Bond, energy strength

Since the hydrogen-element bond energy decreases from sulphur to tellurium they are stronger acids than hydrogen sulphide in aqueous solution but are still classified as weak acids—similar change in acid strength is observed for Group Vll hydrides. 

Some of the evidence indicating that alkyl substituents stabilize free radicals comes from bond energies The strength of a bond is measured by the energy required to break It A covalent bond can be broken m two ways In a homolytic cleavage a bond between two atoms is broken so that each of them retains one of the electrons m the bond... 

Extensive reviews of the effects of fluonnation on stmeture and bonding are available [75, 76, 77], and only the charactenstic trends in bond strengths will be covered here. The bond energies cited are average values corrected for the revised heats of formation of alkyl radicals [78], but their precision is seldom better than 2 kcal/mol for the fluoro compounds. 

The C-F bonds in 1 fluoroalkenes and fluorobenzenes also are very strong (Table 17), but alkene k bond strengths vary with the level of fluonnabon (Table 18) Both CHF=CF2 and CF2=Cp2 have significantly weaker 7t bonds than CH2= CH2, CH7=CHF, and CH2=Cp2, consistent with other data indicatuig that tn- and tetrafluonnation thermodynamically destabilize double bonds [75] The low n bond energy in Cp2=CF2 underlies its propensity to undergo thermal biradical [2-1-2] cycloaddibons [103] (see p 767 )... 

Fig. 4.—Calculated one-electron bond energy-values (D/k, full curves) and squared bond strength values (S, dashed curves) for s-p hybridization.

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