stable... 

In each of the chapters of this text, we will explore the use of different models to explain and predict the structures and reactions of organic compounds. For example, we will consider alternative explanations for the hybridization of orbitals, the c,7c description of the carbon-carbon double bond, the effect of branching on the stability of alkanes, the electronic nature of substitution reactions, the acid-base properties of organic compounds, and the nature of concerted reactions. The complementary models presented in these discussions will give new perspectives on the structures and reactions of organic compounds. 

B. Heats of Combustion and Relative Stability of Alkanes and Cycloalkanes... 

Relative to hydrogen, all the first-row substituents, with the exception of fluorine, stabilize the silicenium ion. The greater stability of a-substituted silicenium cations versus their corresponding carbocations can be attributed in part to the lower electronegativity of silicon (1.7) compared to carbon (2.5). Equation 3 also takes into account the relative stability of alkanes versus silanes, i.e. the stability of C—H versus Si—H bonds. The Si—H bond is weaker than the C—H bond by ca 13 kcalmoU in the gas phase, e.g. the bond dissociation energy D//2Qg(C—H) in CH4 is 104.9 kcalmoU, compared... 

Stabilities of alkanes (and other compounds) can be derived from thermodynamic parameters such as heat of combustion or heat of formation. 

FIG. 11 Effect of carbon number of the oil on the stability of alkane ultrafine emulsions at 25°C. Lp, liquid parafin S, squalane. (From Ref. 10.)... 

Heats of combustion can be used to measure the relative stability of isomeric hydrocarbons They tell us not only which isomer is more stable than another but by how much Consider a group of C His alkanes... 

In this section you have seen how heats of com bustion can be used to determine relative stabilities of isomeric alkanes In later sections we shall expand our scope to include the experimentally determined heats of certain other reactions such as bond dissociation energies (Section 4 16) and heats of hydrogenation (Section 6 2) to see how AH° values from various sources can aid our understanding of structure and reactivity... 

The heat evolved on burning an alkane increases with the number of car bon atoms The relative stability of isomers may be determined by com paring their respective heats of combustion The more stable of two iso mers has the lower heat of combustion... 

We assess the relative stability of alkyl radicals by measuring the enthalpy change (AH°) for the homolytic cleavage of a C—H bond m an alkane... 

The alkanes have low reactivities as compared to other hydrocarbons. Much alkane chemistry involves free-radical chain reactions that occur under vigorous conditions, eg, combustion and pyrolysis. Isobutane exhibits a different chemical behavior than / -butane, owing in part to the presence of a tertiary carbon atom and to the stability of the associated free radical. 

The haloform reaction of unsymmetrical perfluoroalkyl and co-hydroper-fluoroalkyl trifluororaethyl ketones gives the alkane corresponding to the longer alkyl chain [54] (equation 53) If the methyl group contains chlorine, the reaction can take different pathways, leading to loss of chlorine (equation 54), because of the variable stability of the chlorine-substituted methyl carbanions in alkali. 

Earlier (Sections 2.18, 3.11) we saw how to use heats of combustion to compare the stabilities of isomeric alkanes. We can do the sane thing with isomeric alkenes. Consider the heats of combustion of the four isomeric alkenes of molecular fonnula C4Hj5. All undergo combustion according to the equation... 

Thermal stabilization of polyolefins has been first demonstrated for low-molecular models-normal structure alkanes [29]. It has been shown that metallic sodium and potassium hydroxide with absorbent birch carbon (ABC) as a carrier are efficient retardants of thermal destruction of n-heptane during a contact time of 12-15 s up to the temperature of 800°C [130]. Olefins and nitrous protoxide, previously reported as inhibitors of the hydrocarbon thermal destruction, are ineffective in this conditions. 

The enhanced selectivity of alkane bromination over chlorination can be explained by turning once again to the Hammond postulate. In comparing the abstractions of an alkane hydrogen by Cl- and Br- radicals, reaction with Br- is less exergonic. As a result, the transition state for bromination resembles the alkyl radical more closely than does the transition state for chlorination, and the stability of that radical is therefore more important for bromination than for chlorination. 

Simple alkyl halides can be prepared by radical halogenation of alkanes, but mixtures of products usually result. The reactivity order of alkanes toward halogenation is identical to the stability order of radicals R3C- > R2CH- > RCH2-. Alkyl halides can also be prepared from alkenes by reaction with /V-bromo-succinimide (NBS) to give the product of allylic bromination. The NBS bromi-nation of alkenes takes place through an intermediate allylic radical, which is stabilized by resonance. 

What accounts for the stability of conjugated dienes According to valence bond theory (Sections 1.5 and 1.8), the stability is due to orbital hybridization. Typical C—C bonds like those in alkanes result from a overlap of 5p3 orbitals on both carbons. In a conjugated diene, however, the central C—C bond results from conjugated diene results in part from the greater amount of s character in the orbitals forming the C-C bond. 

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