1.7 Acidity and basicity 13 The cation can be stabilised by +Iand +M groups.which can delocalise the Mesomeric effects are introduced positive charge.(The more'spread out'the positive charge,the more stable it is.) in Section 1.6.3 1.7.2.1 Inductive effects and aliphatic(or alkyl)amines Aliphatic amines have nitre ge On protonation of amines (e.g.RNH2),ammonium salts are formed. R一iH2 aryl groups primary amine ammonium ion The greater the +Ieffect of the R group,the greater the electron density at nitrogen and the more basic the amine.The greater the +I effect,the more stable (RaNH)and tertary (RN)amines the ammonium ion and the more basic the amine. are introduced in Section 2.1 no+l group three +l groups H E× EI● Triethvlamine -H EI N-HE9 -H (Section 5.2.2) H Et pK9.3 10.7 10.9 10.9 se steadily as more +I alkyl groups ogen.Ho in wo p ing N-H l （hich nts cannot solvate the cation)the order of pK's is RaN R2NH RNH2 NHa (R=+lalkyl group) most basic least basic The presence of-I and/or-M groups on nitrogen reduces the basicity and so, for example,primary amides(RCONH2)are poor bases. Secondary amides(RCONHR)and Ethanamide The C-group stabilises hoealoaePaseabiedby resonance M If ethanamide was protonated on nitrogen,the positive charge could not be Reactions of amides are discussed stabilised by delocalisation.Protonation therefore occurs onoxyg en as the charge in Section 9.8 can be delocalised on to the nitrogen atom
The cation can be stabilised by þI and þM groups, which can delocalise the positive charge. (The more ‘spread out’ the positive charge, the more stable it is.) 1.7.2.1 Inductive effects and aliphatic (or alkyl) amines On protonation of amines (e.g. RNH2), ammonium salts are formed. R NH2 + H R NH3 primary amine ammonium ion The greater the þI effect of the R group, the greater the electron density at nitrogen and the more basic the amine. The greater the þI effect, the more stable the ammonium ion and the more basic the amine. H NH H H Et NEt H H Et NEt Et H Et NH H H pKa 9.3 10.7 10.9 10.9 no +I group three +I groups The pKa values should increase steadily as more þI alkyl groups are introduced on nitrogen. However, the pKa values are determined in water, and the more hydrogen atoms on the positively charged nitrogen, the greater the extent of hydrogen bonding between water and the cation. This solvation leads to the stabilisation of the cations containing NH bonds. In organic solvents (which cannot solvate the cation) the order of pKa’s is expected to be as follows. R3N>R2NH > RNH2 > NH3 most basic least basic (R = +I alkyl group) tertiary amine secondary amine primary amine ammonia The presence of I and/or M groups on nitrogen reduces the basicity and so, for example, primary amides (RCONH2) are poor bases. C O H3C NH2 C O H3C NH2 Ethanamide –M, –I The C=O group stabilises the lone pair on nitrogen by resonance – this reduces the electron density on nitrogen If ethanamide was protonated on nitrogen, the positive charge could not be stabilised by delocalisation. Protonation therefore occurs on oxygen as the charge can be delocalised on to the nitrogen atom. Mesomeric effects are introduced in Section 1.6.3 Aliphatic amines have nitrogen bonded to one or more alkyl groups; aromatic amines have nitrogen bonded to one or more aryl groups Primary (RNH2), secondary (R2NH) and tertiary (R3N) amines are introduced in Section 2.1 Triethylamine (Et3N) is commonly used as a base in organic synthesis (Section 5.2.2) Hydrogen bonds are introduced in Section 1.1 Secondary amides (RCONHR) and tertiary amides (RCONR2) are also very weak bases because the nitrogen lone pairs are stabilised by resonance Reactions of amides are discussed in Section 9.8 1.7 Acidity and basicity 13
14 Structure and bonding The conjugate acid has a low pof-0.5 .H NH2 HgC 1.7.2.2 Mesomeric effects and aryl(or aromatic)amines The lone pair of electrons on the nitrogen atom of aminobenzene (or aniline PhNH)can be stabilised by delocalisation of the electrons onto the 2-.4-and 6- positions of the benzene ring.Aromatic amines are therefore less basic than aliphatic amines. H .If-Mgroups are introduced at the 2-.4-and/or 6-positions(but not the 3-or5- positions)the anion can be further stabilised by delocalisation,as the negative charge can be spread on to the -M group.This reduces the basicity of the amine. .If-I groups are introduced on the benzene ring,the order of-I stabilisation is 2-position>3-position>4-position.This reduces the basicity of the amine. NO o, pKa 4.6 2.45 -0.28 Most basic as no The NO can Least basic-the NO on the ring pair inductively (These are pK values of the conjugate acids) by resonance .If +M groups (e.g.OMe)are introduced at the 2-,4-or 6-position of aminobenzene (PhNH2),then the basicity is increased.This is because the +M group donates electron density to the carbon atom bearing the amine group.Note that the nitrogen atom,not the oxygen atom,is protonated-this is because nitrogen is less electronegative than oxygen and is a better electron donor
