Organic Chemistry II: Carboxylic Acids (Edexcel A Level Chemistry): Flashcards
Exam code: 9CHO
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Define carboxylic acid.
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Define carboxylic acid.
A carboxylic acid is an organic compound containing the carboxyl functional group, -COOH, with general formula RCOOH.
At what chain length do carboxylic acids typically become solids at room temperature?
Carboxylic acids with approximately ten or more carbon atoms are solids at room temperature, becoming very slightly soluble in cold water but more soluble in hot water.
True or False?
Carboxylic acids can form hydrogen bonds because of both the C=O and O-H bonds.
True.
Both the C=O and O-H bonds are polarised, allowing carboxylic acids to act as hydrogen bond donors and acceptors, which contributes to their relatively high melting and boiling points.
The general formula of a carboxylic acid is ..........
The general formula of a carboxylic acid is CnH(2n+1)COOH.
Why do carboxylic acids have exceptionally high boiling points compared to other organic compounds of similar Mr?
Carboxylic acids can form hydrogen-bonded dimers — pairs of molecules linked by two O–H···O hydrogen bonds between their -COOH groups.
Breaking these dimers requires significantly more energy, giving carboxylic acids anomalously high boiling points compared to alcohols or aldehydes of similar molecular mass.
What is the carboxyl group?
The carboxyl group is the -COOH functional group found in carboxylic acids, consisting of a carbonyl (C=O) and a hydroxyl (O-H) group on the same carbon.
How are carboxylic acids named from their parent alkane?
The suffix -oic acid replaces the final -e of the alkane name, giving names such as ethanoic acid and propanoic acid. No locant is needed because the -COOH group is always on carbon 1.
True or False?
The solubility of carboxylic acids in water increases as the hydrocarbon chain length increases.
False.
Solubility decreases as chain length increases. The longer hydrocarbon chain disrupts hydrogen bonds between water molecules, reducing the ability of the acid to dissolve.
Why are short-chain carboxylic acids soluble in water and other polar solvents?
Short-chain carboxylic acids are soluble because they form hydrogen bonds with polar solvents via both the C=O and O-H groups. Solubility decreases as the hydrocarbon chain lengthens and disrupts the water's hydrogen bonding network.
Carboxylic acids have relatively high boiling points due to .......... and ..........
Carboxylic acids have relatively high boiling points due to hydrogen bonding and permanent dipole–dipole interactions arising from the polarised C=O and O-H bonds.
What is hydrolysis?
Hydrolysis is a reaction in which a compound is broken down by reaction with water (or an aqueous acid or alkali), splitting bonds to form two or more products.
What reagents and conditions are used to oxidise a primary alcohol to a carboxylic acid?
Acidified K2Cr2O7 or acidified KMnO4 under reflux are used. The oxidising agent is reduced: dichromate ions change from orange to green, and manganate ions change from purple to colourless.
True or False?
When acidified KMnO4 oxidises a primary alcohol, the solution turns from purple to colourless.
True.
The purple manganate(VII) ions (MnO4-) are reduced to colourless Mn2+ ions during the oxidation.
What is a nitrile?
A nitrile is an organic compound containing the -CN (cyano) functional group, which can be hydrolysed to produce a carboxylic acid.
What are the products of hydrolysing a nitrile with dilute acid?
A carboxylic acid and an ammonium salt are formed. The -CN group is converted to a -COOH group.
Hydrolysis of a nitrile with dilute acid produces a .......... and an ..........
Hydrolysis of a nitrile with dilute acid produces a carboxylic acid and an ammonium salt.
True or False?
Both primary alcohols and aldehydes can be oxidised to carboxylic acids using acidified K2Cr2O7 under reflux.
True.
Primary alcohols are first oxidised to aldehydes, which are then further oxidised to carboxylic acids. Aldehydes can also be oxidised directly to carboxylic acids by the same reagent under reflux.
What are the products of hydrolysing a nitrile with dilute alkali, and what extra step is needed?
A sodium carboxylate salt and ammonia are formed. Acidification is then required to protonate the carboxylate ion and produce the carboxylic acid.
When acidified K2Cr2O7 acts as an oxidising agent, the colour changes from .......... to ..........
When acidified K2Cr2O7 acts as an oxidising agent, the colour changes from orange to green as dichromate ions are reduced to Cr3+ ions.
Define esterification.
Esterification is the condensation reaction between a carboxylic acid and an alcohol, using concentrated H2SO4 as a catalyst, to form an ester and water.
How can sodium carbonate or sodium hydrogencarbonate be used to test for a carboxylic acid?
Adding aqueous Na2CO3 or NaHCO3 to a carboxylic acid produces CO2 gas (effervescence), along with a carboxylate salt and water. This is a positive test for the presence of a carboxylic acid.
