CHM1022 Week 6 Lecture 1: Acids, Esters, and Amides PDF

Week 6/Lecture 1: Acids, esters and amides Weekly objectives 1. Understand the relationship between the structure and properties of carboxylic acids, esters and amides 2. Examine how esters are made from carboxylic acids and related derivatives 3. Examine how amides are made from carboxylic aci...

Week 6/Lecture 1: Acids, esters and amides Weekly objectives 1. Understand the relationship between the structure and properties of carboxylic acids, esters and amides 2. Examine how esters are made from carboxylic acids and related derivatives 3. Examine how amides are made from carboxylic acids and related derivatives 4. Write a general mechanism for nucleophilic substitution at a carbonyl group  Carbonyls: Week 5, ketones and aldehydes. O C O H C H 3C CH3 HO O H 3C aldehydes ketones O O C C Class of Carbonyl R H R R 1-carbon group (R) 2-carbon group (R) 1-hydrogen (H) http://molview.org/?cid=1183 http://molview.org/?cid=180  Carbonyls: Week 6, acids and derivatives. O CH3 C H 3C O O H N C C C OH H 3C O CH3 O CH3 N H n carboxylic acids esters amides O O O C C C Class of Carbonyl R OH R OR R NH 2 1-carbon group (R) 1-carbon group (R) 1-carbon group (R) 1-hydroxyl (OH) 1-ether (OR) 1-amine (N)  Carboxylic acids in your world. O O H 3C OH H 3C OH ethanoic acid butanoic acid (acetic acid) O OH O O O H 3C HO OH OH OH 2-hydroxypropane-1,2,3- OH tricarboxylic acid (S)-2-hydroxypropanoic (citric acid) acid (lactic acid) Carboxylic acids – why are they acidic? Why are carboxylic acids more acidic than alcohols? H 2O H 3O Ka = [A-][H+]/[AH] H 3C OH H 3C O Ka = 1.3 x 10-16 pKa = 15.9 AH A H O O Ka = [A-][H+]/[AH] H 2O H 3C O H 3O Ka = 1.5 x 10-5 H 3C OH pKa = 4.8 AH A H Carboxylic acids – why are they acidic? Why are carboxylic acids more acidic than alcohols? O O H 3C O H 3C O The carboxylate anion is resonance stabilised. Hydrogen bonding, properties of water. O O H H H 3C CH3 MW = 18 MW = 46 b.p. = 100 °C b.p. = -24 °C Water is a liquid at room temperature while dimethyl ether is a gas. Why? The difference is due to hydrogen bonding. Covalent O-H bond 492 kJ/mol Hydrogen bond δ O Up to 23 kJ/mol O H H δ H δ O H H H The O-H bond is polarised. Hydrogen bonding is a (relatively) (Oxygen is more electronegative) weak stabilising interaction. Water solubility of carboxylic acids. H-bond acceptor δ O H R O δ H-bond donor H-bond acceptor δ Carboxylic acids can H-bond to water, or each other. O O H H H O H O H H H H O H O R O H O H O H H H O O R O O H R O H O H H H H H O R O H H H O O H H O H H Neat acid Aqueous acid Aqueous acid (protonated) (deprotonated)  Carboxylic acids and water solubility. Larger acids (n>4) Not soluble in water O H 3C n OH O H 3C n OH Small acids n = 0 - 4 soluble in water O O NaOH H 3C OH H 3C O Na not soluble in water Sodium salt, common detergent  Individual ACTIVITY 1: Properties of Carboxylic Acids Rank the following compounds (A, B and C) from highest to lowest boiling point. 5 mins. O O O HO O OH H 3C OH H 3C OH O O A B C  Individual ACTIVITY 1: Discussion and feedback FLUX poll 5 mins. O O O HO O OH > H 3C OH > H 3C OH O O A C B 235 °C 213 °C 164 °C More COOH groups, more H bonds, hence higher boiling point.  Esters, naming and smell. O ethyl acetate H 3C O CH3 ethyl acetate O O O O H 3C H 3C O H 3C O CH3 H 3C O CH3 O CH3 butyl butyrate butyl acetate propyl acetate ethyl benzoate  Individual ACTIVITY 2: Properties of esters Rank the following compounds from from most soluble to least soluble in water. 5 mins. O O O H 3C O CH3 H 3C O CH3 H O CH3 A B C  Individual ACTIVITY 2: Discussion and feedback Solubility (g/100 g water) 5 mins. 1.7 8.7 10.5 O O O H 3C O CH3 H 3C O CH3 H O CH3 A B C Esters can H bond with water. Longer hydrocarbon chain, less soluble in water. Polyesters are common polymers…in the water! O HO O O O O ester O OH n O polyethylene terephthalate (PET) Polyethylene terephlalate (PET) is a polymers where the monomers are linked by esters. Globally 1,000,000 bottles bought per minute. 91% will not be recycled, and decompose naturally over 400 years. By 2050 the ocean will be filled with more PET than fish. How are esters made? They can be made from an carboxylic acids and alcohols with an acid catalyst. BUT more often activated acids are used. O O H catalyst HO R R’ R OH R O H 2O carboxylic alcohol ester acid O O HO R R R Cl R O HCl acid chloride alcohol ester Activated acids O O O O HO R R R O R R O HO R carboxylic acid alcohol ester anhydride Nucleophilic substitution at the carbonyl group. Nucleophilic addition to the carbonyl group (Week 5) δ H 3C δ H 3C H 3C H H 3C O H 3C O H H 3C O δ H 3C MgBr H 3C H 3C Nucleophilic substitution at the carbonyl substitution R δ H R R H O O O O O Cl δ R Cl R Cl O R The “Cl” is called a leaving group. It can stabilise negative charge. Any atom, or groups of atoms, that can stabilise negative charge are good leaving groups.  