Isomerism and Carbonyl compunds

Question 1

Optical isomerism

Answer 1

• a chiral carbon is a carbon atom with 4 different functional groups around it
• chiral carbons can create two enantiomers that are mirror images of each other but cannot be superimposed
• a racemic mixture contains equal amounts of both enantiomers
• optically active isomers rotate plane polarised light, each enantiomer in a different direction
• racemic mixtures cancel out this effect

Question 2

Aldehydes and ketones tests

Answer 2

• Tollen’s reagent forms a silver mirrored precipitate in presence of aldehydes not ketones
• Ag(NH3)2 + e- -> Ag + 2NH3 (is reduced by e- from oxidation of aldehyde)
• Fehling’s solution is reduced from blue to brick red by aldehydes
• Cu2+ + e- -> Cu+
• as ketones cannot be oxidised, it doesn’t have the same effects

Question 3

Reduction of aldehydes/ketones

Answer 3

• done by nucleophillic addition, in aqueous solution
• [H] = NaBH4
• H- acts as nucelophile and attacks the Carbon in the C=O bond, while electrons from that bond attack the Oxygen
• Now the oxygen has a lone pair, these electrons attack a H+ to form an -OH

Question 4

Hydroxynitriles

Answer 4

• contain both a -CN and an -OH, formed by nucelophillic addition of aldehyde or ketone where nucleophile is -CN
• H+ comes from the acid reagent
• as the -CN can attack from either side of the C=O bond, it forms two enantiomers when it reacts with aldehydes or asymmetrical ketones
• reaction should be done in a fume cupboard as KCN is toxic

Question 5

Carboxylic acids and Esters

Answer 5

• as carboxylic acids don’t fully dissociate, they form reversible reactions of carboxylic acid carboxylate ion + H+
• adding a carbonate molecule (e.g CaCO3) forms a carboxylate salt, water and CO2
• esterification is carboxylic acid + alcohol ester + water
• requires conc. H2SO4 to provide H+, and reflux conditions

Question 6

Esters

Answer 6

• used in flavourings, perfumes, solvents and plasticisers
• formed by carboxylic acid reacting with alcohol
• Hydrolysis is the decomposition of esters into their components with either water and acid or a strong base
• Acid hydrolysis is reversible and uses acid as a catalyst while the ester reacts with water in reflux conditions
• Base hydrolysis is non-reversible and is just a strong base reacting with an ester in reflux

Question 7

Fats/Oils

Answer 7

• fatty acids = long chain carboxylic acids (saturated = no double bonds, unsaturated = double bonds)
• fats = unsaturated (no double bonds means chains fit better together and have stronger IMF), oils = saturated (chains are further apart due to structure)
• fat or oil molecule = ester of glycerol with 3 carboxylic acids bonded to it
• hydrolysis of fats/oils requires heating with 3NaOH, produces glycerol molecule and three sodium carboxylate ions (soap)
• add acid to the soap to make a fatty acid and Na+

Question 8

Biodiesel

Answer 8

• react fat/oil with methanol and KOH catalyst to make a mixture of long chain carboxylic acid and methyl esters
• mixture of methyl esters is biodiesel, which isn’t completely carbon neutral as energy is needed in its production that comes from fossil fuels

Question 9

Acyl Chlorides

Answer 9

• has -COCl functional group, undergoes nucleophillic addition/elimination with water, alcohol, ammonia and primary amines
• Acyl chloride + water -> Carboxylic acid + HCl
• Acyl Chloride + methanol -> Ester + HCl
• Acyl Chloride + ammonia -> Amide + HCl
• Acyl Chloride + amide -> N-substituted Amine + HCl

Question 10

Nucleophillic addition/elimination

Answer 10

• Nucleophile attacks carbon in C=O bond, bond opens up and donates a lone pair to the Oxygen
• Cl takes electrons from its bond and escapes, double bond seals back up and nucleophile substitutes for Cl

Question 11

Acid Anhydrides

Answer 11

• 2 identical carboxylic acids react to form acid anhydride and H2O, as one -OH loses its H+ and the other completely escapes to form the link and the water molecule
• Acid Anhydride + water -> carboxylic acid *2
• acid anhydride + methanol -> ester + carboxylic acid
• acid anhydride + ammonia -> amide + carboxylic acid

Question 12

Aspririn

Answer 12

• salicylic acid + ethanoic anhydride -> aspirin

- or use ethanoyl chloride, however ethanoic anhydride is cheaper and safer as it doesn’t produce HCl which is toxic

Question 13

Separation method of purification

Answer 13

• if organic product is insoluble in water, mixing it with water should form an organic layer and an aqueous layer containing impurities dissolved in the water, so the layer can be released from a tap leaving only the product
• alternatively use a solvent in which the product is more soluble than what it is currently in and shake to mix them then separate the layers
• the organic product should be washed and then dried, using an anhydrous salt to absorb the water
• filter out the solid drying agent after drying

Question 14

Distillation method of purification

Answer 14

• used heating apparatus then a condensing tube
• separates liquids with different boiling points, the liquids must have different enough boiling points that one can be gas while the other stays liquid
• once the desired product is evaporated, it is condensed back into pure liquid product

Question 15

Recrystalisation method of purification

Answer 15

• find a solvent that the solid product is soluble in only when it is hot, but that impurities are always soluble in
• heat the solvent and dissolve the product in, then let it cool until the product crystallises when its solubility falls
• cold filter the crystals out of the mixture, then wash them with ice cold solvent
• test purity by finding the range of melting points of the crystals and comparing it to the known range of the pure product. wider range = less pure