Organic compounds constitutional isomerism

Constitutional isomerism is not limited to alkanes—it occurs widely throughout organic chemistry. Constitutional isomers may have different carbon skeletons (as in isobutane and butane), different functional groups (as in ethanol and dimethyl ether), or different locations of a functional group along the chain (as in isopropylamine and propylamine). Regardless of the reason for the isomerism, constitutional isomers are always different compounds with different properties, but with the same formula. 

No final or absolute decision is therefore yet possible. By analogy with other cases of a similar type amongst organic compounds, the possibility of a dynamic isomerism must not be left out of consideration. It is quite possible that in sulphurous acid solution, molecules of the symmetrical and the unsymmetrical constitution may be present, side by side, in equilibrium with one another, and that even in solutions of the salts a similar condition of equilibrium may exist. Indeed, the distinctly contradictory nature of some of the chemical evidence favours this view. 

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. 

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. 

Isomeric liquid, organic compounds of similar chemical constitution have approximately the same molecular heat Mcp. 

The heat of combustion, and hence also the heat of formation of organic compounds depend in a marked degree upon their constitution, that is to say, they depend not only upon the number and nature of the atoms in the molecule, but also upon the manner in which they are bound together. Isomeric compounds may have totally different heats of formation. Thus acetone CHg. CO. CHg has a heat of combustion equal to 423000 cal., while the heat of combustion of its isomer allyl alcohol CHg. CHCHg(OH) is 465000 cal., and of its other isomer propionic aldehyde CHgCHgCHO is 441000 cal. Similarly, the heat of combustion of polymeric substances (per gram) varies with the size of the molecule, as the following table shows ... 

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 ). 

The diazo-compounds form the starting point of numerous organic compounds, including the azo-dyes, the first representative of which (amino-azobenzene hydrochloride, aniline yellow ) was prepared and shown to be a dye by Mene. What was called aniline yellow is the oxalate of aminoazo-benzene (CeH5 N N-CeH4 NH2), introduced as a dye by Simpson, Maule, and Nicholson in 1863 its constitution was established by C. A. Martius and Griess, and the isomeric change of diazoaminobenzene into aminoazobenzene was explained by Kekule. ... 

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. 

We first encountered the term isomer in the context of molecular geometry and molecular polarity Section 9.2], Isomers are different compounds that have the same chemical formula. In this section, we will discuss the different types of isomerism (namely, constitutional isomerism and stereoisomerism) and their importance to organic chemistry. 

The isomeric optically active substituted ammonium salts which have been reported by organic chemists are not readily explained by an ammonium theory such as has been wrongly assumed to have been established by the experiments of V. Meyer and M. T. Lecco—vide supra. J. C. Cain argued from the above hypothesis of the constitution of the ammonium compounds that the optical activity is the result of a similar spatial configuration as that assumed for the carbon compounds where the quadrivalent nucleus, say NCI", instead of quadrivalent carbon, is at the centre of a tetrahedron so that... 

Structural (or constitutional) isomers are compounds with the same molecular formulas but different structural formulas (that is, different arrangements of the atoms in the molecule). Isomerism is especially important in organic chemistry because of the capacity of carbon atoms to be arranged in so many different ways continuous chains, branched chains, and rings. Structural formulas can be written so that every bond is shown, or in various abbreviated forms. For example, the formula for n-pentane (n stands for normal) can be written as ... 

Stereochemistry.—Compounds of the type POX3 may, as already pointed out, have one or other of two constitutions (see p. 52) or may exist in tautomeric equilibrium, but if the halogens X are replaced by hydrocarbon or other organic radicals R, the compound will be fixed in one or other of the two isomeric forms. This isomerism has been well established in the case of the compounds having the empirical formula OP(C6H5)3.5 One of these, phenoxydiphenylphosphine, is an oily liquid, prepared by the condensation of phenol with diphenyl-chlorophosphine —... 

Nitrogen chemical shifts constitute a formidable means of differentiation between the various isomeric or tautomeric structures which are abundant in both organic and inorganic compounds of nitrogen, since relative differences in nitrogen screening constants are usually large. Sometimes they may reach several hundred ppm. 

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