Chapter 21. Carboxylic Acid Derivatives and Nucleophilic Acyl Substitution Reactions General mechanism: O leaving O O acyl C O group R Y group + Y: C C C R Y R Nu R Nu :Nu Y tetrahedral intermediate Carboxylic acid derivatives: Increasing reactivity
O O O O O O C C C C C C R OH R N R OR' R O R' R Cl carboxylic acid acid amide ester acid anhydride chloride parent O O O C C P Chapter 21.8 function group R SR' R O O O please read thioester acyl phosphate 197
21.1 Naming Carboxylic Acid Derivatives: (Please read) Acid Chlorides: Derived from the carboxylic acid name by replacing the -ic acid ending with -yl choride or replacing the -carboxylic acid ending with -carbonyl chloride O O O C C Cl C Cl H3C Cl
acetyl chloride benzoyl chloride Cyclohexancarbonyl chloride (from acetic acid) (from benzoic acid) (from cyclohexane carboxylic acid) Acid Anhydrides: Name symmetrical anhydrides as the carboxylic acid except the word acid is replaced with anhydride
O O O O C C O C C H3C O CH3
acetic anhydride benzoic anhydride 198
100 Amides:
Primary amides (RCONH2) are named as the carboxylic acid except the -ic acid ending is replaced with -amide or the -carboxylic acid ending is replaced with -carboxamide. O O O C C NH C 2 NH2 H3C NH2
acetamide benzamide Cyclohexancarboxamide (from acetic acid) (from benzoic acid) (from cyclohexane carboxylic acid)
Substituted amides are named by first identifying the substituent(s) with an N- in front of it, then naming the parent amide
O O O C CH2CH3 C CH CH C CH3 N 2 3 H3C N N H CH 3 CH2CH3 N-methylacetamide N-ethyl-N-methylbenzamide N,N-diethylcyclohexancarboxamide 199
Esters: Esters are named by first identify the group on the carboxylate oxygen, then identifying the carboxylic acids and replacing the -oic acid with -ate.
O O O C CH3 C O C H3C OCH2CH3 OC(CH3)3
ethyl acetate methyl benzoate tert-butylcyclohexancarboxylate
21.2 Nucleophilic acyl substitution reactions
O Y = a leaving group O O + Y: -OH, -OR’, -NR , -Cl, C C C 2 R Y R 2- Nu R Nu -O2CR’, -SR’,-OPO3 :Nu Y tetrahedral intermediate 200
101 Relative reactivity of carboxylic acid derivatives: The mechanism of nucleophilic acyl substitution involves two critical steps that can influence the rate of the overall reaction: 1) the initial addition to the carbonyl groups, and 2) the elimination of the leaving group.
The nature of the acyl group: The rate of addition to the carbonyl carbon is slower as the steric demands of the R group increase. Branching at the α-carbon has the largest effect. Increasing reactivity O O O O R C < R C < R C H C C Y C Y C Y < C Y R R R H H H H H
Recall from Chapter 4: -CH3 ΔG°= 7.6 KJ/mol -CH2CH3 8.0 R -CH(CH ) 9.2 Keq 3 2 R H -C(CH3)3 > 18 -CH2C(CH3)3 8.4 H 201 -C6H5 12.6
Nature of the leaving group (Y): In general, the reactivity of acyl derivatives correlate with leaving group ability. Increasing reactivity
O O O O O C C C C < C R N < R OR' < R O R' R Cl
amide ester acid anhydride acid chloride What makes a good leaving group?
