organic II

• Optical isomers are molecules that are non super imposeable mirror images of each other
• Optical isomers are chiral and have four different groups attached
• Optical isomers have identical chemical and physical properties
• Primary significance of optical isomers are in biochemistry as spatial arrangement and shape is often critical
• Optial isomers are difficult to seperate, but it is done via a process called resolution
• Optical isomers rotate plane polarised light in opposite directions by the same amount
• When optical isomers are made synthetically they form in equal amounts, this is known as a racemic mixture and have not effect on plane polarised light
• `Optical isomers form in equal amounts as the nucleophile affects the planar carbocation equally from both sides
• RECRYSTALLIZATION METHOD

  1. Filter crude solid under reduced pressure
  2. Transfer crude solid to concial and dissolve in minimum amount of hot solvent (creates saturated solution from which crystals will form quickly on cooling
  3. If any solid impurities present then filter hot
  4. Allow to cool
  5. Filter crystals under reduced pressure
  6. Wash out with minimum amount of distilled water
• Only one isomer of Ibuprofen has painkilling properties
• Thalidomide was originally taken by pregnant women in 50s to act against morning sickness but one isomer causes feotal abnormalities
• Use the melting point to check purity
• MELTING POINT METHOD

  1. Place 1-2 dry crystals in a small glass capillary tube
  2. Place in melting point apparatus
  3. Heat
  4. Heat more slowly as you near melting point
  5. If pure, melting point should be sharp and close to text book value
• Aldehydes and Ketones both undergo nucleophilic addition reactions, aldehydes generally more reactive
• Extra alkyl group in ketone means that the carbonyl carbon is less electron deficient due to positive inductive effective of the alkyl group therefore is less vulnerable to nucleophilic attack
• C-H bonds in aldehydes are weaker than normal as electron density is withdrawn by carbonyl group therefore aldehydes are easily oxidised to carboxylic acids
• Fehlings solution goes from a blue solution to red precipitate if aldehyde is present
• C=C is a non polar bond and undergoes electrophilic attack therefore main reaction is electrophilic addition
• C=O is a polar bond so undergoes nucleophilic attack therefore main reaction is nucleophilic addition
• KCN/HCN used as reagent (provides nucleophile) in nucleophilic addition of ketones and aldehydes
• Reduction of aldehydes and ketones done using complex metal hydrides such as NaBH4 (LiAlH4 is more powerful but harder to use), which supplies H+ ions
• Ketones are reduced to secondary alcohols, aldehydes reduced to primary alcohols, reaction results in optical isomers as nucleophile can attack equally from above/below the planar C=O group
• Most reactive acid derivative is an acid chloride (...oyl chloride), followed by acid anhydrides (...oic anhydride), carboxylic acid, ester (...yl ...oate), amides (...amide), nitrile (...nitrile), and then carboxylate salt (...oate)
• Acid chloride reaction involve the release of toxic, acidic HCl as it is eliminated from the intermediate
• Acid chlorides most reactives as Cl and O2 are highly EN leaving the carbonyl carbon very electron deficient leaving it highly suspectible to nucleophilic attack
• Overall mechanism of acid chlorides and anhydrides are nucleophilic addition-elimination
• Acid chlorides react with water to form an alcohol and HCl
• Test for acid chlorides by adding AgNO3, and instant white precipitate forms (steamy fumes) Ag+ + Cl- --> AgCl
• Acid chlorides react with alcohols to form esters and HCl (Steamy fumes)
• Acid chlorides react with ammonia to form amides and NH4Cl (white clouds)
• Acid chlorides react with amines to form substituted amides and HCl (eg. N-ethyl propanamide)
• A tertiary amine will not react as there is no replaceable hydrogen on the amine
• Advantages of acid anhydrides: cheaper, no HCl released, easier to handle, less exothermic so more controllable reactions (SAFER)
• Aspirin manufactured from salicylic acid and ethanoic anhydride
• Carboxylic acids are less reactive as carbonyl carbon withdraws electron density from the OH bond, therefore it is less electron deficient making it less susceptible to nucleophilic attack
• In carboxylic acids the OH bond is weakened and H+ is more easily lost, therefore it is able to act as an acid
• Carboxylic acids react with water to form a carboxylate ion and H3O+
• NaHCO3 most commonly used to test for carboxylic acids (effervescence)
• Carboxylic acids react with alcohols to produce esters when heated with conc H2SO4, the reaction is slower than that of chlorides and anhydrides + doesn't go to completion therefore lower yield
• Esters can be used in perfumes, food flavourings, solvents, and plasticisers