O C H3C NH2 H OH C H3C NH2 OH C H3C NH2 O C H3C NH3 not stabilised by resonance stabilised by resonance The conjugate acid has a low pKa of –0.5 1.7.2.2 Mesomeric effects and aryl (or aromatic) amines The lone pair of electrons on the nitrogen atom of aminobenzene (or aniline, PhNH2) can be stabilised by delocalisation of the electrons onto the 2-, 4- and 6- positions of the benzene ring. Aromatic amines are therefore less basic than aliphatic amines. NH2 NH2 NH2 NH2 6 2 4 If M groups are introduced at the 2-, 4- and/or 6-positions (but not the 3- or 5- positions) the anion can be further stabilised by delocalisation, as the negative charge can be spread on to the M group. This reduces the basicity of the amine. If I groups are introduced on the benzene ring, the order of I stabilisation is 2-position > 3-position > 4-position. This reduces the basicity of the amine. NH2 NH2 NO2 NH2 NO2 pKa 4.6 2.45 –0.28 Most basic as no –I or –M groups on the ring The NO2 can stabilise the lone pair inductively Least basic – the NO2 can stabilise the lone pair inductively and by resonance (These are pKa values of the conjugate acids) If þM groups (e.g. OMe) are introduced at the 2-, 4- or 6-position of aminobenzene (PhNH2), then the basicity is increased. This is because the þM group donates electron density to the carbon atom bearing the amine group. Note that the nitrogen atom, not the oxygen atom, is protonated – this is because nitrogen is less electronegative than oxygen and is a better electron donor. For the preparation and reactions of aniline (PhNH2), see Section 7.8 For the Pauling electronegativity scale see Section 1.6.1 14 Structure and bonding
1.7 Acidity and basicity 15 The OMe group is-I but +M NH2 OMe :OMe The OM 42 5.3 roup (e Section 24 for naming ic as the Most OMe organic compounds) o the sity to the carbon o put it has a onheeca6ntagakdstegtawe Curly arrows can be used to show the delocalisation of electrons on to the carbon atom bearing the nitrogen. NH2 electron density the basicity OMe 1.7.3 Lewis acids and bases in f mple filled vale shells and so can accept electron pairs elone pair(s)of electrons on the hcteroom (N es in H2O.RC ketone aluminium chloride coordinate bond R C=0Al-CI 个 C=0-AICI Lewis base Lewis acid Coordination complex 1.7.4 Basicity and hybridisation The greater the's'character of an orbital,the lowerin energy the electrons and the more tightly the electre re held to the .The are therefore less available fo hence the those or sp compounds are
NH2 OMe NH2 OMe NH2 OMe The OMe group is –I but +M pKa 4.2 4.5 5.3 Least basic as the OMe group cannot donate electron density to the carbon atom bearing the nitrogen The OMe group can donate electron density to the nitrogen but it has a strong –I effect as it is in the 2-position Most basic as the OMe group can donate electron density to the nitrogen and it has a weak –I effect (as well apart from the nitrogen) (These are pKa values of the conjugate acids formed by protonation of the –NH2 group) Curly arrows can be used to show the delocalisation of electrons on to the carbon atom bearing the nitrogen. NH2 OMe NH2 OMe NH2 OMe electron density adjacent to the nitrogen increases the basicity 1.7.3 Lewis acids and bases A Lewis acid is any substance that accepts an electron pair in forming a coordinate bond (Section 1.1). Examples include Hþ, BF3, AlCl3, TiCl4, ZnCl2 and SnCl4. They have unfilled valence shells and so can accept electron pairs. A Lewis base is any substance that donates an electron pair in forming a coordinate bond. Examples include H2O, ROH, RCHO, R2CO, R3N and R2S. They all have a lone pair(s) of electrons on the heteroatom (O, N or S). C O R R Cl Al Cl Cl C O R R Cl Al Cl Cl Lewis base Lewis acid ketone aluminium chloride Coordination complex + coordinate bond 1.7.4 Basicity and hybridisation The greater the ‘s’ character of an orbital, the lower in energy the electrons and the more tightly the electrons are held to the nucleus. The electrons in an sp-orbital are therefore less available for protonation than those in an sp2 - or sp3 -orbital, and hence the compounds are less basic. The OMe group is called a methoxy group (see Section 2.4 for naming organic compounds) Curly arrows are introduced in Section 4.1 A heteroatom is any atom that is not carbon or hydrogen Reactions of ketones are discussed in Chapter 8 1.7 Acidity and basicity 15
16 Structure and bonding tertiary amine nitrile most basic RN RC=NH RC least basic alkyl anion alkenyl anion alkynyl anion most basle Rc RC-CH RC=least basic 0%s 1.7.5 Acidity and aromaticity Aromatic compounds are planar.coniugated systems which have 4n +2 electrons (Huickel's rule)(Section 7.1).If.on deprotonation,the anion is part of an aromatic -system then the negative charge will be stabilised.Aromaticity will therefore the acidity of the compound. HH Toluene isa commo n solvent The anion CH2 Resonance stabilisation of e 8n more stable less stable Ifa lone pair of electrons on a heteroatom is part of an aromatic -system,then these electrons will not be available for protonation.Aromaticity will therefore decrease the basicity of the compound. s to Reactions of aromatic heterocycles c rin Pyrrole is therefore not basic(pK-4) Pyrrole s-- 2 electrons Pyridine Rtgnpkaese 1.7.6 Acid-base reactions The pk values can be used to predict if an acid-base reaction can take place.An acid will donate a proton to the conjugate base of any acid with a higher pK value
