6 TUTORIAL CHEMISTRY TEXTS RS·C ROL SOCETY OF CHEMSTRY Functional Group Chemistry by JAMES.R.HANSON
Preface The aim of this book is to provide an introduction to the characteristic properties of functional groups.It is written for first-year undergraduates. The book is in four chapters.The first is devoted to a general considera- tion of the bonding in functional groups.the classes of reagent and the types of r first of the se are p ay be divided into eral broad functional groups which the tions,mainly substitution and elimination,are those chemistry of these functional groups forms the second chapter.A second class of functional groups is those in which a n-bond is a characteristic feature.The initial step in many of their reactions is an addition.These functional groups are described in the third chapter.The electrons within a -bond may be symmetrically distributed as in an alkene,or unsymmetri- cally distribute as in the carbo alCOntcmgp n The ys a o. part in funct group che mistry. The chapter is voted to escription of the interaction between functional groups and ring.Heteroaromatic compounds are considered in terms of the perturba- tion of the n-system brought about by the insertion of the heteroatom. A balance has to be drawn in the use of systematic and trivial names. The IUPAC rules recognize that the use of trivial names for many simple ofter referred.Students will meet these names in the currer nt lite on ttles in the labo Howe because the systematic nomencla ture basis docum nting more complex structures,students need to be familiar with the use of sys tematic nomenclature by applying it to simple molecules.Both names will be given for compounds at appropriate stages in the text.An Appendix of Common and Systematic Names can be found on the RSC website (http://www.chemsoc.org/pdf/tct/functionalappendix.pdf),as well as a Further Reading list(http://www.chemsoc.org/pdf/tct/functionalreading am indebted to Martyn Berry and Professor Sir John Cornforth AC FRS for their many valuable comments on the draft manuscript and par. ticularly to Professor Alwyn Davies FRS for his substantial help and encouragement throughout the preparation of the manuscript and dia- grams. J.R.Hanson Sussex
Preface The aim of this book is to provide an introduction to the characteristic properties of functional groups. It is written for first-year undergraduates. The book is in four chapters. The first is devoted to a general consideration of the bonding in functional groups, the classes of reagent and the types of reaction. Functional groups may be divided into several broad classes. The first of these are those functional groups in which the reactions, mainly substitution and elimination, are those of the o-bond. The chemistry of these functional groups forms the second chapter. A second class of functional groups is those in which a x-bond is a characteristic feature. The initial step in many of their reactions is an addition. These functional groups are described in the third chapter. The electrons within a n-bond may be symmetrically distributed as in an alkene, or unsymmetrically distributed as in the carbonyl group. The aromatic ring plays a major part in functional group chemistry. The final chapter is devoted to a description of the interaction between functional groups and the aromatic ring. Heteroaromatic compounds are considered in terms of the perturbation of the x-system brought about by the insertion of the heteroatom. A balance has to be drawn in the use of systematic and trivial names. The IUPAC rules recognize that the use of trivial names for many simple compounds is often preferred. Students will meet these names in reading the current literature and on the bottles in the laboratory. However, because the systematic nomenclature forms the basis of documenting more complex structures, students need to be familiar with the use of systematic nomenclature by applying it to simple molecules. Both names will be given for compounds at appropriate stages in the text. An Appendix of Common and Systematic Names can be found on the RSC website (h t tp://www.chemsoc, org/pdf/tct/functionalappendix. pdf), as well as a Further Reading list (http://www.chemsoc.org/pdf/tct/functionalreading. I am indebted to Martyn Berry and Professor Sir John Cornforth AC FRS for their many valuable comments on the draft manuscript and particularly to Professor Alwyn Davies FRS for his substantial help and encouragement throughout the preparation of the manuscript and diagrams. J. R. Hanson Sussex PdQ