Summary Discussed how the properties of carboxylic acids and esters are related to their structure. Examined how esters are made. Introduced a general mechanism for nucleophilic substitution at a carbonyl group. Week 6/Lecture 2: Acids, esters and amides Weekly objectives 1. Understand the relationship between the structure and properties of carboxylic acids, esters and amides 2. Examine how esters are made from carboxylic acids and related derivatives 3. Examine how amides are made from carboxylic acids and related derivatives 4. Write a general mechanism for nucleophilic substitution at a carbonyl group Amides and hydrogen bonding. Esters lack H-bond donors Primary and secondary amides have H-bond donors O O O CH3 CH3 H 3C NH 2 H 3C N H 3C N H CH3 primary (1°) secondary (2°) tertiary (3°) bp = 222 °C bp = 206 °C bp = 165 °C Primary amine has the greatest opportunity for H-bonding H O CH3 O N H H H 3C N H O N CH3 H H  Individual ACTIVITY 3: Identify the functional groups Identify the ketones, aldehydes, amides, esters and acids in each molecule. Label the amides as primary (1º), secondary (2º) or tertiary (3º). O H O 5 mins O O a) H b) H2N CH3 OH c) O H O O CO2H O CH3 O O OH H 3C CH3 H NH2 O O H N NH2 CH3 H O H H d) HO H H3C CH3 e) N OH H 3C CH3 HN N H O CH3 O O CH3 CH3 O CH3  Individual ACTIVITY 3: Discussion and feedback Identify the ketones, aldehydes, amides, esters and acids in each molecule. Label the amides as primary (1º), secondary (2º) or tertiary (3º). O H O 5 mins O O a) H b) H2N CH3 OH c) O H O O CO2H O CH3 O O OH H 3C CH3 H NH2 O O H N NH2 CH3 H O H H d) HO H H3C CH3 e) N OH H 3C CH3 HN N H O CH3 O O CH3 CH3 O CH3 Amides and hydrogen bonding. Proteins are natural polymers based on amide bonds. H-bonding leads to this a-helix. H O CH3 H O CH3 H O CH3 H O N N N N N N N N CH3 H O CH3 H O CH3 H O CH3 H H-bond 3.6 amino acids away How are amides made? Like esters they are generally made using activated acids such as acid chlorides. The reaction with ammonia is shown, similar reactions with primary and secondary amines occur. The mechanism is very similar to that for esterification. O O R Cl NH3 R NH2 HCl acid chloride ammonia primary amide Nucleophilic substitution at the carbonyl substitution R R δ H R H O N O H N O H Cl H δ H N H Cl H Cl H The “Cl” is called a leaving group. It can stabilise negative charge. Any atom, or groups of atoms, that can stabilise negative charge are good leaving groups.  Individual ACTIVITY 4: Polymer synthesis Identify potential starting materials for the following polymer (Kevlar). What by-products are produced? 5 mins H 2N O N H H N O O N H n OH Kevlar O  Individual ACTIVITY 4: Discussion and feedback Identify potential starting materials for the following polymer (Kevlar). What by-products are produced? 5 mins H 2N O N O H H N H 2N O Cl O Cl HCl is the byproduct NH 2 N H O n OH O  Demo ACTIVITY 5: Synthesis of esters and amides Feed Code: V6SPFH Nylon is a common synthetic amide polymer. The synthesis of nylon 6,10 is shown. The synthesis of nylon 6,10 is very fast and will be demonstrated by in the following video. O 10 mins H 2N Cl NH 2 Cl Demo 6-carbons 10-carbons O O H N + HCl N H O n nylon 6,10  Demo ACTIVITY 5: Synthesis of esters and amides Feed Code: V6SPFH Nylon is a common synthetic amide polymer. The synthesis of nylon 6,10 is shown. The synthesis of nylon 6,10 is very fast and will be demonstrated by your instructor. 10 mins Demo Nucleophilic substitution at the carbonyl and rate. A number of carboxylic acids derivatives engage in substitution reactions but with different rate. O O R X OH R OH X acid derivative hydroxide acid Nucleophilic substitution at the carbonyl and rate. A number of carboxylic acids derivatives engage in substitution reactions but with different rate. O O R X OH R OH X acid derivative hydroxide acid Whether a reaction occurs depends on how stable the X- group is. How do we how stable it is? Nucleophilic substitution at the carbonyl and rate. O O R Cl OH R OH Cl H Cl acid chloride pKa = -8 O O O O O R O R OH R OH O R H O R carboxylic acid increasing acid anhydride pKa = ~4.76 strength ∴ increasingly "happy" O O as conjugate base. R OR OH R OH OR H OR ester pKa = ~14 H OH pKa = 16 Summary Today we have: examined the synthesis of amides. been introduced to polyamides. discussed differences in rate with the hydrolysis of carboxylic acid derivatives.