R’2N-H R’O-H R’CO2-H Cl-H pKa: 30-40 16-18 4-5 -7 The reactivity of the acyl derivative inversely correlates with their resonance electron-donating ability
Resonance effect of the Y atom reduces the O O electrophilicity of the carbonyl carbon. C C R NH2 R NH2 Partial double bond character of the C-N bond makes it harder to break. 202
102 All acyl derivatives are prepared directly from the carboxylic acid Less reactive acyl derivative (amides and esters) are more readily prepared from more reactive acyl derivatives (acid chlorides and anhydrides)
acid chloride
acid anhydride ester amide y t
acid anhydride i v i t c a
carboxylic e r
acid ester amide g n i s a e r
ester c n I
amide
amide 203
Nucleophilic acyl substitution reactions O hydrolysis C Y= -Cl, -O2CR', -OR', -NR'2 R OH H2O carboxylic acid
O aminolysis C Y= -OH, -Cl, -O2CR', -OR' R NR' R'2NH 2 amide
O O C alcoholysis R Y C Y= -OH, -Cl, -O2CR' R OR' Carboxylic acid R'OH ester derivative Y= -OH, -Cl, -O CR', O H H 2 reduction Y= -OH, -Cl, -O CR', -OR' -OR', -NR'2 C C 2 H R H R OH H H
C Y= -NR2 R NR2
O R'' R'' C C R''-M R R'' R OH
R''-M = R''2CuLi R''-M = R''-MgBr, R''-Li Y= -Cl Y= -Cl, -OR' 204
103 21.3 Nucleophilic acyl substitution reactions of carboxylic acids Nucleophilic acyl substitution of carboxylic acids are slow because -OH is a poor leaving group Reactivity is enhanced by converting the -OH into a better leaving groups However, acid chlorides, anhydrides, esters and amides can be prepared directly from carboxylic acids
O C R OH carboxylic acid
O O O O O C C C C R Cl R O R' R OR' R NR'2 acid chloride acid anhydride ester amide
205
Conversion of carboxylic acids into acid chlorides
Reagent: SOCl2 (thionyl chloride)
O SOCl2, !" O C C + SO2 + HCl R OH R Cl
Recall the conversion of 1° and 2° alcohols to alkyl chlorides Mechanism (p. 780), please read
206
104 Conversion of carboxylic acids into acid anhydrides (please read) Reagent: heat
Reaction is best for the preparation of cyclic anhydrides from di-carboxylic acids
Conversion of carboxylic acids into esters 1. Reaction of a carboxylate salt with 1° oXXXr 2° alkyl halides Reagents: NaH, R’-X, THF O O O H CH C Br C NaH, THF C 3 2 OH C O O CH2CH3 SN2 Na
Note the similarity to the Williamson ether synthesis (Chapter 18.3) 207
Conversion of carboxylic acids into esters 2. Fisher esterification reaction: acid-catalyzed reaction of carboxylic acids with 1° or 2° alcohols to give esters Reagents: ROH (usually solvent), HCl (strong acid)
HO H HO H H3CH2C OH C OH C OCH2CH3 C C + H2O HCl O O Mandelic acid Ethyl mandelate Mechanism: (Fig. 21.5, p. 782)
208
105 Conversion of carboxylic acids into amides Not a particularly good reaction for the preparation of amides.
Amines are organic bases; the major reaction between a carboxylic acid and an amine is an acid-base reaction (proton transfer)
21.4: Chemistry of Acid Halides Preparation of acid halides (Chapter 21.3)
Reaction of a carboxylic acid with thionyl chloride (SOCl2)
O SOCl2, !" O C C + SO2 + HCl R OH R Cl
209
Reactions of acid halides Friedel-Crafts acylation (Chapter 16.4): reaction of an acid chloride with a benzene derivative to give an aryl alkyl ketone.
O O SOCl O C 2 R C C + HCl R OH R Cl AlCl3 carboxylic acid acid chloride
Nucleophilic acyl addition reactions of acid halides 1. hydrolysis O H2O O C C R Cl NaHCO3 R OH acid chloride -or- carboxylic acid NaOH
210
106 2. Alcoholysis: Acid chlorides react with alcohols to give esters. reactivity: 1° alcohols react faster than 2° alcohols, which react faster than 3° alcohols
O pyridine O Cl OH + C + C N R Cl R O H acid chloride ester
3. Aminolysis: Reaction of acid chlorides with ammonia, 1° or 2° amines gives amides.
O O + H2NR2 Cl C + HNR2 C R Cl R NR2 amine acid chloride (two equiv) ester
211
4. Reduction: Acid chlorides are reduced to primary alcohols
with lithium aluminium hydride (LiAlH4)
5. Reaction of acid chlorides with organometallic reagents: Reacts with two equivalents of a Grignard reagent to give a 3° alcohols (recall reaction with esters) a) THF O + OH b) H3O + MgXCl + H C MgX C CH C 3 R 3 R Cl CH3 Grignard reagent acid chloride 3° alcohol (two equiv.)
