Comparing Isomers

When turning to transition-metal-containing compounds the situation changes somewhat. Yoshizawa et al.40 studied the benzene hydroxylation by iron-oxo species. As an initial study to this, they considered the dissociation energy of FeO+. Whereas the experimental value lies about 81 kcal mol 1, various Cl-like methods yielded values between 25 and 75 kcal mol 1, and a hybrid method gave 75 kcal mol 1. Thus, the error is larger, but the density-functional calculations gave some of the most accurate results. Therefore, 

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We admit some cowardice. Most of our earlier estimates of entropy invoked symmetry numbers and/or were studies of bond cleavage reactions dominated by translational effects. We hesitate to compare isomers with the same carbon or heavy atom skeleton when effects of a few kJ mol-1 are crucial. 

Comparing isomers in Table 4-4, we see that 2-methylpropane and 2,2,3,3-tetramethylbutane give off less heat when burned than do butane and octane, and this is a rather general characteristic result of chain branching. 

To build and compare isomers having a given molecular formula. 

Since one usually is interested in energy differences rather than the total energy of a single system, it is useful to compare isomer energy differences computed with the various procedures. 

The concentrations of tetrabutyl tin range from 130 pg/kg TOC at sample site B to 14 pg/kg TOC at sample sites E and F and is below the LOQ at sample site D. The emission of this well known Elbe-specific compound was formerly linked to an industrial point source situated near the confluence of the Mulde and the Elbe rivers (Wilken et al., 1994, Schwarzbauer, 1997). Mono- and disubstituted chloronaphthalenes were also detected with concentrations between the LOQ at sample sites not influenced by Elbe derived contaminations and 70 and 100 pg/kg TOC at sample site B. These low chlorinated naphthalenes occur in the sediments with patterns similar to those of technical agents (e.g. Halowax 1000) and were formerly identified with comparable isomer distributions in sediments of the Elbe river and its tributaries (Schwarzbauer 1997, Schwarzbauer et al. 2001). On the contraiy the origin of the Elbe specific contaminant 4,4 -d ic h Iorodiphcnylsul tidc detected at sample sites A,B,C,E and F with concentrations ranging from 7 to 45 pg/kg TOC is still unknown. 

Pentakis(pentafluoroethyl)pyridine yields very stable pant-hooded and prismane isomers (see Vol. 1, p. 268), and these have now been described in fuU. No comparable isomers are formed from peifluoro-2,4,5-triethyl- and -2,3,4,6-tetraethyl-pyridines. The poro-bonded pyridine (12S), formed photochemically from its parent pyridine, formed complexes with palladiumfn) and platinum(u) chlorides. ... 

The Figure 4 presents two comparable isomers of heptane (2-me-thylhexane and heptane) the former precedes the latter. Also, two noncomparable isomers of octane are presented, according to Muirhead s criterion,... 

Figure 39. Benzene to benzvalene reaction, (a) Assuming that the prebenzvalene structure is a transition state. The two benzvalene isomers are anchors, (b) Assuming that prebenzvalene is an intermediate, A two-anchor loop results, compare Figure 12.

Nitrations are usually carried out at comparatively low temperatures at higher temperatures there may be loss of material because of the oxidising action of the nitric acid. For substances which do not nitrate readily with a mixture of concentrated nitric and sulphuric acids ( mixed acid ), the intensity of the reaction may be increased inler alia by the use of fuming sulphuric acid (containing up to 60 per cent, of sulphur trioxide) or by fuming nitric acid. Thus nitrobenzene is converted by a mixture of fuming nitric acid and concentrated sulphuric acid into about 90 per cent, of wi-dinitrobenzene and small amounts of the o- and p-isomers the latter are eliminated in the process of recrystallisation ... 

The isomer proportions for the nitration of the chlorotoluenes, to be expected from the additivity principle, have been calculated from the partial rate factors for the nitration of toluene and chlorobenzene and compared with experimental results for nitration with nitric acid at o °C. The calculated values are indicated in brackets beside the experimental values on the following structural formulae. In general, it can be... 

The nitration of phenylpyridines and related compounds has attracted attention for a long time, and measurements of isomer proportions have been made for several compounds of this type. Nitration occurs in the phenyl ring. For 2-phenylpyridine and 2-phenylpyridine i-oxide measurements of the dependence of rate of nitration upon acidity in 75-81 % sulphuric acid at 25 °C show that both compounds are nitrated as their cations (table 8.1). The isomer distribution did not depend significantly upon the acidity, and by comparison with the kinetic data for quinolinium ( 10.4.2) the partial rate factors illustrated below were obtained.They should be compared with those for the nitration of 2-nitrobiphenyl ( 10.1). The protonated heterocyclic groups are much... 

As noted earlier m this section branched alkanes have lower boiling points than their unbranched isomers Isomers have of course the same number of atoms and elec Irons but a molecule of a branched alkane has a smaller surface area than an unbranched one The extended shape of an unbranched alkane permits more points of contact for mtermolecular associations Compare the boiling points of pentane and its isomers... 

Figure 2 14 compares the heats of combustion of these isomers on a potential... 

The CIS and trans forms of 1 2 dimethylcyclopropane are stereoisomers Stereoisomers are isomers that have their atoms bonded m the same order—that is they have the same constitution but they differ m the arrangement of atoms m space Stereoiso mers of the cis-trans type are sometimes referred to as geometric isomers You learned m Section 2 18 that constitutional isomers could differ m stability What about stereoisomers We can measure the energy difference between as and trans 1 2 dimethylcyclo propane by comparing their heats of combustion As illustrated m Figure 3 20 the two compounds are isomers and so the difference m their heats of combustion is a direct measure of the difference m their energies Because the heat of combustion of trans 1 2 dimethylcyclopropane is 5 kJ/mol (12 kcal/mol) less than that of its cis stereoisomer it follows that trans 1 2 dimethylcyclopropane is 5 kJ/mol (12 kcal/mol) more stable than as 1 2 dimethylcyclopropane... 

The difference m stability between stereoisomeric alkenes is even more pronounced with larger alkyl groups on the double bond A particularly striking example compares as and trans 22 5 5 tetramethyl 3 hexene m which the heat of combustion of the cis stereoisomer is 44 kJ/mol (10 5 kcal/mol) higher than that of the trans The cis isomer IS destabilized by the large van der Waals strain between the bulky tert butyl groups on the same side of the double bond... 

Further insight into the E2 mechanism comes from stereochemical studies One such experiment compares the rates of elimination of the cis and trans isomers of 4 tert butyl cyclohexyl bromide... 

Mesitylene (13 5 trimethylbenzene) is the most stable of the tnmethylbenzene isomers Why2 Which isomer do you think is the least stable" Make a molecular model of each isomer and compare their calculated strain energies with your predictions Do space filling models support your explanation" ... 

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