Isomerism in Organic Compounds

Isomers are distinctly different compounds, with different properties, that have the same molecular formula. In Section 22.1, we considered structural isomers of alkanes. You will recall that butane and 2-methylpropane have the same molecular formula, C4H10, but different structural formulas. In these, as in all structural isomers, the order in which the atoms are bonded to each other differs. 

Structural isomerism is common among all types of organic compounds. For example, there are 

Three structural isomers with the molecular formula C3H80. Two of these are alco- 

Consider the molecule C3H6C12, which is derived from propane, C3H8, by substituting two Cl atoms for H atoms. Draw the structural isomers of CsH.  

Strategy All of these compounds have the same carbon skeleton, — C—C—C—. 

All these compounds have three carbon atoms, hence the same skeleton. 

More than one stable substance can correspond to a given molecular formula. Examples are butane and 2-methylpropane (isobutane), each of which has the molecular formula C4H10. Similarly, methoxymethane (dimethyl ether) and ethanol have the same formula, C2H60  

Compounds having the same number and kind of atoms but having different bonding arrangements between the atoms are called position isomers. Butane and 2-methylpropane are examples of position isomers. The atoms are connected differently in the two structures because the carbon chain in butane is a straight or continuous chain, whereas in 2-methylpropane it is branched  

Therefore these two molecules are structurally different and, accordingly, do not have the same chemical and physical properties. They cannot be converted one into the other without breaking and remaking C—C and C—H bonds. Methoxymethane and ethanol are also position isomers because the oxygen clearly is connected differently in the two molecules  

The term position isomer means the same as constitutional isomer. The designation structural isomer also is used, but this term is taken by some to include both position isomers and stereoisomers that is, structure can mean both the way in which atoms are connected and their different arrangements in space. 

The number of position isomers possible for a given formula rapidly increases with the increasing number of carbon atoms, as can be seen from the number of theoretically possible structures of formula CnH2n+2 up to n = 10 given in Table 2-3. In 1946, it was reported that all of the 75 compounds with values of n = 1 to n = 9 had been prepared in the laboratory. Before we can begin to discuss the chemistry of these compounds it is necessary to know how 

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FIGURE 18.1 A summary of the types of isomerism in organic compounds. 

Isomerism types of isomerism in organic compounds, and we will cover them in detail in Chapter 5 (Stereochemistry). For now, we need to recognize the two large classes of isomers constitutional isomers and stereoisomers. 

Stereoisomerism Ster-e-o- l-s9-m9r [ISV] (1894) n. A kind of isomerism in organic compounds arising from the fact that a carbon atom hnked to four different groups can exist in two spatially different forms that, though chemically identical, are not superimposable. A simple example is lactic acid, CHaCHCOH-COOH). 

Isomers are compounds with the same chemical formula but different properties (Section 3.2). Recall the discussion of isomerism in organic compounds (Section 15.2) coordination compounds exhibit the same two broad categories—constitutional isomers and stereoisomers. 

Geometric isomerism in organic compounds refers to the existence of nonequivalent structures cis and trans) that differ in the positioning of substituent groups relative to a double bond. In complexes, the nonequivalent structures are based on the positions at which ligands are attached to the metal center. 

Figure 18.1 summarizes the types of isomerism that are found in organic compounds. Molecules that are structural isomers are built from the same atoms, but the atoms are connected differently that is, the molecules have a different connectivity. For example, we can insert a —CH2— group into the C,HS molecule in two different ways to give two different compounds with the formula C4H 0 ... 

So, geometric isomerism can occur in organic compounds that contain a C=C double bond. However, in addition to the double bond, the molecule must have two different groups attached to each of the carbon atoms of the double bond. Propene, for example, would not exhibit geometric isomerism because it has two identical hydrogen atoms attached to one of the carbon atoms of the double bond. 

Polynierism.—This may be regarded as similar to polymerism in organic compounds. The best example of this type of isomerism is shown in triammino-trinitro-cobalt, [Co(NH3)3(N02)3]. Polymers have been prepared which are dimoleeular, trimolecular, up to pentamole-eular, thus ... 

Polymerisation isomerism may be compared with polymerisation in organic compounds, and just as benzene may be regarded as a polymer of acetylene, so in the polynuclear cobalt-ammines the same type of isomerism may occur. For example, dodecammino-hexol-tetracobaltic salts are polymers of hexammino-triol-dicobaltic salts thus ... 

When a molecule is chiral, then it will have two isomeric forms called enantiomers, each of which is the nonsuperimposable mirror image of the other. Enantiomers are distinct stereoisomers because they are compounds that have die same molecular formula and sequence of bonded elements but which differ in tile spatial arrangement of groups in the molecule. If a molecule is chiral, and thus has two enantiomers, it usually (but not always) contains at least one chiral center. In organic compounds a chiral center usually corresponds to an asymmetric tetrahedral carbon atom. 

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. 

Intramolecular rearrangements or isomerizations of organic compounds primarily important in organic syntheses " produce new C—H bonds, e.g. ... 

The decompositions and isomerizations of organic compounds are discussed in this volume, aliphatic, homocyclic, aromatic and heterocyclic being considered together, where appropriate, under the general headings of (1) hydrocarbons (2) halogen compounds of all types (3) aldehydes and ketones (4) other com-... 

J. Wislicenus, The space arrangement of the atoms in organic molecules and the resulting geometric isomerism in unsaturated compounds. The Foundations of Stereo Oiemisfry Memoirs by Pasteur. Van t Hoff, Le M, and Wislkenus (G. 

Lunn and Senior35 proposed many years ago a mathematical model of isomerism in organic chemistry which Iliev generalized in recent years.36 38 Lunn and Senior considered three types of isomerism (i) univalent substitution isomerism (positional isomerism), (ii) skeletal (structural) isomerism and (iii) enantiomorphism. Univalent substitution isomerism is the relationship existing between any two compounds Ai and A2 with the same empirical formula in case that structural formula of A can be converted into that of A2 by a permutation of the univalent substituents without disturbing the skeleton. Then the molecules Ai and A2 are said to be univalent substitution isomers. For example, 1-chloropropane and 2-chloropropane are univalent substitution (positional) isomers. Skeletal isomers are any two compounds Ai and A2 with... 

Isomers can be broadly divided into two major classes constitutional isomers and stereoisomers. In Chapter 25 we discussed isomerism in coordination compounds, and in Chapter 27 we learned about some isomeric organic compounds. In this chapter we will take a more systematic look at some three-dimensional aspects of organic structures—a subject known as stereochemistry ( spatial chemistry ). 

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