1. Optical isomerism is a type of stereoisomerism; isomers that have the same structural formula but a different arrangement in space
2. A chiral carbon is a carbon atom that has 4 different groups attached to it; this means that there are two possible arrangements of the molecules around it; these are called enantiomers
3. Enantiomers are mirror images of each other; they cannot be superimposed no matter what
1. Normal light is made up of different wavelengths and vibrates in all directions. Monochromatic, plane-polarised light has a single wavelength and only vibrates in one direction
2. Optical isomers are optically active - they rotate the plane of polarisation of plane polarised monochromatic light
3. One enantiomer rotates it clockwise, the other rotates it anti-clockwise
A racemic mixture is a mixture of both enantiomers
A racemic mixture (a racemate) contains equal quantities of each enantiomer of a chiral compound
They do not rotate plane polarised light; the enantiomers cancel each other out
Chemists often act two achiral things together to get a racemate of a chiral product
Use optical activity to work out a reaction mechanism:
Sn1 mechanism:
Start off with single enantiomer reactant; the product will by a racemic mixture of the two optical isomers of the product because
Step 1: a group breaks off, leaving a planar ion
Step 2: planar ion can be attacked from either side; results in two optical isomers
Sn2 mechanism: Single enantiomer produces a single enantiomer product because there is only one step, and in this step:
The nucleophile always attacks from the opposite side to the leaving group; the product will rotate plane polarised light in the opposite direction to the reactant
Aldehydes and ketones don't H-bond with themselves
They do not have a polar O-H group, so cannot form h-bonds with other aldehyde or ketone molecules; this means that they have lower boiling points than their equivalent alcohols
Still bond together due to London forces; have permanent dipoles due to their carbonyl groups
1. They have a lone pair on the O of their C=O group
2. Can use this lone pair to form H-bonds with the hydrogen atoms on water molecules, so small aldehydes and ketones are water-soluble
3. Large aldehydes and ketones have longer carbon chains which are not able to form h-bonds with water; they can disrupt h-bonds between water molecules but cannot form h-bonds themselves
4. Therefore, if an aldehyde or ketone is long enough, the London forces between the molecules and the hydrogen bonds between the water molecules are stronger than the bonds that could form between the carbonyl and the water
A colourless solution of silver nitrate dissolved in aqueous ammonia; heat this in a test tube with the substance being tested. If an aldehyde is present then a silver mirror is formed
2. Fehling's/Benedict's solution
Fehling's is a blue solution of copper(II) ions dissolved in NaOH. If heated with an aldehyde then a brick-red precipitate is formed (copper(I) oxide). Benedict's is the same, but the copper(II) ions are dissolved in Na₂CO₃ instead
3. Acidified dichromate(VI) ions; oxidising agent, so if an aldehyde is present it will be oxidised to a carboxylic acid, reducing the dichromate(VI) to chromium(III) ions; solution turns from orange to green
Hydrogen cyanide will react with carbonyls by nucleophilic addition
HCN reacts with carbonyl compounds to produce hydroxynitriles; it is a nucleophilic addition reaction; a nucleophile attacks the molecule and adds itself
HCN is a weak acid; partially dissociates in water:
HCN ⇌ H⁺ + CN⁻
1. CN⁻ ion attacks slightly positive carbon and donates an electron pair; both electrons from the double bond transfer to the oxygen
2. H⁺ bonds to the oxygen to form a hydroxyl group
Carbonyls that contain a methyl carbonyl react with iodine when heated in the presence of an alkali; if there is a methyl carbonyl then a yellow iodoform precipitate will be formed
1. Can form hydrogen bonds with each other, giving them relatively high boiling points
2. The ability to form H-bonds makes small carboxylic acids very soluble in water
3. As the chain lengthens, the water solubility decreases due to the proportionally less soluble area
4. In pure liquid carboxylic acids, dimers can form; where one h-bonds with just one other molecule, effectively increasing the size of the molecule, increasing the intermolecular forces; raises boiling point
Can make a carboxylic acid by oxidising a primary alcohol into a an aldehyde, and then further into a carboxylic acid. Use acidified potassium dichromate
Hydrolysis of nitriles:
Reflux the nitrile with dilute HCl, then distil off the carboxylic acid
1. If you heat a carboxylic acid with an alcohol in the presence of an acid catalyst, an ester is formed (esterification reaction)
2. Example; make ethyl ethanoate by refluxing ethanol with ethanoic acid
3. Reaction is reversible, so you need to separate out the product as its formed. This is done by distillation, collecting the liquid that comes out just below 80°C
4. The product is then mixed with sodium carbonate to react with any carboxylic acid that might remain; the ester forms on top of the aqueous layer so can easily be separated with a separating funnel