Formulas, Isomers

Isomers (Section 1.8) Different compounds that have the same molecular formula. Isomers may be either constitutional isomers or stereoisomers. 

Following are four formula isomers first drawn in the more conventional straight-chain manner and below in more conformationally correct forms drawn for a compound, 2,3-dichlorohexane, with two stereocenters. 

Name Alkane Prefix Molecular Formula Structural Formula Isomers... 

As noted earlier, more than one compound may have the same molecular formula (isomers), but a structural formula is unique to one compound. In addition, there are many chemicals which possess more than one chemical name, for the same reason mentioned above. The most common organic chemicals are those that have the shortest carbon chains. This fact is also true of their derivatives. The inclusion of a double bond in the structural formula has a profound effect on the properties of a compound. Table 2 illustrates those differences through the properties of alkenes. The presence of a double bond (and, indeed, a triple bond) between two carbon atoms in a hydrocarbon increases the chemical activity of the compound tremendously over its corresponding saturated hydrocarbon. The smaller the molecule (that is, the shorter the chain), the more pronounced this activity is. A case in point is the unsaturated hydrocarbon ethylene. Disregarding... 

In contrast to chemical formula isomers, positional isomers are a narrower subclass where the compounds contain the same appending groups, but those groups may be connected in a different manner one from another. Positional isomers must have the same substituents, but those substituents occupy a different position on the molecule. 

A compound that has two or more alternate structures but the same molecular formula. Isomers differ in physical properties from each other. There are many different types of isomerism, e.g. structural, optical, functional group and geometrical isomerism. The term is most widely used in organic chemistry. 

Molecular formula. Isomer number. Kekul structure... 

Despite their identical molecular formulas, isomers have different properties. The boiling and melting points of 2-methylpropane and butane are different, as are their densities and solubilities in water. In addition, their chemical reactivity is different. Figure 18.3 shows some property differences in the isomers of pentane. 

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... 

An isomer is one of several species that have the same atomic composition (molecular formula), but have different constitutional formulae (atom connectivities) or different stereochemical formulae. Isomers exhibit different physical and/or chemical properties. 

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. 

Isomers are different compounds that have the same molecular formula. Isomers that have their atoms connected in a different order (branched versus unbranched chain, for example) or a different sequence of bond types (C=CCC versus CC=CC,... 

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

Formula Isomer Point, °C Formula Isomer Point, °C... 

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