FUNCTIONAL GROUPS AND NOMENCLATURE 1 FUNCTIONAL GROUPS • A functional group is a portion of an organic molecule which consists of atoms other than carbon and hydrogen, or which contains bonds other than C–C and C–H bonds. For example, ethane (Fig 1a) is an alkane and has no functional group. All the atoms are carbon and hydrogen and all the bonds are C–C and C–H. Ethanoic acid (Fig 1b) on the other hand has a portion of the molecule which contains atoms other than carbon and hydrogen, and bonds other than C—H and C—C. This portion of the molecule is called a functional group – • in this case a carboxylic acid. 2 Functional Groups 3 Common functional groups • The following are some of the more common functional groups in organic chemistry. • These are rarely classified as functional groups 4 Common functional groups 5 Common functional groups 6 Common functional groups 7 NOMENCLATURE • Nomenclature of alkanes • Alkanes are composed of only carbon atoms and hydrogen atoms and contain only single bonds. Compounds that contain only carbon and hydrogen are called hydrocarbons, so an alkane is a hydrocarbon that has only single bonds. Alkanes in which the carbons form a continuous chain with no branches are called straight-chain alkanes. 8 Alkanes • As the number of carbons in an alkane increases beyond three, the number of possible structures increases. There are two possible structures for an alkane with molecular formula In addition to butane—a straight-chain alkane—there is a branched butane called isobutane. Both of these structures fulfill the requirement that each carbon forms four bonds and each hydrogen forms only one bond. 9 Alkanes • Compounds such as butane and isobutane that have the same molecular formula but differ in the order in which the atoms are connected are called constitutional isomers—their molecules have different constitutions. In fact, isobutane got its name because it is an “iso”mer of butane. The structural unit—a carbon bonded to a hydrogen and two groups—that occurs in isobutane has come to be called “iso.” Thus, the name isobutane tells you that the compound is a four- carbon alkane with an iso structural unit. 10 Alkanes 11 Alkanes • There are three alkanes with molecular formula Pentane is the straightchain alkane. Isopentane, as its name indicates, has an iso structural unit and five carbon atoms. The third isomer is called neopentane. The structural unit with a carbon surrounded by four other carbons is called “neo.” 12 Alkanes 13 Alkanes • There are five constitutional isomers with molecular formula C6H14 14 Alkanes • There are nine alkanes with molecular formula C7H16 • Notice that neoheptane cannot be used as a name because three different heptanes have a carbon that is bonded to four other carbons and a name must specify only one compound. • The number of constitutional isomers increases rapidly as the number of carbons in an alkane increases. For example, there are 75 alkanes with molecular formula C10H22 15 Alkanes 16 17 18 IUPAC nomenclature • This method of nomenclature is called systematic nomenclature. It is also called IUPAC nomenclature because it was designed by a commission of the International Union of Pure and Applied Chemistry (abbreviated IUPAC and pronounced “eye-youpack”) at a meeting in Geneva, Switzerland, in 1892. The IUPAC rules have been continually revised by the commission since then. Names such as isobutane and neopentane—nonsystematic names—are called common names 19 Nomenclature of Alkyl Substituents • Removing a hydrogen from an alkane results in an alkyl substituent (or an alkyl group). Alkyl substituents are named by replacing the “ane” ending of the alkane with “yl.” The letter “R” is used to indicate any alkyl group. 20 Nomenclature of Alkyl Substituents • If a hydrogen of an alkane is replaced by an OH, the compound becomes an alcohol; if it is replaced by an the compound becomes an amine; and if it is replaced by a halogen, the compound becomes an alkyl halide. 21 Nomenclature of Alkyl Substituents • An alkyl group name followed by the name of the class of the compound (alcohol, amine, etc.) yields the common name of the compound. The following examples show how alkyl group names are used to build common names: 22 Nomenclature of Alkyl Substituents • Two alkyl groups—a propyl group and an isopropyl group—contain three carbon atoms. A propyl group is obtained when a hydrogen is removed from a primary carbon of propane. A primary carbon is a carbon that is bonded to only one other carbon. An isopropyl group is obtained when a hydrogen is removed from the secondary carbon of propane. A secondary carbon is a carbon that is bonded to two other carbons. Notice that an isopropyl group, as its name indicates, has its three carbon atoms arranged as an iso structural unit. 23 Nomenclature of Alkyl