Carboxylic Acids Flashcards
Explain the high melting temperatures of carboxylic acids.
Boiling temperature increase with carbon chain length due to an increase in the number of Van der Waals. Boiling temperatures are unexpectedly high due to carboxylic acid molecules being able to hydrogen bond with each other.
Explain the acidity of carboxylic acids.
Carboxylic acids produce hydrogen ions when dissolved in water and are thus considered as acids. These are able to act as strong acids because they can delocalise their negative charge over both oxygen atoms.
Carboxylic acids, along with phenols may be neutralised by sodium hydroxide to produce a salt.
Carboxylic acids are the only ones strong enough to liberate CO2 when hydrogen carbonate is added.
Test for carboxylic acids
Add to a solution of sodium hydrogen carbonate, effervescence indicates the presence of a carboxylic acid.
Explain the solubility of carboxylic acids in water and other polar solvents.
Carboxylic acids can be soluble in water because they are able to hydrogen bond. The polar atoms of the hydroxyl group are attracted to the polar atoms of the water molecule.
Solubility decreases as the length of the carbon chain increases because the polar acid group is becoming a smaller part of the molecule.
Formation of carboxylic acids by oxidation of 1° alcohols
Observation, R+C
> Acidified potassium dichromate(VI) or manganate(VII)
Refux
~ Colour change orange –> green or purple –> colourless
Production of aromatic carboxylic acids by oxidation of alkylbenzenes
(Observations, R+C)
> Reflux > Alkaline potassium manganate(VII) ~ Purple solution reduced to brown sludge (brown manganese(IV) oxide) > Acidify with a dilute acid ~ White crystals produced
Define decarboxylation
The loss of a carboxyl group (CO2), leads to a reduction in the carbon chain length.
Reduction of carboxylic acids using LiAlH4
Product, R+C
= 1° alcohol (reaction cannot be stopped to produce aldehyde)
> Non-aqueous solvent such as ethoxyethane
Decarboxylation of a carboxylic acid (or more usually its sodium salt)
(Product, R+C)
> Heat strongly
Solid sodium hydroxide or sodalime
= Alkane + Carbonate
How is sodalime prepared?
Heat a mixture of sodium hydroxide and calcium oxide
Used as NaOH in equations
What happens when the calcium salt of the carboxylic acid is heated strongly?
(Reaction type and product)
*Decarboxylation
= Aldehyde/Ketone + Calcium carbonate
Carboxylic acid + PCl5 –>
Acyl Chloride + POCl3 + HCl (steamy fumes)
3Carboxylic acid + PCl3 –>
3Acyl chloride + H3PO3
Carboxylic acid + SOCl2 –>
Acyl chloride(l) + SO2(g) + HCl(g) Advantageous because two of the products are gases, therefore acyl chloride can be easily separated
How would you produce carboxylic acids from acyl chlorides?
By-product, R+C
= HCl (steamy fumes - fume cupboard)
> Water
Salt of carboxylic acid + Acyl chloride –>
Name one use of product
Acid anhydride + HCl
Use - production of cellulose based polymers
What is observed when the ammonium salt of a carboxylic acid is reacted with FeCl3?
Reddish brown solution
Producing an ester from a carboxylic acid
R+C
> Alcohol
Conc. sulphuric acid catalyst
Heat
Dry ether to prevent hydrolysis
Hydrolysis of an ester using water or dilute acid
Product, R+C
= Alcohol + Carboxylic acid
> Dilute acid catalyst (if water is used)
> Reflux
Hydrolysis of an ester using dilute alkali (sodium hydroxide)
(Product and advantage over hydrolysis with dilute alkali)
= Sodium salt of carboxylic acid + Alcohol
Advantage - Reaction is not reversible OR products are easier to separate
Dehydration of carboxylic acids
Products, R+C
= Mixture of CO, CO2 and H2O
> Conc. sulphuric acid
> Warm
How would you convert a carboxylic acid to an amide without using an ester or acyl chloride?
(Stages, R+C)
1. Form ammonium salt of carboxylic acid > Ammonium carbonate > Excess carboxylic acid 2. Dehydration of ammonium salt > Reflux
Production of an amide from an acyl chloride
By-product, R+C
= HCl
> Ammonia
> Room temp.
Production of an amide from an ester
By-product, R+C
= Alcohol
> Ammonia
Formation of nitriles from halogenoalkanes
Mechanism, R+C
*Nucleophilic substitution
> KCN
> Ethanol solvent
> Reflux
Reduction of nitriles
Product, R+C
= Primary Amine
> LiAlH4 dissolved in ethoxyethane
> Followed by treatment with dilute acid
Acid hydrolysis of Nitriles
Product, R+C
= Carboxylic acid + NH4+
> Dilute acid
> Reflux
Alkaline hydrolysis of nitriles using NaOH
Product, R+C
= Sodium salt of carboxylic acid + ammonia
> Reflux
> Dilute NaOH
