3.5 - Isomerism And Carbonyl Compounds Flashcards
What are optical isomers?
Optical isomerism is a type of stereoisomerism. Stereoisomers have the same structural formula, but their atoms are arranged differently in space. A chiral (asymmetric) carbon atom is one that has four different groups attached to it. You can arrange them in two different ways called enantiomers or optical isomers. The enantiomers are mirror images and cannot be superimposed. Locate the chiral centre and draw two mirror images, including mirroring the bonds.
What happens with optical isomers and plane-polarised light?
Plane-polarised light only vibrates in one direction. Optical isomers are optically active - they rotate plane-polarised light. One enantiomer rotates it in a clockwise direction and the other anticlockwise. A racemate (or racemic mixture) contains equal quantities of each enantiomer of an optically active compound. Racemates don’t show any optical activity. Chemists often react two achiral things together and get a racemic mixture of a chiral product, because there is an equal chance of forming each of the enantiomers.
Why do reactions involving planar bonds often produce racemates?
Double bonds are planar. The products of reactions that happen at the carbonyl end of aldehydes and unsymmetrical ketones are often enantiomers present as a racemic mixture. For example, propanal with potassium cyanide. The CN^- ion attack on the delta positive carbon can come from above the plane of the molecule, or below it. Because the C=O bond is planar, there is an equal chance that the nucleophile will attack from either of these directions. So you get a racemic mixture of products. If you start with a symmetrical ketone, you’ll make a product that doesn’t have a chiral centre, so it won’t display optical isomerism.
How do you reduce aldehydes and ketones back to alcohols? Mechanism?
NaBH4 dissolved in water with methanol is usually the reducing agent used. Aldehyde + 2[H] gives primary alcohol. Also add 2[H] to ketone to give secondary alcohol. The mechanism is a H^- ion donating pair of electrons to a delta positive carbon, and an arrow from the double bond to the O delta negative. Then you have an arrow from the O^- to a H^+. Finally you have the alcohol.
How does potassium cyanide react with carbonyls?
Potassium cyanide reacts with carbonyl compounds to produce hydroxynitriles (molecules with a CN and an OH). Nucleophilic addition reaction. Potassium cyanide dissociates in water to form K^+ and CN^-. Mechanism: pair of electrons from CN^- goes to delta positive carbon, and arrow from double bond to O delta negative. Then arrow from O^- to H^+ to create a hydroxynitrile. Explanation: CN^- group attacks the partially positive carbon atom and donates a pair of electrons. Both electrons from the double bond transfer to the oxygen. H^+ ions add to the oxygen to form the hydroxyl group. Acidified KCN is usually used so there is a source of H^+ ions. Overall reaction for an aldehyde is RCHO + KCN -> RCH(OH)CN + K^+
Ketone is RCOR’ + KCN -> RCR’(OH)CN + K^+
If you start with an unsymmetrical ketone or any aldehyde except methanal you will produce a mixture of enantiomers.
Why is potassium cyanide dangerous?
It is an irritant and dangerous in inhaled or ingested. It can react with moisture to produce hydrogen cyanide, a toxic gas. Wear gloves, safety goggles, lab coat and perform the experiment in a fume cupboard.
What are features of carboxylic acids?
Most important functional group. They are weak acids and partially dissociate into a carboxylate ion and a H^+ ion. Equilibrium lies to the left because most of the molecules don’t dissociate. They react with carbonates to form a salt, carbon dioxide, and water. 2CH3COOH(aq) + Na2CO3(s) -> 2CH3COONa(aq) + H2O(l) + CO2 (g)
CH3COOH(aq) + NaHCO3(s) -> CH3COONa(aq) + H2O(l) + CO2(g)
