ESTERIFICATION of an alcohol and a carboxylic acid:
An alcohol molecule and a carboxylic acid molecule are combined alongside a water molecule being removed.
OH taken from the alcohol (longest from shortest)
H taken from carb acid (shortest taken from longest)
Shorthand structural formula:
-COOC- (in centre of the whole molecule)
Naming an ester:
R₁ comes from the carboxylic acid
R₂ comes from the alcohol
Put the alkyl bit then carboxyloate when naming.`
Lower than expected as cannot form hydrogen bonds as no hydrogens bonded to oxygen/nitrogen/fluorine
Relatively insoluble as cannot form hydrogen bonds with the water
Preparation of esters:
There are two methods to prepare esters:
1) esterification of carb acids and alcohols
2) Reaction between acid anhydrides and alcohols
ESTERIFICATION: type of reaction
Esterification is a condensation reaction as...
Two molecules are joined to make a larger product molecule but a small molecule is eliminated in the process (in this case water)
Esterification requires a concentrated sulphuric acid catalyst and heat under reflux
ACID ANHYDRIDE + ALCOHOL: reaction
Anhydride + alcohol → Ester + carboxylic acid
ACID ANHYDRIDE + ALCOHOL: conditions
Gentle heat but no catalyst needed (big advantage to this method)
Production of acid anhydrides:
Acid anyhydrides are compounds formed from two carboxylic acid molecules with the elimination of water.
Naming acid anhydrides:
Ethanoic anyhdride / butanoic anhydride are examples.
HYDROLYSIS of esters: definition
A hydrolysis reaction is where a compound is broken down by reaction with water
Problem with HYDROLYSIS:
Reaction with water alone is very slow so to get the hydrolysis to occur at a useful rate an aqueous aid or base catalyst is used.
BASE catalysed HYDROLYSIS: reaction
Due to the alkaline conditions the products are the alcohol and the salt of the carboxylic acid.
Ester + Base → Ionic salt of carboxylic acid + Alcohol
BASE catalysed HYDROLYSIS: conditions
Reflux the ester with aqueous NaOH
BASE catalysed HYDROLYSIS: information
Under these conditions hydrolysis is irreversible.
So base catalysis is a good method giving HIGH yields.
BASE catalysed HYDROLYSIS: to convert salt to parent carboxylic acid-
1) Neutralise the alkali with aqueous acid
2) Distill of the carboxylic acid
ACID catalysed HYDROLYSIS: reaction
Simply the reverse of esterification.
Ester + Water → Carboxylic acid + alcohol
ACID catalysed HYDROLYSIS: conditions
Reflux the ester with aqueous HCl
ACID catalysed HYDROLYSIS: information
A reversible reaction (with esterification) so doesnt go to completion so gives relatively POOR yields.
USES of esters:
Simple esters are volatile and have pleasant, fruity smells so are used widely in flavourings and perfumes.
What are TRIGLYCERIDES?
Triglycerides make up fats and oils which are found in animals and plants.
How are TRIGLYCERIDES made?
Triglycerides are formed when a glycerol molecule undergoes 3 esterifications and forms 3 ester links with carboxylic acids.
In fats the carboxylic acids are reffered to as fatty acids.
An alcohol with 3 -OH groups.
Two main types of fatty acids, saturated and unsaturated.
SATURATED fatty acids:
NO C=C, carbon carbon double bonds.
UNSATURATED fatty acids:
Have C=C, carbon carbon double bonds, in their structure.
Two types of C=C double bond, trans // or cis |_|
POLYUNSATURATED fatty acids:
Fatty acids which contain several C=C double bonds.
Naming fatty acids:
Long hand method:
cis-9-octadecenoic acid / trans,trans-3,5-nonadienoic acid
SO type,place(double bonds),carbons,number of doub bonds
Short hand method:
trans, trans, 9, 2 (3,5)
SO types of double bond, number of carbons, number of double bonds, and position of double bonds
When counting where double bond is located:
START at carboxylic acid end - first carbon attatched to oxygens.
SATURATED or UNSATURATED triglecerides:
As fatty acids can be either saturated or unsaturated then the triglycerides which they form can also be.
Higher melting points?
SATURATED fats (without double bonds) have higher melting points because UNSATURATED fats do have double bonds which makes the fatty chains less flexible and less able to pack together efficiently in a solid.
What is MARGARINE?
Vegetable oils are typically highly unsaturated so are liquid at room temperature, if the vegetable oil is hydrogenated - H₂ reacts with some of the double bonds to from single linkages. SO melting point increases and solid materials are formed.
Bubble H₂ gas through vegetable oil at HIGH TEMPERATURE with a NICKEL CATALYST.
1 molecule of H₂ per double bond
Eating TOO much SATURATED fat:
Can lead to increased risk of circulatory, heart and obesity problems.
DUE TO...a high intake of saturated fats causing our bodies to produce excessive amounts of cholestoral - deposited on walls of arteries
When fats are HYDROGENATED:
Some of the cis double bonds ISOMERISE to form trans double bonds instead - resulting material called 'TRANS FATS'
Health risks of TRANS FATS:
Lead to high levels of cholestoral in blood as causes our bodies to produce high levels of cholestoral
Vegetable oils (triglycerides) can be used directly as fuels in some engines - engines need however to be modified. Bio diesel fuel is the term used for this - need to undergo transesterification first though.
The 3 fatty acid chains can be removed from the glycerol molecule by reacting with short chain alcohol molecules.
Three separate methyl esters are formed - a mixture of these esters make up biodiesel (glycerol is the by product)
BIODIESEL: what actually is it?
The three methyl esters make up biodiesel (produced in transeterification)
Importance of BIODIESEL?
We need to reduce the amount of fossil fuels we burn as they are non-renewable and will run out.
We also need to reduce the amount of CO2 we release into the atmosphere - to combat climate change.
Carbon neutral BIODIESEL:
The SAME amount of carbon dioxide is taken in when the plants grow than what is released when they burn so no MORE carbon dioxide is released - same quantities of atmospheric CO2
Advantages of BIODIESEL:
- Renewable as more plants can be grown
- Carbon neutral
Disadvantages of BIODIESEL:
- Takes up space/land needed to grow food for ever increasing population