3.3.9 Carboxylic acids and derivatives

Question 1

What is a carboxylic acid, how do you name them?

Answer 1

Carboxylic acids contain the carboxyl functional group -COOH

To name them, name the longest chain, take the “e” off and add “-oic acid”

The carboxyl group is always at the end of the molecule and when naming, it’s more important than the other functional groups - so all other functional groups are numbered starting from this carbon

Question 2

Why are carboxylic acids weak acids?

Answer 2

Carboxylic acids are weak acids because they partialliy dissociate into a carboxylate ion and an H⁺ ion:

RCOOH <=> COO^- + H⁺

The equilibrium lies to the left because most of the molecules don’t dissociate

Question 3

How do carboxylic acids react with carbonates to form CO₂?

Answer 3

Carboxylic acids react with carbonates (which contain the CO₃^2- ion) to form a salt, CO₂ and H₂O

Question 4

Give the equation where ethanoic acid reacts with sodium carbonate or sodium hydrogen carbonate (baking soda)

Answer 4

Sodium carbonate:

2CH₃COOH_(aq) + Na₂CO_3(s) -> 2CH₃COONa_(aq) + H₂O_(l) + CO_2(g)

Sodium hydrocarbonate:

CH₃COOH_(aq) + NaHCO_3(s) -> CH₃COONa_(aq) + H₂O_(l) + CO_2(g)

CO₂ fizzes as gas

Question 5

What is an ester?

Answer 5

• Organic compounds that contain the functional group -COO-

Question 6

How do carboxylic acids react with alcohols to form esters?

Answer 6

Esters are frequently made by heating a carboxylic acid with an alcohol in the presense of a strong acid catalyst (usually sulfuric acid) by reflux, this is called a esterification reaction:

RCOOH + R’OH <=H⁺/reflux=> RCOOR’ + H₂O

It’s a condestation reaction as water is released

Question 7

How do you name an ester?

Answer 7

1. The alkyl group of the alcohol is the first part of the esters name - e.g. propanol is propyl
2. Look at the part that came from the carboxylic acid, swap the “-oic acid” suffix for “oate” - e.g. ehtanoic acid would be ethanoate
3. Put the two together - e.g. propyl ethanoate
4. The name is written the opposite way to the formula - e.g. propyl ethanotate is written in formula as CH₃COOCH₂CH₂CH₃

Question 8

How do you name a branched ester?

Answer 8

Number the carbons from the C atoms it the ester bond - e.g. C₁-O-C₂

Question 9

What are esters used for, why?

Answer 9

1. Esters have a very sweet smell, varying from gluey sweet for smaller esters to a fruity “pear drop” smell for longer ones. This makes them very useful in perfumes. The food industry uses esters to flavour things like drinks and sweets
2. Esters are polar liquids so lots of polar organic compounds will dissolve in them. They also have a low boiling point, so they evaporate easily from mixtures. This makes them good solvents in glues and printing inks
3. Esters are often used as plasticisers - they’re added to plastics during polymerisation to make the plastic more flexible. Over time the plasticiser molescules escape through, and the plastic becomes brittle and stiff

Question 10

Explain how esters are split by acid hydrolysis

Answer 10

Splits the acid into an acid and an alcohol. You have to reflux the ester with a dilute acid, such as hydrochloric or sulfuric.

For example:

CH₃COOCH₂CH₃ + H₂O <=H⁺/reflux=> CH₃COOH + CH₃CH₂OH

As it’s a reversible reaction lots of water needs to be added to push the equilbrium to the right

Question 11

Explain how esters are split by base hydrolysis

Answer 11

Involves refluxing the ester with a dilute alkali (e.g. NaOH). You get a caboxylate ion instead and an alcohol.

For example: CH₃COOCH₂CH₃ + OH^- –Reflux–> CH₃COO^- + CH₃CH₂OH

Question 12

Explain how fats and oils are esters of glycerol and fatty acids and how the properties may differ and why

Answer 12

• Fatty acids are long chain carboxylic acids
• They combine with glycerol (propane-1,2,3-triol) to make esters. These esters of gycerol are fats and oils. The fatty acids can be saturated (no double bonds) or unsaturated (have C=C double bonds)
• Most of a fat or oil is made from fatty acid chains - which gives them the biggest infulence over many of the properties
  ~ Animal fats have mainly saturated hydrocarbon chains - they fit neatly together, increasing van der waals forces between them. This means a higher temperature is needed to melt them, so they’re solid at room temperature
  ~ Vegetable oils have unsaturated hydrocarbons chains - the double bonds mean that the chains are bent and don’t pack well together, decreasing VDW forces between them. Therefore they are easier to melt and are liquids at room temperature

