Friday 27 May 2011

Chemistry : Organic synthesis

Organic synthesis

I just want to say that the procedures for different organic reactions are different, it needs your own practice. Also, method (2,3) are out syb. in HKDSE.
In organic reactions the percentage yield is not 100% every step (about 70%), so we should use the least steps to produce one product.
Some techniques:
1)       Switching position of a functional group
RCH2CH2X → RCH2CH2OH (NaOH, reflux) → RCH=CH2 (dehydration)→ RCHXCH3 (HX)
Similar techniques can be applied on alcohol.
RCH2CH2OH → RCH=CH2 (dehydration) → RCHXCH3 (HX) → RCH(OH)CH3 (NaOH, reflux)
2)       Inserting a carbon atom in benzene
Apply alkylation on aromatic compounds, but rearrangement between carbon atoms occurs for longer chain, so we can use acylation (acylium ion won't rearrangement since it's stabilized by resonance)

3)       Inserting a carbon atom into alkane by HCN
Since HCN is extremely toxic and volatile, it's prepared during the experiments by 2KCN + H2SO4 → 2HCN + K2SO4.
From the hydroxyalkanenitriles we can also produce amine by reducing the cyanide group into CH2NH2 (by LiAlH4)

General experimental procedure in organic synthesis
1)       Planning the reaction steps
2)       Produce the desired product (carrying out the reaction)
3)       Separation, purification and drying of product
4)       Calculate the percentage yield (actual/theoretical product * 100%)
We can separate the unreacted reagents, by-products and catalysts by distillation if they have a large difference in volatility or boiling point. For example, if the unwanted substance has high boiling point (like carboxylic acid), we can use water bath to heat the solution and distillate the products. Besides water bath, oil bath or Bunsen flames can also be applied for higher temperature.
Fractional distillation
We only collect the distillate at a certain range about the b.p. of the products. This separate most of the unwanted substances. Glass beads are applied to give higher surface area for condensation and vaporization of the products.
Liquid-liquid extraction
By the like-dissolve-like principle, we can put the solution in a separating funnel. We can hold the stopper and tap firmly when shaking the separating funnel (open the cap regular to release pressure), allow the two layers to separate and run off the lower layer. Then organic and aqueous layers are separated.
1)       Organic solvent (dry ether) can be added. Upon shaking the organic product preferentially dissolves in dry ether, then dry ether can be removed by distillation (b.p. of ethoxyethane is 35ºC)
2)       Aqueous solvent can be added to dissolve unreacted or by-products. For example, unreacted alcohol dissolves in HCl, then only the reacted product are left in the organic layer.
We use a solvent that has no reaction with the product, dissolve the product in minimum amount of hot, conc. solvent, filter the impurities, then allow it to cool down slowly while impurities may stay in the solvent. Collect the crystals by filtrations.
Take BuOH + HBr → BuBr + H2O under reflux as example (Bu is the butyl group)
1)       We prepare HBr by conc. H2SO4 and NaBr.
2)       Distillate the crude product (oily drops) from the distillate.
3)       Allow aqueous and organic layer to separate, remove the aqueous later by teat pipette (crude product might be slight denser than aqueous product).
4)       Put the distillate into separating funnel and add HCl to remove unreacted BuOH. Treat the organic layer with NaHCO3 to remove unwanted acids and separate them by separating funnel again.
5)       Use anhydrous Na2SO4 to dry the solution until the liquid is clear.
6)       Filter the solution. Collect BuBr by fractional distillation.
Note that the percentage yield is usually quite low because:
1)       Uncompleted reactions
2)       Side reactions, especially acid reaction vs redox reaction
3)       Product is lost during purification

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