carboxylic acid & derivatives

Question 1

why do carboxylic acids have higher boiling points that corresponding alcohols (with similar size of electron cloud)?

Answer 1

1. hydrogen bonding between carboxylic acid molecules is stronger than that between alcohol molecules. the presence of the electron-withdrawing C=O group causing the δ+ on the H atom to be intensified -> the O-H bond is more polarised in the carboxylic acid
2. carboxylic acids can dimerise (in the pure liquid) through hydrogen bonding -> doubles the number of electrons (hence electron cloud) in the dimer -> stronger dispersion forces between dimers

Question 2

why does solubility of carboxylic acids in water decrease as hydrocarbon chain gets longer?

Answer 2

longer non-polar hydrocarbon chain -> stronger dispersion forces between carboxylic acid molecules -> energy released from hydrogen bonding between -COOH group & water molecules is less than energy needed to overcome these dispersion forces, as well as energy existing hydrogen bonding between water molecules

carboxylic acid does NOT dimerise in water

Question 3

how can carboxylic acids be prepared?

Answer 3

1. oxidative cleavage of alkenes
2. side chain oxidation of alkylbenzenes
3. acid & alkaline hydrolysis of nitriles
4. oxidation of primary alcohols
5. oxidation of aldehydes
6. hydrolysis of acyl chlorides
7. acid & alkaline hydrolysis of esters/amides

Question 3

why are carboxylic acids with longer hydrocarbon chains generally soluble in non-polar solvents?

Answer 3

dispersion forces can be formed between the solvent molecules and the hydrocarbon chain of the dimers

Question 4

why can’t carboxylic acids undergo nucleophilic addition (like aldehydes & ketones)?

Answer 4

the delocalisation of electrons from the O-H group into the C=O group makes the carboxyl carbon less electron deficient and less readily attacked by nucleophiles

Question 5

why are carboxylic acids stronger acids than phenols?

Answer 5

• in the carboxylate ion (conjugate base of carboxylic acid), the negative charge is delocalised equally over 2 highly electronegative oxygen atoms -> the negative charge is dispersed and the carboxylate anion is greatly stabilised
• in the phenoxide ion, the negative charge is deloclised over the benzene ring, but delocalisation is less than in the carboxylate ion. bc the carbon atoms in the benzene ring do not bear the negative charge as well as the 2 electronegative O atoms in the carboxylate ion -> carboxylate ions are more stabilised than the phenoxide ion, and phenols are weaker acids than carboxylic acids

Question 6

why are carboxylic acids stronger acids than alipathic alcohols?

in order of decreasing acid strength: carboxylic acid, phenol, alcohol

Answer 6

• the electron-donating alkyl group intensifies the negative charge on the alkoxide ion, destablising the alkoxide ion
• but in the carboxylate ion, the negative charge is delocalised equally over 2 highly electronegative oxygen atoms. the negative charge is dispersed and the carboxylate anion is greatly stabilised

Question 7

how do electron-donating groups affect the acidity of carboxylic acids?

Answer 7

electron-donating groups intensify the negative charge on the carboxylate anion, destabilising the anion and decreasing the acidity of the carboxylic acid

Question 8

LO: how do electron-withdrawing groups affect the acidity of carboxylic acids?

Answer 8

electron-withdrawing groups disperse the negative charge on the carboxylate anion, stabilising the anion and increasing the acidity of the carboxylic acid

more electron-withdrawing groups -> increasing acidity (stronger acid)

for halogen substituents, the more electronegative the halogen atom is, the greater the electron-withdrawing effect will be, so the stronger the acid is.

Question 9

how does the proximity of the electron-withdrawing group to the -COOH group affect acidity?

Answer 9

the nearer the electron-withdrawing group is to the carboxyl carbon, the greater the inductive effect, and the stronger the acid

as inductive effect decreases with increasing distance, inductive effect becomes insignificant when the electron-withdrawing effect is acting through more than 4 atoms (since electrons are further and exert less effect on strength of the acid)

Question 10

how to distinguish carboxylic acids from phenols & alcohols?

Answer 10

react all 3 compounds with sodium carbonate. for the carboxylic acid, effervescence of a colourless gas that produces white ppt with limewater (CO2) will be observed. for phenols & alcohols, there will be no visible reaction

Question 11

what are the reagents, conditions & observations for the nucleophilic substitution of PCl5 with carboxylic acids?

Answer 11

reagents & conditions: PCl5, rt
observation: white fumes of HCl produced

Question 12

what are the reagents & conditions for nucleophilic substitution of PCl3 with carboxylic acids?

Answer 12

reagents & conditions: PCl3, rt

can use PBr3 or P with I2 to obtain acyl bromides or acyl iodides

Question 12

what are the reagents & conditions for nucleophilic substitution of SOCl2 with carboxylic acids?

Answer 12

reagents & conditions: SOCl2, warm

Question 13

what are the reagents & conditions for condensation of carboxylic acids with esters?

Answer 13

reagents & conditions: alcohol, carboxylic acid, few drops of H2SO4, heat

Question 14

what are the reagents & conditions for the reduction of carboxylic acids to primary alcohols?

Answer 14

reagents & conditions: LiAlH4 in dry ether, rt

carboxylic acids can only be reduced back to alcohols, not to aldehydes

consider 2-methyl, 3-oxobutanoic acid (1 carboxyl group & 1 ketone group). if a weaker reducing agent, eg NaBH4 in methanol, is used, only the ketone group will be reduced. but if a stronger reducing agent, eg LiAlH4 in dry ether is used, both the acid & the ketone are reduced

Question 15

which 2 carboxylic acids can undergo further oxidation, and what are the reagents & conditions for this to occur?

