FUNCTIONAL GROUPS AND NOMENCLATURE

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FUNCTIONAL GROUPSAND NOMENCLATURE1

FUNCTIONAL GROUPS A functional group is a portion of an organic moleculewhich consists of atoms other than carbon andhydrogen, or which contains bonds other than C–C andC–H bonds. For example, ethane (Fig 1a) is an alkaneand has no functional group. All the atoms are carbonand hydrogen and all the bonds are C–C and C–H.Ethanoic acid (Fig 1b) on the other hand has a portionof the molecule which contains atoms other thancarbon and hydrogen, and bonds other than C—H andC—C. This portion of the molecule is called a functionalgroup – in this case a carboxylic acid.2

Functional Groups3

Common functional groups The following are some of the more commonfunctional groups in organic chemistry. These are rarely classified as functional groups4

Common functional groups5

Common functional groups6

Common functional groups7

NOMENCLATURE Nomenclature of alkanes Alkanes are composed of only carbon atomsand hydrogen atoms and contain only singlebonds. Compounds that contain only carbonand hydrogen are called hydrocarbons, so analkane is a hydrocarbon that has only singlebonds. Alkanes in which the carbons form acontinuous chain with no branches are calledstraight-chain alkanes.8

Alkanes As the number of carbons in an alkaneincreases beyond three, the number ofpossible structures increases. There are twopossible structures for an alkane withmolecular formula In addition to butane—astraight-chain alkane—there is a branchedbutane called isobutane. Both of thesestructures fulfill the requirement that eachcarbon forms four bonds and each hydrogenforms only one bond.9

Alkanes Compounds such as butane and isobutane thathave the same molecular formula but differ in theorder in which the atoms are connected arecalled constitutional isomers—their moleculeshave different constitutions. In fact, isobutane gotits name because it is an “iso”mer of butane. Thestructural unit—a carbon bonded to a hydrogenand two groups—that occurs in isobutane hascome to be called “iso.” Thus, the nameisobutane tells you that the compound is a fourcarbon alkane with an iso structural unit.10

Alkanes11

Alkanes There are three alkanes with molecularformula Pentane is the straightchain alkane.Isopentane, as its name indicates, has an isostructural unit and five carbon atoms. Thethird isomer is called neopentane. Thestructural unit with a carbon surrounded byfour other carbons is called “neo.”12

Alkanes13

Alkanes There are five constitutional isomers withmolecular formula C6H1414

Alkanes There are nine alkanes with molecular formulaC7H16 Notice that neoheptane cannot be used as aname because three different heptanes have acarbon that is bonded to four other carbonsand a name must specify only one compound. The number of constitutional isomersincreases rapidly as the number of carbons inan alkane increases. For example, there are 75alkanes with molecular formula C10H2215

Alkanes16

17

18

IUPAC nomenclature This method of nomenclature is called systematicnomenclature. It is also called IUPACnomenclature because it was designed by acommission of the International Union of Pureand Applied Chemistry (abbreviated IUPAC andpronounced “eye-youpack”) at a meeting inGeneva, Switzerland, in 1892. The IUPAC ruleshave been continually revised by the commissionsince then. Names such as isobutane andneopentane—nonsystematic names—are calledcommon names19

Nomenclature of Alkyl Substituents Removing a hydrogen from an alkane resultsin an alkyl substituent (or an alkyl group). Alkylsubstituents are named by replacing the “ane”ending of the alkane with “yl.” The letter “R” isused to indicate any alkyl group.20

Nomenclature of Alkyl Substituents If a hydrogen of an alkane is replaced by anOH, the compound becomes an alcohol; if it isreplaced by an the compound becomes anamine; and if it is replaced by a halogen, thecompound becomes an alkyl halide.21

Nomenclature of Alkyl Substituents An alkyl group name followed by the name ofthe class of the compound (alcohol, amine,etc.) yields the common name of thecompound. The following examples show howalkyl group names are used to build commonnames:22

Nomenclature of Alkyl Substituents Two alkyl groups—a propyl group and anisopropyl group—contain three carbon atoms. Apropyl group is obtained when a hydrogen isremoved from a primary carbon of propane. Aprimary carbon is a carbon that is bonded to onlyone other carbon. An isopropyl group is obtainedwhen a hydrogen is removed from the secondarycarbon of propane. A secondary carbon is acarbon that is bonded to two other carbons.Notice that an isopropyl group, as its nameindicates, has its three carbon atoms arranged asan iso structural unit.23

Nomenclature of Alkyl Substituents24

Nomenclature of Alkyl Substituents There are four alkyl groups that contain fourcarbon atoms. The butyl and isobutyl groups havea hydrogen removed from a primary carbon. Asec-butyl group has a hydrogen removed from asecondary carbon (sec-, often abbreviated s-,stands for secondary), and a tert-butyl group hasa hydrogen removed from a tertiary carbon (tert-,sometimes abbreviated t-, stands for tertiary). Atertiary carbon is a carbon that is bonded tothree other carbons. Notice that the isobutylgroup is the only group with an iso structuralunit.25

