Carboxylic Acids And Derivatives

Question 1

What is the functional group of carboxylic acids?

Answer 1

-COOH

Question 2

Which carboxylic acids are water soluble?

Answer 2

The ones with fewer than 6 carbon atoms per molecule. This is because water molecules can H-bond with the functional group

Question 3

Why are carboxylic acids weak acids?

Answer 3

In aqueous solution they are only partially ionised, to give low conc. of hydronium ions and alkanoate/carboxylate ions.

Question 4

How can you test for the presence of a carboxylic acid?

Answer 4

React an aqueous solution of carboxylic acids with an aqueous solution of sodium carbonate to produce carbon dioxide

Question 5

How is an ester formed? What is the by-product of the reaction? What is this reaction referred to as?

Answer 5

1) When an alcohol and a carboxylic acid react together in the presence of a concentrated, strong acid catalyst e.g. sulfuric acid or hydrochloric acid
2) Water
3) An esterification, it is an example of a condensation reaction

Question 6

Radioactive labelling has shown that which C-O bond breaks in the formation of an ester?

Answer 6

In most cases the C-O bond in the acid

Question 7

How can higher yields of ester be produced from ester formation reactions?

Answer 7

1) Equilibrium can be shifted towards product if water is distilled from reaction mixture as it is formed or if a large excess of alcohol is used
2) Higher yields of esters obtained by reacting the alcohol with the acyl chloride or acid anhydride in reactions called acylations

Question 8

Give some uses for esters

Answer 8

1) Fragrances and flavourings - fruity sweet smell
2) Plasticisers - increase flexibility of polymers
3) Solvents for organic compounds - e.g in glue or nail varnish

Question 9

How can the fruity smell of an ester be used as a test for the presence of an alcohol or carboxylic acid?

Answer 9

If we suspect compound is an alcohol:

1) It can be warmed with a carboxylic acid, e.g. ethanoic acid, in the presence of conc sulfuric acid (catalyst).
2) Excess acid can be removed by adding warm aqueous sodium carbonate solution
3) If remaining mixture has sweet smell of an ester, this confirms presence of alcohol. Warmth of the solution causes ester to evaporate so sweet smell is easily detected

Question 10

Give an example of a polymer that is mixed with esters to increase flexibility, how does ester make polymer more flexible?

Answer 10

1) PVC ( polychloroethene) - strong and rigid due to strong IM attraction between polar C-Cl bonds within the chains
2) Plasticisers (esters) penetrate between polymer chains and increase distance between the chains, weakening the effects of the polar C-Cl bond. As a result polymer chains can slide over each other making plastic soft and pliable

Question 11

Name a common solvent, how does a solvent work?

Answer 11

1) Ethyl ethanoate - relatively cheap, low toxicity, boiling point 77°
2) The ingredients are dissolved in the ester, which evaporates into the air and leaves the varnish or scent behind

Question 12

What are triglycerides? How are they formed?

Answer 12

1) Naturally occurring fats and oils. They are tri-esters of glycerol (propane-1,2,3-triol) and fatty acids (long-chain hydrocarbon carboxylic acids)
2) When 1 mole of glycerol reacts with 3 moles of a fatty acid in the presence of an acid catalyst, this reaction is an esterification reaction where 3 moles of water are eliminated

Question 13

What is the hydrolysis of esters? How does it work?

Answer 13

1) The reverse of an esterification reaction - acid and alcohol formed from reaction between ester and water
2) Ester is hydrolysed by action of water to re-form the acid and the alcohol. This hydrolysis requires heating and a strong acid or alkali such as sulphuric acid or sodium hydroxide catalyst. Normally carried out with excess water to drive equilibrium to desired carboxylic acid

Question 14

Describe alkaline hydrolysis

Answer 14

An excess of sodium hydroxide is used. The carboxylic acid that forms reacts with the excess alkali to form a carboxylate salt. This reaction continually removes carboxylic acid from the equilibrium mixture. The alkaline solution containing the carboxylate salt is neutralised to give a high yield of carboxylic acid

Question 15

What are soaps?

