Carboxylic Acid Derivatives (A2) PDF

# Derivative of Carboxylic acids (A2) ## Acyl Chloride eg. * CH₃C=O * CI * ethanoyl chloride * CH₃C=O * OH * ethanoic acid ## Formation of Acyl Chloride 1. CH₃COOH + PCI₅ → CH₃COCl + POCI₃ + HCl * ethanoic acid *...

# Derivative of Carboxylic acids (A2) ## Acyl Chloride eg. * CH₃C=O * CI * ethanoyl chloride * CH₃C=O * OH * ethanoic acid ## Formation of Acyl Chloride 1. CH₃COOH + PCI₅ → CH₃COCl + POCI₃ + HCl * ethanoic acid * phosphorus (V) chloride * ethanoyl chloride * steamy fumes 2. CH₃COOH + SOCl₂ → CH₃COCl + SO₂ + HCl * sulfur dichloride oxide 3. CH₃COOH + PCI₃ heat → CH₃COCl + H₃PO₃ * phosphorous (III) chloride ## Acyl Chlorides - reactive compounds +δ δ- R - C= O * CI both Cl atom and O atom withdrawing electrons from carbon atom → relatively large partial positive charge → highly attractive to attack by nucleophiles ## Hydrolysis of Acyl Chloride * reagent : water * condition: room temperature * products: Carboxylic acids * observation: heat released (reaction is vigorous) steamy white fumes (HCl) CH₃C=O + H₂0 → CH₃C=O + HCl * CI * ethanoyl chloride * OH * ethanoic acid ## Ease of hydrolysis in the order of R - C=O * CI * acyl chloride * ethnaoyl chloride > R - CI * alkyl chloride * chloroethane > R - CI * aryl chloride * chlorobenzene * ethanoyl chloride reacts vigorously with water * chloroethane reacts slowly with water * chlorobenzene does not react with water * The relative ease of hydrolysis can be followed by warming each chloride with NaOH_(ag), then by excess HNO_3(ag) and AgNO_3(ag) * Positive observation with white precipitate: * Ag⁺_(ag) + Cl^-_(ag) → AgCl_(s) * Ethanoyl chloride gives an immediate ppt of AgCl. The reaction is more vigorous than directly with water alone. * CH₃COCI + OH^- → CH₃CO₂^- + H⁺ + Cl^- * Chloroethane gives white ppt after some time. ## Explanation * The ease of hydrolysis is depend upon the attack by nucleophiles on the δ+ carbon atoms. * Acyl chloride * R - C=O * CI * Acyl chlorides are rapidly hydrolysed by water because the chloride atom is high in electronegativity and pulls electrons away from the carbon atom of the C=O group. * The oxygen atom also pulls electrons from this carbon atom, so it becomes quite positive. Hence a water molecule can easily from a dative bonds with this carbon atom. * e.g. ethanoyl chloride: δ+ δ- CH₃ - C = O → CH₃ - C = O + Cl^- * CI * δ+ δ- * H *OH * H (Addition Elimination Reaction) δ+ δ- CH₃ - C - OH + HCl * The attack is enhanced by the possibility of a shift of the π electrons in the C=O bond, which also makes it possible for the carbon atom to bond with a nucleophile before the Cl atom is released. Hence the intermediate is formed more readily. ## Alkyl chloride * e.g. chloroethane δ+ δ- CH₃ - C - CI * Carbon atom in a C-Cl group is attached to only one highly electronegative atom (Cl atom) and, therefore, carries a smaller partial positive charge than the carbon atom in a COCI group, which is attached to two strongly electronegative atoms. * The methyl group has electron donating property and will reduce the partial positive charge on the carbon atom. * Since alkyl chlorides carry a smaller positive charge on the carbon atom of the C-Cl group, they will be less susceptible to nucleophilic attack than acyl chlorides. * Hydrolysis with water alone is very slow unless under improved conditions like added NaOH_(aq) + heat. ## Aryl chloride * O - CI. chlorobenzene. * Benzene ring has delocalised π electrons built from overlapped of p electrons by 6 carbon atoms in the ring. * With overlap of all p-orbitals including Cl atom, delocalised π bonding extends over Cl atom and benzene ring: * benzene ring: C / \ C C / \ / \ C C C \ / \ / C C \ / C | CI * overlap of p-orbitals C / \ C C / \ / \ C C C \ / \ / C C \ / C | CI π bonding extends over Cl atom and benzene ring * The carbon atom attached to Cl atom is in the benzene ring. The delocalised π electron cloud is ready to compensate any partial positive charge created on the carbon atom by the Cl atom (electronegativity difference) * The benzene ring repel any nucleophile from getting near and therefore prevent any attack by a nucleophile (e.g. water molecule). # Reactions of Acyl Chloride * Due to highly partial positive charge of carbon atom at C=O group, Acyl chlorides are very reactive. * They fumes in moist air * They are very reactive towards nucleophilic reagents. 