A - Carboxylic Acids

Question 1

What functional group do carboxylic acids contain?

Answer 1

The carboxyl group -COOH.

Question 2

What two functional groups make up the carboxyl group?

Answer 2

A carbonyl group -C=O and a hydroxyl group -OH.

Question 3

When is the carboxyl group found in a carboxylic acid?

Answer 3

Only ever at the end of the carbon chain.

Question 4

What is the suffix for carboxylic acids?

Answer 4

-oic acid.

Question 5

How are the carbons numbered in a carboxylic acid?

Answer 5

The carbon in the carboxyl group is counted as carbon one.

Question 6

How are carboxylic acids named when the carboxyl group is attached to a benzene ring?

Answer 6

The suffix -carboxylic acid is used and the carbon of the functional group is not counted as part if the root.

Question 7

Are carboxylic acids strong or weak?

Answer 7

Weak acids.

Question 8

What happens when carboxylic acids are dissolved in water?

Answer 8

They partially dissociate into a carboxylate ion and an H+ ion. This sets up an equilibrium which lies to the left as most of the molecules don’t dissociate.

Question 9

What happens when carboxylic acids react with carbonates?

Answer 9

They liberate CO2.

The other products are a salt and water.

Carbon dioxide fizzes out of the solution in these reactions.

Question 10

How are esters formed from carboxylic acids?

What is this type of reaction called?

Answer 10

Carboxylic acids and alcohols react, in the presence if an acid catalyst to give esters.

Called an esterification reaction.

Question 11

What are esters?

Answer 11

Organic compounds that contain a -COO- group. An alkyl or aryl group replaces the H from the hydroxyl group on the carboxylic acid it is derived from.

Question 12

How are esters named?

Answer 12

The names of esters are based on the name pf the parent acid. Esters from ethanoic acid are called ethanoates, those from methanoic acid are called methanoates and so on.

But the name of the ester always starts with the alkyl or aryl group that has replaced the H of the acid.

For example:

Methyl ethanoate or ethyl methanoate.

Question 13

What else is produced in the reaction producing esters from carboxylic acids and alcohols? What type of reaction is it also therefore called?

Answer 13

Water.

Condensation reaction.

Question 14

How are the carbons numbered in esters?

Answer 14

The carbons in the C-O-C are both numbered 1 and the carbons in their respective chains are numbered from there.

Question 15

Give some common uses of esters.

Answer 15

1. Esters have a sweet smell so are used in perfumes.
2. Used by the food industry to flavour drinks and sweets.
3. Esters are polar liquids so lots of polar organic compounds will dissolve in them. This makes them good solvents in glued and printing inks.
4. Used as plasticisers - they’re added to plastics during polymerisation to make the plastic more flexible. (But over time, the plasticiser molecules escape and the plastic can become brittle and stiff).

Question 16

Vegetable oils and animal fats are esters of what?

Answer 16

Propane-1,2,3-triol (glycerol).

Question 17

What are fatty acids?

Answer 17

Long chain carboxylic acids.

Question 18

What do fatty acids combine with to make esters?

Answer 18

Glycerol (propane-1,2,3-triol)

Question 19

What is it that gives fats and oils most of their properties and why?

Answer 19

The fatty acid chains as these make up the majority of the molecules.

Question 20

Describe the structure and melting point of animal fats.

Answer 20

They have mainly saturated hydrocarbon fatty acid chains.

They fit neatly together, increasing the VDWs forces between them.

This means you need higher temperatures to melt them and they’re solid at room temperature.

Question 21

Describe the structure and melting point of vegetable oils.

Answer 21

They have unsaturated hydrocarbon chains. The double bonds mean that the chains are bent and don’t pack together well, decreasing VDWs forces.

They’re easier to melt than animal fats and are liquids at room temperature.

Question 22

What is hydrolysis?

Answer 22

When a substance is split up by water.

Question 23

Esters are hydrolysed to what?

Answer 23

Alcohols and either carboxylic acids or salts of carboxylic acids.

Question 24

What are the two types of hydrolysis of esters? Why do they exist?

Answer 24

1. Acid hydrolysis.
2. Base hydrolysis.

Just using water for this reaction is often very slow so an acid or alkali is used to speed it up.

Question 25

State the conditions for acid hydrolysis of an ester and the products formed.

How can you push the equilibrium to the right?

Answer 25

You have to reflux the water with a dilute acid, such as hydrochloric or sulfuric acid.

The products are a carboxylic acid and an alcohol.

The reaction produces an equilibrium so you can add lots of water to push this to the right.

Question 26

What are the conditions of base hydrolysis? What products are formed?

Answer 26

Reflux the water with a dilute alkali, such as sodium hydroxide. You get a carboxylate ion and an alcohol.

