Carbonyls and Structure Determination

• Geometric isomers have the same structural formulas but a different spatial arrangement.
• Optical isomerism occurs when a molecule has a chiral centre.
• A chiral centre is a carbon with 4 different groups arranged tetrahedrally around it.
• Optical isomers are also called enantiomers.
• Optical isomers are mirror images but non-superimposable.
• Optical isomers have identical chemical and physical properties, so are very difficult to separate. They can, however, be distinguished using plane polarised light.
• Plane polarised light:

  • Light source produces light vibrating in all directions
  • Polarising filter only allows light vibrating in one direction
  • Plane polarised light passes through sample
  • If substrate is optically active it rotates the plane polarised light
  • Analysing filter is turned so that light reaches a maximum
  • Direction of rotation is measured coming towards the observer
• Plane polarised light:

  If the light appears to have turned right - dextorotatory
• Plane polarised light:

  If the light appears to have turned left - laevorotatory
• Racemic mixtures - sometimes we have optical isomers and not see rotation.
• Most laboratory syntheses tend to give a 50:50 mixture of the two enantiomers. In this case, the rotation effect cancels out. We call this 50:50 mixture, a racemate or racemic mixture.
• Aldehydes and ketones can be reduced to alcohols, using [H].

  The mechanism is nucleophilic addition.
• [H]: The reducing agent used is sodium tetraborohydride in aqueous conditions. It is used because it provides a supply of hydride ions(H-).
• Nucleophilic Addition with Cyanide (:CN-)

  All aldehydes except methanal, will form a chiral centre at the carbon from the carbonyl group. This is because the original molecule is planar, so the cyanide could attack the carbon from the top or bottom, forming 2 enantiomers. The probability of attack from either side is 50:50, so a racemic mixture is formed.
• Nucleophilic Addition with Cyanide (:CN-)

  HCN is not often used as it is a highly toxic gas. KCN is also toxic but it is either solid or solution at room temperature so the risks are less.
• Esters

  Esters are formed from a carboxylic acid (or derivative) and an alcohol. Water is lost - this is a condensation reaction.
• Hydrolysis of Esters

  Esters can be broken down by the addition of water (also involves addition of an acid or base).
• Acid hydrolysis returns the ester to the alcohol and carboxlic acid.

  The acid acts as a catalyst.
• Base hydrolysis turns the ester into the alcohol and the salt of the carboxylic acid.

  The base acts as a reagent.
• Triglycerides are also used to make biodiesel for cars. They are reacted with methanol using NaOH as a catalyst. The methyl esters produced are the biodiesel.
• Acyl Chlorides

  Both the oxygen and chlorine are very electronegative, so both pull electrons away from the carbon. so the carbon is significantly delta positive - it will undergo nucleophilic attack.
• Nucleophilic Addition-Elimination

  Acyl chlorides react with water to form a carboxylic acid.
• Nucleophilic Addition-Elimination

  Acyl chlorides react with alcohols to form an ester.
• Acid Anhydrides are formed by dehydrating carboxylic acids.
• Acid anhydrides are preferred to acyl chlorides:

  •cheaper

  •less violent reactions

  •less hazardous (as acyl chlorides produce corrosive HCl fumes)
• Acid anhydrides react with alcohols to form an ester and a carboxylic acid as a byproduct.