Mod 7.5 - Reaction of Organic Acids and Bases

Question 1

Explain the properties for alkanoic acids

Answer 1

Alkanoic acids have greater molecular weights than their corresponding alkanols or alkanes, and thus the dispersion forces between their molecules are greater

They are more polar than alkanols and therefore have greater dipole dipole forces.

Thus, the MP and BP of alkanoic acids are higher than alkanols, due to more extensive hydrogen bonding .

Carboxylic acid can form a structure called a dimer which forms two hydrogen bonds between each pair of molecule –> higher molecular weight compound –> increase in BP

Question 2

Explain the properties for aldehydes and ketones

Answer 2

Smaller molecules of these compounds are soluble because they can form hydrogen bonds with water, but solubility decreases with increasing length of the non polar chain

These molecules cant hydrogen bond with each other, but the polarity of the molecule means that the BP is higher than for similar sized alkanes but lower than for alcohols and carboxylic acids, which have hydrogen bonding between hydroxyl groups

Question 3

Explain the properties for amines

Answer 3

Hydrogen bonding is possible in amines due to the presence of N-H bonds. The BP of amines are lower than corresponding alcohols as the hydrogen bonds in amines are weaker than those in alcohols

As with the other polar compounds containing hydrogen bonding, the solubility decreases with chain length

Question 4

Explain the properties for amides

Answer 4

Amides have higher MP and BP than similar sized organic compounds due to capacity to form multiple hydrogen bonds between molecules (even more than the dimers formed in the carboxylic acids)

SMaller amides are soluble but less soluble than comparable amines and carboxylic acids; their solubility is similar to that of esters

Question 5

Why does increased chain length decrease solubility?

Answer 5

Increased carbon chain length –> increased non polar nature of compound –> increased dispersion forces domination –> water cannot solvate the long hydrocarbon chain due to cohesive bonds –> decreased solubility

SImilar to alcohols, the trend is reversed for their solubility in organic solvents

Question 6

WHat are esters?

Answer 6

Esters are organic compounds with the functional group -COO-

They are formed from the reaction of a carboxylic acid and an alcohol.

Question 7

How is an ester named?

Answer 7

The alkanol is changed to ‘alkyl’ and is the first word of the esters name

The alkanoic acid becomes the alkanoate and is the second word of the esters name

Question 8

What is esterification?

Answer 8

It is a condensation reaction (refers to a reaction where two reactants combine together with the elimination of a smaller molecule such as water), between an alkanoic acid and an alcohol

This is a slow process at room temp and does not proceed to completion

Question 9

What is the general equation for an esterification reaction?

Answer 9

Alkanol + alkanoic acid ⇌ alkyl alkanoate + water

with conc H2so4 on top of the equilibrium sign, and reflux at the bottom of the equilibrium sign

Question 10

If it is a slow process at room temp, how is esterification designed to ensure that it occurs at a reasonable rate? (i.e. equipment)

Answer 10

(REMEMBER TO MEMORISE DIAGRAM DRAWING)

Add a suitable catalyst (e.g. 1-3ml of conc H2So4 which acts as a dehydrating agent by removing water and causing the equilibrium to shift to favour formation of the ester)

Heat reaction mixture to increase kinetic energy of molecules via reflux. This allows substances to be heated, then the vapours are condensed, ensuring that reactions dont escape and thus maintaing forward reaction

Water bath or an electric heating mantle can be used as a means of heating the reaction mixture

Boiling chips are used to prevent the reaction mixture from superheating, by providing uneven surfaces where gases can form bubbles to escape the mixture

Question 11

What is reflux? What is the importance of doing so?

Answer 11

Reflux is a technique that involves heating a reaction mixture in a vessel fitted with a cooling condenser so that the volatile reactants and products are returned to the reaction mixture without any loss

Ultimately, it increases the rate of esterification

Question 12

How is an ester extracted and purified after esterification?

