A Review of Isomerism

ACTIVE FIGURE 5.14 A summary of tine different kinds of isomers. Co to this book s student companion site at www.cengage. com/chemistry/mcmurry to explore an interactive version of this figure. 

There are two fundamental types of isomers, both of which we ve now encountered constitutional isomers and stereoisomers. 

Stereoisomers (Section 4.2) are compounds whose atoms are connected in the same order hut with a different arrangement in space. Among the kinds of stereoisomers we ve seen are enantiomers, diastereomers, and cis-trans isomers of cycloalkanes. Actually, cis-trans isomers are just one class of diastereomers because they are non-mirror-image stereoisomers  

Cis-trans diastereomers (substituents on same side or opposite side of double bond or ring) 

Constitutional isomers (Section 3.2) are compounds whose atoms are connected differently. Among the kinds of constitutional isomers we ve seen are skeletal, functional, and positional isomers. 

Different functional groups CH3CH2OH Ethyl alcohol and CH3OCH3 Dimethyl ether 

Different position of functional groups NH, 1 CH3CHCH3 and CH3CH2CH2NH2 

We ll see in Section 21.3 that carboxylic acids (RCO2H) react with alcohols (R OH) to form esters (RC02R )- Suppose that ( )-lactic acid reacts with CH3OH to form the ester, methyl lactate. What stereochemistry would you expect the product(s) to have What is the relationship of the products  

Reaction of a racemic acid with an achiral alcohol such as methanol yields a racemic mixture of mirror-image (enantiomeric) products. 

What stereoisomers would result from reaction of (z )-laclic acid with (S)-l-phenyl-ethylamine, and what is the relationship between them  

As noted on several previous occasions, isomers are compounds that have the same chemical formula but different structures. We ve seen several kinds of isomers in the past few chapters, and it s a good idea at this point to see how they relate to one another (f igure 9.14). 

---

A review of isomerization processes involving N—X bonds contains numerous examples of N—alkyl conformational processes (Martin et al., 1985). 

The clay-cataly2ed iatermolecular condensation of oleic and/or linoleic acid mixtures on a commercial scale produces approximately a 60 40 mixture of dimer acids and higher polycarboxyUc acids) and monomer acids (C g isomerized fatty acids). The polycarboxyUc acid and monomer fractions are usually separated by wiped-film evaporation. The monomer fraction, after hydrogenation, can be fed to a solvent separative process that produces commercial isostearic acid, a complex mixture of saturated fatty acids that is Hquid at 10°C. Dimer acids can be further separated, also by wiped-film evaporation, iato distilled dimer acids and trimer acids. A review of dimerization gives a comprehensive discussion of the subject (10). 

Most of the subsequent work on this reagent was concerned with the formation of aryl radicals (see review by Cadogan, 1971). However, 2-terf-butyl-A-nitrosoacet-anilide was found to decompose in benzene to give, instead of 2-tert-butylbiphenyl, as expected for a substitution of benzene by a 2-tert-butylphenyl radical, a mixture of isomeric tert-butylphenyl acetates. A careful reexamination (Cadogan and Hib-bert, 1964) suggested that the ratio of 2- and 3-tert-butylphenyl acetates was consistent with the involvement of 2-tert-butylbenzyne, i.e., the product of an ionic dediazoniation, as an intermediate. This was later confirmed by trapping experiments designed to detect aryne intermediates. 

A process similar to that of Scheme 23, but involving 3-alkynyl derivatives of 76, viz. 78, leads to the isomeric series of pyrrolopyrimidopyridazines represented by 79 (Scheme 25) <2003RCB441>, and imidazopyrimidopyrid-azines such as 80 and 81 are similarly accessible via 3-(alkylamino) derivatives of 76 <2003T7669> (Equations 18 and 19). A review of the range and scope of these reactions is available <2005JHC375>. 

Only a trace of the corresponding cubane 167 is formed on irradiation of the tricy-clooctadiene 168 in pentane at ambient temperatures using a 125-watt mercury arc lamp. The principal product 169 is the result of rearrangement within a biradical intermediate79. A review of the synthetic approaches to cubane and to its reactions has been published77. The diene 170 photochemically converts on irradiation in pentane solution at 254 nm to yield a photostationary mixture of the cubane 171, the starting material 170 and the isomeric diene 17280. Other additions of this type have been used for synthesis of the propellaprismane 173, essentially a heavily substituted cubane, by the intramolecular (2 + 2)-photocycloaddition of the diene 17481. 

G. K. Anderson and R. J. Cross, Chem. Soc. Revs. 9, 185 (1980) for a comprehensive and incisive review of isomerization mechanisms of square-planar complexes. 

Dolbier WR, Sellers SF (1982) J. Am. Chem. Soc. 104 2494. A review of the authors works on thermal rearrangements of gem-difluorocyclopropanes covering, inter alia, cyclopropane thermolysis, methylene cyclopropane and spiropentane rearrangements, vinyl-cyclopropane and cyclopropylcarbinyl isomerizations has been published Dolbier WR (1981) Acc. Chem. Res. 14 195... 

The work of Nazarov on vinyl ethynyl carbinols involves condensation of vinylacetylene with ketones in the presence of caustic potash and also their conversions, many of which are catalytic in nature. A review of his work involving polymerization, isomerization, hydrogenation, and other conversions was published by him (252). Hydration of divinylacetylenes in methanol solution in the presence of mercuric sulfate and sulfuric acid gave vinyl alkyl ketones. These can be reacted with hydrogen sulfide, amines, etc., to yield heterocyclic compounds. Substituted vinyl alkyl ketones underwent spontaneous cyclization to cyclopentenones. Nazarov summarized a decade of this research in this field in 1951 (253). His general review of organic syntheses based on acetylene is also of interest in this connection (254). 

An indium-mediated ring-expansion reaction of a cyclic thioether derivative 31 in aqueous medium leads to the eight-membered cyclic thioether derivative 32, which isomerizes to the more stable conjugated product 33 in 62% yield (referred to 31) (Scheme 10) <1999SL735>. For a review of such Barbier-Grignard type reactions in water see <1996T5643>. 

The focus of this chapter is a review of the methodologies employed in a priori implementations of RRKM theory for the collisionless dissociation/ isomerization steps in gas-phase unimolecular reactions. Special attention will be paid to recent developments, particularly those that have proven their utility through substantive applications. With microscopic reversibility, RRKM treatments of the dissociation process are directly applicable to the reverse bimolecular associations. Furthermore, some of the more interesting illustrations are for bimolecular reactions and so we do not limit our discussion of RRKM theory to unimolecular reactions. However, one should bear in mind that TST was originally derived for bimolecular reactions and the specific term RRKM theory is really only applicable to the unimolecular direction. 

---