Contents 1 General Principles 1.1 The Strueture of Functional Groups Reagents and Rea tion Learning Organic Functional Group Chemistry 1218 2 Chemistry of the c-Bond 24 24 Alkyl Halides 2.3 Alcohols 33 2.4 Epoxides and Ethers 43 Organosulfur Compounds 2.6 Aliphatic Amines 51 3 Chemistry of the n-Bond Alkenes Alkynes 3.3 Carbonyl Compounds 469 3.4 Carboxylic Acids and their Relatives 3.5 Enolate and Related Carbanion Chemistry 100 3.6 Nitriles,Imines and Nitro Compounds 4 Chemistry of Aromatic Compounds 115 4.1 Aromatic Substitution 115
Contents 1.1 The Structure of Functional Groups 1.2 Reagents and Reactions 1.3 Learning Organic Functional Group Chemistry 2.1 Alkanes 2.2 Alkyl Halides 2.3 Alcohols 2.4 Epoxides and Ethers 2.5 Organosulfur Compounds 2.6 Aliphatic Amines 3.1 Alkenes 3.2 Alkynes 3.3 Carbonyl Compounds 3.4 Carboxylic Acids and their Relatives 3.5 Enolate and Related Carbanion Chemistry 3.6 Nitriles, Imines and Nitro Compounds 4.1 Aromatic Substitution 1 12 18 24 27 33 43 48 51 64 76 79 92 100 105 115 V
vi Contents 4.2 Aromatic Functional Groups 123 4.3 Heteroaromatic Compounds 133 Answers to Problems 147 Subject Index 164
vi Contents 4.2 Aromatic Functional Groups 4.3 Heteroaromatic Compounds 123 133
1 General Principles Aims The aims of the first chapter of this book are to provide the foun. The relationship between bonding and structure of organic compounds 。 The oxidative and substitutive relationship between functional groups The relationship between electronegativity differences and the reactivity of functional groups The kinetic and thermodynamic control of reaction products 1.1 The Structure of Functional Groups A functional group isa chemically reactive group of atoms within molecule.Each may be modified by its position within the molecule or by the presence of other neighbouring functional groups. 1.1.1 Hybridization and Bonding aiolaicdet2o0roiCaPGaoekceoetronsini1sooa of pomding found arbon compounds eomrou ybid of the 2s and 2p orbitals.Combination of one 2s and three 2p orbitals
General Principles 1.1 The Structure of Functional Groups A is a chemically reactive group of atoms within a molecule. Each functional group has its characteristic reactivity, which may be modified by its position within the molecule or by the presence of other neighbouring functional groups. I =l =I Hybridization and Bonding An isolated carbon atom possesses two electrons in its 1s orbital, two electrons in its 2s orbital and two electrons in its 2p orbitals. The types of bonding found in carbon compounds arise from various hybrids of the 2s and 2p orbitals. Combination of one 2s and three 2p orbitals 1
Functional Group Chemistry leads to four equivalent sp'hybridized orbitals directed towards the H apices of a tetrahedron(see 1.1).Each of these orbitals can be occupied H置H b carbon atom. Thes atom to produce,fo 1.1 example methane (1.2).The energy required for the reorganization of the orbitals is gained from the formation of the four covalent bonds. Alternatively,the 2s and two 2p orbitals may be hybridized to give a planar sp2system accommodating three electrons from the carbon,one in each hybrid orbital.Three bonds may then be formed with other atoms (see 1.3).The remaining electron,which is in a p orbital at 90 to the plane of the spsystem,may overlap with a compa atom to toa double bond between th carbon and this atom as in ethene (1.4). A further way of making four bonds from the carbon is to hybridize a-bon the 2s and one 2p orbital to give two sp hybrids in which the orbitals are at 180 to each other.The remaining two 2p orbitals are used to form two -bonds at 90 each other (see 1.5).In this case there is a triple bond between the carbon and another atom as,for example,in ethyne (acetylene,1.6). These hybridizations have several consequences.Since an s orbital i closer to the nucleus than the corresponding p orbital,the increasing s character in the orbitals in changing from sp to sp2and then sp leads H-C=C-H to a decrease in bond length:sp3 C-C,0.154 nm;sp2 C=C,0.134 nm;sp 1.6 C=C.0.120 nm.Secondly,the increasing s character means that the bond- ing electrons in the sp and sp2 orbitals are held more tightly to the car bon than in spbond.This is reflected in the