Nomenclature of Alkyl Substituents26

Nomenclature of Alkyl Substituents A name of a straight-chain alkyl group oftenhas the prefix “n” (for “normal”), toemphasize that its carbon atoms are in anunbranched chain. If the name does not havea prefix such as “n” or “iso,” it is assumed thatthe carbons are in an unbranched chain.27

Nomenclature of Alkyl Substituents Like the carbons, the hydrogens in a moleculeare also referred to as primary, secondary, andtertiary. Primary hydrogens are attached toprimary carbons, secondary hydrogens tosecondary carbons, and tertiary hydrogens totertiary carbons.28

Nomenclature of Alkanes The systematic name of an alkane is obtainedusing the following rules: 1. Determine the number of carbons in thelongest continuous carbon chain. This chain iscalled the parent hydrocarbon.29

TWO The name of any alkyl substituent that hangs offthe parent hydrocarbon is cited before the nameof the parent hydrocarbon, together with anumber to designate the carbon to which thealkyl substituent is attached. The chain isnumbered in the direction that gives thesubstituent as low a number as possible. Thesubstituent’s name and the name of the parenthydrocarbon are joined in one word, and there isa hyphen between the number and thesubstituent’s name.30

Number the chain so that the substituentgets the lowest possible number.31

THREE If more than one substituent is attached to theparent hydrocarbon, the chain is numbered in thedirection that will result in the lowest possiblenumber in the name of the compound. Thesubstituents are listed in alphabetical (notnumerical) order, with each substituent gettingthe appropriate number. In the followingexample,the correct name (5-ethyl-3methyloctane) contains a 3 as its lowestnumber,while the incorrect name (4-ethyl-6methyloctane) contains a 4 as its lowest number:32

THREE If two or more substituents are the same, theprefixes “di,”“tri,” and “tetra” are used toindicate how many identical substituents thecompound has.33

THREE The prefixes di, tri, tetra, sec, and tert areignored in alphabetizing substituent groups,but the prefixes iso, neo, and cyclo are notignored.34

FOUR When both directions lead to the samelowest number for one of the substituents,the direction is chosen that gives the lowestpossible number to one of the remainingsubstituents.35

FIVE If the same substituent numbers are obtainedin both directions, the first group citedreceives the lower number.36

SIX If a compound has two or more chains of thesame length, the parent hydrocarbon is thechain with the greatest number ofsubstituents37

NOTE In a compound such as 4-(1methylethyl)octane, the substituent name is inparentheses; the number inside theparentheses indicates a position on thesubstituent, whereas the number outside theparentheses indicates a position on the parenthydrocarbon.38

EXAMPLES39

EXAMPLES40

Nomenclature of Cycloalkanes Cycloalkanes are alkanes with their carbonatoms arranged in a ring. Because of the ring,a cycloalkane has two fewer hydrogens thanan acyclic (noncyclic) alkane with the samenumber of carbons. This means that thegeneral molecular formula for a cycloalkane isCycloalkanes are named by adding the prefix“cyclo” to the alkane name that signifies thenumber of carbon atoms in the ring.41

EXAMPLES42

Cycloalkanes Cycloalkanes are almost always written asskeletal structures. Skeletal structures showthe carbon–carbon bonds as lines, but do notshow the carbons or the hydrogens bonded tocarbons.43

Acyclic molecules Acyclic molecules can also be represented byskeletal structures. In a skeletal structure of anacyclic molecule, the carbon chains arerepresented by zigzag lines. Again, each vertexrepresents a carbon, and carbons are assumedto be present where a line begins or ends.44

Acyclic molecules45

The rules for naming cycloalkanes They resemble the rules for naming acyclicalkanes:1. In the case of a cycloalkane with an attachedalkyl substituent, the ring is the parenthydrocarbon unless the substituent has morecarbon atoms than the ring. In that case, thesubstituent is the parent hydrocarbon and thering is named as a substituent. There is no needto number the position of a single substituent ona ring.46

The rules for naming cycloalkanes47

The rules for naming cycloalkanes If the ring has two different substituents, theyare cited in alphabetical order and thenumber 1 position is given to the substituentcited first.48

The rules for naming cycloalkanes If there are more than two substituents on the ring,they are cited in alphabetical order. The substituentgiven the number 1 position is the one that results in asecond substituent getting as low a number aspossible. If two substituents have the same lownumber, the ring is numbered—either clockwise orcounterclockwise—in the direction that gives the thirdsubstituent the lowest possible number. For example,the correct name of the following compound is 4-ethyl2-methyl1-propylcyclohexane, not 5-ethyl-1-methyl-2propylcyclohexane:49

