Carboxylic Acids & Derivatives (AQA A Level Chemistry): Flashcards
Exam code: 7405
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- FrontCarboxylic Acids
What functional group defines a carboxylic acid?
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What functional group defines a carboxylic acid?
The carboxyl group, –COOH. It consists of a carbonyl (C=O) and a hydroxyl (–OH) on the same carbon. The general formula is R–COOH.
True or False?
Carboxylic acids fully dissociate in water and are therefore strong acids.
False.
Carboxylic acids are weak acids. They only partially dissociate in water — the equilibrium lies to the left. The low concentration of H+ produced is still sufficient to react with carbonates.
The reaction of a carboxylic acid with an alcohol to form an ester is called .......... . It requires a .......... acid catalyst and is a .......... reaction because water is eliminated.
The reaction of a carboxylic acid with an alcohol to form an ester is called esterification. It requires a concentrated strong acid catalyst and is a condensation reaction because water is eliminated.
How are carboxylic acids named?
Using the stem of the parent alkane with the suffix –oic acid (e.g. propanoic acid). No number is needed because the –COOH group is always on carbon 1.
What gas is produced when a carboxylic acid reacts with sodium carbonate?
Carbon dioxide (CO2 (g)). The ionic equation is: 2RCOOH + CO32- → 2RCOO- + CO2 + H2O.
Carboxylic acids with fewer than .......... carbon atoms are water-soluble because water molecules can .......... with the functional group. In solution they partially ionise to give .......... ions and carboxylate ions.
Carboxylic acids with fewer than six carbon atoms are water-soluble because water molecules can hydrogen-bond with the functional group. In solution they partially ionise to give H+ ions and carboxylate ions.
True or False?
Esterification of a carboxylic acid with an alcohol is a reversible reaction.
True.
Esterification is a reversible condensation reaction (shown with ⇌). Heating under reflux with an acid catalyst achieves an equilibrium mixture of ester, water, alcohol and acid.
Carboxylate ion
A carboxylate ion (RCOO-) is the anion formed when a carboxylic acid partially dissociates in water, losing a proton (H+) from the –COOH group. The negative charge is delocalised across both oxygen atoms of the –COO- group.
What functional group is present in an ester?
The ester group, –COO–. It is formed by a condensation reaction between a carboxylic acid and an alcohol, with loss of water.
True or False?
The name of an ester is written with the alcohol-derived part first, followed by the acid-derived part.
True.
For example: ethanol + propanoic acid gives ethyl propanoate. The alkyl part (from the alcohol) is named first; the –oic acid suffix is changed to –oate.
Triglycerides are tri-esters of .......... and .......... . Hydrolysis of triglycerides gives .......... and fatty acids.
Triglycerides are tri-esters of glycerol (propane-1,2,3-triol) and fatty acids (long-chain carboxylic acids). Hydrolysis of triglycerides gives glycerol and fatty acids.
What is a plasticiser in the context of esters?
An ester additive mixed into a polymer to increase flexibility. It penetrates between polymer chains, increasing the distance between them and reducing intermolecular attractions.
To test for an alcohol, warm an unknown substance with a carboxylic acid and concentrated .......... catalyst. Remove excess acid with warm aqueous .......... . A .......... smell confirms an alcohol is present.
To test for an alcohol, warm an unknown substance with a carboxylic acid and concentrated H2SO4 catalyst. Remove excess acid with warm aqueous sodium carbonate. A sweet/fruity (ester) smell confirms an alcohol is present.
True or False?
Esters are commonly used as solvents, flavourings and plasticisers.
True.
Esters have a fruity smell and are used as artificial flavourings and fragrances. They are also used as solvents (e.g. ethyl ethanoate in nail varnish) and plasticisers in polymers such as PVC.
What is the systematic name of the ester CH3CH2CH2COOCH2CH3?
Ethyl butanoate.
The –CH2CH3 group on the oxygen comes from ethanol (ethyl–). The remaining –CH2CH2CH2COO– chain comes from butanoic acid (–butanoate).
What is transesterification?
A reaction in which a triglyceride reacts with methanol to form fatty acid methyl esters (FAME) and glycerol. It is used to produce biodiesel from vegetable oils.
True or False?
Biodiesel is produced from non-renewable petrochemicals.
False.
Biodiesel is produced from renewable vegetable oils by transesterification. This makes it more sustainable than conventional diesel.
In biodiesel production, a triglyceride reacts with .......... in the presence of an alkaline catalyst. The products are .......... and .......... . An excess of .......... is used to push the equilibrium to the right.
In biodiesel production, a triglyceride reacts with methanol in the presence of an alkaline catalyst. The products are fatty acid methyl esters (FAME) and glycerol. An excess of methanol is used to push the equilibrium to the right.
Why is methanol used to convert triglycerides into biodiesel rather than using the triglycerides directly as fuel?
Transesterification reduces the viscosity of the oil, making it suitable for use in diesel engines. Unmodified vegetable oil is too viscous to work effectively.
True or False?
An alkaline catalyst (NaOH or KOH) for transesterification works by protonating the carbonyl group.
False.
An acid catalyst works by protonating the carbonyl group. An alkaline catalyst works by generating a stronger nucleophile from methanol. In industry, alkaline catalysts are more commonly used.
Under optimal conditions, biodiesel production by transesterification achieves yields of around .......... %. The reaction is .......... , which is why excess methanol is used.
Under optimal conditions, biodiesel production by transesterification achieves yields of around 98%. The reaction is reversible, which is why excess methanol is used.
