Structural formula isomers

Name Alkane Prefix Molecular Formula Structural Formula Isomers... 

Isomers A molecular formula tells the actual number of each type of atom, providing as much information as possible from mass analysis. Yet different compounds can have the same molecular formula because the atoms can bond to each other in different arrangements to give more than one structural formula. Isomers are compounds with the same molecular formula but different properties. The simplest type of isomerism, called constitutional, or structural, isomerism, occurs when the atoms link together in different arrangements. The pair of constitutional isomers shown in Table 3.2 (on the next page) share the molecular formula C2HgO... 

You have learned that isomers are compounds that have the same molecular formula but different structural formulas. Isomers can be further classified by structure and geometry. 

PRACTICE EXAM PLE B Are the pairs of molecules represented by the following structural formulas isomers ... 

The isomer proportions for the nitration of the chlorotoluenes, to be expected from the additivity principle, have been calculated from the partial rate factors for the nitration of toluene and chlorobenzene and compared with experimental results for nitration with nitric acid at o °C. The calculated values are indicated in brackets beside the experimental values on the following structural formulae. In general, it can be... 

Wnte a structural formula for the CFI3NO isomer characterized by the structural unit indi cated None of the atoms in the final structure should have a formal charge... 

Wnte structural formulas for all the constitutional isomers of molecular formula C3HgO that contain... 

Now write structural formulas for the remaining three isomers Be sure that each one IS a unique compound and not simply a different representation of one writ ten previously... 

In Problem 2 5 you were asked to write structural formulas for the five isomeric alkanes of molecular formula C6H14 In the next section you will see how the lUPAC rules generate a unique name for each isomer... 

From among the 18 constitutional isomers of CgHig wnte structural formulas and give the lUPAC names for those that are named as derivatives of... 

Excluding compounds that contain methyl or ethyl groups write structural formulas for all the bicychc isomers of (a) CsHg and (b) CgHjo... 

Photochemical chlorination of pentane gave a mixture of three isomenc monochlorides The pnncipal monochlonde constituted 46% of the total and the remaining 54% was approximately a 1 1 mixture of the other two isomers Write structural formulas for the three monochlonde iso mers and specify which one was formed in greatest amount (Recall that a secondary hydrogen is abstracted three times faster by a chlonne atom than a pnmary hydrogen)... 

Each of the reactions shown involves nucleophilic substitution The product of reaction (a) IS an isomer of the product of reaction (b) What kind of isomer" By what mechanism does nude ophilic substitution occur" Wnte the structural formula of the product of each reaction... 

How many isomers of CsHs can you write" An article in the March 1994 issue of the Journal of Chemical Educa tion (pp 222-224) claims that there are several hun dred and draws structural formulas for 25 of them... 

The hydrogen at C 8 (the one shown in the structural formulas) crowds the —SO3H group in the less stable isomer... 

Within each analogous series of hydrocarbons there exist isomers of the compounds within that series. An isomer is defined as a compound with the same molecular formula as another compound but with a different structural formula. In other words, if there is a different way in which the carbon atoms can align themselves in the molecule, a different compound with different properties will exist. 

With the five-carbon alkane, pentane, there are three ways to draw the structural formula of this compound with five carbon atoms and twelve hydrogen atoms. The isomers of normal pentane are isopentane and neopentane. The structural formulas of these compounds are illustrated in Table 2, while typical properties are given in Table 1. 

Table 2. Illustrates the Structural Formulas for Isomers of Pentane.

As in the alkanes, it is possible for carbon atoms to align themselves in different orders to form isomers. Not only is it possible for the carbon atoms to form branches which produce isomers, but it is also possible for the double bond to be situated between different carbon atoms in different compounds. This different position of the double bond also results in different structural formulas, which, of course, are isomers. Just as in the alkanes, isomers of the alkenes have different properties. The unsaturated hydrocarbons and their derivatives are more active chemically than the saturated hydrocarbons and their derivatives. 

The substitution of one hydroxyl radical for a hydrogen atom in propane produces propyl alcohol, or propanol, which has several uses. Its molecular formula is C3H7OH. Propyl alcohol has a flash point of 77°F and, like all the alcohols, bums with a pale blue flame. More commonly known is the isomer of propyl alcohol, isopropyl alcohol. Since it is an isomer, it has the same molecular formula as propyl alcohol but a different structural formula. Isopropyl alcohol has a flash point of 53 F. Its ignition temperamre is 850°F, while propyl alcohol s ignition temperature is 700 F, another effect of the different stmcture. Isopropyl alcohol, or 2-propanol (its proper name) is used in the manufacture of many different chemicals, but is best known as rubbing alcohol. 

The answer to the first question is that you cannot easily calculate the number of isomers. The data in Table 2.1 were determined by a mathematician who concluded that no simple expression can calculate the number of isomers. The best way to ensure that you have written all the isomers of a par ticular- molecular- formula is to work systematically, beginning with the unbranched chain and then shortening it while adding branches one by one. It is essential that you be able to recognize when two different-looking structural formulas are actually the sane molecule written in different ways. The key point is the connectivity of the car bon chain. For example, the following group of structural... 

Recently, Nesmeyanov and co-workers have published definitive evidence that dwaZ reactivity (the formation of derivatives of two different structural formulas) extends beyond tautomerism (isomers in equilibrium or reversible isomeric transformation). A single molecular species can form two series of derivatives, in one of which a transfer of the reaction center occurs in the reaction. 

Two pictures of two spatial (three-dimensional) models can represent the same structural formula without representing the same stereoformula they describe the same structural formula if they exhibit the same relationships (if they are topologically congruent, i.e., they satisfy conditions (I), (II), (III)). In order to describe the same stereoformula they must display the same relationships and the same spatial orientation [they satisfy (I), (II), (III), and in addition (IV) (with A ), that is, be spatially congruent]. If two formulas viewed as stereoformulas are equal then they are certainly equal when they are treated as structural formulas. Consequently there are at least as many stereoisomers as there are structural isomers. This fact is reflected by (2.8). It is true particularly for paraffins and monosubstituted paraffins. 

In the realm of chemical enumeration we note Polya s equation (4.4) which gives the generating function for stereoisomers of the alkyl radicals, or equivalently, alcohols — that is, equation (5.2) of this article. His equation (4.3) gives the corresponding result for the structural isomers of these compounds. His equations (4.2) and (4.5) correspond, respectively, to the cases of alcohols without any asymmetric carbon atoms and the number of embeddings in the plane of structural formulae for alcohols in general. The latter problem is not chemically very significant. 

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. 

Draw structural formulas for all the isomers of ethylene (QH ) in which two of the hydrogen atoms have been replaced by deuterium atoms. Label the cis and the irons isomers. 

We have now found all possible structural formulas for the ethanol molecule. The oxygen atom is either directly bonded to one carbon atom or to two carbon atoms. Once a choice between these two possibilities is made, the structure of the rest of the molecule can be determined from the molecular formula and the bonding rules. The two possible structures are shown in Figure 18-2. Such compounds with the same molecular formula but different structural formulas are called structural isomers. The existence of the two compounds I and 2 was known long before their structures were clarified. Hence the existence of these isomers perplexed chemists for decades. Now we recognize the crucial impor-... 

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