Organic Nomenclature and Functional Groups Essay

Learning how to name and draw the structure of the various compounds is the first step in learning to speak the language of organic chemistry. Chapter 1 presented organic chemistry as the chemistry of the carbon atom. However, many organic compounds contain other atoms besides carbon that contribute significantly to the physical and chemical properties of the compound. Chemists call these atoms heteroatoms, and the groups they form, functional groups. This chapter provides an overview of the rules for naming organic www. ochem4free. com 5 July 2005 Organic Chemistry – Ch 2 73 Daley & Daley compounds. It also introduces the major functional groups that you will encounter as you study organic chemistry along with the rules of how to name them and draw their structures. The presence of heteroatoms radically changes the physical and chemical properties of the compounds to which they are bonded. In fact, the carbon— heteroatom bonds and the carbon—carbon multiple bonds are the main sites where chemical reactions take place. Organic compounds are arranged into classes according to the particular functional groups that they contain. Members of each class of compounds share common chemical and physical characteristics. The names of organic compounds are assigned according to the class of the compound as determined by the functional groups. This chapter also shows how to draw the structural representations of these compounds. 2. 1 Drawing Organic Structures A two-dimensional structural formula of a hydrocarbon shows all of the atoms with all of their bonds in the plane of the page. Molecules are actual, three-dimensional entities. Their structure is a major factor that determines their physical properties and the way one molecule interacts with another molecule. These bonds are combinations of single bonds with hydrogen atoms and single or multiple bonds with other carbon atoms. For molecules that contain a large number of atoms or complex structures, drawing every bond and every atom is time and space consuming. A common notation developed to abbreviate the drawing without sacrificing the clarity of the structure is the condensed structural formula shown below for heptane: CH3 CH2 CH2 CH2 CH2 CH2 CH3 Heptane www. ochem4free. com 5 July 2005 Organic Chemistry – Ch 2 74 Daley & Daley Taking out the lines representing the carbon—carbon bonds condenses this formula still more: CH3CH2CH2CH2CH2CH2CH3 Heptane Heptane has five repeating —CH2— groups, called methylene groups. Because many organic molecules have such repetitive groups, an even more condensed notation shows these repeating units. Using this notation, the formula for heptane is as follows: CH3(CH2)5CH3 Heptane Bond-line formulas represent the carbon atoms as the intersection of lines and as line ends. You assume all the hydrogens needed to complete carbon’s valences. The bond-line structural formula is the notation that most organic chemists prefer to use. Bond-line formulas are easy to draw and quickly convey the essential structure of a molecule. Both the ends and the angles of the structure represent the carbon atoms. C—H bonds are not shown, but you should assume that the appropriate number of hydrogen atoms is present to complete the four bonds required by carbon to have its octet of electrons. The bond-line formula for heptane looks like this: Heptane Not all hydrocarbons are straight chains; many are rings. Chemists use the same structural formulas for them. Because the illustration of the two-dimensional structural formula of methylcyclopentane is so cluttered, it does not clearly show the ring. H H H C C H C C C H H CH HH H HH Methylcyclopentane The condensed structural formula is clearer. www. ochem4free. com 5 July 2005 Organic Chemistry – Ch 2 75 Daley & Daley CH2 CH2 CH CH2 CH3 CH2 Methylcyclopentane The bond-line structural formula is even clearer. Thus, chemists use it most frequently. Methylcyclopentane Often, chemists combine the bond-line and condensed notations to clarify a structure or emphasize specific features. This formula also represents methylcyclopentane. CH3 Methylcyclopentane Exercise 2. 1 Redraw each of the following condensed structural formulas using the bond-line notation.