212
107 Reaction of acid halides with diorganocopper reagents
a) ether O + O b) H3O C + (H3C)2 CuLi C R Cl R CH3 diorganocopper acid chloride ketone reagent Mechanism (page 787), not typical, specific to diorganocopper reagents. please look it over if you like Diorganocopper reagent can be alkyl, aryl or vinyl O a) 4 Li(0) H C O H H H b) CuI H3CH2CH2C Cl H C C C 2 C C H CH CH C C H C Cu Li 3 2 2 H Br H H 2 Fill in the reagents: O O OH O O
OCH2CH3 OCH2CH3 OCH2CH3
O O O
OH Cl CH2CH3 Ch. 21.6 213
21.5 Chemistry of acid anhydrides Preparation of acid anhydrides
O Na O O O O O + NaCl C + C C C C C R Cl O R' R O R' R O R' Cl acid chloride sodium acid anhydride carboxylate
Reactions of acid anhydrides Acid anhydrides are slightly less reactive reactive that acid chlorides; however, the overall reactions are nearly identical and they can often be used interchangeably.
They do not react with diorganocopper reagents to give ketone
214
108 21.6 Chemistry of esters Preparation of esters Na 1. O O O NaH, THF H3C I Limited to 1° alkyl halides C C C R OH R O R O CH and tosylates SN2 3
2. O O H3CH2C-OH Limited to simple 1° and 2° C R OH C alcohols (used as solvent) HCl R O CH2CH3
H3C CH 3. O SOCl2 O HC OH O 3 Very general method, 1°, 2° and C C H3C C CH 3° alcohols usually work fine R OH R Cl R O CH pyridine 3
Reactions of esters O C R OR' ester
H2O + (NaOH, H2O or H3O ) R'2NH RMgX H- O O OH O H- H H C C C C R OH R NR' R'' C 2 R R'' R H R OH carboxylic acid amide 3° alcohol aldehyde 1° alcohol 215
Hydrolysis of esters: esters can be hydrolyzed to the carboxylic acids with aqueous hydroxide (saponification) or aqueous acid.
Mechanism of the base-promoted hydrolysis (Figure 21.9)
O a) NaOH, H2O O b) H O+ C 3 C R OCH3 R OH ester carboxylic acid
216
109 Acid-catalyzed hydrolysis of esters (Figure 21.10, p. 793) (reverse of the Fischer esterification)
O + O H3O C C R OCH3 R OH ester carboxylic acid
217
Aminolysis: Conversion of esters to amides
O HN O C C R OCH3 R N ester amide
Reduction of esters
LiAlH4 reduces esters to primary alcohols
O O LiAlH4 O LiAlH4 H H C R C OCH3 R OCH3 C C H R H fast R OH 218 H
110 Reduction of esters Diisobutylaluminium hydride (DIBAH) can reduce an ester to aldehyde
iBu
Al iBu O O + O DIBAH H3O O C R C OCH3 R C OCH3 R OCH3 C H R H H H
Esters do not react with NaBH4 or BH3 Reaction of esters with Grignard reagents: Esters will react with two equivalents of a Grignard reagent to give 3° alcohols
O 1) 2 H3CMgBr + 2) H3O HO C CH3 OCH CH 2 3 CH _ 3 H3C H3CMgBr + the H3O
MgX _ _ O - CH CH O O CH 3 2 C 3 C CH OCH2CH3 _ 3 H3C The Grignard reagent can be alkyl, vinyl or aryl 219
21.7 Chemistry of Amides Preparation of amides: amides are prepared from the reaction of an acid chloride with ammonia, 1° or 2° amines
O SOCl2 O C C R OH R Cl carboxylic acid acid chloride
