Carboxylic acids Flashcards
(36 cards)
Functional group
R-COOH
Physical states at room temp
C1-C8 –> liquids
C8+ –> solids
benzoic acid –> solid
Physical properties
Soluble (form hydrogen bonds with water)
High bp (hydrogen bonds between molecules of itself)
Why do carboxylic acids have a higher boiling point than alcohols of the same mass?
- They form hydrogen bonds between molecules which are stronger than alcohols due to the increased polarity of the OH bond, as the C=O bond has electron withdrawing properties
- They form two hydrogen bonds between each two molecules, forming a dimer which doubles the size of the molecule and increases London forces between each dimer
Carboxylic acids in nature
Methanoic acid –> irritant in ant bites
Ethanedioic acid –> toxin in rhubarb leaves
2-hydroxypropanoic acid –> lalctic acid –> sour milk
Citric acid
How do carboxylic acids dissociate in water?
form carboxylate ion –> R-COO^-
What is special about the carboxylate ion?
The C=O and C-O^- bonds are equal lengths. This is because the electrons are delocalised and spread across the carboxylate group. This makes the ion more stable and less likely to be protonated, making carboxylic acids stronger acids than similar molecules eg alcohols
What effect does increasing the carbon chain have on the strength of the carboxylic acid
As the carbon chain increases, the acid becomes weaker. This is because there is an increased positive inductive effect. The alkyl groups have a tendency to push electrons away from themselves. This increases the negative charge on the carboxylate group, making it less stable and more attractive to H+ ions, making it more likely to reprotonate, shifting the equilibrium to the undissociated side meaning there are fewer H+ ions in solution
What effect do halogens have on the strength of carboxylic acid
With a halogen added to the chain, the acid becomes stronger. Halogens are very electronegative, meaning they attract electrons (electron withdrawing) so they pull the negative charge away from the carboxylate group. This increases stability, making it less attractive for H+ ions, leaving more H+ ions in solution and making it less likely to reprotonate
How to form carboxylic acids
- Oxidation of primary alcohol (heat under reflux with acidified potassium dichromate)
- Oxidation of aldehyde
- Hydrolysis of nitriles:
Heat ethanenitrile under reflux with HCl (catalyst) and water –> ethanoic acid and NH4Cl
OR
Heat under reflux with strong alkali (NaOH) then use a strong acid to liberate carboxylic acid
Why is the OH bond more polar
The C=O is electron withdrawing and attracts electrons away from the OH group, making it easier for the OH bond to be broken and H+ to be lost, making it a stronger acid than alcohols (dissociation of electrons).
The flow of electrons from the OH group toward the C=O group reduces the partially positive charge on the C, making it less likely to be attacked by nucleophiles that attack carbonyl compounds
acid + metal
salt + hydrogen
acid + carbonate
salt + CO2 +H2O
only common organic compound that releases CO2 when reacting with carbonates - useful test
acid + alkali
salt + water
salt only partially dissociates
Reduction of carboxylic acids
Use powerful reducing agent of LiAlH4 in dry ether
Abscence of water needed –> destroys reducing agent by reacting with hydride ions produced
Form an acyl chloride
React with PCl5 at room temperature
Reaction with PCl5
Form acyl chloride eg ethanoyl chloride
and HCl (misty fumes)
+ phosphorus trichloride oxide (POCl3)
OH replaced by Cl
Carboxylic acid –> ester
React with alcohol
Sped up by using strong acid catalyst and heat under reflux
Esterification/elimination
Reversible
React with alcohol
Form ester and water
esterification/elimination
How to increase yield of ester
Use a higher concentration of H2SO4 than needed for catalystic action –> reacts with OH- of water, removing water, equil shifts to make more
Use excess acid/alcohol
Distill off ester as it forms –> prevents backwards reaction and decreases conc of ester, causing equil shift to RHS
Derivitives of carboxylic acids
OH group has been replaced by an atom or group of atoms
Esters
Acyl chlorides
Amides
Why do acyl chlorides not exist naturally
They are highly reactive and so would react with any water and be converted into carboxylic acid
Reactivity of acyl chlorides
Very reactive due to the electron withdrawing effects of the chlorine atom and carbonyl oxygen. This produces a very partially positive carbon atom to which nucleophiles are very attracted to
ethanoyl chloride + water
Add ethanoyl chloride dropwise
Effervescence
Steamy fumes as HCl gas is produced - turn to misty fumes when NH3 is added
Exothermic
Carboxylic acid forms