Question 27

Oils and animal fats can be hydrolysed to form what?

Answer 27

Soap and glycerol.

Question 28

What is a soap?

Answer 28

The salt of a long-chain carboxylic acid.

Question 29

What conditions do oils and fats need to hydrolysed in to form a soap and glycerol?

Answer 29

Alkaline conditions - need to. E heated with an alkali such as sodium hydroxide.

Question 30

What is biodiesel formed from?

Answer 30

It’s a mixture if methyl esters of long-chain carboxylic acids.

Question 31

How is biodiesel produced?

Answer 31

By reacting vegetable oils with methanol in the presence of a catalyst (potassium hydroxide).

This gives the methyl ester and glycerol as the products.

Question 32

What is another name for acyl chlorides?

Answer 32

Acid chlorides.

Question 33

What functional group do acyl chlorides have?

How are they named? How are the carbons numbered?

Answer 33

COCl.

All the names end in -oyl chloride.

The carbon atoms are numbered from the end of with the acyl group (as with carboxylic acids).

Question 34

What type of reaction occurs with acyl chlorides a d acid anhydrides with the following:

1. Water
2. Alcohols
3. Ammonia
4. Primary amines?

Answer 34

Nucleophilic addition-elimination reactions.

Question 35

Describe the reaction between an acyl chloride and water.

Answer 35

A vigorous reaction with cold water, producing a carboxylic acid and HCl.

Question 36

Describe the reaction between an acyl chloride and an alcohol.

Answer 36

A vigorous reaction at room temperature, producing an ester and HCl.

This irreversible reaction is a much easier, faster way to produce an ester than esterification.

Question 37

Describe the reaction between an acyl chloride and ammonia.

Answer 37

A violent reaction at room temperature, producing an amide and HCl.

Question 38

Describe the reaction between an acyl chloride and a primary amine.

Answer 38

A violent reaction at room temperature, producing an N-substituted amide and HCl.

Question 39

What is an acid anhydride?

Answer 39

A molecule consisting of two identical carboxylic acid molecules.

Question 40

How do you name an acid anhydride?

Answer 40

Name the carboxylic acid and replace the word acid with anhydride.

Question 41

How do acid anydrides react with the following:

1. Water
2. Alcohols
3. Ammonia
4. Primary amines?

Answer 41

Almost the same as those of acyl chlorides but less vigorous and producing a carboxylic acid in place of HCl.

1. 2 molecules of carboxylic acid
2. An ester and a carboxylic acid
3. An amide and a carboxylic acid
4. An N-substituted amide and a carboxylic acid.

Question 42

Which is more reactive, an acyl chloride or an acid anhydride? How is this shown in their reactions?

Answer 42

Acyl chloride is more reactive. Reactions with acyl chlorides will be more vigorous/violent.

Question 43

Give the order of reactivity for the following nucleophiles:

1. Water
2. Alcohols
3. Amines
4. Ammonia

Answer 43

Most to least reactive:

1. Ammonia
2. Amines
3. Alcohols
4. Water

Question 44

What is acylation?

Answer 44

The process by which an acyl group is introduced into another molecule.

Question 45

What is the acyl group?

Answer 45

R — C = O
\
Z

Question 46

Why are acid anhydrides and acyl chlorides both good acylating agents?

Answer 46

The oxygen and Z groups of acyl chlorides and acid anhydrides both withdraw electrons from the carbonyl carbon, giving this carbon atom a slightly positive charge, therefore it can be easily attacked by nucleophiles.

Question 47

Describe the nucleophilic addition-elimination reaction between ethanoyl chloride and methanol.

How do other nucleophilic addition-elimination reactions with an acyl chloride/acid anhydride and water, ammonia or a primary amine?

Answer 47

1. Methanol is the nucleophile. It attacks the partially positive carbon atom on the acyl chloride, and the por of electrons from the double bond are transferred to the oxygen. (The oxygen from methanol which has attacked the carbon atom, is left with a positive charge).
2. The pair of electrons on the oxygen atom reform the double bond with the carbon atom and chlorine leaves the molecule with a pair of electrons.
3. The chlorine bonds with the H in the hydroxyl group from the methanol. Electrons are transferred to the oxygen atom which still had the positive charge to counteract this and HCl is eliminated.

The reaction is the same with the other reactants mentioned, just different nucleophiles and products.

Question 48

What type of organic compound is aspirin?

Answer 48

An ester.

Question 49

How can aspirin be made using an acid anhydride or acyl chloride? Which method is preferred, and why?

Answer 49

It’s made by reacting salicylic acid with ethanoic anhydride or ethanoyl chloride.

