8 Structure and bonding Res ance forms(sometimes positive charge,a lone pair of electrons or an adjacent bond by resonance (i.e delocalisation or'spreading out'of the electrons).Curly arrows (Section 4.1)are in a molecule or an ion used to represent the movement of or non-bonding electrons to give differen resonance forms.It is only the electrons.not the nuclei,that move in the resonance forms,and a double-headed arrow is used to show their relationship. 1.6.3.1 Positive mesomeric effect .When a t-system donates electrons.the -system has a positive mesomeric effect,+M. This carbocation is called an allylic cation (see Section 5.3.1.2) e∠CHCHR C-CH-CHR donge8estoas .When a lone pair of electrons is donated,the group donating the electrons has a positive mesomeric effect,+M. The OR gr COR c-0R tona8eprons 1.6.3.2 Negative mesomeric effect .When a t-system accepts electrons,the t-system has a negative mesomeric effect.-M. CH-CCHR C=CH-HR C-CH-8 52psaeiodh The actual structures of the cations or anions lie somewhere between the two resonance forms.All resonance forms must have the same overall charge and obey the same rules of valency. -M groups generally contain an electronegative atom(s)and/or a -bond(s) CHO,C(O)R,CO,H,CO,Me,NO,,CN,aromatics,alkenes +M groups generally contain a lone pair of electrons or a bond(s) ClBr.OH,OR,SH.SR,NH,NHR,NRz aromatics,alkenes Aromatic (or aryl)groups and alkenes can be both +M and -M
positive charge, a lone pair of electrons or an adjacent bond by resonance (i.e. delocalisation or ‘spreading out’ of the electrons). Curly arrows (Section 4.1) are used to represent the movement of p- or non-bonding electrons to give different resonance forms. It is only the electrons, not the nuclei, that move in the resonance forms, and a double-headed arrow is used to show their relationship. 1.6.3.1 Positive mesomeric effect When a p-system donates electrons, the p-system has a positive mesomeric effect, þM. C CH CHR C CH CHR donates electrons: +M group When a lone pair of electrons is donated, the group donating the electrons has a positive mesomeric effect, þM. C OR ORC donates electrons: +M group 1.6.3.2 Negative mesomeric effect When a p-system accepts electrons, the p-system has a negative mesomeric effect, M. C CH CHR CHC CHR accepts electrons: –M groups C OCH OCHC The actual structures of the cations or anions lie somewhere between the two resonance forms. All resonance forms must have the same overall charge and obey the same rules of valency. –M groups generally contain an electronegative atom(s) and/or a π-bond(s): +M groups generally contain a lone pair of electrons or a π-bond(s): Aromatic (or aryl) groups and alkenes can be both +M and –M. CHO, C(O)R, CO2H, CO2Me, NO2, CN, aromatics, alkenes Cl, Br, OH, OR, SH, SR, NH2, NHR, NR2, aromatics, alkenes Resonance forms (sometimes called canonical forms) show all possible distributions of electrons in a molecule or an ion This carbocation is called an allylic cation (see Section 5.3.1.2) The OR group is called an alkoxy group (see Section 2.4) This anion, formed by deprotonating an aldehyde at the a-position, is called an enolate ion (Section 8.4.3) Functional groups are discussed in Section 2.1 8 Structure and bonding
1.7 Acidity and basicity 9 group donates(+M)the electrons,the other group(s)accepts the electrons(-M). An amide.such as RCONH2,also ROCH-CHR RO-CH-CHR a-M ) Sections 1.72and93. M group -M group All resonance fomms arenot of the same energy.Generally,the most stable reso aere shel of elhe greast umber of cvaentond.h complt and/or an aromatic ring.In phenol (PhOH).for example the resonance form with the intact aromatic benzene ring is expected to predominate H---+M group H 一一 Ben. ompounds,including phenolare -M group discussed in Chapter ringinta As a rule of thumb,the more resonance structures an anion,cation or neutral -system can have,the more stable it is. 1.6.3.3 Inductive versus mesomeric effects Mesomeric effects are generally stronger than inductive effects.A+M group is Mesomeric effects can be effective over much long effects provided thatco ent (i.e.alternating single and double bonds) s inductive effe are det ined by distance.m ric effects are deter mined by the relative positions of +M and-M groups in a molecule(Section 1.7). 1.7 Acidity and basicity 1.7.1 Acids An acid s substance that (Bronsted-Lowry).Acidiccom pou es ar the anions (or Equilibriaandequilibrium are elatively stable. Section In water: acidity constant HA+ H+A Acid Base Conjugate Cope The more stable the conjugate base the stronger the acid k=H,oa吗 pKa=-10910K [HA] The higher the value of Ka.the As H2O is in excess mor
