Stability of constitutional isomers

You have seen that measurements of heats of reaction such as heats of combustion can pro vide quantitative information concerning the relative stability of constitutional isomers (Section 2 18) and stereoisomers (Section 3 11) The box in Section 2 18 described how heats of reaction can be manipulated arithmetically to generate heats of formation (AH ) for many molecules The following material shows how two different sources of thermo chemical information heats of formation and bond dissociation energies (see Table 4 3) can reveal whether a particular reaction is exothermic or en dothermic and by how much... 

The most accurate experimental means of determining the relative stability of constitutional isomers is often by direct equilibration but this is constrained by the limit of precision of the analytical tool employed. For the most common methods (e.g., glc, nmr, ir) where the practical limit of detectability is 0.1%, AAG (25 C) —RT ln[.4]/[B]) must generally be in the 0-4 kcal mole" range. 

In order to compare the stability of constitutional isomers, we look at the heat liberated when they each undergo combustion. For an alkane, combustion describes a reaction in which the alkane reacts with oxygen to produce CO2 and water. Consider the following example ... 

Because HCN has formal charges of zero on all the atoms, whereas HNC has a formal charge on nitrogen of +1 and a formal charge on carbon of — 1, we would expect HCN to be more stable than HNC. Indeed, this is the case. We will encounter many cases of constitutional isomers in which the difference in stability is much less than in this example. In such cases, much more subtle arguments must be used to predict which is more stable, if a prediction can be made at all without experimental measurements. 

It is known that the photochromic properties of constitutional isomers of dithienylethenes differ in quantum yields for photocyclization/cyclor-eversion and thermal stability. The potential energy surfaces for switching in several dithienylethene derivatives were demonstrated by theoretical calculations and/or spectroscopic studies. " The ring crossing pathway of dithienylethene in single crystals was directly observed by using femto-second electron ciystallography. ... 

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 cis and trans forms of 1,2-dimethylcyclopropane are stereoisomers. Stereoisomers are isomers that have then- atoms bonded in the sane order—that is, they have the sane constitution, but they differ in the anangernent of atoms in space. Stereoisomers of the cis-trans type are sometimes refened to as geometric isomers. You learned in Section 2.18 that constitutional isomers could differ in stability. What about stereoisomers ... 

Mithramycin (24) bears the terminal sugar unit -mycarose. By a similar preparation as for the methyl-branched glycals and the -enantiomer of namely the labile g-mycaral 40 w s obtained ( ). Its NIS condensation with the 3, 4 -Saol disaccharide proceeds smoothly, but the reaction turned out to be rather slow with respect to the stability of the g-mycaral. This results in only a modest yield (12%) of the trisaccharide which was hydrogenolyzed to compound 4. This in turn constitutes an isomer of the trisaccharide sequence in... 

Which of these two structures is correct Both of them satisfy the octet rule and neither has formal charges, so both are predicted to be of comparable stability. On the basis of what we have discussed so far, we cannot predict which is more stable. In fact, both of these compounds are quite stable and can be put in a bottle. But they are different compounds. Ethyl alcohol is the alcohol found in beverages. It is a liquid at room temperature. In contrast, dimethyl ether is a gas at room temperature and is quite poisonous. As was mentioned in Section 1.7, compounds such as these, with the same molecular formula but different arrangements of bonded atoms (different structures or different connectivities), are called constitutional isomers (or structural isomers). Constitutional isomerism is very common in organic compounds. This is another reason why it is necessary to show the structure of the compound under discussion rather than just the molecular formula. 

Stability of methylenecyclopropene (stable below — 75°C) stands in contrast to cyclobutadiene, an isomer, which appears to dimerize in a diffusion controlled process The triapentafulvenes or calicenes " constitute another type of methylenecyclopropene which gives some indication of dipolar character and thus a degree of aromaticity. These compounds link the positively charged cyclopropenyl cation to a cyclopentadienyl... 

Halophenols constitute the simplest structural model for the analysis of an intramolecular hydrogen bond since the syn-anti isomerization involves only the rotation of the OH proton around the single C—O bond. Furthermore, the two forms coexist in apolar solvents and give two characteristic absorptions in the OH stretching region of the IR spectrum with the notable exception of the fluoro derivative. In a series of papers Okuyama and Ikawa ° "° have re-examined by FTIR the relative stability of the syn-anti isomers by varying the temperature and the pressure. The enthalpies of isomerization are reported in Table 10. 

---