• Optical isomers are mirror images of each other


  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

  4. Must be able to draw optical isomers
• Rotation of plane-polarised light

  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
• Contain a carbonyl group


  Carbonyl group is the C=O

  
  

  Aldehydes have it at the end of their chain

  
  

  Ketones have it in the middle of their chain
• 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
• They can H-bond with water though

  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
• Testing for aldehydes

  1. Tollen's reagent

  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
• Reactions of aldehydes and ketones
• Can reduce them back to alcohols

  Using a reducing agent, [H], can

  1. Reduce an aldehyde to a primary alcohol

  2. Reduce a ketone to a secondary alcohol

  
  

  Usual reducing agent is LiAlH₄ in dry ether; it is a very powerful reducing agent, but reacts violently with water
• 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
• 2,4-DNPH tests for carbonyl group


  Dissolve 2,4-DNPH in methanol and conc. sulfuric acid; the DNPH then reacts to form a bright orange precipitate if a carbonyl group is present

  
  

  This only happens with C=O groups, not with more complex ones like COOH, so only tests for aldeyhydes and ketones
• Some carbonyl compounds will react with iodine


  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
• Carboxylic acids
• Solubility of Carboxylic acids

  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
• Making carboxylic acids


  Oxidation of primary alcohols:

  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
• Reaction with bases


  Are neutralised by aqueous bases (alkalis) to form salts and water

  e.g. CH₃COOH + NaOH → CH₃COONa + H₂O

  
  

  React with carbonates or hydrogencarbonates to form salt, CO₂ and water

  e.g. CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂
• Other carboxylic acid reactions

  Reduce to a primary alcohol (skips carboxylic acid stage) with LiAlH₄ in dry ether

  
  

  Carboxylic acid + phosphorous(V) chloride to make an acyl chloride, POCl₃ and HCl
• Esters


  Functional group -COO-
• Making esters


  Can be made from alcohols and carboxylic acids:

  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
• Hydrolysis of esters


  Acid hydrolysis:

  Splits an ester into an acid and an alcohol; the reverse of the condensation reaction that joins them. Reflux the ester with a dilute acid.

  
  

  Base hydrolysis:

  This time you have to reflux the ester with a dilute alkali, e.g. NaOH, forms a carboxylate ion and an alcohol; irreversible reaction
• Polyesters


  1. Diols contain two -OH functional groups and dicarboxylic acids contain 2 -COOH functional groups

  2. The dcarboxylic acids and diols can react together to form long ester chains, called polyesters; this a condensation polymerisation reaction
• Acyl chlorides


  Have the functional group -COCl

  
  

  Easily lose their chlorine; they react with:

  1. Water - a vigorous reaction occurs with cold water, forming a carboxylic acid

  2. Alcohols - vigorous reaction at room temperature, forming esters

  3. Conc. ammonia - violent reaction at room temperature, producing an amide

  4. Amines - violent reaction at room temp producing an N-substituted amide