True or False?
LiAlH4 in dry ether reduces a carboxylic acid to an aldehyde.
False.
LiAlH4 reduces a carboxylic acid fully to a primary alcohol, not an aldehyde. The reduction goes beyond the aldehyde stage.
What is meant by saying carboxylic acids are weak acids?
Carboxylic acids are weak acids because they only partially ionise in aqueous solution: the equilibrium lies to the left, giving a low concentration of H+ relative to the concentration of the undissociated acid.
Define ester.
An ester is an organic compound containing the -COOR functional group, formed from a carboxylic acid and an alcohol. Esters are characterised by sweet, fruity smells.
What product is formed when a carboxylic acid is reduced by LiAlH4, and what conditions are required?
A primary alcohol is formed. LiAlH4 is suspended in dry ether at room temperature; water is added at the end to destroy excess LiAlH4.
Reaction of a carboxylic acid with PCl5 produces an .........., .......... and ..........
Reaction of a carboxylic acid with PCl5 produces an acyl chloride, POCl3 and HCl.
How is an acyl chloride made from a carboxylic acid?
The carboxylic acid is reacted with solid PCl5 (phosphorus(V) chloride). The products are the acyl chloride, POCl3, and HCl (steamy fumes). Liquid products are separated by fractional distillation.
True or False?
The first part of an ester's name comes from the alcohol and the second part from the carboxylic acid.
True.
For example, propanol reacted with ethanoic acid gives propyl ethanoate: propyl from the alcohol, ethanoate from the acid.
Esterification uses concentrated .......... as a catalyst and is an example of a .......... reaction.
Esterification uses concentrated H2SO4 as a catalyst and is an example of a condensation reaction.
How is an acyl chloride made from a carboxylic acid using thionyl chloride (SOCl2), and why is this method preferred over using PCl5?
The carboxylic acid is warmed with SOCl2 (thionyl chloride, also called sulfur dichloride oxide):
RCOOH + SOCl2 → RCOCl + SO2 + HCl
Both by-products (SO2 and HCl) are gases and leave the reaction mixture automatically, making purification of the acyl chloride simpler than the PCl5 route (which gives the liquid by-product POCl3).
Define acyl chloride.
An acyl chloride is a derivative of a carboxylic acid in which the -OH group is replaced by a chlorine atom, giving the functional group -COCl. Acyl chlorides are named using the suffix -oyl chloride.
Why are two moles of ammonia needed when an acyl chloride reacts with ammonia?
The first mole of ammonia attacks the carbonyl carbon to form the amide. The second mole neutralises the HCl produced, forming ammonium chloride (NH4Cl). Overall: RCOCl + 2NH3 → RCONH2 + NH4Cl.
True or False?
Heating an ester under reflux with dilute acid gives complete hydrolysis.
False.
Acid hydrolysis is reversible, forming an equilibrium mixture. The reaction does not go to completion. Alkaline hydrolysis with NaOH is the irreversible route that gives complete hydrolysis.
Define polyester.
A polyester is a condensation polymer in which monomers are linked by ester bonds, formed by the reaction between dicarboxylic acid and diol monomers (or a single hydroxycarboxylic acid monomer) with elimination of water.
A polyester is formed by condensation polymerisation of a .......... and a .........., with .......... eliminated at each step.
A polyester is formed by condensation polymerisation of a dicarboxylic acid and a diol, with water eliminated at each step.
Why is alkaline hydrolysis of an ester preferred over acid hydrolysis to ensure complete reaction?
Acid hydrolysis is reversible and establishes an equilibrium, so the reaction does not go to completion. Alkaline hydrolysis is irreversible because the ester reacts directly with NaOH to form a carboxylate salt, driving the reaction to completion.
What is saponification?
Saponification is the alkaline hydrolysis of an ester using dilute alkali (e.g. NaOH) under reflux. It is an irreversible reaction producing a carboxylate salt and an alcohol. Acidification converts the carboxylate salt to a carboxylic acid.
What monomers are used to make PET (polyethylene terephthalate), and what type of polymerisation is involved?
PET is made by condensation polymerisation of ethane-1,2-diol (a diol) and benzene-1,4-dicarboxylic acid (a dicarboxylic acid). Water is eliminated with each ester bond formed.
True or False?
Hydrolysis of an acyl chloride produces a carboxylic acid and HCl.
True.
A water molecule undergoes nucleophilic addition across the C=O bond of the acyl chloride, and HCl is eliminated. For example, propanoyl chloride hydrolyses to give propanoic acid and HCl.
Alkaline hydrolysis of an ester with NaOH produces a .......... salt and ..........
Alkaline hydrolysis of an ester with NaOH produces a carboxylate salt and an alcohol.
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