4.8 Further Organic Chemistry
Isomers
Structural: compounds with the same molecular formulae but different structural formulae
A racemic mixture contains equal quantities of each enantiomer
of an optically active compound (rotates plane polarised light).
Optical Activity can be used to work out a reaction mechanism. For example, nucleophillic substitution can occur in two different ways;
If a reaction is SN1 and you start with one enantiomer, the product will be a racemic mixture of two optical isomers. The electrons move in the polar bond (Cδ+ — Xδ-) move heterolytically to the Xδ- (1 stage)
If a reaction is SN2 and you start with one enantiomer, the product will be a single enantiomer which will rotate the polarised light. First the nucleophile attacks a carbon and then the electrons in the polar bond (Cδ+ — Xδ-) move heterolytically to the Xδ- (2 stages)
*Remember: from rates of reaction; SN1 means only 1 molecule will be involved in the rate determining step and SN2 means there are 2 molecules in the rate determining step
Aldehydes and Ketones
They do not hydrogen bond with themselves as they don’t have a polar Oδ-—Hδ+ bond. For this reason, aldehydes and ketones have lower boiling points than alcohols (which can hydrogen bond)
They can hydrogen bond with water due to their polar Cδ+=Oδ- bond. Oxygen uses its lone pair to form hydrogen bonds with Hδ+ atoms on the water molecules.
Note: small ketones/Aldehydes will dissolve due to the polarity mentioned above, however large ketones/Aldehydes will have very strong intermolecular forces and will not dissolve.
NUCLEOPHILIC ADDITION
Hydrogen cyanide is a weak acid – it partially dissociates in water
HCN 
H+ + CN–
CN– is a nucleophile and attacks the slightly positive carbon atom and donates its electrons to it. The electrons in the C=O bond move to the oxygen. H+ from water/hydrogen cyanide bond to the oxygen forming OH.
NOTE: HCN is a very toxic gas; acidified potassium cyanide is used to reduce the risk. Experiment must be conducted in fume cupboard.
Evidence of optical activity: carbonyl group is planar; nucleophile can attack from either side. Asymmetric (not symmetrical) ketone/aldehyde + CN– —> racemic mixture/two optical isomers.
This is what you expect if the CN can attack either side, producing two different isomers.
Tests to identify
| TEST | Info | Ketone | Aldehyde |
| Bradys reagent | 2,4-dinitropheylhydrazine
|
Orange | Orange |
| Tollen’s reagent
+ heat (water bath – not flame as flammable!) |
Colourless solution of silver nitrate dissolved in ammonia which gets reduced and changes colour;
Ag(NH3)2+(aq) + e– —> Ag(s) +2NH3 (aq) |
No change | Silver mirror
(Ag(s)) Aldehyde oxidised |
| Fehlings/Benedicts solutions | Blue solution of copper(II) ions dissolved in NaOH(aq) become Cu+ ions;
Cu2+(aq) + e– —> Cu+(aq) |
No change | Brick red precipitate (Cu+ ions)
Aldehyde oxidised |
| Iodine in alkali + heat
(tests for CH3 on carbon attached to oxygen) |
Positive test = yellow precipitate
If aldehyde positive = ethanal If ketone positive = one end is CH3 |
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*NOTE: Brady’s: you can identify carbonyl compounds by the melting point of the orange precipitate against known values
OXIDISING
Aldehyde —> Carboxylic acid √ [heat with acidified potassium dichromate (VI) ions (oxidising agent)]
colour change: ORANGE to GREEN
Ketone —> Carboxylic acid X [acidified dichromate (VI) ions are not a strong enough oxidising agent]
REDUCING: [LiAlH4 in dry ether]
Aldehyde —> Primary alcohol Ketone —> Secondary alcohol
Carboxylic Acids
They hydrogen bond with themselves as they do have a polar Oδ-—Hδ+ bond. For this reason, carboxylic acids have very high boiling points.
They can hydrogen bond with water due to their polar Cδ+=Oδ- bond. Oxygen uses its lone pair to form hydrogen bonds with Hδ+ atoms on the water molecules. Therefore, carboxylic acids are soluble, however as they get bigger they become less soluble as the intermolecular forces get too strong.
Dimer: 
when a molecule hydrogen bonds with just one other molecule, increasing the size and intermolecular forces of the molecule, meaning the boiling point is also higher.
Making a carboxylic acid:
- Primary alcohol – oxidised – aldehyde – oxidised – carboxylic acid
- Nitrile – hydrolysed (reflux with HCl then distil) – distilled product is carboxylic acid
REACTIONS OF CARBOXYLIC ACIDS:
Neutralisation; 1) CH3COOH + NaOH —> CH3COONa + H2O
ethanoic acid sodium ethanoate *NOTE: CO2 causes effervescence
2) 2CH3COOH + Na2CO3 —> 2CH3COONa + H2O + CO2
E.G. 12.5 ml of 0.1 moldm-3 of NaOH exactly neutralises 25ml of orange juice. What is the concentration of citric acid in the juice?
3NaOH + C6H8O7 —> Na3C6H5O7 + 3H2O
1) Find moles mols = conc x vol = 0.0125 x 0.1 = 0.00125mols
2) Find ratio/moles 3mol NaOH neutralised 1mol citric acid; 3:1
0.00125 ÷ 3 = 0.000417mol
3) Find concentration conc = mols ÷ vol
0.025 ÷ 0.000417 = 0.017 moldm-3
Reduction; 1) CH3COOH —LiAlH4 (in dry ether)—> 2CH3OH
2) CH3COOH + PCl5 —> CH3COCl + POCl3 + HCl
ethanoic acid ethyl chloride
Making an ester:
Carboxylic acid + alcohol (heat/reflux/acid catalyst) ester
It is a reversible reaction so in order to get the ester you must distil off the liquid at 80˚C, and then mix with sodium hydrogen carbonate solution to remove any acid. Then separate the top layer (ester) using a funnel.
USES: ethyl ethanoate is used as a solvent in chromatography as well as pineapple flavouring.
Naming; the alcohol that was added comes first i.e. ethanol + methanoic acid will produce an ester call ethyl methanoate
Acyl chlorides and Esters
REACTIONS OF ACYL CHLORIDES
WATER (produce carboxylic acid)
– Vigorous reaction with cold water
acyl chloride + H2O —> COOH + HCl
ALCOHOL (produce ester)
– Violent reaction @298K
acyl chloride + OH —> COOCH + HCl
AMMONIA (produce amide)
– Violent reaction at 298K
acyl chloride + NH3 —> CONH2 + HCl
AMINE (produce Nsub-amide)
Violent reaction at 298K
acyl chloride + CNH2 —> CONH2C + HCl
*NOTE: HCl gas is always given off (observation)
REACTIONS OF ESTERS
Acid hydrolysis – adding water so that the ester splits into an acid and an alcohol (reverse of making ester) using reflux/heat/acid catalyst.
Base Hydrolysis
Reflux an ester with DILUTE ALKALI (e.g. NaOH) producing a carboxylate ion (H3COO–) and an alcohol.
USES: making soaps; hydrolysing vegetable oils and animal fats (trimesters) and heating them with NaOH produces glycerol (tri-ol) and sodium salt (soap) that we use every day
Trans-Esterification (TE)
Hydrogenation: adding hydrogen to remove the double bonds.
Use: making low fat spread from butter, biodiesel
Problem: some trans isomers have been linked to various diseases
Solution: to hydrogenation:
trans-esterification;
Ester + Alcohol —> New ester
Forming a polyester