In neutral compounds, there will always be a þM and M group(s): one group donates (þM) the electrons, the other group(s) accepts the electrons (M). CHRO CHR CHRO CHR +M group –M group All resonance forms are not of the same energy. Generally, the most stable resonance forms have the greatest number of covalent bonds, atoms with a complete valence shell of electrons, and/or an aromatic ring. In phenol (PhOH), for example, the resonance form withtheintact aromatic benzene ringis expectedto predominate. OH +M group OH OH OH –M group aromatic ring is intact As a rule of thumb, the more resonance structures an anion, cation or neutral p-system can have, the more stable it is. 1.6.3.3 Inductive versus mesomeric effects Mesomeric effects are generally stronger than inductive effects. A þM group is likely to stabilise a cation more effectively than a þI group. Mesomeric effects can be effective over much longer distances than inductive effects provided that conjugation is present (i.e. alternating single and double bonds). Whereas inductive effects are determined by distance, mesomeric effects are determined by the relative positions of þM and M groups in a molecule (Section 1.7). 1.7 Acidity and basicity 1.7.1 Acids An acid is a substance that donates a proton (Brønsted-Lowry). Acidic compounds have low pKa values and are good proton donors as the anions (or conjugate bases), formed on deprotonation, are relatively stable. HA [HA] H2O H3O + A Ka pKa = –log10Ka acidity constant + Acid Base Conjugate acid Conjugate base In water: [H3O ] [A ] Ka As H2O is in excess ≈ The higher the value of Ka, the The more stable the conjugate base the stronger the acid lower the pKa value and the more acidic is HA An amide, such as RCONH2, also contains both a þM group (NH2) and a M group (CO). See Sections 1.7.2 and 9.3.1 Benzene and other aromatic compounds, including phenol, are discussed in Chapter 7 Conjugated enones, containing a CCCO group, are discussed in Section 8.5.1 Equilibria and equilibrium constants are discussed in Section 4.9.1.1 1.7 Acidity and basicity 9
10 Structure and bonding The pk value equals the pH of the acid when it is half ionised.at pH's above the pk the acid (HA)exists predominantly as the conjugate base (A)in water At pH's below the p it exists predominantly as HA. pH-O.strongly acidic pH =7,neutral The infuece of sovent polarityon pH=14,strongly basic ction The pKa values are influenced by the solvent.Polar solvents will stabilise cations and/or anions by solvation in which the charge is delocalised over the solvent (e.g.by hydrogen-bonding in water). HO-OH HH The more electronegative the atom bearing the negative charge,the more stable the conjugate base (which is negatively charged). K. 3 16 33 48 most acidic HF HO NHa CHa least acidic decreasing electronegativityon going from Fto Therefore.F-is more stable than HC The conjugate base can also be stabilised by-I and-M groups which can delocalise the negative charge.(The more 'spread out'the negative charge.the more stable it is). s therefore lower th e pKa while +l and 1.7.1.1 Inductive effects and carboxylic acids The carboxylate ion(RCO2)is formed on deprotonation of a carboxylic acid (RCO2H).The anion is stabilised by resonance(i.e.the charge is spread over both oxygen atoms)but can also be stabilised by the R group if this has a-I effect. carboxylic acid 0 Base -00 R-C OH (-BaseH) R-cO一R- The greater the-I effect,the more stable the carboxylate ion (e.g.FCH2CO2 is more stable than BrCHCO)and the more acidic the carboxylic acid (e.g. FCH2CO,H is more acidic than BrCH,CO,H)
The pKa value equals the pH of the acid when it is half ionised. At pH’s above the pKa the acid (HA) exists predominantly as the conjugate base (A) in water. At pH’s below the pKa it exists predominantly as HA. pH = 7, neutral pH = 14, strongly basic pH = 0, strongly acidic The pKa values are influenced by the solvent. Polar solvents will stabilise cations and/or anions by solvation in which the charge is delocalised over the solvent (e.g. by hydrogen-bonding in water). HO H A H OH H2O O H H H OH2 δ+ δ+ δ– δ– The more electronegative the atom bearing the negative charge, the more stable the conjugate base (which is negatively charged). HF H2O NH3 CH4 decreasing electronegativity on going from F to C most acidic pKa 3 16 33 48 least acidic Therefore, F is more stable than H3C. The conjugate base can also be stabilised by I and M groups which can delocalise the negative charge. (The more ‘spread out’ the negative charge, the more stable it is). –I and –M groups therefore lower the pKa while +I and +M groups raise the pKa 1.7.1.1 Inductive effects and carboxylic acids The carboxylate ion (RCO2 ) is formed on deprotonation of a carboxylic acid (RCO2H). The anion is stabilised by resonance (i.e. the charge is spread over both oxygen atoms) but can also be stabilised by the R group if this has a I effect. R C O OH R C O O R C O O Base (–BaseH) carboxylic acid carboxylate ion The greater the I effect, the more stable the carboxylate ion (e.g. FCH2CO2 is more stable than BrCH2CO2 ) and the more acidic the carboxylic acid (e.g. FCH2CO2H is more acidic than BrCH2CO2H). The influence of solvent polarity on substitution and elimination reactions is discussed in Sections 5.3.1.3 and 5.3.2.3 Inductive effects are introduced in Section 1.6.1 Mesomeric effects are introduced in Section 1.6.3 The reactions of carboxylic acids are discussed in Chapter 9 10 Structure and bonding