inrease in the polarity and acidity of a C-H*bond and in the ease of formation of organome compounds containing. the order sp3<sp2<sp.On the other hand,there is an increase in the difficulty of breaking a carbon-halogen bond in the ionic sense C*-X, in changing from an alkyl to an alkenyl(vinyl)halide.Thirdly,whereas the maximum electron density of a o-bond lies between the atoms form- ing the bond,that of a r-bond lies above and below the plane of the onding atoms,i.e.a n-bond is more exposed for reaction e lies in the anity for one n-bond to inter a c onjugate multiple bond by one carbon-carbon single bond so that overlap is possible between the r-bonds,they are said to be conjugated.Thus in butadiene (1.7)molecular orbitals may be written embracing all four atoms.These conjugated double bonds often behave as one functional rather than as two isolated double bonds.Electronic effects are conjugated system
2 Functional Group Chemistry leads to four equivalent orbitals directed towards the apices of a tetrahedron (see 1.1). Each of these orbitals can be occupied by one of the four available electrons from the carbon atom. These can each pair with one electron from another atom to produce, for example, methane (1.2). The energy required for the reorganization of the orbitals is gained from the formation of the four covalent bonds. Alternatively, the 2s and two 2p orbitals may be hybridized to give a planar system accommodating three electrons from the carbon, one in each hybrid orbital. Three bonds may then be formed with other atoms (see 1.3). The remaining electron, which is in a p orbital at 90" to the plane of the sp2 system, may overlap with a comparable p orbital from a second atom to form a , leading to a double bond between the carbon and this atom as in ethene (1.4). A further way of making four bonds from the carbon is to hybridize the 2s and one 2p orbital to give two hybrids in which the orbitals are at 180" to each other. The remaining two 2p orbitals are used to form two n;-bonds at 90" each other (see 1.5). In this case there is a triple bond between the carbon and another atom as, for example, in ethyne (acetylene, 1.6). These hybridizations have several consequences. Since an s orbital is closer to the nucleus than the corresponding p orbital, the increasing s character in the orbitals in changing from sp3 to sp2 and then sp leads to a decrease in bond length: sp3 C-C, 0.154 nm; sp2 C=C, 0.134 nm; sp CeC, 0.120 nm. Secondly, the increasing s character means that the bonding electrons in the sp and sp2 orbitals are held more tightly to the carbon than in an sp3 bond. This is reflected in the increase in the polarity and acidity of a C--H+ bond and in the ease of formation of organometallic compounds containing the C--M' bond. These follow the order sp3 < sp2 < sp. On the other hand, there is an increase in the difficulty of breaking a carbon-halogen bond in the ionic sense C+-X-, in changing from an alkyl to an alkenyl (vinyl) halide. Thirdly, whereas the maximum electron density of a 0-bond lies between the atoms forming the bond, that of a n;-bond lies above and below the plane of the bonding atoms, i.e. a n-bond is more exposed for reaction. A further consequence lies in the opportunity for one n;-bond to interact with another suitably oriented n-bond to give a system. When a carbon-carbon double bond is separated from another carbon multiple bond by one carbon-carbon single bond so that overlap is possible between the n;-bonds, they are said to be conjugated. Thus in butadiene (1.7) molecular orbitals may be written embracing all four atoms. These conjugated double bonds often behave as one Functional group rather than as two isolated double bonds. Electronic effects are relayed through the conjugated system. H I ,,\$ \ HH 1.2 ,A ,'I '\ 4+ - =: = ' 1.1 H-CCC-H 1.6