Question 13

How can oils an fats be hydrolysed to produce gycerol, soap and fatty acids

Answer 13

Like any ester, you can hydrolyse vegatable oils and animal oils by heating them with NaOH, and a sodium salt (soap) is produced:

For example:

CH₂(OO(CH₂)₁₆CH₃)CH(OO(CH₂)₁₆CH₃)CH₂(OO(CH₂)₁₆CH₃) (Fat) + 3 NaOH -> CH₂OHCHOHCH₂OH (Glycerol) + 3CH₃(CH₂)₁₆COO^-Na⁺ (Sodium salt (Soap))

Structural formulas for fat and glycerol may be incorrect

Question 14

Explain how biodiesel is made from a mixture of methyl esters of fatty acids

Answer 14

1. Vegetable oils, e.g. rapeseed oil, make good vehicle fuels, but they can’t be burnt directly in engines
2. The oils must be converted to biodiesel first. This involves reacting them with methanol, using potassium hydroxide as a catalyst
3. You get a mixture of methyl esters of fatty acids - this is bidiesel

Triester CH₂(OOCR¹)CH(OOR²)CH₂(OOCR) (Triester) + 3CH₃OH –KOH(catalyst)–> CH₂OHCHOHCH₂OH (Glycerol) + 3CH₃OOCR (Methyl ester)

Structural formulae for triester, glycerol and methyl ester may be wrong

Question 15

What is an acyl chloride?

Answer 15

• Acyl (or acid) chlorides have the funtional group COCl.

Question 16

How do you name acyl chlorides?

Answer 16

They end in the suffix “-oyl chloride”

The carbons are numbered from the end with the acyl functional group

Question 17

What can acyl chlorides react with due to being able to easily lose the chlorine

Answer 17

• Water
• Alcohols
• Ammonia
• Primary amines

Each time, Cl is substituted by an oxygen or nitrogen group and misty fumes of hydrogen chloride are given off

Question 18

Give the equation for when ethanoyl chloride reacts vigourously with cold water

Answer 18

CH₃COCl +H₂O -> CH₃COOH + HCl

Produces a carboxylic acid

Question 19

Give the equation for when ethanoyl chloride vigourously reacts with an alcohol at room temperature

Answer 19

CH₃COCl +CH₃OH -> CH₃COOCH₃ + HCl

Produces an ester

Question 20

Give the equation for when ethanoyl chloride violently reacts with ammonia at room temperature

Answer 20

CH₃COCl + NH₃ -> CH₃ -> CH₃CONH₂ + HCl

Produces an amide (ethanamide)

Question 21

Give the equation for when ethanoyl chloride violently reacts with a primary amine at room temperature

Answer 21

CH₃COCl + CH₃NH₂ -> CH₃CONHCH₃ +HCl

Produces an N-substituted amide (N-methylethanamide)

Question 22

What is an acid anhydride?

Answer 22

An acid anhydride is made of two identical carboxylic acid molecules, they are made in a condensation reaction

Question 23

How do you name acid anhydrides?

Answer 23

Take the name of the carboxylic acid and replace the suffix “acid” with “anhydride”

Question 24

How do acid anhydride react in the same way as acyl chlorides, how are they different?

Give an example

Answer 24

• They react with, water, alcohol, ammonia, and amines
• The reactions are less vigorous
• A carboxylic acid is produced instead of HCl

E.g. (CH₃CO)₂O_(l) + CH₃OH_(aq) -> CH₃COOCH_3(aq) + CH₃COOH_(aq)

Question 25

Explain how acyl chloride undergos nucleophillic addition-elimination

Answer 25

In acyl chlorides, both the chlorine and oxygen atoms draw electons towards themselves, so the carbon has a slight positive charge - meaning it’s easily attacked by nucleophiles.

Example of nucleophillic addition elimination between enthanoyl chloride and methanol:
* Methanol is the nucleophile, the lone pair of electrons on the oxygen are attracted to the partially positive carbon on the acyl chloride, and a pair of electrons from the C=O bond are transferred to the oxygen
* The oxygen of methanol become positive since it’s bonded to three atoms
* The pair of electrons on the oxygen reform the double bond, causeing the single bond between the carbon and the chlorine to break
* The chlorine now bonds with the hydrogen in the hydroxyl group, the electrons from the O-H bond go to the δ+ oxygen

The other reactions of acyl chlorides e.g. H₂O, NH₃ or an amine, work the same way

Question 26

Explain how ethanoic anhydride is used in the manufacture of aspirin

Answer 26

Aspirin is an ester, it is made by reacting salacylic acid with ethanoic anhydride or ethanoyl chloride

Question 27

Why is ethanoic anhydride used instead of ethanoyl chloride in industry?

