Carboxylic Acids and Derivatives Flashcards
(24 cards)
Describe the hybridisation of the carbon atom in a COOH group.
The C is sp2 hybridised and molecular geometry is trigonal planar with respect to carboxyl C.
Why does carboxylic acids have higher boiling points than their corresponding alkanes and alcohols of the similar Mr?
Carboxylic acids can form hydrogen bonds with each other. Carboxylic acids form stronger hydrogen bonds than alcohols because the -OH group is more polarised due to the presence of electron withdrawing carbonyl group. Carboxylic acid molecules dimerise in the vapour state and in non-polar solvents, forming two hydrogen bonds between each pair of molecules
The H aom is more electron decficient and has a greater +dipole in acid.
They exist as monomers or dissociate to some extent into H+ and RCO2- in polar solvents.
Why does the boiling points of carboxylic acids increase wiht inceasing relative molecular mass?
Boiling points increase iwth increasing relative molecular mass because this leads to an increase in the number of electrons the molecules have and thus lead to stronger id-id interactions between the molecules.
Why lower aliphatic acids completely miscible with water?
This is because the -COOH group is able to form hydrogen bonds with water molecules.
Why are higher aliphatic acids not miscible with water?
As the hydrocarbon chain lengthens, the pd-pd interactions between the larger non-polar alkyl group and water molecules do not release sufficient energy to overcome the strong hydrogen bonds between the water molecules. In addition, the larger alkyl group also hinders the formation of hydrogen bonds between the carboxylic acid functional group and water molecules.
How do you prepare carboxylic acids from alcohols and aldehydes?
Carboxylic acids an be prepared by oxidation of primary alcohols and aldehydes.
Reagents and conditions:
KMnO4(aq), H2SO4(aq), heat OR K2Cr2O7(aq), H2SO4(aq), heat
Observation:
puple KMnO4 turns colourless OR orange K2r2O7 turns green
How do you prepare carboxylic acids from alkenes?
Carboxylic acids an be prepared by oxidative cleavage of alkenes
Reagents and conditions:
KMnO4(aq), H2SO4(aq), heat
Observation:
purple KMnO4 decolourises
K2Cr2O7 cannot be used as it is a weaker oxidising agent
How do you prepare carboxylic acids from alkylbenzene?
Carboxylic acids an be prepared by side-chain oxidation of an alkylbenzene to form benzoic acid.
Reagents and conditions:
KMnO4(aq), H2SO4(aq), heat
Observation:
puple KMnO4 decolourises.
K2Cr2O7 CANNOT be used
How do you prepare carboxylic acids from nitriles?
Carboxylic acid can be prepared by acidic/alkaline hydrolysis from nitriles
Acidic hydrolysis
Reagents and conditions: HCl(aq) or H2SO4(aq), heat
Alkaline hydrolysis
Reagents and conditions: NaOH(aq), heat, followed by HCL(aq) or H2SO4(aq)
State the reagents and conditions needed to oxidise carboxylic acids and any observations.
Carboxylic acids are generally stable to oxidation except for methanoic acid and ethanedioic acid.
For methanoic acid:
Reagents and conditions: K2Cr2O7(aq), H2SO4(aq), heat OR KMnO4 (aq). H2SO4(aq), heat
Observations: Orange K2Cr2O7 tuns green OR Purple KMnO4 is decolourised
For ethandioic acid:
Reagents and conditions: KMnO4(aq) ONLY, H2SO4 (aq), heat
Observations: Purple KMnO4 is decolourised
State the reagents and conditions to form esters from carboxylic acids and alcohols ONLY.
Esters are formed from the condensation reaction with an alcohol and a carboxylic acid
Reagent and conditions: R’OH, conc H2SO4, heat
This reaction is REVERSIBLE.
Stte the reagents and conditions needed to form acyl chlorides from carboxylic acids and any observations.
Reagents and conditions: PCl5 at room temperature OR PCl3/SOCl2 with heat
Observations: (For PCl3 and SOCl2) White fumes of HCl
PCl5 cannot be used to distinguish between alcohols and carboxylic acids
Using water as reference, why are carboxylic acids stronger than phenols and alcohols?
A carboxylic acid dissociates to give the carboxylate ion RCO2-, which can form 2 equilavent resonance structures with delocalisation of the negative charge over 2 high electronegative O atoms (resonance effect), his results in the carboxylate anion being greatly stabilised compared to phenoxide and alkoxide ions and its fomration is energetically favoured.
