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. H^- ions come from the reducing agent, the H^+ ions usually come from water.
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 if 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)
How are esters formed?
Heating a carboxylic acid with an alcohol in the presence of a strong acid catalyst. Esterification. Concentrated sulfuric acid is usually used as the acid catalyst. Bit after O is first part, second part is O double bond bit.
How do you name esters? What are they used for?
After the O you can count the carbons and it will end it yl. Carboxylic acid ends with oate. Esters have a sweet smell, varying from gluey sweet for smaller esters to a fruity pear drop smell for larger ones. This makes them useful in perfumes. The food industry uses esters to flavour things like drinks and sweets. Esters are polar liquids so lots of polar organic compounds will dissolve in them. They’ve also got quite low boiling points, so they evaporate easily from mixtures. This makes them good solvents in glues and printing inks. Esters are used as plasticisers - they’re added to plastics during polymerisation to make the plastic more flexible. Over time, the plasticiser molecules escape though, and the plastic becomes brittle and stiff.
What happens to esters?
An acid or an alkali is used to speed up hydrolysis. There is acid hydrolysis and base hydrolysis. With both times you get an alcohol, but the second product differs.
Acid hydrolysis:
Splits the ester into an acid and alcohol. You have to reflux the ester with a dilute acid, such as hydrochloric acid or sulfuric.
Ethyl ethanoate + H2O <=> ethanoic acid + ethanol
Add lots of ester as it’s reversible so you need to push the equilibrium to the right.
Base hydrolysis:
Involves refluxing the ester with a dilute alkali, such as sodium hydroxide. You get a carboxylate ion and an alcohol.
Reflux:
Ethyl ethanoate + OH- -> ethanoate- + ethanol
What are fats and oils?
Fatty acids are long chain carboxylic acids. They combine with glycerol (propane-1,2,3-triol) to make esters. These esters of glycerol are fats and oils. The fatty acids can be saturated or unsaturated. Most of a fat or oil is made from fatty acid chains. Animal fats have mainly saturated hydrocarbon chains - they fit neatly together, increasing the van der waals forces between them. This means you need higher temperatures to melt them, so they’re solid at room temperature. Vegetable oils have unsaturated hydrocarbon chains - the double bonds mean the chains are bent and don’t pack together, so they’re easier to melt and are liquid at room temperature.
How do you make glycerol, soap and fatty acids?
Like any ester, you can hydrolyse vegetable oils and animal fats by heating them with sodium hydroxide. The sodium salt produced is a soap, and glycerol is also produced.
What is biodiesel?
Vegetable oils must be converted into biodiesel to make it a useful vehicle fuel. This involves reacting them with methanol, using potassium hydroxide as a catalyst. You get a mixture of methyl esters of fatty acids - this is biodiesel.
What are acyl chlorides?
Acyl (or acid) chlorides have the functional group COCl. All their names end in oyl chloride. It’s like a COOH but instead of the OH you have a cl.
Ethanoyl chloride
What do acyl chlorides react with?
Cold water to produce a carboxylic acid and HCl. Alcohols to produce an ester. Ammonia to produce an amide. Primary amines to produce an N-substituted amide. Each time misty fumes of hydrogen chloride are given off.
