Chapter 18 - Carboxylic Acids & Derivatives Flashcards
(30 cards)
Why do carboxylic acids have higher boiling points than its corresponding alcohols? (2)
1) hydrogen bonding between carboxylic acid molecules is stronger than that between alcohol molecules. This is due to the presence of electron withdrawing C=O group that causes the d+ on the H atom to be intensified, and the O-H bond to be more polarised in the carboxylic acid.
2) Carboxylic acids can dimerise (forming a dimer, a chemical structure formed by 2 similar sub units) through hydrogen bonding, effectively doubling the number of electrons and thus the size of electron cloud in the dimer. This results in stronger dispersion forces between dimers.
Why can carboxylic acids dissolve in non-hydrogen-bonding solvents?
Carboxylic acids can form hydrogen-bonded dimers in such solvents, especially if they have longer carbon chains. This is because dispersion forces can be formed between solvent molecules and the hydrocarbon chain of the dimers.
How do carboxylic acids dissolve in water and how does the solubility of carboxylic acids change in water (as the carbon chain grows longer)?
In water, the carboxylic acid does not dimerise, but instead form hydrogen bonding with the water molecules.
Solubility in water decreases as the hydrocarbon chain grows longer. The longer the non-polar hydrocarbon chain, the stronger the dispersion forces between the carboxylic acid molecules. Energy released from the hydrogen bonding between the -COOH group and water molecules is less than the energy needed to overcome these dispersion forces and the existing hydrogen bonding between molecules.
How can carboxylic acids be prepared? (7)
1) oxidative cleavage of alkenes
2) side chain oxidation of alkylbenzenes
3) acid and alkaline hydrolysis of nitriles
4) oxidation of primary alcohols
5) oxidation of aldehydes
6) hydrolysis of acyl chlorides
7) acid and alkaline hydrolysis of esters/amides
What are the 4 types of reactions that carboxylic acids can undergo?
1) acid-metal/acid-base reaction (where O-H bond is broken to form salts)
2) nucleophilic substitution (replacement of -OH group)
3) reduction of the COOH group
4) oxidation (only methanoic and ethanedioic acid)
Why are carboxylic acids unable to undergo nucleophilic addition?
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.
Why are carboxylic acids stronger acids than phenols or aliphatic alcohols? (2)
1) The negative charge on the carboxylate ion (the conjugate base) is delocalised equally over two highly electronegative oxygen atoms. With a dispersed negative charge, the carboxylate anion is greatly stabilised. It is thus less likely to accept a proton to reform the acid.
2) although the negative charge on the phenoxide ion is also delocalised, it is less than that on the carboxylate ion as the carbon atoms in the benzene ring do not bear the negative charge as well as the two electronegative O atoms in the carboxylate ion. Hence the carboxylate ions are more stabilised and makes it more stable.
What are 4 factors of different substituents that can affect the acidity of carboxylic acids?
1) electron-donating groups: intensify the negative charge on the carboxylate anion, destabilise the anion and decrease acidity
2) electron-withdrawing groups: disperse the negative charge on the carboxylate anion, stabilise the anion and increase acidity
3) number of electron-withdrawing groups: increasing the number of groups will increase acidity
4) proximity of electron-withdrawing groups to the -COOH group: the nearer the electron-withdrawing groups to the carboxyl groups, the greater the inductive effect, and the stronger the acid. (However, the electron withdrawing effect becomes ineffective when it is acting through more than 4 atoms)
Why can carboxylic acids and its derivatives undergo nucleophilic (acyl) substitution reactions? (2)
1) the C=O carbon has a partial positive charge due to the electronegative O
2) the C=O carbon has a planar geometry and is relatively unhindered, which makes it susceptible toward nucleophilic attack.
What are the 2 nucleophilic substitution reactions carboxylic acids can undergo？
1) formation of acyl chloride (-OH group replaced with Cl)
2) condensation to form esters (C-O bond of the -COOH group is broken)
What are the reagents, conditions and observations for the formation of acyl chlorides through carboxylic acids?
Reagents and conditions: PCl5/PCl3/SOCl2, room temperature (warm for SOCl2)
Observations: white fumes of HCl produced, H3PO3 for PCl3
What are the reagents and conditions needed for the reduction of carboxylic acids into primary alcohols?
LiAlH4, dry ether, room temperature. LiAlH4 must be used in anhydrous conditions
What are the reagents and conditions needed for the reduction of carboxylic acids into ketones?
NaBH4 in methanol
What are the reagents, conditions and observations for the oxidation of carboxylic acids?
Reagents and conditions: KMnO4, dilute H2SO4, heat under reflux
Observations: Purple solution decolourises, effervescence of CO2 observed.
What are carboxylic acid derivatives?
They are compounds in which the -OH group is replaced by a halogen, -OR’ or -NH2.
Why do esters and acyl chlorides (carboxylic acid derivatives) have lower boiling points than their corresponding carboxylic acids?
Their intermolecular pd-pd attractions are weaker than the hydrogen bonding for carboxylic acids, and require less energy to break. They are unable to form intermolecular hydrogen bonding as they lack a hydrogen atom that is directly attached to electronegative O.
What is the solubility of esters in water compared to carboxylic acids and alcohols?
Esters are much less soluble in water and many esters are immiscible with water.
How does acyl chloride smell like?
They are colourless but have a sharp and irritating odour.
Compare the relative reactivity of the 4 types of carboxylic acid derivatives (including carboxylic acid itself) towards nucleophilic substitution. Explain the trend. (2)
In descending order of reactivity,
Acyl chloride > carboxylic acid > ester > amide
1) electron donating/withdrawing: a halogen substituent withdraws electrons through inductive effect from the C=O group, thus increasing the partial positive charge on the carbon atom. This makes acyl chlorides more readily attacked by nucleophiles. However, an amino group or an alkoxy group (-OR’) donates electrons to the carbonyl carbon through resonance effect. This decreases the partial positive charge on the carbon atom, and makes amides and esters less readily attacked by nucleophiles.
2) leaving group: the weaker the base, the better the leaving group. Halide anions are better leaving groups compared to the hydroxide ion, as halides are weaker bases.
What are the 3 nucleophilic substitution reactions acyl chlorides undergo?
1) hydrolysis in water to form carboxylic acids
2) condensation to form amides
3) condensation to form esters by reacting with alcohol (topic 6 hydroxy compounds)
What are the reagents and conditions needed and the observations for the hydrolysis of acyl chlorides?
Reagents and conditions: water, room temperature
Observations: rapid reaction forming white fumes of HCl
What are the reagents and conditions needed for the condensation of acyl chlorides to form amides?
Ammonia/primary amines/secondary amines in excess, room temperature
Why are excess ammonia/amines added during condensation of acyl chlorides to form amides?
To ensure complete reaction, as some ammonia or amines reacts with acidic HCl to form an ammonium salt (acid base)
What is the relative rate of hydrolysis of acyl, alkyl and aryl chlorides respectively?
In descending order of reactivity,
Acyl chloride > alkyl chloride > aryl chloride