General Principles 3 1.7 18 1.9 H2C=C=CH2 1.10 1.11 1.12 1.13 A cyclic conjugated system containing(4n+2)n electrons has an extra stability over that of a comparable number of isolated double bonds. This extra stabilization,known as aromaticity,leads to a characteristic pattern of reactivity which distinguishes the reactions of benzene(1.8) from,for example,the linear hexatriene(1.9)or cyclooctatetraene(1.10) (4n electrons,n =2).The aromatic sextet may arise not just from the overlap f oub bonds s in benzene (or pyridine(11)bu also from the participa ion of the lone pair of electrons on a heteroatom Thus pyrrole(1.12),with effectively six -electrons,shows some aromatic character.In allene(1.13)the double bonds are at 90 to each other and conjugation does not occur. 1.1.2 Bonding and Structure The tetrahedral arangement of the bonds of an sp hybridized carbon atom,the planar trigonal sparrangement and the linear sp system each have structural and geometrical consequences.The existence of free rotation about a single bond means that in a molecule such as ethane the methyl groups are free to take up a range of different conformations relative to each other.There are two extreme conformations,one in which the hydrogen atoms are staggered (see 1.14)and the other in which see 1.15).In the former the in the H .-= H H H 1.14 H H H ■H H 1.15
General Principles 3 1.7 1.8 1.9 1.10 1.11 1.12 1.13 A cyclic conjugated system containing (4n + 2)n electrons has an extra stability over that of a comparable number of isolated double bonds. This extra stabilization, known as , leads to a characteristic pattern of reactivity which distinguishes the reactions of benzene (1.8) from, for example, the linear hexatriene (1.9) or cyclooctatetraene (1.10) (4n electrons, n = 2). The aromatic sextet may arise not just from the overlap of three double bonds as in benzene (1.8) or pyridine (1.11) but also from the participation of the lone pair of electrons on a heteroatom. Thus pyrrole (1.12), with effectively six n-electrons, shows some aromatic character. In allene (1.13) the double bonds are at 90" to each other and conjugation does not occur. I. 1.2 Bonding and Structure The tetrahedral arrangement of the bonds of an sp3 hybridized carbon atom, the planar trigonal sp2 arrangement and the linear sp system each have structural and geometrical consequences. The existence of free rotation about a single bond means that in a molecule such as ethane the methyl groups are free to take up a range of different relative to each other. There are two extreme conformations, one in which the hydrogen atoms are staggered (see 1.14) and the other in which they are eclipsed (see 1.15). In the former the interactions between the hydrogen atoms of the methyl group are minimized, and the structure is of a lower energy than that in which the hydrogen atoms are eclipsed
4 Functional Group Chemistry When these ideas are extended to butane,there are three rotamers abou the central CC bond which need to be the e Not only are conformations (1.17) mum respectively,but there is also a gauche conformation (1.18)which is intermediate between these. H Me Me MeMe Me Me 1.16 1.17 1.18 When the carbon chain is constrained in a cyclohexane ring,there are two extreme conformations known as the boat (1.19)and chair (1.20) 1.19 boat 1.20 chair forms.The former is destabilized by eclipsed interactions whilst in the latter the interactions are gauche.This conformation is more stable. The C-H bonds on the chair cyclohexane ring are of two types.One set of six C-H bonds are parallel to the axis of the e know H xia bonds (see 11)The other set of sir bondso out of the horizontal plane of the ring and are known as the equatorial bonds H (see 1.22).When these C-H bonds are replaced by substituents,the sub stituents experience different interactions depending on their conforma- 1.21 tion.These steric relationships play an important role in the influence of the carbon skeleton of a particular compound on the reactivity of its H functional groups. In an alkene uch as ethene,the presence of the r-bond prevents rota tion about the C=Cbond.The the separate subs 122 on the separate carbon atoms,these are cis or trans to each other.Distinct geometric isomers are possible.These compounds have different prop- erties.Thus cis-ethenedicarboxylic acid is maleic acid (1.23).The car- boxyl groups are close together in space and react together to form a cyclic anhydride(1.24).On the other hand,trans-ethenedicarboxylic acid is fumaric acid(1.25)and no such interaction is possible
