Isomerism and Chirality
Stereoisomerism occurs when there are the same number of molecule in a compund but the molecules are all arranged in different ways.
Structural isomerism occurs when the moecules in a compound are arranged in different ways.
Optical isomerism occurs when two objects are mirror images of each other and non-superimposible. Therefore, they are said to chiral. If a carbon containing molecule has 4 different groups attached to it is is said to have a chiral centre and displays optical isomerism.
Isomers that are mirror images of each other are called enantiomers. They cannot be told appart by differences in their physical properties but by how they act on plane polarised light.
Molecules that are optical isomers are said to be optically active.
Optical Activity of Chiral Molecules
Light oscillates in different directions and when this is the case it is called unpolarised light but when it is only traveling in one direction it is said to be polarised light.
Enantiomers are able to rotate polaried light and the extent to which they are optically active is measured by to what degree the light is rotated by the enantiomer.
To measure this two polaroids are set up with a sample of the enantiomer in the middle. One of the polaroids is placed with the slit vertical and the other polaroid is placed at a right angle to the first with the slits in a horizontal position. The enantiomer will rotate the light and the degree with which the light is rotated is measured and if the light is rotated by 90 degrees it will exit through the second polaroid.
When an enatiomer is formed two of them are form and they form a racemic mixture where there is an equal amount of each enantiomer. This forms a mixture with a 1:1 ratio so there is no rotation of the light. One enantiomer will rotate the light one way and the other enantiomer will rotate the light the other way so there is no overall rotation of the light.
Aldehydes consist of a C=C bond called a carbonyl group. The double bond consists of a pi bond and a sigma bond. however the oxygen in the aldehyde has a higher electronegativity than the carbon so the electron density is pulled towards the oxygen so the oxygen has a delta- charge and the carbon has a delta+ charge.
Aldehydes has a carbonyl carbon with a hydrogen atom attached to it. They can be produced by the oxidation of primary alcohols.
Aldehydes have a high boiling point. This is because straight chais form more intermolecular forces than branched chains because they can fit closer together.
Aldehydes are soluble in water because the O in the C=O can form hydrogen bonds with the water. However, as the chain length increases the solubility decreases.
Ketones consist of a C=C bond called a carbonyl group. The double bond consists of a pi bond and a sigma bond. however the oxygen in the aldehyde has a higher electronegativity than the carbon so the electron density is pulled towards the oxygen so the oxygen has a delta- charge and the carbon has a delta+ charge.
Ketones have two hydrocarbon groups attached to the carbon. They can be produced by the oxidation of secondary alcohols.
Ketones have a high boiling point. This is because straight chais form more intermolecular forces than branched chains because they can fit closer together.
Ketones are soluble in water because the O in the C=O can form hydrogen bonds with the water. However, as the chain length increases the solubility decreases.
Addition Reaction With Hydrogen Cyanide
Two CN- are used to provide enough charge to `dissociate the C=O bond.
The CN- can attack from above or below wich means that an equal reacemic mixture of the product is formed so the product is optically active.
The Triiodomethane Reaction
This reaction is specific to organic compounds that contain a methyl group next to a carbonyl group or can react to give a carbonyl group next to a methyl group.
Synthesis of Esters
An ester is formed when a carboxylic acid is mixed with an acid in the presence of a strong acid catalyst. This reaction is called esterification.
They are named with the alcohol first and the acid second. e.g. methyl ethanoate
Esters are volatile compounds that don't form hydrogen bonds, so are not very soluble in water. The boiling and melting temperatures are very low and they are responsible for nice smells.
Esters can be split up by either acid hyrolysis or base hydrolysis. The acid provides a catalyst for the reaction with water. With the base hydrolysis the acid that is produced is neutralised by the alkali in the solution.
When a polyester is formed it is formed from a condensation polymer with the removal of water.
Acyl Chloride Reactions
Acyl chlorides are formed when phosphorus(V) chloride reacts with a carboxylic acid. Reactions involve addition followed by an elimination.
Reactions with water
Acyl chlorides are hyrolised rapidly in water and a carboxylic acid and HCl are formed.
Reaction with alcohols
Acyl chlorides react with alcohols to produce esters without heating.
Reaction with concentrated ammonia
Acyl chlorides react with concentrated ammonia to produce amides. The reaction takes place without heating.
Reaction with amines
acyl chlorides react with amines to produce a substituted amide.