1.7 Acidity and basicity 11 FCH.CO.H Br-CH2-CO2H HaCCOH 2.7 2.9 4.8 etteasFs 1.7.1.2 Inductive and mesomeric effects and phenols Mesomeric effects can also stabilise positive and negative charges. Te nae non aom On deprotonation of phenol (PhOH)the phenoxide ion(PhO-)is formed.This anion is stab d by the delocalisation of the negative charge on to the 2-.4-and 6-positions of the benzene ring. OH If-M groups are introduced at the 2-,4-and/or 6-positions,the anion can be can use stabilised b deocalisatio the negative o the M ay ofuing curlydelecais e are on to group oups e introduced on the ben zene ring the effect will de end on their stance rom the e nega - ep the harge.the greater th The c ng effect will be.The ordero position ·The- much stronger(Section 16.). Examples The NO2 group is strongly electron-withdrawing:-I and-M
F CH2 CO2H Br CH2 CO2H H3C CO2H pKa 2.7 2.9 4.8 Most acidic as F is more electronegative than Br and has a greater –I effect Least acidic as the CH3 group is a +I group 1.7.1.2 Inductive and mesomeric effects and phenols Mesomeric effects can also stabilise positive and negative charges. The negative charge needs to be on an adjacent carbon atom for a –M group to stabilise it The positive charge needs to be on an adjacent carbon atom for a +M group to stabilise it On deprotonation of phenol (PhOH) the phenoxide ion (PhO) is formed. This anion is stabilised by the delocalisation of the negative charge on to the 2-, 4- and 6-positions of the benzene ring. OH O O O O Base 26 4 (–BaseH) If M groups are introduced at the 2-, 4- and/or 6-positions, the anion can be further stabilised by delocalisation through the p-system as the negative charge can be spread onto the M group. We can use double-headed curly arrows to show this process. If M groups are introduced at the 3- and/or 5-positions, the anion cannot be stabilised by delocalisation, as the negative charge cannot be spread onto the M group. There is no way of using curly arrows to delocalise the charge on to the M group. If I groups are introduced on the benzene ring, the effect will depend on their distance from the negative charge. The closer the I group is to the negative charge, the greater the stabilising effect will be. The order of I stabilisation is therefore 2-position > 3-position > 4-position. The M effects are much stronger than I effects (Section 1.6.3). Examples The NO2 group is strongly electron-withdrawing; –I and –M Double-headed curly arrows are introduced in Section 4.1 1.7 Acidity and basicity 11
12 Structure and bonding OH OH OH OH NO2 NO, pK9.9 84 73 40 as The NO2 Most acidic as the ring the by resonance and by reson 009 00 0 09 N N N 6w9 1.7.2 Bases A base is a substance that accepts a proton(Bronsted-Lowry).Basic compounds are good proton acceptors as the co njugate acids,formed on protonation.are relatively stable.bases (B:rB)give cojugate aids (BH+or BH)with high pK values. Equilibriaand qubrium In water: basicity constant +H20 8H Base Acid Congate Copugate The strength of bases is usually described by the K and pk values of the conjugate acid. H20 K For reactions of bases with [B][Hg K As H,O is in excess If B is a strong base then BH+will be relatively stable and not easily deprotonated.BH+will therefore have a high pK value. If B is a weak base then BH+will be relatively unstable and easily deproto- nated.BH will therefore have a low pK value
OH OH NO2 OH NO2 OH NO2 NO2 pKa 9.9 8.4 7.2 4.0 Most acidic as both NO2 groups can stabilise the anion inductively and by resonance Least acidic as no –I or –M groups on the ring The NO2 can only stabilise the anion inductively 2 4 6 The NO2 can stabilise the anion inductively and by resonance O N N O O O O O N O O N O O O N O O N O O O N N O O O O 1.7.2 Bases A base is a substance that accepts a proton (Brønsted-Lowry). Basic compounds are good proton acceptors as the conjugate acids, formed on protonation, are relatively stable. Consequently, strong bases (B: or B) give conjugate acids (BHþ or BH) with high pKa values. B H2O BH + HO Kb basicity constant + Base Acid Conjugate acid Conjugate base In water: The strength of bases is usually described by the Ka and pKa values of the conjugate acid. BH H2O [ BH] B H3O [B ] [H3O ] Ka As H2O is in excess ≈ + Ka + If B is a strong base then BHþ will be relatively stable and not easily deprotonated. BHþ will therefore have a high pKa value. If B is a weak base then BHþ will be relatively unstable and easily deprotonated. BHþ will therefore have a low pKa value. Naming substituted benzenes is discussed in Section 2.4 Equilibria and equilibrium constants are discussed in Section 4.9.1.1 For the use of bases in elimination reactions of halogenoalkanes, see Section 5.3.2 For reactions of bases with carbonyl compounds see Sections 8.4.3 and 9.11 Inductive effects are introduced in Section 1.6.1 12 Structure and bonding