18 Structure and bonding CH3 -CH2 2 8 beomanes2oe H2C The benzyl anion 4 is more stable than the methyl anion2 ause it is stabilise by resonance-the negative charge is delocalised on to the 2,4 and 6 positions of theo i the most sable the o one resonance form has the negative charge on oxygen-a negative charge on oxygen is more stable than a negative charge on carbon (b)Hydrogen atoms bonded to oxygen are more acidic than those bonded to carbon.As oxygen is more electronegative than carbon,the conjugate base is more stable.The carboxylic acid group is more acidic than the alcohol group in 5 because deprotonation of the carboxylic acid gives a conjugate base that is stabilised by resonance. Formation of carboxylate ions is discussed in Section 1.7.1 0) HO OH OH 0 OH 5 carboxylate ion is stabilised by resonance (c)The tertiary amine is the most basic g oun in 6 The lone pairs on the atoms in the tertiary amide a aniline hoth delo onation (the ox om of the tertiary amide is less basic than the tertiary amine be n is mor electronegative than nitro hence the o yge cid is stabilised by three+effects iline-lone pa on toe N、 十 h船amine N® basic group +l groups Problems Sections 1.6.1 and 1.6.3 1.Using the I and M notations,identify the electronic effects of the following substituents
CH3 H3C CH3 CH3 O H2C CH3 CH2 1 3 4 2 CH2 O H2C CH3 the more stable resonance form The benzyl anion 4 is more stable than the methyl anion 2 because it is stabilised by resonance – the negative charge is delocalised on to the 2, 4 and 6 positions of the ring. Enolate ion 1 is the most stable because the anion is stabilised by resonance and one resonance form has the negative charge on oxygen – a negative charge on oxygen is more stable than a negative charge on carbon. (b) Hydrogen atoms bonded to oxygen are more acidic than those bonded to carbon. As oxygen is more electronegative than carbon, the conjugate base is more stable. The carboxylic acid group is more acidic than the alcohol group in 5 because deprotonation of the carboxylic acid gives a conjugate base that is stabilised by resonance. HO O OH 5 O O OH most acidic O O OH carboxylate ion is stabilised by resonance (c) The tertiary amine is the most basic group in 6. The lone pairs on the nitrogen atoms in the tertiary amide and aniline groups are both delocalised and less available for protonation (the oxygen atom of the tertiary amide is less basic than the tertiary amine because oxygen is more electronegative than nitrogen, hence the oxygen lone pairs are less available). On protonation of the tertiary amine, the conjugate acid is stabilised by three þI effects. NH2 N N O 6 tertiary amide – lone pair delocalised on to O aniline – lone pair delocalised on to the benzene ring N H ammonium ion stabilised by three +I groups tertiary amine is the most basic group (partial structure) Problems 1. Using the I and M notations, identify the electronic effects of the following substituents. Sections 1.6.1 and 1.6.3 For the preparation and reactions of enolate ions, see Section 8.4.3 Formation of carboxylate ions is discussed in Section 1.7.1 Functional groups are discussed in Section 2.1 Formation of ammonium ions is discussed in Section 1.7.2 18 Structure and bonding
Problems 19 (a)-Me (b)-CI (©-NH(d-OH (e)-Br (f)-CO2Me (g)-NO2 (h)-CN 2.(a)Use curly arrows to show how cations A.B and C(shown below)are Section 1.6.3 stabilised by resonance,and draw the alternative resonance structure(s). ◇8H2 Hsc久8tHc0人8 A (b)Would you expect A.B or C to be the more stable?Briefly explain your reasoning. 3.Provide explanations for the following statements. Sections 1.6.3 and 1.7.1 CH. The c o t =CH-CN. ), 4.Why is cyclopentadiene (pK.15.5)a stronger acid than cycloheptatriene Section 1.7.5 (pK.36)? 0 cyclopentadiene cycloheptatriene Section 1.7.1 (a)4-Methylphenol (or p-cresol,4-HOCH4CH3) (b)4-Hydroxybenzoic acid (4-HOC HaCO2H) (c)H2C=CHCH2CH2C=CH (d)HOCHCH,CH2C=CH Section 1.7.2 (a)1-Aminopropane,ethanamide(CH3CONH2),guanidine [HN=C(NH2)2]. aniline (C6HsNH2). (b)Aniline (C.HsNH2),4-nitroaniline,4-methoxyaniline,4-methylaniline. .Poreach hllwin compounds D.identify the most acidichydrogen ator in your r OH OCH3 OCH E