Answer 12

1. Mixture is washed with water in a separating funnel to remove water soluble substances. After settling, the aqueous layer is removed (contains water, sulfuric acid and any soluble short chain alcohol and carboxylic acid molecules)
2. Sodium carbonate solution is added to neutralise any acid present. Flask is swirled until carbon dioxide production is no longer vigorous. Mixture is allowed to stand and the aqueous layer is removed.

Net equation: CO3^2- (aq) + H^+ (aq) –> CO2 (g) + H2O (l)

3. Mixture is washed with water and the aqueous layer discarded. This removes any water soluble salt formed
4. Anhydrous magnesium sulfate (drying agent) is added to remove water from the mixture)
5. The impure ester is then purified through distillation, collecting the ester at its boiling point

Question 13

What are the industrial applications of esters?

Answer 13

Short chain esters are known for their distinctive, fruit like odours and may occur naturally in fruits and the essential oils for plants

Due to pleasant odours, they are commonly used as flavouring agents in processed foods, as well as fragrances in perfumes and cosmetics

Fats and oils are also naturally occuring triesters derived from glycerol and fatty acids

Question 14

What is the purpose of the reaction flask in refluxing for esterification?

Answer 14

Contains volatile reactants and products

Question 15

What is the purpose of the condenser in refluxing for esterification?

Answer 15

Prevents the volatile reactant or product from escaping before the reaction has reached equilibrium by cooling the reactant vapour into a liquid. Water enters at the base and leave from the top

Question 16

What is the purpose of the boiling chips in refluxing for esterification?

Answer 16

To provide a surface upon which bubbles form, promoting even boiling

Question 17

What is the purpose of the open top in refluxing for esterification?

Answer 17

To avoid dangerous pressure build up inside the apparatus

Question 18

What are organic acids? Examples?

Answer 18

These refer to organic substances which exhibit acidic properties. For example, carboxylic acids.

Carboxylic acids are weak acids because they hyrdolyse to a small extent in water, and have a pH less than 7

Question 19

What are organic bases? Examples

Answer 19

Refer to organic substances which exhibit basic properties.

An example of organic bases are amines. They contain the amine (-NH2) functional group

Amines:
Are weak bases because they hydrolyse to a small extent in water
Show typical reactions of bases (i.e. react readily with acids)
Have a pH more than 7

Question 20

What is the reaction between organic acids and bases?

Answer 20

Carboxylic acid + amine –> ammonium salt –> heat + dehydration = amide + water

Question 21

What is saponification?

Answer 21

It is a process to make soaps from natural fats and oils. This involves an ester being hydrolysed under basic conditions to form an alcohol and the salt of a carboxylic acid

Question 22

What are the esters that make up natural oils and fats? What is glycerol?

Answer 22

These esters are called triglycerides and they are the natural esters of fatty acids and glycerol

Glycerol (1,2,3-propanetriol) forms the backbone of fats/oils and is an important by product of saponification that is used in the pharmaceutical and cosmetic industries

Question 23

What is the general equation of a saponification reaction?

Answer 23

Triglycerides + NaOH –> glycerol + salt of carboxylic acid (in this case sodium carboxylates, because it reacts with Na) (this is the soap)

Question 24

What is the basic structure of a soap molecule?

Answer 24

It has a long hydrophobic non polar tail, and a hydrophilic (water loving) negatively charged polar head

Draw it as a zigzag which goes for a while, with a polar head

Question 25

What is a surfactant?

Answer 25

Surfactants are chemical compounds that decrease the surface tension or interfacial tension between two liquids

Question 26

Is a soap a surfactant?

