Carbonyls Flashcards
Organic chemistry A2
What is optical isomerism?
Occurs in compounds with 4 different groups of atoms attached to a carbon (called an asymmetric carbon).
What are enantiomers?
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).
How can stereoisomers be disttinguished?
How they rotate plane polarised light.
How does one enantiomer rotate plane polarised light?
An enantiopure mixture will rotate plane polarised light. The direction depends on the enantiomer.
What do you call a mixture of 2 enantiomers in a 50:50 ratio?
How do they rotate plane polarised light?
Racemic mixture/ Racemate
They rotate plane polarised light equally in different directions, cancelling each other out.
So they don’t rotate plane polarised light.
What do you call a mixture of 2 enantiomers that are not in a 50:50 ratio?
How do they rotate plane polarised light?
Enantiomeric excess
Will rotate plane polarised light in the direction of the excess enantiomer.
How will an achiral compound rotate plane polarised light?
It will not.
Drugs can be optically active molecules in which one or both isomers are active:
What are the issues with this?
- 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.
How do the physical and chemical properties of the enantiomer differ?
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.
What kind of acids are carboxylic acids?
(Strong or weak)
Gove an equation to show how they disassociate:
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.
Write a general equation for carboxylic acids reacting with bases:
carboxylic acid + base –> salt + water
Write a general equation for carboxylic acids reacting with carbonates:
carboxylic acid + carbonate –> salt + water + carbon dioxide
Write a general equation for forming an ester:
Carboxylic acid + alcohol –> ester + water
Only in the presence of a FEW DROPS of strong acid such as H2SO4
What happens if we add too many drops of our strong acid catalyst?
Alcohol could get dehydrated into an alkene.
What are some uses of esters?
Short chain esters are fairly volatile (vapourises quickly) and so are used in:
- Perfumes
- Food flavourings
- Solvents and plasticisers (to improve flexibility in resins).
What happens to the boiling point as the chain length of esters increases?
- Boiling point increases as chain length increases
- This is due to increases Van der Waals forces between molecules which requires more energy to overcome.
Write a general equation for the hydrolysis of an ester under acidic conditions:
Ester + water –> carboxylic acid + alcohol
This is a reversible reaction that will never go to completion unless a basic catalyst is used.
What is the difference between acidic and basic hydrolysis?
- 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)
Give the general equation to form a triglyceride:
glycerol + 3 fatty acids –> triglyceride + 3 H2O
Which one of fats or oils is saturated and which one is unsaturated?
Fats are usually saturated (no c=c bond)
Oils are usually unsaturated (contain c=c bond).
How does a saturated fat affect its melting point?
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.
How does an unsaturated oil affect its melting point?
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.
Explain why vegetable oil is a liquid at room temperature but butter is a solid at room temperature:
- 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.
How can we make soap?
By hydrolysing naturally occurring fats and oils with sodium hydroxide
When making soap, how would the reaction be different if it was under acidic conditions?
You would make a carboxylic acid (fatty acid) instead of the salt, so it doesn’t make soap.
How does soap work?
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.
What is biodiesel?
A mixture of methyl esters of long chain carboxylic acids.
Is biodiesel a carbon-neutral fuel?
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.
Why are acyl chlorides so reactive?
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.
Name 2 acylating groups:
Acyl chlorides
Acid anhydride
Acid anhydride + water –>
Carboxylic acid + carboxylic acid
Acid anhydride + Ammonia/Amine
Amide + carboxylic acid
Acid anhydride + alcohol –>
Ester + carboxylic acid
What are the industrial advantages of using ethanoic anhydride over ethanoyl chloride in the manufacture of the drug aspirin?
- 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