(a) Me (b) Cl (c) NH2 (d) OH (e) Br (f) CO2Me (g) NO2 (h) CN 2. (a) Use curly arrows to show how cations A, B and C (shown below) are stabilised by resonance, and draw the alternative resonance structure(s). A CH2 B CH2 H3C CH2 H3CO C (b) Would you expect A, B or C to be the more stable? Briefly explain your reasoning. 3. Provide explanations for the following statements. (a) The carbocation CH3OCH2 þ is more stable than CH3CH2 þ. (b) 4-Nitrophenol is a much stronger acid than phenol (C6H5OH). (c) The pKa of CH3COCH3 is much lower than that of CH3CH3. (d) The CC single bond in CH3CN is longer than that in CH2CHCN. (e) The cation CH2CHCH2 þ is resonance stabilised whereas the cation CH2CHNMe3 þ is not. 4. Why is cyclopentadiene (pKa 15.5) a stronger acid than cycloheptatriene (pKa 36)? cyclopentadiene cycloheptatriene 5. Which hydrogen atom would you expect to be the most acidic in each of the following compounds? (a) 4-Methylphenol (or p-cresol, 4-HOC6H4CH3) (b) 4-Hydroxybenzoic acid (4-HOC6H4CO2H) (c) H2CCHCH2CH2CCH (d) HOCH2CH2CH2CCH 6. Arrange the following sets of compounds in order of decreasing basicity. Briefly explain your reasoning. (a) 1-Aminopropane, ethanamide (CH3CONH2), guanidine [HNC(NH2)2], aniline (C6H5NH2). (b) Aniline (C6H5NH2), 4-nitroaniline, 4-methoxyaniline, 4-methylaniline. 7. For each of the following compounds DF, identify the most acidic hydrogen atom(s). Briefly explain your reasoning. O OH O O OCH3 O DE F OH OCH3 O Section 1.6.3 Sections 1.6.3 and 1.7.1 Section 1.7.5 Section 1.7.1 Section 1.7.2 Section 1.7.1 Problems 19
20 Structure and bonding Section 1.7.2 8.For each of the following compounds G-I,identify the most basic group Briefly explain your reasoning. HN. o H Section 1.7.6 9.Given the approximate pKa values shown below,for the following acid-base reactions(a)-(e).determine whether the position of the equilibrium lies over to the reactant side or the product side. Acid pK value PhOH HO 157 CHCOCHs 2 H 35 WuC-CH. (a)NaH PhOH -Pho Na+H2 间CHCOCH3+NaOH一CHCo8HN°+H,C HC-Ch+NaNh一HC8HN°，NH 间CHCo品HNa°+PhOH一CH,COCH3+Pho Na e)H,C=是HNa°+H,0一H,C=CH2+NaO
8. For each of the following compounds GI, identify the most basic group. Briefly explain your reasoning. H N H2N NH2 H2N O N H2N G I H 9. Given the approximate pKa values shown below, for the following acid-base reactions (a)(e), determine whether the position of the equilibrium lies over to the reactant side or the product side. Acid pKa value PhOH 9.9 H2O 15.7 CH3COCH3 20 H2 35 NH3 38 H2CCH2 44 NaH + PhOH PhO Na + H2 (a) (b) CH3COCH3 + NaOH CH3COCH2Na + H2O H2C=CH2 + NaNH2 H2C=CH Na + NH3 (c) (d) CH3COCH2 Na + PhOH CH3COCH3 + PhO Na (e) H2C=CH Na + H2O H2C=CH2 + NaOH Section 1.7.2 Section 1.7.6 20 Structure and bonding
Functional groups,nomenclature and drawing organic compounds Key point.Organic compounds are classified by functional groups,which determine their chemistry.The names of organic compounds are derived from the functional group (or groups)and the main carbon chain.From the name,the structure of organic compounds can be drawn using full structural formulae, condensed structural formulae or skeletal structures. 2.1 Functional groups with charac Hydrocarbons(only hydrogen and carbon are present) HH H、 - C-H H- -H C=C H-C三C-H HH H H H c-C ethane ethene ethyne ene The lengths of all six CCb is in C-CandaCC bond (Section 7.1) CC bonds