4 Functional Group Chemistry When these ideas are extended to butane, there are three about the central C-C bond which need to be considered. Not only are there the extremes of the staggered (1.16) and eclipsed conformations (1.17), in which the methyl group interactions are at a minimum and a maximum respectively, but there is also a gauche conformation (1.18) which is intermediate between these. 1.16 1.17 1.18 When the carbon chain is constrained in a cyclohexane ring, there are forms. The former is destabilized by eclipsed interactions whilst in the latter the interactions are gauche. This conformation is more stable. The C-H bonds on the chair cyclohexane ring are of two types. One set of six C-H bonds are parallel to the axis of the ring and are known as bonds (see 1.21). The other set of six bonds point out of the horizontal plane of the ring and are known as the bonds (see 1.22). When these C-H bonds are replaced by substituents, the substituents experience different interactions depending on their conformation. These steric relationships play an important role in the influence of the carbon skeleton of a particular compound on the reactivity of its functional groups. In an alkene such as ethene, the presence of the .n-bond prevents rotation about the C=C bond. The hydrogen atoms on the separate carbons are either cis or trans to each other. When the alkene bears substituents on the separate carbon atoms, these are cis or trans to each other. Distinct are possible. These compounds have different properties. Thus cis-ethenedicarboxylic acid is maleic acid (1.23). The carboxyl groups are close together in space and react together to form a cyclic anhydride (1.24). On the other hand, trans-ethenedicarboxylic acid is fumaric acid (1.25) and no such interaction is possible. # two extreme conformations known as the (1.19) and (1.20) 1.19 boat 1.20 chair 0 II H\C/C, OH 0 1.23 1.24 0 II I1 0 1.25
General Principles 5 1.1.3 The Inter-relationship of Functional Groups Functional groups may be regarded in a systematic,formal sense to be related by a series of redox and substitutive transformations. Replacement of a hydrogen atom on the carbon atom at the end of the four-carbon chain of butane(1.26)by a hydroxyl (OH)group gives the primary alcohol butan-1-o1 (1.27).and when one of the ene(CH) hydrogen atoms (1.26)is repla aced we obta in the alcohol butan-2-ol(1.28).Replacement of the central hydrogen of the C isomer 2-methylpropane (1.29)by an OH group gives 2-methylpropan- 2-ol(1.30),a tertiary alcohol.In each of these isomeric alcohols there is a hydroxyl(OH)group conferring similar properties.However,the alco- hols differ in the number of hydrogen atoms attached to the carbon atom and hence in the oxygen bet entral carbon a further cH o mer,eth ne(diethy I ether, 1.31),lacking the character istic OH of the alcohol and thus containing a different functional group,the ether group. CHCH-CHCH 1.26 1.27 1.28 CH CH-CH3 OH CH3CH2OCH2CH3 CH 1.29 1.30 1.31 Oxidation of butan-1-ol gives butanal (1.32)which is characterized by 0 thesaroroup,in this case an aldchyde group.Oxidation of CH;CH2CH2C one(133).a There are 1.32 many comme prope and others that differ because of the =0 Oxidation of butanal leads to a carboxylic acid.butanoic acid(1.34) CH.CHC The distinctive properties of a carboxylic acid [C(=O)OH]can be con- CHa sidered as combining those of a carbonyl group modified by an attached 133 hydroxyl group and those of a hydroxyl group modified by an attached carbonyl group.Replacement of the hydrogen atom of a carboxylic acid alkyl example ethyl acetate (ethyl CH;CH2CH2C OH 0ate,1.3 roup gives an ester.for The functional group which contains two alkoxy groups attache 134 the same carbon atom occurs,for example,in dimethoxymethane(1.36), and is known as an acetal.A compound containing three alkoxy groups (see 1.37)is an ortho ester. If another atom such as a halogen,a sulfur or a nitrogen is substi- 135
General Principles 5 1 .I .3 The Inter-relationship of Functional Groups Functional groups may be regarded in a systematic, formal sense to be related by a series of redox and substitutive transformations. Replacement of a hydrogen atom on the carbon atom at the end of the four-carbon chain of butane (1.26) by a (OH) group gives the butan-1-01 (1.27), and when one of the methylene (CH,) hydrogen atoms of butane (1.26) is replaced we obtain the butan-2-01 (1.28). Replacement of the central hydrogen of the C, isomer 2-methylpropane (1.29) by an OH group gives 2-methylpropan- 2-01 (1.30), a . In each of these isomeric alcohols there is a hydroxyl (OH) group conferring similar properties. However, the alcohols differ in the number of hydrogen atoms attached to the carbon atom and hence in the properties associated