Carboxylic Acids And Their Derivatives Flashcards
Describe the trend in acidity between: Carboxylic acids, phenols, water and ethanol.
Strongest to weakest:
Carboxylic acids, phenols, water and then ethanol.
Remember the greater the value of Kpa, the greater the acidity.
Why are carboxylic acids stronger acids than alcohol?
The OH bond in the carboxylic acid is weakened by the carbonyl group.
The carboxylic ion is stabilised by the delocalisation of electrons around the COO- group. This delocalisation spreads out a negative charge on the carboxylate ion reducing its charge density. This makes it less likely to bond with an H+ ion to reform the unassociated acid molecule with its COOH group.
What is the effect of electron withdrawing groups bonded to the carbon atom next to the COOH group on acidity?
Electron withdrawing groups bonded to the carbon atom next to the COOH group make the acid stronger.
Electron withdrawing groups further weaken the OH bond in the unassociated acid molecule.
Electron withdrawing groups extend the delocalisation of the negative charge on the COO- group of the carboxylate ion further increasing the stabilisation of the COO- group and making it less likely to bond with an H plus ion . This is because once the OH bond is broken the resulting anion is also stabilised more effectively as the negative charge is further spread out by the electron withdrawing groups making it less attractive to H plus ions.
Chlorine atoms are an example of electron withdrawing groups. The more electron withdrawing groups on the carbon atom bonded to the C atom in the COOH group the stronger the acid.
What is the effect of electron donating groups on acidity?
Electron donating groups such as a methyl group has the opposite affect of electron withdrawing groups.
The electron donating group strengthens the OH bond in the acids -COOH group.
It donates negative charge towards the COO- group of the carboxylate ion concentrating its negative charge and making it more likely to accept an H+ ion.
Describe and explain the oxidation of methanoic acid
Primary alcohols can be oxidised by heating with acidified potassium dichromate solution to form aldehydes and then further oxidation produces carboxylic acids. The carboxylic acid prepared is not usually oxidised any further.
However, methanoic acid (HCOOH) is a stronger reducing agent than so it can undergo further oxidation.
The oxidation can be carried out by warming with mild oxidising agents such as failing or towing reagent used to distinguish between aldehydes and ketones. When methanoic acid is oxidised by fehlings solution the Cu2+ ion is reduced to Cu+ ion.
When methanoic acid is oxidised by tollens reagent the silver ion present Ag+ is reduced to silver metal Ag.
Half equation for oxidation of methanoic acid:
HCOOH ➡️ CO2 + 2H+(aq) + 2e-
The oxidation number of carbon goes from +2 to +4
Addition of oxygen from an oxidising agent :
HCOOH + [O] ➡️ CO2 + H2O
This oxidation of methanoic acid will also occur with stronger oxidising agent such as potassium manganate (decolorizing the purple solution ) or potassium dichromate (turning orange solution green)
Describe the oxidation of ethanedioc acid
Ethanedioc acid can also be oxidised by stronger oxidising agents. Its structural formula is (COOH)2.
As with methanoic acid the oxidation results in the formation of carbon dioxide and water .
What is an acyl chloride?
An acyl chloride is a reactive organic compound which is characterised by the functional group -COCl.
How can we prepare acyl chlorides?
We can prepare acyl chlorides from their corresponding carboxylic acids using:
- CH3COOH + PCL5 ➡️ CH3COCL + POCL3 + HCL
- 3CH3COOH + PCl3 ➡️ 3CH3COCl + H3PO3
(Heat is required for this reaction to take place)
- CH3COOH + SOCl2 ➡️ CH3COCl + SO2 + HCl
Third reaction with SOCL 2 is the one reaction to produce the acyl chloride as the only liquid product as both SO2 and HCL are gases .
Note: you can observe steamy white fumes when hydrogen chloride gas is given off.
Describe the reactivity of acyl chlorides
Acyl chlorides are much more reactive then carboxylic acids. The carbon carbon has electrons drawn away from it by the CL atom as well as by its oxygen atom as both are strongly electronegative atoms. This gives the carbonyl carbon a relatively large partial positive charge and makes it particularly open to attack from nucleophiles. Remember that nuclear files can donate a loan pair of electrons to an electron deficient carbon atom in the mechanism of a reaction. The chloride are reactive liquids when they react with nucleophiles the Cl bond breaks and white fumes of HCL are given off.