Answer 26

Yes it is. This means it functions by reducing the surface tension of water and binding to grease and dirt to emulsify them

Question 27

How do soaps work? in terms of reduicng surface tension

Answer 27

The hydrophobic part of the soap molecule is long non polar hydrocarbon chain. It is strongly repelled by water molecules

WHen soap molecules are added into water, they form an oriented monolayer (with tails sticking out of the water) at the surface in order to satisfy the interaction of both the hydrophobic and hydrophillic portions of the soap molecule

This effectively breaks the hydrogen bonding between molecules of water and thus reduces the surface tension of water

The components self assemble into the most stable arrangements which consist of spherical structures with the carboxylate groups forming a negatively charged spherical surface, with hydrocarbon chains inside the core of the sphere. These spheres are called micelles. The hydrophobic tails are shielded from the water by the polar heads, which minimises the repulsive forces in the system

Question 28

Explain the cleansing action of soaps

Answer 28

1. Dissolution - soap molecules first dissolve in water. The hydrophilic head of a soap ion interacts with water molecules via ion dipole interactions and hydrogen bonding
2. Absorption - The hydrophobic tails of the soap ions dissolve in the grease due to the dispersion forces and reorientates themselves. Surfactant molecules continue to absorb into the grease, decreasing the surface tension of water at the interface between the grease and water. After enough hydrophilic heads are on the grease, it interacts with water via ion dipole forces, effectively pulling the grease off the surface
3. Emulsification (separation of oil/fat from water)- with agitation (i.e. shaking), the grease layer breaks into smaller, spherical droplets (micelles), with the hydrophilic surfactant head groups interacting with the water via ion dipole forces, and the hydrophobic surfactant tails absorbed into the grease. This forms a dispersion of grease droplets in water (an emulsion is a stable dispersion of small droplets of one liquid throughout another liquid)

The negative charged heads on the soap repeal each other, preventing the grease and dirt from joining together and keeping them dispersed throughout the solution. Therefore, the grease and oil can be simply rinsed away, leaving a clean surface

(REMEMBER TO COPY DIAGRAMS)

Question 29

What is the general way of describing the cleaning action of soaps?

Answer 29

1. Describe structure of soap
2. Explain its role as surfactant and breaking down surface tension in water
3. Draw the three step process to describe its action
4. Explain emulsion and soap/detergent forming micelles during cleaning
5. Agitation and rinsing to remove dirt containing micelles

Question 30

What is the difference between soaps and detergents?

Answer 30

Soaps are commonly used for personal hygiene and are non-synthetic, whereas detergents act as cleansing agents and are synthetic

Question 31

What is non-synthetic vs synthetic definition and examples?

Answer 31

Definition of non-synthetic: Made from natural ingreadients such as plant oils or acids derived from animal fats

Types of non-synthetic: Soaps

Definition of synthetic: Made from petroleum products

Types of synthetic: Anionic, cationic, non ionic

Question 32

What is hard water?

Answer 32

It is water that contains minerals such as calcium and magnesium.

Question 33

What is soft water?

Answer 33

It is water that is treated to contain only sodium ions

Question 34

What is the effects of soaps on hard and soft water?

Answer 34

Hard water: The calcium and magnesium ions can form precipitates with the soap

Soft water: Produces a lather of bubbles

Question 35

What are the problems arising from using soap?

Answer 35

The soap ion must be dissolved in water to have a cleaning effect

As salts of weak acids, in low Ph, soaps are converted into uncharged fatty acids

Soaps can also form insoluble salts in hard water

Question 36

As a resullt of these problems what is the solution?

Answer 36

Synthetic surfactants known as detergents are now more widely used than soap

Question 37

What are detergents?

Answer 37

These are derived from fossil fuels. they are synthetic surfactants

They have a similar molecular structure and cleaning action to soap, but their hydrophilic head differs, as it can be anionic (a sulfonate group), cationic (an alkylated quaternary ammonium group) or non-ionic (repeating ethoxy groups and a -OH group)

Detergents do NOT form precipitates with Ca and Mg ions so they are more effective cleaning actions in hard water

Question 38

What are anionic detergents?

