Cyclohexane proton affinity

A resolution into two or more components always occurs if the solvent has a high proton affinity, so that a solvent molecule can form a particularly stable association with a phenol molecule as a result of an energetically favourable mutual orientation. This is the case, for example, if benzene and toluene are used as the solvents. However, this effect is even more pronounced in the case of cyclohexene. Dielectric constant measurements for phenol in various solvents agree with this observation. In particular, the dipole moments in benzene and cyclohexene (1-45 and 1-79 D respectively), are considerably greater than the value of 1-32 in cyclohexane. Liittke and Mecke (1949) attributed this effect to the ability of this unsaturated solvent to act as a proton acceptor, i.e. to form 7r-complexes. 

Class (3) reactions include proton-transfer reactions of solvent holes in cyclohexane and methylcyclohexane [71,74,75]. The corresponding rate constants are 10-30% of the fastest class (1) reactions. Class (4) reactions include proton-transfer reactions in trans-decalin and cis-trans decalin mixtures [77]. Proton transfer from the decalin hole to aliphatic alcohol results in the formation of a C-centered decalyl radical. The proton affinity of this radical is comparable to that of a single alcohol molecule. However, it is less than the proton affinity of an alcohol dimer. Consequently, a complex of the radical cation and alcohol monomer is relatively stable toward proton transfer when such a complex encounters a second alcohol molecule, the radical cation rapidly deprotonates. Metastable complexes with natural lifetimes between 24 nsec (2-propanol) and 90 nsec (tert-butanol) were observed in liquid cis- and tra 5-decalins at 25°C [77]. The rate of the complexation is one-half of that for class (1) reactions the overall decay rate is limited by slow proton transfer in the 1 1 complex. The rate constant of unimolecular decay is (5-10) x 10 sec for primary alcohols, bimolecular decay via proton transfer to the alcohol dimer prevails. Only for secondary and ternary alcohols is the equilibrium reached sufficiently slowly that it can be observed at 25 °C on a time scale of > 10 nsec. There is a striking similarity between the formation of alcohol complexes with the solvent holes (in decalins) and solvent anions (in sc CO2). 

The formation of either the radical cation M,+ or the protonated [M + H]+ molecule, or both together, will depend on the relative ionization energies or proton affinities of the sample molecules and the solvent components. Concerning the solvent, the charge exchange is favoured for solvents with low proton affinity (water, chloroform, cyclohexane, etc.), while solvents with higher proton affinities (methanol, acetonitrile, etc.) will favour proton transfer. 

The proton affinities of alicyclic carboxylic acids are identical within 5 kj mol-1, with cyclohexane >cyclopropane >cyclopentane >cyclobutane [205]. This quite surprising situation seems not to be fully explained yet. 

The use of ammonia generates other types of data. When the adduct ion [M + NH4] is stable the measurement of the ratio MH /M -t- NH4] gives direct access to the distinction between diastereomeric diols, for example, the cyclopentane and cyclohexane diols [37,38]. This behavior is due to slight variations of proton affinity, in these cases certainly resulting from the possibility of forming hydrogen bonds (Table 2). 

Like protons, transition-metal ions are strongly acidic and they can, in principle, add to both the C—H and C—C bonds of alkanes. As already noted in the section on proton affinities (Table 1) strained cycloalkanes are intrinsically more basic than open-chain alkanes, and the reaction of cyclopropane with Pt((II) to form a platinacyclo-butane (equation 14) was the first reaction of a formally saturated hydrocarbon with a transition-metal ion . The driving force in this reaction is relief of the strain associated with the small ring. The resulting metallacyclobutane is essentially free of ring strain. Many low-valent transition-metal complexes have been found to react with cycloalkanes. Metal ions convet the strained hydrocarbons quadricyclane , cubane , bicyclo-[2.1.0]pentanes , bicyclo[3.1.0]hexanes , bicyclo[4.1.0]heptanes and bicyclo-butanes into less strained isomers (usually cyclohexanes). 

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