the actual shape of the molecule, the chain of atoms is not at all straight. Also of impor- tance is this: Atoms joined by single bonds can rotate relatively freely with respect to one another . (We shall discuss the reason for this in Section 1.12B.) This relatively free rotation means that the chain of atoms in propyl alcohol can assume a variety of arrangements like these: Equivalent dash formulas for propyl alcohol or C C H C H H H H H H O H C C H C O H H H H H H H H H H or C C H C O H H H H It also means that all of the structural formulas above are equivalent and all repre- sent propyl alcohol. Dash structural formulas such as these indicate the way in which the atoms are attached to each other and are not representations of the actual shapes of the molecule. (Propyl alcohol does not have 90 8 bond angles . It has tetrahedral bond angles.) Dash structural formulas show what is called the connectivity of the atoms. Constitutional isomers (Section 1.6A) have different connectivities and, therefore, must have different structural formulas . Consider the compound called isopropyl alcohol, whose formula we might write in a variety of ways: Equivalent dash formulas for isopropyl alcohol H 9 C 9 C 9 C 9 H or H H H H H H O H 9 C 9 C 9 C 9 H H H H H O H H or H 9 C 9 C 9 H H H H O 9 H H 9 C 9 H Isopropyl alcohol is a constitutional isomer (Section 1.6A) of propyl alcohol because its atoms are connected in a different order and both compounds have the same molecular formula, C 3 H 8 O . In isopropyl alcohol the OH group is attached to the central carbon; in propyl alcohol it is attached to an end carbon. L In problems you will often be asked to write structural formulas for all the isomers that have a given molecular formula. Do not make the error of writing several equivalent formulas, like those that we have just shown, mistaking them for di ff erent constitutional isomers. Although electron-dot formulas account explicitly for all of the valence electrons in a molecule, they are tedious and time-consuming to write. Dash, condensed, and bond-line formulas are therefore used more often. Generally it is best to draw unshared electron pairs in chemical formulas, though some- times they are omitted if we are not considering the chemical properties or reactivity of a com- pound. When we write chemical reactions, however, we shall see that it is necessary to include the unshared electron pairs when they participate in a reaction. It is a good idea, therefore, to be in the habit of writing unshared electrons pairs. Helpful Hint It is important that you be able to recognize when a set of structural formulas has the same connectivity versus when they are constitutional isomers.
1.7 HOW TO WRITE AND INTERPRET STRUCTURAL FORMULAS 17 1.7B Condensed Structural Formulas Condensed structural formulas are somewhat faster to write than dash formulas and, when we become familiar with them, they will impart all the information that is con- tained in the dash structure. In condensed formulas all of the hydrogen atoms that are attached to a particular carbon are usually written immediately after the carbon. In fully
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