What is ester hydrolysis?
The reverse of esterification: an ester is cleaved by water (with acid or alkali catalyst) to reform the carboxylic acid (or carboxylate salt) and alcohol.
True or False?
Acid hydrolysis of an ester goes to completion.
False.
Acid hydrolysis establishes an equilibrium and does not go to completion. Alkaline hydrolysis is irreversible and goes to completion because the carboxylic acid formed reacts with the alkali.
In alkaline hydrolysis, an ester heated under reflux with dilute NaOH gives a .......... salt and an .......... . To obtain the carboxylic acid from the salt, it must be .......... with an acid such as HCl.
In alkaline hydrolysis, an ester heated under reflux with dilute NaOH gives a carboxylate salt and an alcohol. To obtain the carboxylic acid from the salt, it must be acidified with an acid such as HCl.
What is saponification?
The alkaline hydrolysis of triglycerides (fats/oils) using hot aqueous NaOH to produce glycerol and the sodium salts of fatty acids (soaps).
True or False?
Soaps are the sodium or potassium salts of long-chain carboxylic acids (fatty acids).
True.
Saponification of a triglyceride with NaOH produces glycerol and soap, where the soap is the sodium carboxylate salt of the fatty acid chains.
Acid hydrolysis of CH3COOCH2CH3 with dilute H2SO4 gives .......... and .......... . The reaction is .......... (shown with ⇌).
Acid hydrolysis of CH3COOCH2CH3 with dilute H2SO4 gives CH3COOH (ethanoic acid) and CH3CH2OH (ethanol). The reaction is reversible (shown with ⇌).
What are the two products of saponification of a triglyceride with NaOH?
Glycerol (propane-1,2,3-triol) and the sodium salts of fatty acids (soaps).
What is an acyl chloride?
A carboxylic acid derivative in which the –OH group is replaced by a chlorine atom. Named by replacing –oic acid with –oyl chloride (e.g. ethanoyl chloride, CH3COCl).
In nucleophilic addition–elimination, a .......... adds across the C=O bond in step 1, then a .......... molecule is eliminated in step 2. Reactions of acyl chlorides with water produce .......... and HCl.
In nucleophilic addition–elimination, a nucleophile adds across the C=O bond in step 1, then a small molecule is eliminated in step 2. Reactions of acyl chlorides with water produce carboxylic acids and HCl.
True or False?
When an acyl chloride reacts with ammonia, only one mole of ammonia is needed per mole of acyl chloride.
False.
Two moles of ammonia are needed: the first forms the amide, and the second neutralises the HCl produced (giving NH4Cl).
What are the two steps in the general mechanism of nucleophilic addition–elimination?
Step 1: Nucleophilic addition — the nucleophile's lone pair attacks the carbonyl carbon.
Step 2: Elimination — a small molecule (HCl or H2O) is expelled, restoring the C=O.
Acid anhydrides are preferred over acyl chlorides in the manufacture of aspirin because they are less .......... , produce a less hazardous by-product (a .......... rather than HCl) and are .......... .
Acid anhydrides are preferred over acyl chlorides in the manufacture of aspirin because they are less reactive, produce a less hazardous by-product (a carboxylic acid rather than HCl) and are cheaper.
What is an acid anhydride?
A carboxylic acid derivative formed by replacing the –OH with an alkanoate group. Named using the suffix –oic anhydride (e.g. ethanoic anhydride, (CH3CO)2O).
True or False?
In the mechanism of ester formation from an acyl chloride and alcohol, the alcohol acts as the nucleophile attacking the carbonyl carbon.
True.
The lone pair on the oxygen atom of the alcohol attacks the δ+ carbonyl carbon (nucleophilic addition), followed by elimination of HCl to give the ester.
What are the two reactants in the industrial manufacture of aspirin?
Salicylic acid and ethanoic anhydride. They react in an acylation reaction to form aspirin (acetylsalicylic acid) and ethanoic acid.
In aspirin manufacture, .......... is preferred over .......... as the acylating agent because it is less reactive, the by-product .......... is less hazardous than HCl, and it is cheaper.
In aspirin manufacture, ethanoic anhydride is preferred over ethanoyl chloride (an acyl chloride) as the acylating agent because it is less reactive, the by-product ethanoic acid is less hazardous than HCl, and it is cheaper.
True or False?
The reaction of salicylic acid with ethanoic anhydride is a nucleophilic addition–elimination reaction.
True.
The –OH group on salicylic acid acts as the nucleophile, attacking the carbonyl of ethanoic anhydride. After addition, a carboxylate group is eliminated, forming the ester bond in aspirin.
What type of reaction converts salicylic acid to aspirin?
An acylation reaction — specifically nucleophilic addition–elimination in which an acyl group from ethanoic anhydride is introduced onto the –OH of salicylic acid, forming the ester bond in aspirin.
True or False?
Acyl chlorides are preferred over acid anhydrides for aspirin manufacture because they react faster.
False.
Although acyl chlorides react faster and are more exothermic, acid anhydrides are preferred. They are easier to control, cheaper and produce ethanoic acid (not corrosive HCl gas) as a by-product.
The overall equation for aspirin manufacture is: salicylic acid + .......... → aspirin + .......... .
The overall equation for aspirin manufacture is: salicylic acid + ethanoic anhydride → aspirin + ethanoic acid.
Acylation
An acid dissociation constant (Ka) is the equilibrium constant for the partial dissociation of a weak acid in water. A larger Ka indicates a stronger weak acid that dissociates to a greater extent.
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