Ethanoic anhydride is used in industry because:

1. It’s cheaper than ethanoyl chloride.
2. It’s safer to use than ethanoyl chloride as it’s less corrosive, reacts more slowly with water and produces ethanoic acid instead of HCl, which produces dangerous fumes.

Question 50

In most cases, once a product of an organic reaction has been produced, what will need to happen to the product and why?

Answer 50

It will need to be purified as the product nearly always contains impurities.

Question 51

If a product is insoluble in water, what method can be used to remove impurities that do dissolve in water? Outline this method.

Answer 51

1. Once the reaction to form the product has been completed, pour the mixture into a separating funnel, and add water.
2. Shale the funnel and allow it to settle. The organic layer, and the aqueous later containing any water soluble impurities, are immiscible (do not mix), so separate into two distinct layers.
3. You can then open the tap of the separating funnel and run each layer off into a separate container.

Question 52

If the product and any impurities are both soluble in water, what method can be used, which is similar to the separation method, to remove impurities?

Answer 52

It’s called solvent extraction.

1. Take an organic solvent in which the product is more soluble than it is in water.
2. Add it to the impure product solution and shake well.
3. The product will dissolve into the organic solvent, leaving the impurities dissolved in the water.
4. The solvent containing the product can then be run off using a separating funnel using the separation technique.

Question 53

If you use separation to purify a product, the organic layer will end up containing trace amounts of water and so has to be dried. How is the product dried?

Answer 53

1. Add an anhydrous salt, such as magnesium sulfate (MgSO4) or calcium chloride (CaCl2). The salt is used as a drying agent - it bonds to any water present to become hydrated.
2. When you first add the salt to the organic layer, it will clump together. You keep adding the drying agent until it dispersed evenly when you swirl the flask.
3. Filter the mixture to remove any solid drying agent (put a piece of filter paper into a funnel that feeds into the flask and pour the mixture into the filter paper).

Question 54

The product of a reaction can be contaminated with leftover reagents. You can remove some of these by washing the product (adding another liquid and shaking). Outline an example of this method.

Answer 54

1. Aqueous sodium hydrogencarbonate can be added to an impure product in solution to remove acid from it.
2. The acid reacts with the sodium hydrogencarbonate to give CO2 Gad, and the organic product can be removed using a separating funnel.

Question 55

Volatile liquids can be purified by distillation. Briefly explain how distillation works.

Answer 55

Distillation separated out liquids with different boiling points.

It works by gently heating a mixture in distillation apparatus. The substances will evaporate out of the mixture in order of increasing boiling point.

Question 56

Outline the method of distillation in full.

Answer 56

1. Connect a condenser to a round bottomed flask containing the impure product in solution.
2. Place a thermometer in the neck of the flask so that the bulb of the thermometer sits next to the entrance of the condenser. The temperature on 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 you should use an electric heater).
4. When the product that you want to collect boils (when the thermometer shows it’s boiling point), place a flask at the open end of the condenser to collect the pure product.

Question 57

What’s the simplest way to purify a solid organic product?

Briefly describe how this method works.

Answer 57

By recrystallisation.

First, you dissolve your solid in a hot solvent to make a saturated solution.

A saturated solution is one in which the maximum possible amount of solid is dissolved in the solvent.

Then you let it cool. As the solution cools, the solubility of the product falls. When it reached a point where it can’t stay in solution, it forms pure crystals.

Question 58

Outline the method of recrystallisation in full.

Answer 58

1. Add very hot solvent to the impure solid until it just dissolves (it’s important to not add too much solvent). This will give a saturated solution of the impure product.
2. Filter the hot solution through a heated gu. El to remove any insoluble impurities.
3. Leave the solution to cool down slowly. Crystals of the product will form as it cools.
4. Remove the liquid containing the soluble impurities from the crystals by filtering the mixture under reduced pressure. (To do this, you pour the mixture into a filter paper lined Büchner funnel - a flat bottomed funnel with holes in the base that’s sitting in a side-arm flask attached to a vacuum line).
5. Wash the crystals with ice-cold solvent to remove any soluble impurities from their surface. Leave the purified crystals to dry.

Question 59

What is unique about the melting and boiling points of pure substances?

What will happen to melting and boiling points of the substance is impure?

Answer 59

They have specific melting and boiling points.

If impure, the melting point is lowered, and boiling point is raised.

If very impure, melting and boiling occurs across a wide range of temperatures.

Question 60

What is the structure of an amide?

Answer 60

Contains the functional group -CONH2 where the CO is the carbonyl group.

Question 61

What is the structure of an N-substituted amide?

Answer 61

The same as an amide however one of the H’s from the NH2 group is replaced with an alkyl group (R).

Question 62

What is an alkyl group?

Answer 62

An alkane missing one hydrogen atom.