Answer 15

Carboxylate salts of long-chain carboxylic acids (fatty acids). They are made by hydrolysing naturally occurring fat and oil triglycerides

Question 16

How is soap made?

Answer 16

1) Fats and oils boiled in aqueous sodium hydroxide - process called saponification
2) NaCl is added to precipitate the soap
3) Since hydrolysis done under alkaline conditions, product is a mixture of glycerol and the salts of the fatty acids, soaps

Question 17

What is the useful by-product of soap manufacture?

Answer 17

Glycerol - uses in pharmaceutical and cosmetic preparations

Question 18

Soaps are anionic detergents, what does this mean?

Answer 18

The polar negatively charged carboxyl group of a soap is attracted to water, it is hydrophilic. The long, non-polar hydrocarbon chains are attracted to oil rather than water, they are hydrophobic

Question 19

How do soap or detergent molecules get rid of dirt, oil and grease?

Answer 19

They accumulate at the oil/water surface with the polar ‘head’ of the soap interacting with the water layer and the non-polar hydrocarbon chain interacting with the non-polar organic oil. When mixture stirred or shaken, the oil breaks up into tiny droplets (emulsification) and is dispersed throughout the water. The droplets are stable because carboxyl groups make up ‘outside’ and ‘inside’ is made up of hydrocarbon segments of the soap

Question 20

How is biodiesel produced?

Answer 20

By transesterification. The triglyceride (in the oil) is converted into the less viscous methyl ester using a sodium hydroxide catalyst and methanol. The transesterification is reversible, so an excess of methanol is used to drive the equilibrium to the right. Can produce 98% yield under optimum conditions

Question 21

What is biodiesel made from?

Answer 21

Renewable vegetable oils

Question 22

How can acyl groups be built into molecules?

Answer 22

Using acyl chlorides or acid anhydrides (acylating agents)

Question 23

How are acyl chlorides and acid anhydrides derived from carboxylic acids?

Answer 23

By substitution of -OH group by a chlorine atom (acyl chlorides) or an alkanoate (acid anhydrides)

Question 24

What are acylations? How are they used in industry?

Answer 24

1) Reactions Where acyl groups are introduced into molecules

2) Used in pharmaceutical and textile industries to make drugs such as aspirin and textiles such as cellulose acetate

Question 25

In general, why are acid anhydrides better acylating agents than acyl chlorides? Give an example

Answer 25

• Acyl chlorides react with water or alcohol in a fast exothermic process that generates highly corrosive hydrogen chloride gas
• Acid anhydrides are cheaper to produced and as they are less reactive, their reactions can be more easily controlled
• E.g. Industrial processes to produce aspirin and cellulose acetate use ethanoic anhydride instead of ethanoyl chloride

Question 26

What does the reaction between acylating agents and ammonia or primary amines produce?

Answer 26

Amides and N-substituted amides, respectively. Because the by-products are acidic, two moles of ammonia or amine are required for each mole of acylating agent. The first acts as a nucleophile, the second as a base and reacts with the generated acid

Question 27

All acylation reactions are examples of which mechanism?

Answer 27

Nucleophilic addition-elimination

Question 28

What are the four stages of the nucleophilic addition-elimination mechanism for acylation reactions?

Answer 28

1) Attraction: the δ+ carbon atom of the polar C=O bond attracts the lone pair of the nucleophile
2) Addition: the lone pair of the nucleophile molecule forms a new bond to the carbon atom and the C=O bond breaks, producing a saturated ion that has both positive and negative charges
3) Elimination: the C-Cl (acyl chloride) or the C-O (anhydride) breaks, with the electron pair liberating a chloride ion or ethanoate ion, simultaneously the C=O bond re-forms
4) Deprotonation: the new compound is formed by removal of an H+. With ammonia or amine 2 moles required, one acts as a nucleophile and one acts as a base and reacts with the acid produced