The rules for naming cycloalkanes50

Nomenclature of Alkyl Halides Alkyl halides are compounds in which ahydrogen of an alkane has been replaced by ahalogen. Alkyl halides are classified as primary,secondary, or tertiary, depending on thecarbon to which the halogen is attached.Primary alkyl halides have a halogen bondedto a primary carbon, secondary alkyl halideshave a halogen bonded to a secondarycarbon, and tertiary alkyl halides have ahalogen bonded to a tertiary carbon51

Nomenclature of Alkyl Halides52

Nomenclature of Alkyl Halides The common names of alkyl halides consist ofthe name of the alkyl group, followed by thename of the halogen—with the “ine” endingof the halogen name replaced by “ide” (i.e.,fluoride, chloride, bromide, iodide).53

Nomenclature of Alkyl Halides In the IUPAC system, alkyl halides are named assubstituted alkanes. The substituent prefix namesfor the halogens end with “o” o”). Therefore, alkylhalides are often called haloalkanes.54

Nomenclature of Ethers Ethers are compounds in which an oxygen isbonded to two alkyl substituents. If the alkylsubstituents are identical, the ether is asymmetrical ether. If the substituents aredifferent, the ether is an unsymmetrical ether.55

Nomenclature of Ethers The common name of an ether consists of thenames of the two alkyl substituents (inalphabetical order), followed by the word“ether.” The smallest ethers are almost alwaysnamed by their common names.56

Nomenclature of Ethers57

Nomenclature of Ethers The IUPAC system names an ether as analkane with an RO substituent. Thesubstituents are named by replacing the “yl”ending in the name of the alkyl substituentwith “oxy.”58

Nomenclature of Ethers59

Nomenclature of Alcohols Alcohols are compounds in which a hydrogenof an alkane has been replaced by an OHgroup. Alcohols are classified as primary,secondary, or tertiary, depending on whetherthe OH group is bonded to a primary,secondary, or tertiary carbon—the same wayalkyl halides are classified.60

Nomenclature of Alcohols The common name of an alcohol consists ofthe name of the alkyl group to which the OHgroup is attached, followed by the word“alcohol.”61

Nomenclature of Alcohols The functional group is the center of reactivityin a molecule. In an alcohol, the OH is thefunctional group. The IUPAC system uses asuffix to denote certain functional groups. Thesystematic name of an alcohol, for example, isobtained by replacing the “e” at the end of thename of the parent hydrocarbon with thesuffix “ol.”62

Nomenclature of Alcohols When necessary, the position of the functionalgroup is indicated by a number immediatelypreceding the name of the alcohol orimmediately preceding the suffix. The mostrecently approved IUPAC names are thosewith the number immediately preceding thesuffix.63

Nomenclature of Alcohols The following rules are used to name acompound that has a functional group suffix:1. The parent hydrocarbon is the longestcontinuous chain containing the functionalgroup. The parent hydrocarbon is numbered in thedirection that gives the functional group suffixthe lowest possible number.64

Example65

Nomenclature of Alcohols If there is a functional group suffix and asubstituent, the functional group suffix getsthe lowest possible number.66

Nomenclature of Alcohols If the same number for the functional groupsuffix is obtained in both directions, the chainis numbered in the direction that gives asubstituent the lowest possible number.Notice that a number is not needed todesignate the position of a functional groupsuffix in a cyclic compound, because it isassumed to be at the 1-position.67

Nomenclature of Alcohols68

Nomenclature of Alcohols If there is more than one substituent, thesubstituents are cited in alphabetical order.69

Nomenclature of Amines Amines are compounds in which one or moreof the hydrogens of ammonia have beenreplaced by alkyl groups. Smaller amines arecharacterized by their fishy odors. Fermentedshark, for example, a traditional dish inIceland, smells exactly like triethylamine.70

Nomenclature of Amine There are primary amines, secondary amines,and tertiary amines. The classificationdepends on how many alkyl groups arebonded to the nitrogen. Primary amines haveone alkyl group bonded to the nitrogen,secondary amines have two, and tertiaryamines have three.71

Nomenclature of Amine The common name of an amine consists ofthe names of the alkyl groups bonded to thenitrogen, in alphabetical order, followed by“amine.” The entire name is written as oneword (unlike the common names of alcohols,ethers, and alkyl halides, in which“alcohol,”“ether,” and “halide” are separatewords).72

Examples73

Nomenclature of Amine Nitrogen compounds with four alkyl groupsbonded to the nitrogen—thereby giving thenitrogen a positive formal charge—are calledquaternary ammonium salts. Their namesconsist of the names of the alkyl groups inalphabetical order, followed by “ammonium”(all in one word), and then the name of thecounterion as a separate word.74

Nomenclature of Amine75

Reference Textbooks Organic Chemistry Clayden Organic Chemistry Klein Organic Chemistry Paula Bruice76

Nomenclature of Alkyl Halides Alkyl halides are compounds in which a hydrogen of an alkane has been replaced by a halogen. Alkyl halides are classified as primary, secondary, or tertiary, depending on the carbon to which the halogen is attached. Primary alkyl halides have a halogen bonded to a primary carbon, secondary alkyl halides

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