Carbonyls

Question 1

What is optical isomerism?

Answer 1

Occurs in compounds with 4 different groups of atoms attached to a carbon (called an asymmetric carbon).

Question 2

What are enantiomers?

Answer 2

Molecules that are non-super imposable mirror images.
(Like your hands, you can mirror them but you can’t put them on top of each other).

Question 3

How can stereoisomers be disttinguished?

Answer 3

How they rotate plane polarised light.

Question 4

How does one enantiomer rotate plane polarised light?

Answer 4

An enantiopure mixture will rotate plane polarised light. The direction depends on the enantiomer.

Question 5

What do you call a mixture of 2 enantiomers in a 50:50 ratio?
How do they rotate plane polarised light?

Answer 5

Racemic mixture/ Racemate

They rotate plane polarised light equally in different directions, cancelling each other out.

So they don’t rotate plane polarised light.

Question 6

What do you call a mixture of 2 enantiomers that are not in a 50:50 ratio?
How do they rotate plane polarised light?

Answer 6

Enantiomeric excess

Will rotate plane polarised light in the direction of the excess enantiomer.

Question 7

How will an achiral compound rotate plane polarised light?

Answer 7

It will not.

Question 8

Drugs can be optically active molecules in which one or both isomers are active:

What are the issues with this?

Answer 8

• You might need to separate the 2 isomers, which can be difficult and expensive as optical isomers have very similar properties.
• Selling the drug as a mixture is wasteful because half of it is inactive
• You can consider designing an alternative (where you only have one enantiomer).
• Synthesis of the drug that makes only the required isomer.

Question 9

How do the physical and chemical properties of the enantiomer differ?

Answer 9

Physically, they have the same properties, except their effect on plane polarised light.

Chemically, they differ in chiral environments. But stay the same in achiral environments.

Question 10

What kind of acids are carboxylic acids?
(Strong or weak)

Gove an equation to show how they disassociate:

Answer 10

They are weak acids.

RCOOH –> RCOO- +(H+)

The negative charge is shared over the whole of the carboxylate group. This delocalisation makes the resulting ion more stable.

Question 11

Write a general equation for carboxylic acids reacting with bases:

Answer 11

carboxylic acid + base –> salt + water

Question 12

Write a general equation for carboxylic acids reacting with carbonates:

Answer 12

carboxylic acid + carbonate –> salt + water + carbon dioxide

Question 13

Write a general equation for forming an ester:

Answer 13

Carboxylic acid + alcohol –> ester + water

Only in the presence of a FEW DROPS of strong acid such as H2SO4

Question 14

What happens if we add too many drops of our strong acid catalyst?

Answer 14

Alcohol could get dehydrated into an alkene.

Question 15

What are some uses of esters?

Answer 15

Short chain esters are fairly volatile (vapourises quickly) and so are used in:

• Perfumes
• Food flavourings
• Solvents and plasticisers (to improve flexibility in resins).

Question 16

What happens to the boiling point as the chain length of esters increases?

Answer 16

• Boiling point increases as chain length increases
• This is due to increases Van der Waals forces between molecules which requires more energy to overcome.

Question 17

Write a general equation for the hydrolysis of an ester under acidic conditions:

Answer 17

Ester + water –> carboxylic acid + alcohol

This is a reversible reaction that will never go to completion unless a basic catalyst is used.

Question 18

What is the difference between acidic and basic hydrolysis?

Answer 18

• Under basic conditions, you make the carboxylate salt instead of the carboxylic acid.
• The carboxylic acid can be made by treating the carboxylate ion with an acid afterwards (carboxylate ion + proton –> carboxylic acid)

Question 19

Give the general equation to form a triglyceride:

Answer 19

glycerol + 3 fatty acids –> triglyceride + 3 H2O

Question 20

Which one of fats or oils is saturated and which one is unsaturated?

Answer 20

Fats are usually saturated (no c=c bond)

Oils are usually unsaturated (contain c=c bond).

Question 21

How does a saturated fat affect its melting point?

Answer 21

Saturated hydrocarbon chains mean they fit neatly together so there’s a larger surface area and so stronger Van der Waals forces between molecules.

Therefore, they have a higher melting point and are solids at room temperature.

Question 22

How does an unsaturated oil affect its melting point?

Answer 22

The double bonds means the chains are bent and don’t pack well together.
This decreases the surface area between molecules and so Van der Waal forces are weaker.
Therefore, they have a lower melting point and are usually liquids at room temperature.

Question 23

Explain why vegetable oil is a liquid at room temperature but butter is a solid at room temperature:

Answer 23

• Vegetable oil has many unsaturated hydrocarbon chains.
• This means there are many double bonds preventing the chain from stacking neatly together
• So there’s a reduced surface area of contact
• There’s weaker Van der Waals between the molecules
• Lower melting point
• It’s a liquid at room temperature
• Butter is a fat and so it is saturated, containing no double bond
• The hydrocarbon chains stack together neatly giving a greater surface area of contact between molecules.
• There’s stronger van der waals forces between molecules.
• So there’s a higher melting point.
• It’s a solid at room temperature.

Question 24

How can we make soap?

Answer 24

By hydrolysing naturally occurring fats and oils with sodium hydroxide

Question 25

When making soap, how would the reaction be different if it was under acidic conditions?

Answer 25

You would make a carboxylic acid (fatty acid) instead of the salt, so it doesn’t make soap.

Question 26

How does soap work?

Answer 26

The hydrophobic hydrocarbon chain is non-polar so will mix with the grease (that may be on your hands)

The hydrophilic charged end is attracted to the water.

The soap molecules surround the grease, allowing the grease and water to mix and the grease to be washed away.

Question 27

What is biodiesel?

Answer 27

A mixture of methyl esters of long chain carboxylic acids.

Question 28

Is biodiesel a carbon-neutral fuel?

Answer 28

It’s said to be roughly carbon neutral because the same amount of carbon is taken in when it’s formed compared to the amount released when burnt.

However, the machines and fertilisers used in the farming and manufacturing process (to make biodiesel) often use fossil fuels.

So we can’t say it’s 100% carbon neutral.

Question 29

Why are acyl chlorides so reactive?

Answer 29

The delta positive carbon in the carbonyl is very electron deficient.
The O and Cl are both very electronegative and electron withdrawing.
So the nucleophile is more attracted to delta positive C

The C-Cl bond is easily broken so it is considered a good-leaving group.

The nucleophiles used have very electronegative elements.

Question 30

Name 2 acylating groups:

Answer 30

Acyl chlorides
Acid anhydride

Question 31

Acid anhydride + water –>

Answer 31

Carboxylic acid + carboxylic acid

Question 32

Acid anhydride + Ammonia/Amine

Answer 32

Amide + carboxylic acid

Question 33

Acid anhydride + alcohol –>

Answer 33

Ester + carboxylic acid

Question 34

What are the industrial advantages of using ethanoic anhydride over ethanoyl chloride in the manufacture of the drug aspirin?

Answer 34

• Cheaper
• Less corrosive
• t does not react with water as readily
• It is safer, as the by-product of its reaction is ethanoic acid rather than HCL