Phenol dissociates to give the phenoxide ion. The p-orbital of O overlaps with the pi electron cloud of te benzen ring so that the negatibe charge on O delocaises into the benzene ring. This resonance stabilisation is not effective as that in the carboxylate ion because the negative charge is delocalised over the carbon atoms but in carboxylate ion the negative charge is delocalised over 2 highly electronegative O atoms. The dispersal of negative charge stabilises the phenoxide ion so tjat is it more stable than the alkoxide ion.
An alcohol dissociates to give the alkoxide ion. The electron donating alkyl group intensifies the negative charge on the O atom, hence the charge on the alkoxide ion remains localised on a single electronegative O, and is thus the least stable.
What effect does substituents have on the acidity of carboxylic acids?
1) Nature of substituents
Electron donating substituents such as alkyl groups intensifies the negative charge on the carboxylate anion which destabilises the conjugate base of the acid and cause a decrease in acidity
Electron withdrawing groups like halogens, O and N atoms disperse negative charge on the carboxylate anion which stabilises the conjugate base of the acid and cause a increase in acidity.
2) Number of substituents
Eg. The greater the number of EDW, the more intensified the negatibe charge on the carboxylate anion, hence the conjugate base of the acid is stabilised and the acidity is increased
3) The postiion of substituents
Indutive effects operate through sigma bonds and are dependent on distance hence eg the closer the EWG, the more acidic the carboxylic acid.
State the reagents and conditions needed in the reduction of carboxylic acids.
Carboxylic acids are reduced to primary alcohols.
Reagents and conditions: LiAlH4 in dry ether, r.t
State a distinguishing test for carboxylic acids.
Add aqueous sodium carbonate, Na2CO3(aq)/NaHCO3(aq) at room temperature
Observation: Effervescence of colourless CO2(g) observed, gas evolved forms white ppt in Ca(OH)2 solution
aliphatic alcohols and phenols DO NOT react with NaHCO3/NaCo3 to give CO2 gas
Why do acid chlorides and esters have lower boiling points than their carboxylic acids?
They are unable to form hydrogen bonds with their own molecules as they lack an H atom bonded to a highly electronegative O atom, hence they only have pd-pd interactions which are weaker than the hydrogen bonds.
Why does the solubilities of acrboxylic acid derivatives dependent on the size of the hydrocarbon chain.
As the length of the non-pola hydrocarbon chain increases, solubility in water decreases as the pd-pd interactions between the larger non0polar alkyl group and the water molecules do not released suffieient energy to overcome the strong hydrogen bonds between the water molecules, They also hinder the formation of hydrogen bonds between the functional group and water molecules.
State the solubilities of esters and acid chlorides in non polar and polar solvents.
Esters are insouble in polar solvents but ae soluble in non-polar solvents
Acid chloides ae sluble in non-polar solvens and water as they hydrolyse rapidly to form strong acids which ionises.
Describe he hydrolysis of acyl chlorides.
Acid chlorides are the most reactive of the derivatives of carboxylic acids and hydrolyse immediately upon contact with water. White fumes of HCl are evolved. Since HCl is very soluble, some remain dissolved in solution so that when RCOCl is reacted with water, the resultant solution is trongly acidic.
Explain the relative ease of hydrolysis of acyl chlorides, alkyl chlorides and aryl chlorides.
Aryl chlorides: The overlaping of 3p orbital on Cl atom with pi-electron cloud of the benzene ring results in the compound attracting the nucleophile less strongly. The C-Cl bond has partial double bond chracter and hence no cleavage occurs as the bond is strengthened and the electron-riuch benzene ring repels nucleophiles. Additionally, the attack of the nucleophile blocked by the aromatic ring.
In alkyl chlorides has a lower dipole positive charge on C as it is bonded to only 1 electronegative Cl atom hence it sttracts nucleophile less strongly, hence the C-Cl bond cleaves only with heating. Furthermore, the carbon is sp3 hybridised there is more steric hindrance,
In acyl chlorides has a positive charge on the carbon is bonded to 2 strongly electronegative atoms O and Cl, hence it sttracts nucleophile more stronglt. The highly polarised C-Cl bond cleaves easily without heating. The carbon is sp2 hybridised and there is less steric hindrance.
Describe the reactions acyl chlorides undergo.
Acyl chlorides can react with alcohol and phenols to form esters.
Acyl chlorides can also react with ammonia and amines to give amides.
State the reagents and conditions needed for the hydrolysis of esters.
Acidic hydrolysis
Reagents and conditions: HCl(aq)/H2SO4(aq) and heat
Alkaline hydrolysis
Reagents and conditions: NaOH(aq) and heat
Alkaline hydrolysis is irreversible
State the reagents and conditions needed for the reduction of esters.
Esters can be reduced to primary alcohols.
Reagents and conditions: LiAlH4 in dry ether, r.t
