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Chapter 21

Carboxylic Acid Derivatives - Lecture Textbook Notes Chapter 21

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Cecilia Kutas

CHM247H1 Jasmyn Lee Chapter 21: Carboxylic Acid Derivatives: Nucleophilic Acyl Substitution Reactions 21.1 Naming Carboxylic Acid Derivatives Naming Acid Halides, RCOX Naming Acid Anhydrides, RCO CO2’ Naming Esters, RCO R2 1. Name the alkyl group - ethyl 2. Name the acyl group - acetate (ethanoate) o Replace “ic acid” with “ate” Naming 1° Amides, RCONH 2  For all 1° amides o Replace ic acid, oic acid, ylic acid with amide Naming 2° and 3° Amides 1. Name the alkyl group (groups) bonded to N first 2. Use N as a prefix before each alkyl group name 3. In a 3° amide, if two groups are the same, use “N,N-dialkyl”  Some Interesting Amides Thiesters, RCOSR’ 2- Acyl Phosphates, RCO P2 3 and RCO P2 R3 CHM247H1 Jasmyn Lee 21.10 Spectroscopy of Carboxylic Acid Derivatives Infrared Spectroscopy  IR Absorption of Acid Derivatives o As the C=O π bond becomes more delocalized, adsorption shifts to lower frequency o Conjugation also shifts C=O absorption to lower frequencies  Other IR Adsorptions o 1° and 2° Amines  3200-3400 cm (one or two; stretching)  ~1640 cm (bending)  Nitrile adsorption at 2250 cm Typical NMR Signals  1H NMR o α proton signal at 2-2.5 ppm 1  Identity of the carbonyl group cant be determined by H NMR becauseα hydrogen’s of all acid derivatives absorb in the same range o Protons of 1° and 2° amides adsorb at 7.5-8.5 ppm  13C NMR o C=O peak at 160-180 ppm o C≡N peak at 115-120 ppm 21.2 Nucleophilic Acyl Substitution Reactions  Recall: when a nucleophile adds to an ald/ket, the initially formed tetrahedral intermediate can be protonated to yield an alcohol  When a nucleophile adds to a carboxylic acid derivative, the initially formed tetrahedral intermediate eliminated one of the two substituents originally bonded to the carbonyl carbon – leads to a net nucleophilic acyl substitution reaction  Difference between ald/ket and c. acid derivative is a consequence of structure o Carboxylic acid derivatives have an acyl carbon bonded to a group –Y than can act as a leaving group, often as a stable anion – tetrahedral formed, leaving group is expelled to generate new carbonyl compound o Ald/ket do not have a leaving group – don’t undergo substitution  Nucleophilic Acyl Substitution is characteristic o General Mechanism Oxygen Nucleophiles O _ OH OH 2 ROH C _ R O 1. Nu attacks breaks π bond, new C-Nu bond forms tetrahedral Nitrogen Nucleophiles intermediate 2. Leaving group eliminated, substitution product made NH RNH R NH o Overall: Adding Nu, eliminating Z 3 2 2  i.e. substituting Nu for Z; Z = -X, -OCOR,2-OR, -NR CHM247H1 Jasmyn Lee Reactivity of Acid Derivatives  Any factor that makes the carbonyl group more reactive toward nucleophile favors substitution  Steric and electronic factors are important in determining reactivity o Sterically – unhindered, accessible carbonyl groups react with nucleophiles more readily than sterically hindered groups o Electronically – strongly polarized acyl compounds react more readily than less polar  Acid Halide > Acid Anhydride > Thioester > Ester > Amide  Eg/ Cloride substituent is an EWG – inductively withdraws electrons from an acyl group  Eg/ Amino, methoxyl and methylthio substituents donate electrons to acyl group  What determines derivative reactivity? Leaving Group Stability Interconversion of Acid Derivatives Physical Properties of Acid Derivatives C=O undergoes Nucleophilic Acyl Substitution Types of Reactions  Hydrolysis – reaction wit2 H O to yield a carboxylic acid  Alcoholysis – reaction with an alcohol to yield an ester  Aminolysis – reaction with ammonia or an amine to yield an amide  Reduction – reacation with a hydride reducing agent to yield an ald/ket  Grignard Reaction – reaction with an organometallic reagent to yield an ald/ket 21.3 Nucleophilic Acyl Substitution Reactions of Carboxylic Acids Conversion of Carboxylic Acids in to Acid Chlorides Conversion of Carboxylic Acids into Acid Anhydrides Conversion of Carboxylic Acids into Esters  SN2 reaction of a carboxylate anion with a primary alkyl halide  Fischer Esterification starts with carboxylic acid o Need to force it o Only works with 1° and methyl alcohols CHM247H1 Jasmyn Lee o Mechanism  Part A – addition of nucleophile 1. Protonation makes C=O more electrophilic 2. Nucleophilic addition of R’OH forms tetrahedral intermediate 3. Deprotonation for neutral intermediate  Part B – elimination of leaving group 4. Protonate an OH to make good leaving group (H O2 5. Leaving group eliminated 6. Deprotonation for neutral product ester o All steps are reversible – equilibrium constant close to 1  rea
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