2 Functional groups, nomenclature and drawing organic compounds Key point. Organic compounds are classified by functional groups, which determine their chemistry. The names of organic compounds are derived from the functional group (or groups) and the main carbon chain. From the name, the structure of organic compounds can be drawn using full structural formulae, condensed structural formulae or skeletal structures. HO O OH 3-hydroxybutanoic acid 1 2 3 4 Carboxylic acids have at least one carboxyl group; this functional group has the formula –CO2H (or –COOH) The longest chain has four carbons – it is a derivative of butane The OH functional group has the prefix ‘hydroxy’ 2.1 Functional groups A functional group is made up of an atom or atoms with characteristic chemical properties. The chemistry of organic compounds is determined by the functional groups that are present. Hydrocarbons (only hydrogen and carbon are present) C C H H H H H H C C H H H H H H C C C C C C C C H H H H H H ethane (an alkane) single CC bond ethene (an alkene) double CC bond ethyne (an alkyne) triple CC bond benzene (an arene) single/double CC bonds The common name for ethyne is acetylene The lengths of all six CC bonds in benzene are between those for a CC and a CC bond (Section 7.1) Keynotes in Organic Chemistry, Second Edition. Andrew F. Parsons. 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd
22 Functional groups,nomenclature and drawing organic compounds Alkanes are saturated as they contain the maximum number of hydrogen atoms per carbon(CH+2).Alkenes,alkynes and arenes are all unsaturated. Carbon bonded to an electronegative atom(s) .single bond(R is the carbon framework,typically analkyl group:see Section2.2) Halogenoalkanes X=F,Cl.Br. Ether R R Ne comound R一 prmary (1 The of the nitro groupn R-SH secondary(2) Thiol RaCX tertiary (3) Amines Alcohols R-NH2 primary amine 0 RCH2OH primary alcohol R一NHR secondary amine R2CHOH secondary alcohol tertiary amine R3COH tertiary alcohol .double bond to oxygen (these are called carbonyl compounds) R R OH Cyclic esters are called lactones Ketone Acid Carboxylic acid Cyclic amides are called lactams g NH2=primary amide os in an acid anhydride are same.it is called R NR2 a symmetrical anhydride Ester Acid anhydride Amide triple bond to nitrogen R-CEN Nitrile 2.2 Alkyl and aryl groups R is used to sent a general alkyl group (iea methyl,thyl,propyl etc.group name(symbol) structure name(symbol) structure The IUPACname for tert-butyl is methyl (Me) -CH isopropyl (Pr) -CH(CH) 1.1-dimethylethyl ethyl(Et) -CH2CHg isobutyl('Bu) -CH2CH(CHa)2 propyl (Pr) -CH2CH2CH3 sec-butyl(Bu) -CH(CHa)CH2CH3 butyl(Bu) -CH2CH2CH2CHa tert-butyl ('Bu) -C(CHa)a
Alkanes are saturated as they contain the maximum number of hydrogen atoms per carbon (CnH2nþ2). Alkenes, alkynes and arenes are all unsaturated. Carbon bonded to an electronegative atom(s) single bond (R is the carbon framework, typically an alkyl group; see Section 2.2) ROR R SH R NO2 RSR R NH2 R NHR R NR2 R NR3 Ether Thiol Nitro compound Sulfide (thioether) Amines primary amine secondary amine tertiary amine quaternary ammonium ion RCH2OH R3COH Alcohols primary alcohol secondary alcohol tertiary alcohol R2CHOH RCH2X R3CX Halogenoalkanes (X = F, Cl, Br, I) primary (1°) secondary (2°) tertiary (3°) R2CHX double bond to oxygen (these are called carbonyl compounds) R C O H R C O R R C O X R C O OR R C O O C O R R C O OH R C O NR2 Acid (acyl) halide X = Br, Cl Aldehyde Ketone Carboxylic acid Ester Acid anhydride Amide NH2 = primary amide NHR = secondary amide NR2 = tertiary amide triple bond to nitrogen R C Nitrile N 2.2 Alkyl and aryl groups Whenahydrogenatomis removed fromanalkanethisgivesanalkylgroup.Thesymbol R is used to represent a general alkyl group (i.e. a methyl, ethyl, propyl, etc. group). CH2CH3 CH3 CH2CH2CH2CH3 CH2CH2CH3 methyl (Me) ethyl (Et) name (symbol) structure propyl (Pr) butyl (Bu) CH2CH(CH3)2 CH(CH3)2 C(CH3)3 CH(CH3)CH2CH3 isopropyl ( i Pr) isobutyl ( i Bu) name (symbol) structure sec-butyl ( s Bu) tert-butyl ( t Bu) The structure of the nitro group in a nitro compound: R N O O Cyclic esters are called lactones. Cyclic amides are called lactams. If the two R groups in an acid anhydride are the same, it is called a symmetrical anhydride The IUPAC name for isopropyl is 1-methylethyl The IUPAC name for tert-butyl is 1,1-dimethylethyl 22 Functional groups, nomenclature and drawing organic compounds