Structural isomers Organic compounds

The Molecular Structure of Organic Compounds COAL 2 Given Lewis diagrams, ball-and-stick models, or spacefilling models of two or more organic molecules with the same molecular formula, distinguish between isomers and different orientations of the same molecule. 

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. 

At one time, the structures of organic compounds were deduced by chemical reactions that related a compound of unknown strucmre to compounds whose structures were known. Many chemical reactions had to be carried out to accomphsh this task. It was possible to reason backward to postulate what the structure of the original compound must have been to yield the observed products. Structure determination by chemical reactions is a time-consuming process. For example, if we want to determine the structure of a relatively simple compound with molecular formula, we will find that 88 isomers are possible, including ethers, alcohols, aldehydes, and ketones. Many chemical reactions would be required to identify the functional group and to determine the hydrocarbon skeleton. Structure determination by chemical reactions also has another severe limitation each reaction destroys part of the sample of the unknown compound. Spectroscopic structure determination requires only small amounts of a compound, and the experimental methods require very litde time compared to the arduous process of determining a molecular structure by a series of chemical reactions. 

Protonation of the anion [SN2] by acetic acid in diethyl ether produces the thermally unstable sulfur diimide S(NH)2. Like all sulfur diimides, the parent compound S(NH)2 can exist as three isomers (Scheme 5.5). Ab initio molecular orbital calculations indicate that the (cis,cis) configuration is somewhat more stable than the (cis,trans) isomer, while the (trans,trans) isomer is expected to possess considerably higher energy. The alternative syn,anti or E,Z nomenclatures may also be used to describe these isomers. The structures of organic derivatives S(NR)2 (R = alkyl, aryl) are discussed in Section 10.4.2. 

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. 

Isomers are compounds with the same molecular formula but different structural formulas. Some organic and biochemical compounds may exist in different isomeric forms, and these different isomers have different properties. The two most common types of isomers in organic systems are cis-trans isomers and isomerism due to the presence of a chiral Ccirbon. 

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 ... 

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