1. Reaction with alcohols 2. Reaction with phenols 3. Reaction with ammonia 4. Reaction with primary amines ## Reaction with Alcohols * Reagent: alcohol * Condition: room temperature * Product: ester and HCl CH₃C=O + CH₃CH₂OH → CH₃COOCH₂CH₃ + HCl * CI * ethanoyl chloride * ethanol * ethyl ethanoate Ethnaoyl chloride react vigorously with ethanol to form an ester. ## Reaction with phenols * Reagent: phenol * Condition: warming * Product: ester and HCl CH₃C=O + C₆H₅OH → CH₃COOC₆H₅ + HCl * CI * ethanoyl chloride * phenol * phenyl ethanoate O=O C=O C + C₆H₅OH → C=O + HCl * CI * benzoyl chloride * phenol * phenyl benzoate * To make phenyl esters, acyl chlorides must be used. Because there is noreaction between phenol and carboxylic acids. * For both reactions with alcohols or phenols: * The reaction goes to completion and do not form an equilibrium mixture. * They are useful in the synthesis of esters in the chemical industry. # The reaction of acyl chlorides with water, alcohols and phenols * These reactions have similarity in chemistry. * Comparing the structures of water, ethanol and phenol: * Each substance contains an -OH group. * In water, this is attached to a hydrogen atom. * In an alcohol, it is attached to an alkyl group (R). * In phenol, it is attached to a benzene ring (phenol is C₆H₅OH) * O - H * O - H *O - H * H * R * benzene ring # The reaction of acyl chlorides with ammonia and primary amines * These reactions show similarity in chemistry. * Comparing the structures of ammonia and primary amines: * Each substance contains an -NH₂ group. * In ammonia, this is attached to a hydrogen atom. * In primary amine, it is attached to an alkyl group (R) or a benzene ring * H - N - H * H - N - H * H * R * ammonia * primary amines ## Reaction with Ammonia * Reagent: Ammonia (concentrated) * Condition : Room temperature * Product : Amides CH₃C=O + 2NH₃ → CH₃C=O + NH₄Cl * CI * ethanoyl chloride * NH₂ * ethanamide Violent reaction producing lot of white smoke. ## Reaction with Primary Amines * Reagent: primary amine (concentrated) * Condition: room temperature * Product: N-substituted amides CH₃C=O + 2CH₃CH₂NH₂ → CH₃CN-CH₂CH₃ + CH₃CH₂NH₃Cl * CI *ethanoyl chloride * ethylamine * N-ethyl ethanamide * thylammonium chloride The reaction is vigorous. # The acidity of carboxylic acids * Carboxylic acids are weak acids. * But carboxylic acid is a stronger acid compared to alcohol and phenol. * When ethanoic acid dissolves in water: CH₃C=O + H₂O → CH₃CO₂^- + H₃O⁺ * OH * ethanoic acid * ethanoate ion * Ka (25℃) = 1.7 x 10^-5 moldm^-3, the position of this equilibrium lies over to the left-hand side. Small Ka value → weak acid. * The O-H bond in the carboxylic acid is weakened by the C=O group: * R - C=O * OH * electrons in the C-O bond are drawn toward the C=O bond. * electrons are drawn away from the O-H bond. * The carboxylate ion is stabilised by delocalisation of electrons around the COO group. This delocalisation spread out the negative charge on the carboxylate ion, making it less likely to bond with an H⁺_(aq) ion to reform the undissociated acid molecule. * R - C=O * O- * → *R - C=O * O- * ie R- C=O * O-- negative charge is spread over the whole COO group (the bond lengths of both carbon-oxygen bonds are equal) ## Relative Strength of Carboxylic Acids * The strength of a carboxylic acid is affected by the nature of the substituent group. * Substituent group can be either: 1. electron-donating group - decreases acid strength 2. electron-withdrawing group - increases acid strength ## Electron-donating group decreases the acid strength of carboxylic acid. * Electron-donating group intensifies the negative charge on the O atom, making the carboxylate ion less stable. Carboxylate ion prefers to associate back with H⁺ ion to form undissociated carboxylic acid molecule. * Acidity: * H-C=O > CH₃-C=O > CH₃CH₂-C=O * OH * OH * OH * pKa = 3.77 * pKa = 4.76 * pKa = 4.88 * The greater the electron-donating effect, the weaker is the acid. ## Electron-withdrawing group increases the acid strength of carboxylic acids * Electron-withdrawing group helps to reduce the negative charge gathered on O atom, thus stabilises the carboxylate ion and weakening the O-H bond. * These improve the readiness of carboxylic acid to dissociate and form carboxylate ion and H⁺ ion. * Acidity: * Cl₃C-C=O > Cl₂CH-C=O > ClCH₂- C=O * OH * OH * OH * pKa = 0.65 * pKa = 1.29 * pKa = 2.86 * The greater the electron-withdrawing effect (with higher number of electron withdrawing group), the stronger is the acid. ## The acidity of halogen-substituted ethanoic acid. * The acidity increases when the substituted halogen is more electronegative. * Electronegativity: F > CI > Br > I * Acidity: * FCH₂-C=O > CICH₂-C=O > BrCH₂-C=O > ICH-C=O * OH * OH * OH * OH * pKa = 2.66 * pKa = 2.86 * pKa = 2.89 * pKa = 3.16 ## The distance of the substituent group from carboxyl group. * The distance of the substituent group from the carboxyl group increases, the acidity decreases. * Acidity: * CICH₂-C=O > CICH₂CH₂-C=O > CICH₂CH₂CH₂-C=O * OH * pKa = 4.05 * pKa = 4.53 * OH * The same understanding applies for the aromatic acids. The electron-withdrawing groups increases the acidity of the aromatic acids.