Answer 38

These have a negatively charged polar, hydrophilic head, and a long hydrocarbon tail.

The main structural difference between anionic detergents and soap is the presence of a sulfate (R-SO2-O-) or sulfonate group (R-O-SO2-O-) instead of a carboxylate (R-COO-)

Anionic detergents are able to form soluble complexes with Ca2+ and Mg2+ and prevents the surfactant from doing its intended job (to a small extent)

Question 39

What are anionic detergents used for?

Answer 39

They are used in general cleaning, particularly in laundry and dishwashing detergents.

Question 40

What is the effectiveness of anionic detergents in hard or acidic water?

Answer 40

Anionic detergents dont form insoluble precipitates, but form salts that are soluble. However, the effectiveness is still reduced in hard and acidic water. The positive ions in hard/acidic water will attract the negative head of the detergent, thus they are sort of limited in hard water

Question 41

What are cationic detergents?

Answer 41

These have a positively charged polar head. The positive head is usually a quaternary ammonium ion. They are also known as fatty amine salts

They work optimally in solutions where the pH is above 10

Question 42

What are cationic detergents good for?

Answer 42

Cationic detergents have a strong attraction to negatively charged surfaces (Most surfaces are negatively charged) (i.e. glass, hair, fibre, metals and plastics)

This can thus be helpful in certain situations:

Many fabrics acquire a negative charge when they become wet. Thus, cationic detergents are used in fabric softeners and hair conditioners

They can work in hard water, and work optimally in solutions where the pH is above 10

Question 43

Why might cationic detergents be detrimental?

Answer 43

The strong attraction of cationic detergents to negatively charged surfaces can be detrimental in other situations.

Cationic detergents are particularly toxic to micro organisms. They are attracted to the negative surface of bacteria and damage or kill bacteria that are involved in their decomposition. They therefore have very low biodegradability

Question 44

What are non ionic detergents? How do they work?

Answer 44

These have a long hydrocarbon tail, but their ‘head’ is a long polar segment containing many O atoms, ending in an alcohol group. These have uncharged polar heads.

The presence of oxygen in the head makes it polar.

Although these surfactants are uncharged, the polar head groups are still attracted to the highly polar water molecules forming numerous hydrogen bonds

These detergents arent affected by hard water and dont form complexes or precipitates with Ca or Mg

Question 45

What are the uses of non ionic detergents?

Answer 45

Used in dishwasher powders, dishwashers, paints, pesticides, cosmetics

Especially as these dont foam as much as other detergents

Question 46

Examine the environmental impacts of surfactants such as soaps and detergents

Answer 46

Detergents are synthetic, whereas soaps are made frmo naturally occuring biological materials. Thus, detergents are less biodegradable

The enhanced stability of detergents means that they persist in the environment, causing damage to the mucus membrane in wildlife and resulting in excessive frothing in the water ways –> less sunlight penetration

Toxic to aquatic life

Anionic detergents –> reacts with minerals in hard water and form soluble molecules

High levels of phosphates in anionic detergents –> high levels of phosphates entering rivers and waterways –> eutrophication

Question 47

Contrast soaps to synthetic detergents

Answer 47

Soaps:

Tend to be cheap to make
Made from renewable resources (fats and oils)
Less soluble in water than synthetic detergents
Harder to rinse out of fabrics and hair - leads to a build up of material over time
Form a scum in hard water
Precipitate in acidic solutions
Mild and non - irritating on human skin
Deteriorate more quickly in storage

Detergents:

More expensive to produce
Made from non renewable petroleum
More soluble in water
Easier to rinse out of fabrics and hair
Do not form a scum in hard water
Do not precipitate in acidic conditions
More skin irritant
Longer shelf lifeW

Question 48

What is a scum?

Answer 48

Soap molecules react with the salts of calcium and magnesium and forms a precipitate. This precipitate begins to float as an off-white layer over water. This layer is called scum.