Answer 27

• Cheaper
• Safer:
  ~ less corrosive
  ~reacts slower with water
  ~ doesn’t produce dangerous hydrogen chloride fumes

Question 28

Explain how separation removes water soluble impurities from a product

Answer 28

If any product is insoluble in water then separation can be used to remove soluble impurities.

• One the reaction to form the product is completed, pour the mixture into a separating funnel and add water
• Shake the funnel and allow it to settle. The organic layer and the aqueous layer (which contains any water soluble impurities are immiscible, so will separate into two distinct layers
• Each layer can then be ran off into separate containers

Question 29

How is the product separated from the impurity when both the product and impurities are both water soluble?

Answer 29

Solvent extraction:
* A solvent in which the product is more soluble than it is in water is added to the impure product solution
* It is then shaken well
* The product will dissolve in the organic solvent, leaving the impurities dissolved in the water
* The solvent containing the product can then be run off using the separating funnel

Question 30

Why does a purified product need to be dried after separation?

Answer 30

The organic layer will still contain trace amounts of water - so it has to be dried
1. Anhydrous salt, e.g. magnesium sulfate (MgSO₄) or calcium chloride (CaCl₂, is used as a drying agent - it binds to any water present to become hydrated
2. When the salt is first added the to the organic layer it will clump together. The drying agent continues to be added until it disperses evenly when the flask is swirled
3. Finally the mixture is filtered to remove any drying agent

Question 31

Explain how impurites can be removed via washing

Answer 31

The product of some reactions can be contaminated with leftover reagents or unwanted side products. These can be removed by washing the product, adding another liquid and shaking.

For example, aqueous sodium hydrogencarbonate can be added to an impure product in solution to remove acid from it. The acid reacts with the sodium hydrogencarbonate to give CO₂ gas, and the organic product can be removed using a separating funnel

Question 32

Explain how volitile liquids can be purified by distillation

Answer 32

Distillation separates out liquids with different boiling points. It works by gently heating a mixture in distillation apparatus. The substances will evaporate out in order of increasing boiling point.

1. Connect a condenser to a round bottomed flask containing your impure product in solution
2. Place a thermometer in the neck of the flask so that the bulb sits next to the entrance to the condenser. The temperature of the thermometer will show the boiling point of the substance that’s evaporating at any given time
3. Heat the impure product (many organic chemicals are flammable so use an electric heater or water bath)
4. When the product that you want boils (i.e. when the thermometer shows the boiling point), place a flask under the open end of the condensor to collect the pure product

Question 33

Explain how organic solids can be purified by recrystallisation

Answer 33

If the product of an organic reaction is a solid then the simplest way to purify it is recrystalisation

1. Add very hot solvent to the impure solid until it just dissolves. It’s important to not add to much solvent - this should give a saturated solution of the impure product
2. Filter the hot solution through a heated funnel to remove any insoluble impurities
3. Leave the solution to cool down slowly. Crystals of the product will form as it cools, this is because as the mixture cools the solubility of the product falls
4. Remove the liquid containing the soluble impurites from the crystals by filtering the mixture under reduced preasure. To do this, the mixture is poured onto a filter paper lined Buchner funnel - a flat-bottomed funnel with holes in the base - that’s sitting in a side-arm flask attached to a vacuum line
5. Finally wash the crystals with ice cold solvent to remove any soluble impurities from their surface. Leave your purified crystals to dry

Question 34

How can melting and boiling points be used to indicate purity

Answer 34

Pure substances have a specific melting and boiling point. If they’re impure then the melting point is lowered and the boiling point is raised. if they are very impure the melting and boiling point will occur accross a wide range of temperatures

Question 35

Give the method of using melting and boiling points to determine purity

Answer 35

You can use melting point apparatus to accurately determine the melting point of an organic solid
1. Pack a small sample of the solid into a glass capillary tube and place it inside the heating element
2. Increase the temperature until the sample turns from solid to liquid
3. You usually measure a melting range, which is the. range of temperatures from where the liquid starts to melt to where it has completely melted
4. You can look up the melting point of a substance in data books and compare it to your measurements
5. Impurities in the sample will lower the melting point and broaden the melting range

Boiling point will be raised