R3N R3C R2C NH R2C CH RC C RC N sp2 (33% s) sp3 (25% s) most basic most basic least basic least basic tertiary amine imine nitrile alkyl anion alkenyl anion alkynyl anion sp (50% s) 1.7.5 Acidity and aromaticity Aromatic compounds are planar, conjugated systems which have 4n þ 2 electrons (H€uckel’s rule) (Section 7.1). If, on deprotonation, the anion is part of an aromatic p-system then the negative charge will be stabilised. Aromaticity will therefore increase the acidity of the compound. H H CH2 CH3 Although the anion is stabilised by resonance it does not contribute to the aromaticity (this would give 8π electrons) The anion is stabilised by resonance and it is aromatic (planar and 14π electrons) more stable less stable fluorene pKa = 22 toluene pKa= 40 Each ring contributes 6π electrons If a lone pair of electrons on a heteroatom is part of an aromatic p-system, then these electrons will not be available for protonation. Aromaticity will therefore decrease the basicity of the compound. N H Pyrrole The lone pair of electrons contributes to the 6π-electrons in the aromatic ring. Pyrrole is therefore not basic (pKa –4) Each double bond contributes 2π electrons N Pyridine Each double bond contributes 2π electrons The lone pair of electrons does not contribute to the 6π-electrons in the aromatic ring. Pyridine is therefore basic (pKa 5) 1.7.6 Acid-base reactions The pKa values can be used to predict if an acid-base reaction can take place. An acid will donate a proton to the conjugate base of any acid with a higher pKa value. Toluene is a common solvent. Oxidation of the CH3 group is discussed in Section 7.6 Resonance stabilisation of carbanions is introduced in Section 1.6.3 Reactions of aromatic heterocycles, including pyrrole and pyridine are discussed in Sections 7.10 and 7.11 For a table of pKa values see Appendix 3 16 Structure and bonding
Worked Example 17 This means that the product acid and base will be more stable than the starting ethyne amide lon HC三C一H+ HC三C NH3 Deprotonation of terminal alkynes pK,25 is discussed in Section 6.3.2.5 propanone hydroxide ion enolate ion water CHaCOCH3 OH CHgCOCH2+H-O For deprotonation of carbonyl + pK 20 see Section 8.4. Worked Example ether the (a)Giving your reasons,rank the following carbanions 1-4 in order of ons in 1-4 ca increasing stability. se the lone pair by Iand/or M 0 -CH. H人cH hc合CH, 1 2 3 (b)Identify.giving your reasons.the most acidic hydrogen atom(s)in compound 5. njugate bas HO 5 n:Show all the lone pairsn6 itCompare the stability of possible conjugate acids (c)Identify.giving your reasons,the basic functional group in compound 6. Answer (a) increasing stability- Inductive and mesomericeffects d in For the tert-butyl anion 3.because the three CH3 are electron-donating groups sestions 161 and 163 (+D).this makes 3 less stable than the methyl anion 2
This means that the product acid and base will be more stable than the starting acid and base. HC C H + N NH2 HC C + H3 pKa 25 Ammonia has a higher pK 38 a value than ethyne and so the equilibrium lies to the right pKa ethyne amide ion ethynyl anion ammonia CH3COCH3 + OH CH3COCH2 H2O+ hydroxide ion enolate ion water pKa 20 Water has a lower p pKa 15.7 Ka value than propanone and so the equilibrium lies to the left propanone Worked Example (a) Giving your reasons, rank the following carbanions 1–4 in order of increasing stability. CH3 H3C CH3 CH3 O H2C CH3 CH2 12 3 4 (b) Identify, giving your reasons, the most acidic hydrogen atom(s) in compound 5. HO O OH 5 NH2 N N O 6 (c) Identify, giving your reasons, the basic functional group in compound 6. Answer (a) CH3 H3C CH3 CH3 O H2C CH3 CH3 3 4 2 1 increasing stability For the tert-butyl anion 3, because the three CH3 are electron-donating groups (þI), this makes 3 less stable than the methyl anion 2. Deprotonation of terminal alkynes is discussed in Section 6.3.2.5 For deprotonation of carbonyl compounds to form enolate ions, see Section 8.4.3 Hint: Determine whether the groups attached to the negatively charged carbons in 14 can stabilise the lone pair by I and/or M effects Hint: Consider a dþ hydrogen atom bonded to an electronegative atom that, on deprotonation, gives the more stable conjugate base Hint: Show all the lone pairs in 6 and consider their relative availability. Compare the stability of possible conjugate acids Inductive and mesomeric effects (resonance) are discussed in Sections 1.6.1 and 1.6.3 Worked Example 17