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 fehling or tollens 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 carbonyl 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 nucleophiles 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.
Describe the reaction with water (hydrolysis) of acyl chlorides
you should know mechanism
The hydrolysis can be classified as a condensation reaction as there is an initial addition reaction of water across the C=O bond, followed by elimination of HCl.
This reaction is far more vigorous than the hydrolysis of chloroalkanes which needs a strong alkali, such as aqueous sodium hydroxide. The nucleophile needed to hydrolyse a chloroalkane is the negatively charged hydroxide ion.
However a neutral water molecule is sufficient to hydrolyse an acyl chloride quickly at room temperature. The difference is due to the carbon bonded to the chlorine atom in the chloroalkane not being as strongly δ+ as the carbon atom in an acyl chloride.
Remember that in an acyl chloride the carbon atom bonded to the chlorine atom is also attached to an oxygen atom. So the carbonyl carbon has two strongly electronegative atoms pulling electrons away from it. Therefore the attack by the nucleophile is much more rapid.
A lone pair of electrons on the oxygen atom in a water molecule initiates the attack on the δ+ carbonyl carbon atom. The reaction produces a carboxylic acid and hydrogen chloride gas.
The reaction is intermediate and white fumes of HCl are observed rising from the liquid.
Describe the reaction of an aryl chloride or chloroareneswith water
Aryl chlorides or chloroarenes such as chlorobenzene C6H5Cl will not undergo hydrolysis. The carbon atom bonded directly to chlorine atom is part of the delocalised π bonding system of the benzene ring. A lone pair of electrons from the Cl atom tends to overlap with the delocalised p electrons in the benzene ring. This causes the C-Cl to have some double bond character, making it stronger and more difficult to break so hydrolysis does not occur.
Describe the order of ease of hydrolysis starting with the compound most easily broken down.
Chloroalkane, acyl chloride, aryl chloride or chloroarene
Acyl chloride > chloroalkane > aryl chloride or chloroarene
Describe the reaction of acyl chloride with alcohols and with phenol
When acyl chlorides react with alcohols and phenol they form esters and HCl. The reaction happens more quickly than the reactions of alcohols or phenol with carboxylic acids.
The acyl chloride reactions also go to completion and do not form an equilibrium mixture like the reactions of carboxylic acids.
Ethanoyl chloride will react vigorously with ethanol to form ethyl ethanote and HCl.
With phenol, the reaction with an acyl chloride proceeds if warmed.
There is no reaction between phenol and carboxylic, so acyl chlorides must be used if you want to make phenyl esters.
The reaction takes place in the presence of a base (such as sodium phenoxide). The initial reaction between phenol and the base creates the phenoxide ion C6H5O-. The negatively charged phenoxide ion acts as the nucleophile to attack the carbonyl atom to attack the carbonyl carbon in the acyl chloride.
NaCl is the side product
Describe the reaction of acyl chlorides with ammonia and amines
Ammonia acts as the nucleophile in the usual nucleophilic addition-elimination reaction of acyl chlorides. The concentrated ammonia is added at room temperature by and the reaction is vigorous.
CH3COCl + NH3 ➡️ CH3CONH2 (ethanamide) + HCl
Followed by:
HCl + NH3 ➡️ NH4+Cl-
The organic product is an amide (R-CONH2) in which the Cl atom in the acyl chloride is replaced by an -NH2 group.
Primary amines (R-NH2) can also act as nucleophiles as they contain nitrogen atoms with a lone pair of electrons. This lone pair of electrons is available to attack the carbonyl carbon atom in acyl chlorides.
The reaction of an acyl chloride with an amine is vigorous and the organic product is a substituted amide.
The mechanism of the nucleophilic condensation (addition elimination) reaction of acyl chlorides with amines follows the same steps as the hydrolysis in water and the reaction with ammonia. You can imagine the primary amines as an ammonia molecule with one of its H atoms replaced by an alkyl group.
With secondary amines (R1R2NH) such as dimethylamine the reaction is similar.
In both reactions with amine, most of the the HCl is not given off as white fumes. As with ammonia, most of the HCl undergoes acid-base reaction with unreacted amine to form a white ammonium salt.
For example:
CH3NH2 + HCl ➡️ CH3NH3+Cl-
Base + acid = salt