Substituents 24 Nomenclature of Alkyl Substituents • There are four alkyl groups that contain four carbon atoms. The butyl and isobutyl groups have a hydrogen removed from a primary carbon. A sec-butyl group has a hydrogen removed from a secondary carbon (sec-, often abbreviated s-, stands for secondary), and a tert-butyl group has a hydrogen removed from a tertiary carbon (tert-, sometimes abbreviated t-, stands for tertiary). A tertiary carbon is a carbon that is bonded to three other carbons. Notice that the isobutyl group is the only group with an iso structural unit. 25 Nomenclature of Alkyl Substituents 26 Nomenclature of Alkyl Substituents • A name of a straight-chain alkyl group often has the prefix “n” (for “normal”), to emphasize that its carbon atoms are in an unbranched chain. If the name does not have a prefix such as “n” or “iso,” it is assumed that the carbons are in an unbranched chain. 27 Nomenclature of Alkyl Substituents • Like the carbons, the hydrogens in a molecule are also referred to as primary, secondary, and tertiary. Primary hydrogens are attached to primary carbons, secondary hydrogens to secondary carbons, and tertiary hydrogens to tertiary carbons. 28 Nomenclature of Alkanes • The systematic name of an alkane is obtained using the following rules: • 1. Determine the number of carbons in the longest continuous carbon chain. This chain is called the parent hydrocarbon. 29 TWO • The name of any alkyl substituent that hangs off the parent hydrocarbon is cited before the name of the parent hydrocarbon, together with a number to designate the carbon to which the alkyl substituent is attached. The chain is numbered in the direction that gives the substituent as low a number as possible. The substituent’s name and the name of the parent hydrocarbon are joined in one word, and there is a hyphen between the number and the substituent’s name. 30 Number the chain so that the substituent gets the lowest possible number. 31 THREE • If more than one substituent is attached to the parent hydrocarbon, the chain is numbered in the direction that will result in the lowest possible number in the name of the compound. The substituents are listed in alphabetical (not numerical) order, with each substituent getting the appropriate number. In the following example,the correct name (5-ethyl-3- methyloctane) contains a 3 as its lowest number,while the incorrect name (4-ethyl-6- methyloctane) contains a 4 as its lowest number: 32 THREE • If two or more substituents are the same, the prefixes “di,”“tri,” and “tetra” are used to indicate how many identical substituents the compound has. 33 THREE • The prefixes di, tri, tetra, sec, and tert are ignored in alphabetizing substituent groups, but the prefixes iso, neo, and cyclo are not ignored. 34 FOUR • When both directions lead to the same lowest number for one of the substituents, the direction is chosen that gives the lowest possible number to one of the remaining substituents. 35 FIVE • If the same substituent numbers are obtained in both directions, the first group cited receives the lower number. 36 SIX • If a compound has two or more chains of the same length, the parent hydrocarbon is the chain with the greatest number of substituents 37 NOTE • In a compound such as 4-(1- methylethyl)octane, the substituent name is in parentheses; the number inside the parentheses indicates a position on the substituent, whereas the number outside the parentheses indicates a position on the parent hydrocarbon. 38 EXAMPLES 39 EXAMPLES 40 Nomenclature of Cycloalkanes • Cycloalkanes are alkanes with their carbon atoms arranged in a ring. Because of the ring, a cycloalkane has two fewer hydrogens than an acyclic (noncyclic) alkane with the same number of carbons. This means that the general molecular formula for a cycloalkane is Cycloalkanes are named by adding the prefix “cyclo” to the alkane name that signifies the number of carbon atoms in the ring. 41 EXAMPLES 42 Cycloalkanes • Cycloalkanes are almost always written as skeletal structures. Skeletal structures show the carbon–carbon bonds as lines, but do not show the carbons or the hydrogens bonded to carbons. 43 Acyclic molecules • Acyclic molecules can also be represented by skeletal structures. In a skeletal structure of an acyclic molecule, the carbon chains are represented by zigzag lines. Again, each vertex represents a carbon, and carbons are assumed to be present where a line begins or ends. 44 Acyclic molecules 45 The rules for naming cycloalkanes • They resemble the rules for naming acyclic alkanes: 1. In the case of a cycloalkane with an attached alkyl substituent, the ring is the parent hydrocarbon unless the substituent has more carbon atoms than the ring. In that case, the substituent is the parent hydrocarbon and the ring is named as a substituent.