NH3 R'NH2 R'2NH O O O C C R' C R' R NH2 R N R N H R' unsubstitiuted mono-substitiuted di-substitiuted (1°) amide (2°) amide (3°) amide
Reactions of amides: Amides bonds are very stable do to resonance between the nitrogen lone pair and the π-bond of the carbonyl
220
111 Hydrolysis- Amides are hydrolyzed to the carboxylic acids and amine with either aqueous acid or aqueous hydroxide. Acid promoted mechanism:
O + O H3O C C + NH3 R NH2 R OH
Base promoted mechanism O NaOH, H O O 2 + NH C C 3 R NH2 R OH
221
Proteases: catalyzes the hydrolysis of amide bonds of peptides
O R O R O O R O R O H H protease H H H3N N N H3N N + N N N N CO H O N O H3N N CO H H 2 2 H 2 H O R O R H O R O R
Asp–Arg–Val–Tyr–Ile–His–Pro–Phe–His–Leu–Val–Ile–His–Asn angiotensinogen ACE inhibitors
aspartyl renin protease
N N O N CO2H N NH Asp–Arg–Val–Tyr–Ile–His–Pro–Phe–His–Leu angiotensin I (little biological activity) Diovan HS Zn2+ angiotensin converting O protease N enzyme (ACE) HO2C
Captopril Asp–Arg–Val–Tyr–Ile–His–Pro–Phe angiotensin II (vasoconstriction → high blood pressure) 222
112 Reduction of amides to primary amines: amides can be reduced
by LiAlH4 or BH3 (but not NaBH4) to give primary amines Mechanism:
O SOCl O H C-NH O H H 2 3 2 LiAlH4 C C C CH3 C CH3 R OH R Cl R N R N H H
21.8 Thioesters and Acyl Phosphates: Biological Carboxylic AcidDerivatives (please read) 21.9 Polyamides and Polyesters: Step-Growth Polymers (please read) 223
21.10 Spectroscopy of Carboxylic Acid Derivatives IR: typical C=O stretching frequencies for: carboxylic acid: 1710 cm-1 ester: 1735 cm-1 amide: 1690 cm-1 aldehyde: 1730 cm-1 ketone 1715 cm-1
Conjugation (C=C π-bond or an aromatic ring) moves the C=O -1 absorption to lower energy (right) by ~15 cm O O O
OCH3 OCH3 OCH3
aliphatic ester conjugated ester aromatic ester -1 1735 cm 1725 cm-1 1725 cm-1
O O O
NH NH2 NH2 2
aliphatic amide conjugated amide aromatic amide -1 224 1690 cm 1675 cm-1 1675 cm-1
113 1H NMR: Protons on the α-carbon (next to the C=O) of esters and amides have a typical chemical shift range of δ 2.0 - 2.5 ppm Proton on the carbon attached to the ester oxygen atom have a typical chemical shift range of δ 3.5 - 4.5 ppm
δ 2.0 δ 1.2 3H, s 3H, t, δ 4.1 J= 7.1 2H, q, J= 7.1
δ 2.0 O 3H, s H3C C N CH2CH3 H δ 1.1 δ 3.4 3H, t, J= 7.0 2H, q, J= 7.0 NH 225
13C NMR: very useful for determining the presence and nature of carbonyl groups. The typical chemical shift range for C=O carbon is δ160 - 220 ppm Aldehydes and ketones: δ 190 - 220 ppm Carboxylic acids, esters and amides: δ 160 - 185 ppm
O 14.2 H3C C O CH2CH3 60.3 21.0 CDCl 170.9 3
O 21.0 H C C N CH CH 14.8 3 2 3 34.4 H
170.4 CDCl3
226
114 1 C11H12O2 H NMR δ 1.3 δ 7.5 δ 7.3 3H, t, 2H, m 3H, m J= 7.0 δ 4.2 2H, q, J= 7.0 6.4 δ 7.7 δ 1H, d, 1H, d, J= 15.0 J= 15.0
IR 13C NMR
130.2 128.8 127.9 144.5 60.5 134.9 118.2 14.3
166.9 CDCl3 TMS
227
115