Answer 15

methanoic acid, HCOOH, & ethanedioic acid, (COOH)2 can undergo further oxidation.
reagents & conditions: KMnO4, dilute H2SO4, heat under reflux
observations: purple solution decolourises, and effervescence of CO2 is observed

Question 16

why do esters & acyl chlorides have lower boiling points than their corresponding carboxylic acids?

Answer 16

the intermolecular pd-pd attractions are weaker than hydrogen bonding for carboxylic acids and require less energy to break -> lower boiling point

esters & acyl chlordies cannot form intermolecular hydrogen bonding as they lack a hydrogen atom directly attached to a highly electronegative oxygen atom

Question 17

why does reactivity increase in the order: amide, ester, carboxylic acid, acyl chloride?

Answer 17

1 - electronic effects

• a halogen substituent (on acyl chloride) withdraws electrons through inductive effect from the C=O group, increasing the partial positive charge on the carbon atom -> makes acyl halides more readily attacked by nucleophiles
• an amino group (on amide) or alkoxy group (on ester) donate electrons to the carbonyl carbon through resonance effect, decreasing the partial positive charge on the C=O carbon atom -> amides & esters are less readily attacked by nucleophiles

2 - leaving group

• the weaker the base, the better the leaving group -> substituted more readily
• halide anions are better leaving groups compared to the hydroxide ion as halides are weaker bases while the hydroxide ion is a stronger base

Question 18

what are the reagents, conditions & observations for hydrolysis of acyl chlorides?

Answer 18

reagents & conditions: water, rt
observations: white fumes of HCl formed

Question 19

what are the reagents, conditions & observations for condensation of acyl chlorides with alcohols/phenols?

Answer 19

reagents & conditions: acyl choride, alcohol/phenol, rt
observation: white fumes of HCl produced

Question 20

what are the reagents & conditions for condensation of acyl chlorides with ammonia/amines to form amides?

Answer 20

reagents & conditions: ammonia/primary amines/secondary amines (in excess), rt

acyl chlorides react with ammonia to form amides, and with primary & secondary amines to form N-substituted amides. tertiary amines do NOT form amides as they do not contain a replaceable H atom on the N

ammonia/amines are added in excess to ensure complete reaction

carboxylic acids cannot be used in place of the acyl chloride, as they react with amines/ammonia in an acid-base reaction instead

Question 21

relative rate of hydrolysis for acyl chlorides, alkyl chlorides & aryl chlorides in decreasing order of reactivity?

Answer 21

acyl chloride (most reactive), alkyl chloride, aryl chloride (least reactive)

Question 22

hydrolysis conditions for acyl chlorides, alkyl chloride & aryl chlorides?

Answer 22

acyl chloride: H2O, rt
alkyl chloride: aqueous NaOH/KOH, heat under reflux
aryl chloride: not possible under ordinary lab conditions (only possible under very high T & P with aqueous alkali)

Question 23

LO: why does rate of hydrolysis decrease in the order: acyl chloride, alkyl chloride, aryl chloride?

Answer 23

1 - electronic factor - the C-Cl bond in *acyl chlorides* **cleaves easily** without heating. the C on C-Cl is **highly electron deficient** due to the 2 electronegative atoms, Cl & O -> acyl chlorides attract nucleophiles **more strongly** - the C-Cl bond in *alkyl chlorides* **cleaves only with heating**. the C on C-Cl is **less electron deficient** than in acyl chlorides, as polarisation is only by 1 electronegative atom, Cl -> alkyl chlorides attract nucleophiles **less strongly** - the C-Cl bond in *aryl chlorides* **does not cleave easily**. the C on C-Cl is the **least electron deficient** due to the **delocalisation** of the lone pair of electrons on Cl into the benzene ring, imparting partial double bond character to the C-Cl bond in aryl chlorides, **strengthening the bond** 2 - steric factor - the **planar** *acyl* carbon makes it **less hindered** for the nucleophile to attack - the **tetrahedral** geometry around the chloro-substituted carbon in *alkyl chlorides* is relatively more hindered - the bulky aromatic ring **hinders nucleophilic attack**, and the electron-rich benzene ring repels nucleophiles

Question 24

how do acyl chlorides, alkyl chlorides & aryl chlorides react with aqueous AgNO3?

Answer 24

- acyl chloride: white ppt of AgCl observed **immediately** - alkyl chloride: no ppt formed in the cold. to obtain AgCl ppt, NaOH (aq) & heating are required - aryl chloride: no ppt even after prolonged boiling with aqueous NaOH

Question 25

how can esters be prepared?

Answer 25

1. condensation reaction between carboxylic acid & alcohol 2. condensation reaction between acyl chloride & alcohol

Question 26

what are the reagents & conditions of acid hydrolysis of esters (to form carboxylic acids & alcohols)?

Answer 26

reagents & conditions: dilute H2SO4, heat under reflux | reverse of condensation ## Footnote reversible reaction, so a large excess of water is required to ensure POE is on the right (favouring forward reaction)

Question 27

what are the reagents & conditions for base hydrolysis, or saponification, of esters (to form a sodium carboxylate)?

Answer 27

reagents & conditions: dilute NaOH, heat under reflux (to form sodium carboxylate), followed by acidification with dilute H2SO4 at rtp (to form the carboxylic acid by protonating the carboxylate anion)