Structural isomers Organic compounds atoms

Compounds that have the same molecular formula whose atoms are linked in different ways are called isomers. We call an atomic linkage a structure. As we examine the structure of organic compounds in detail, we will find that subtle structural differences profoundly affect the physical and chemical properties of isomers. 

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 ... 

The molecules of two organic compounds are sometimes composed of the same type and number of atoms, but arranged in different ways. The molecular formula of each one of such compounds, which are known as isomers (for example, isoleucine and alloisoleucine, shown in Fig. 73), is therefore identical to that of the other only the structural formulas of the two isomers show the differences between their molecules (see Textbox 63). 

In writing structural isomers as well as any organic compound, remember that carbon forms four bonds One of the most common mistakes that a chemistry student makes is writing an organic structure with a carbon atom having less or more than four bonds. 

This second branched molecule is called isobutane. Compounds sharing the same molecular formula but having different structures are called structural isomers. Normal butane and isobutane have different physical properties. The number of structural isomers for the alkanes is included in Table 15.1. It can be seen in this table that as the number of carbon atoms increases that the number of possible isomers also increases. The fact that numerous isomers exist for most organic compounds is another reason why there are so many organic compounds. 

Organic compounds show a widespread occurrence of isomers, which are compounds having the same molecular formula but different structural formulas, and therefore possessing different properties. This phenomenon of isomerism is exemplified by isobutane and -butane [Fig. l-l(a) and (b)]. The number of isomers increases as the number of atoms in the organic molecule increases. 

Compounds such as those in Figure 14.4 are known as isomers. Isomers are substances which have the same molecular formula but different structural formulae. The different structures of the compounds shown in Figure 14.4 have different melting and boiling points. Molecule b contains a branched chain and has a lower melting point than molecule a, which has no branched chain. All the alkane molecules with four or more carbon atoms possess isomers. Perhaps now you can see why there are so many different organic compounds ... 

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 ... 

As an example, consider the molecule formed from one atom each of carbon, hydrogen, and nitrogen. We can assemble these atoms in two different ways and still satisfy the octet rule. (This is another example of compounds that have the same molecular formula but a different arrangement of bonded atoms. Such compounds are called constitutional isomers or structural isomers and are very common in organic chemistry.) The structure on the left was the one used in Figure 1.6. The structure on the right is obtained if the hydrogen is bonded to the N rather than the C. 

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. 

Figure 13.6 showed you that an organic compound can be arranged in different structural shapes, called isomers. All the isomers of a compound have the same molecular formula. In this investigation, you will make two-dimensional and three-dimensional models of isomers. Your models will help you explore the arrangements of the atoms in organic compounds. 

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