Lecture 3 Isomerism

STRUCTURE CHANGE (ISOMERIZATION) and CONFORMATION CHANGE COMPOSITION C k H l O m STRUCTURE 1STRUCTURE 2STRUCTURE 3 Reaction 1→2 Reaction 2→3 3n-6=12-6=6

Reactivity TS 1→2 TS 2→3 STABILITY 2 STABILITY ΔE = f(x 1, x 2, x 3, x 4, x 5 )

ΔE = f(x 6 )

A molecule may change its structure without bound breaking and with bound breaking. The former one leads to different conformers the latter change produces isomers. If σ-bounds are broken then we may have structural isomers and if π-bounds are broken then we may have geometrical isomers. Geometrical IsomersStructural IsomersConformers CO 2 + H 2 O C + H 2 + O 2 ̶ ̶ ̶ 3.2 ̶ 3.8 ̶ 2877 ̶ 125 ̶ 2875 ̶ 3000 ̶ CO 2 +H 2 O Δ ‡ H= Δ c H=

7. 4 Relative Stabilities of Alkenes Figure 7.3 Enthalpies (  H) of hydrogenation (in units of Kcal/mol) for the isomeric n-butenes. Figure 7.3 shows the heats or enthalpies of hydrogenation of three isomeric butenes. The enthalpies of hydrogenation are a measure of the molecular stabilities of unsaturated hydrocarbons.

Two things should be noted: 1)Disubstituted carbon-carbon double bonds (at the middle and right hand side of Figure 7.3) are more stable than mono-substituted olefinic double bonds. 2)The trans isomer (at the extreme right in Figure 7.3) is more stable than the cis isomer. In Chapter 4, we saw that the stabilities of isomeric saturated hydrocarbons (C n H 2n+2 ) can be assessed from their heat or enthalpy of combustion (c.f. Figure 4.2). In this chapter, we see that the heat or enthalpy of hydrogenation may be used to measure the stability of unsaturated hydrocarbons (C n H 2n ), as shown in Figure 7.3.

The results of these two methods can be put together, as illustrated for cis - and trans butene and n - butane in Figure 7.4. As far as stabilities are concerned, the greater the substitution, the more stable the alkene. Of the disubstituted cases, 1,1 - disubstituted ethylene is more stable than 1,2-disubstituted ethylene. For the latter, the trans - isomer is more stable than the cis - isomer. These stabilities are presented in Figure 7.5 in terms of enthalpies of hydrogenation in a schematic fashion.

Figure 7.4 Comparison of enthalpies of formation, enthalpies of hydrogenation and enthalpies of combustion for selected hydrocarbons.

Figure 7.5 A schematic illustration of the relative stabilities of olefinic double bonds with the extent of substitution as measured by the enthalpies of hydrogenation.

One may wonder if an alkyl substitution on an olefinic double bond can be carried out that will make it more stable. The answer lies in the concept of hyperconjugation. We saw in Chapter 1 that conjugation of two carbon-carbon double bonds leads to stabilization (c.f. [1.42] and Figure 1.28). We can now assess the magnitude of such conjugative stabilization to be of a few Kcal/mol. The example in Figure 7.6 shows the stabilization for butadiene to be 3.5 Kcal/mol. Figure 7.6 Conjugative stabilization (-3.5 Kcal/mol) of 1, 3 - butadiene with respect to two moles of 1-butene as measured by the difference in enthalpies of hydrogenation.

The structural isomerism requires a large initial energy input even though the energy is recovered at the end of the reaction. Clearly this is not a spontaneous isomerization. „Tautomers” represent a special structural isomers which earn be formed quite easily without large energy investment. The two isomers differ from each other only in the position of a proton. 0 Appr. 650 kJ/mol 9kJ

The ketoo-enol tautomerization is formed easily with acid catalysis.

In general, most chemical reaction starts with a complex formation which is called the REACTANT COMPLEX. Usually the in the pen-ultimate step a PRODUCT COMPLEX is formed. These two coplexes separated by a transition state (TS) as shown below. A + B → A···B → [TS] ± → C···D → C + D ReactantsReactant Complex Transition State Product Complex Products

A schematic energy change is shown below A+B A···B TS C···D C+D Reaction Coordinata Energy or Free Energy Product complex Reactant complex

This situation dominantly occur in biological reactions which involve specific enzyme. The first step is the ENZYME-SUBSTRATE complex formation which is converted via a TS to an ENZYME-PRODUCT comlex. The assiciated energy profile and mechanistic equation is analogons to the shown below.

The following figure shows a bacterial enzyme which is able to oxidise CH 4 to H 3 C-OH. The figure also shows the ENZYME-PRODUCT comlex. Since the H atoms are not shown H 3 C-OH is shown in the the complex as C-O.

Methane Monooxygenase CH 4 + NADH + H + + O 2 → CH 3 OH + H 2 O + NAD +

A given molecular structure, such as C 4 H 6 has numerous isomers, as few of them are illustrated below. They may have numerous molecular fragments some of them a complete organic molecules while some of them contein two valent carbons, called carbenes.

Nevertheless the potential energy surface associated with the particular chemical composition (like C 4 H 6 ) does contain all these entities as local minimum. The function of the total PEHS is: If the number of atoms have the following form: The degrees of freedom (D) of the system given of N number of atoms: where the N is: The number of stretches, bends and the torsion add up to 3N-6.

E[R(a)] = E[S(a)][5.10] E[R(g + )] = E[S(g - )][5.11a] E[R(g - )] = E[S(g + )][5.11b]