Strength of Alkane Bonds Radicals

Consequently, breaking such a bond requires heat—in fact, the same amount of heat that was released when the bond was made. This energy is called bond-dissociation energy, DH°, and is a quantitative measure of the bond strength. 

In our example, the bond breaks in such a way that the two bonding electrons divide equally between the two participating atoms or fragments. This process is called homolytic cleavage or bond homolysis. The separation of the two bonding electrons is denoted by two single-barbed or fishhook arrows that point from the bond to each of the atoms. 

A single-barbed arrow f shows the movement of a single electron. 

The fragments that form have an unpaired electron, for example, H% Cl% CHj-, and CH3CH2 . When these species are composed of more than one atom, they are called radicals. Because of the unpaired electron, radicals and free atoms are very reactive and usually cannot be isolated. However, radicals and free atoms are present in low concentration as unobserved intermediates in many reactions, such as the production of polymers (Chapter 12) and the oxidation of fats that leads to the spoilage of perishable foods (Chapter 22). 

In Section 2-2 we introduced an alternative way of breaking a bond, in which the entire bonding electron pair is donated to one of the atoms. This process is heterolytic cleavage and results in the formation of ions. 

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The figures for fluorination reflect the weakness of the F—F [150kJ (36 kcal) mol ],and the strength of the H—F [560 kJ(134 kcal) mol" ], bonds. Fluorination normally requires no specific initiation (cf. p.324), and is explosive unless carried out at high dilution. That fluorination does proceed by a radical pathway, despite not requiring specific initiation, is demonstrated by the fact that chlorination may be initiated in the dark, and at room temperature, by the addition of small traces of F2. Bromination is a good deal slower than chlorination, under comparable conditions, as step (1)—H-abstraction by Br—is commonly endothermic. This step is usually so endothermic for I that direct iodination of alkanes does not normally take place. 

This kind of scission is typical of polyethylene (PE). The backbone of the polymer is broken randomly as all C-C bonds are of the same strength (Figure 27.5). Hence, the hydrocarbon chain breaks randomly and the resulting products are of the form of alkanes, alkenes and alkadienes of smaller size. This is a free radical mechanism. The covalent bond between two carbon atoms is cleaved homolytically to form fragments carrying one... 

Photochemical homolysis of the carbon-carbon bond of alkanes is difficult. The carbon-carbon bond of ethane has a bond strength of 83 kcal moT (347.4 kJ mol ) and it fragments to methyl radicals ( CH3) only at temperatures approaching 600°C. Absorption of a photon of light is also difficult since alkanes absorb only... 

Activities of metals for alkane exchange increase with metal-metal bond strength (i.e. with sublimation energy), presumably because of stronger bonding of the radicals formed when the alkane dissociates, and their consequent higher coverages. ... 

If one looks at the average bond dissociation energies for X2, C-X, H-X, and C-H bonds (Table 2.2), an average heat of reaction for the halogenation of alkanes can be calculated. The results in kcal/mol are as follows F = -101, Cl = -22, Br = -4, and I = 16. The variation in these numbers comes from a continual decrease in H-X and C-X bond strengths in the series F, Cl, Br, and I. These heats of reaction reflect a dramatic change in reactivity. Free radical fluorination is so exothermic that it occurs spontaneously and very explosively. Chlorination and bromination can be controlled and are useful reactions. Free radical iodination rarely occurs. 

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