with these atoms. Insertion of the oxygen between the two central carbon atoms gives a further C,H,,O isomer, ethoxyethane (diethyl ether, 1.31), lacking the characteristic OH of the alcohol and thus containing a different functional group, the ether group. CH3CH2CHCH3 CH3CH2CH2CH3 CH3CH2CH2CH20H I OH 1.26 1.27 1.28 CH3 CH? \ CH3, ,CH3 ,CH-CH3 C CH~CHZOCH~CH~ CH3’ ‘OH 1.29 1.30 1.31 Oxidation of butan- 1-01 gives butanal (1.32) which is characterized by a C=O, a group. Oxidation of the secondary alcohol butan-2-01 (1.28) gives butan-2-one (1.33), a . There are many common properties of aldehydes and ketones, and others that differ because of the aldehydic C-H. , butanoic acid (1.34). The distinctive properties of a carboxylic acid [C(=O)OH] can be considered as combining those of a carbonyl group modified by an attached hydroxyl group and those of a hydroxyl group modified by an attached carbonyl group. Replacement of the hydrogen atom of a carboxylic acid by an alkyl group gives an ester, for example ethyl acetate (ethyl ethanoate, 1.35). The functional group which contains two alkoxy groups attached to the same carbon atom occurs, for example, in dimethoxymethane (1.36), and is known as an . A compound containing three alkoxy groups (see 1.37) is an If another atom such as a halogen, a sulfur or a nitrogen is substigroup, in this case an Oxidation of butanal leads to a 0 I/ CH~CHZCH~C \ H 1.32 0 // \ CH3 CH:;CH2C 1.33 0 // \ OH CH3CH2CH2C 1.34 OCH2CH3 CH3 -- d \\ 0 1.35
6 Functional Group Chemistry OCH OCH3 tuted in place of the hydroxyl group,further functional groups are gen HC-OCHs erated(see Box 1.1). OCH3 1.36 137 Box 1.1 Functional Groups Derived from Alcohols and Carboxylic Acids by Substitution R-OH alcohol RCGOLOH kvlic acid R-CI alkyl chloride RC(=O)-CI acyl chloride (alkyl halide) R-SH thiol (sulfane) RC(=O)-SH thioacid R-NH,amine RCeONH. amide where R is an alkyl group. The alkyl halides (halogenoalkanes),thiols and amines are at the same oxidation level as the alcohols.while acyl halides.thioacids and amides are similarly related to the carboxylic a acids Like ox yg sulfur cane The tervalency of nitrogen does not permit simple insertion;another group such as hydrogen or an alkyl group must be added to nitrogen, producing for example(CH),NH(dimethylamine,a secondary amine) or(CH,),N(trimethylamine,a tertiary amine).Note that the terms pri- mary,secondary and tertiary are used in different ways when referring to alcohols and amines. ary can form stable nonium salt (R,N thioethers form sulfonium salts().but stable ox nium salts (RO*X are less common. →CH2=CH2—CHCH,O 1.39 1.40 Dehydrogenation of alkanes such as ethane (1.38)relates them to alken ch a ethe ne (ethylene.1.39).The sa e functi al roup ma H-C=C-H 1.41 of ethene would generate an alkyne,ethyne (acetylene,1.41).In terms of oxidation level,the alkene is related to the alcohol and the alkyne is related to the ketone. 1.1.4 Electronegativity
6 Functional Group Chemistry OCH3 OCH3 1 OCH3 OCH~ tuted in place of the hydroxyl group, further functional groups are gen- / HC-OCH3 erated (see Box 1.1). H2C, I 1.36 1.37 The (halogenoalkanes), and are at the same are similarly related to the carboxylic acids. Like oxygen, sulfur can be inserted into a chain to generate the equivalent of an ether such as the oxidation level as the alcohols, while 7 and The tervalency of nitrogen does not permit simple insertion; another group such as hydrogen or an alkyl group must be added to nitrogen, producing for example (CH,),NH (dimethylamine, a or (CH,),N (trimethylamine, a ). Note that the terms primary, secondary and tertiary are used in different ways when referring to alcohols and amines. (R,N+X-), while thioethers form Tertiary amines can form stable (R3SfX-), but stable (R,O+X-) are less common. CH3CH3 - CH2=CH2 - CH3CH2OH 1.38 1.39 1.40 Dehydrogenation of alkanes such as ethane (1.38) relates them to such as ethene (ethylene, 1.39). The same functional group may be obtained by dehydration of ethanol (1.40). Further dehydrogenation of ethene would generate an , ethyne (acetylene, 1.41). In terms of oxidation level, the alkene is related to the alcohol and the alkyne is related to the ketone. H-CzC-H 1.41 I .I D4 Electronegativity Having considered the oxidative and substitutive relationships 'between functional groups, we now consider the factors that contribute to their