28 Organic Synthesis

Question 1

Nitriles from haloalkanes

Answer 1

Length of carbon chain increases.
React with sodium cyanide, NaCN, or potassium cyanide KCN, in ethanol.
Nucleophilic substitution
SEE CARD 1

Question 2

Nitriles from aldehydes and ketones

Answer 2

Ald/ket react with HCN, hydrogen cyanide.
Nucleophilic addition reaction.
Increases number of C atoms.
HCN far too poisonous to use and increased reaction rate obtained in presence of cyanide ions.
Mix of sodium cyanide and sulphuric acid used, improving safety and increasing reaction rate.
See card 2

Question 3

Reduction of nitriles

Answer 3

Can be reduced to amines.
React with hydrogen with Ni, nickel catalyst
See card 3

Question 4

Hydrolysis of nitriles

Answer 4

Form carboxylic acids.
Heat with water and DILUTE aqueous acid e.g. HCL
See card 4

Question 5

Alkylation of benzene

Answer 5

Transfer alkyl group from haloalkane to benzene ring.
Takes place in presence of Friedel-Crafts catalyst e.g. aluminium chloride e.g. AlCL3
See card 5

Question 6

Acylation of benzene

Answer 6

React with acyl chloride to form ketone.
In presence of aluminium chloride catalyst.
Useful in synthesis because organic product undergoes typical reactions of ketone
See card 6

Question 7

Preparation of aspirin

Answer 7

Reflux salicylic acid, ethanoic anhydride and ethanoic acid using quickfit apparatus. Quickfit apparatus can be used to reflux methyl benzoate with aq sodium hydroxide, followed by acidification.

Question 8

Filtration under reduced pressure

Answer 8

Used to separate solid product from solvent or liquid reaction mix.
Need:
Büchner flask, Büchner funnel, pressure tubing, filter paper access to filter or vacuum pump.
1. Connect one end of pressure tubing to vacuum outlet or to filter pump whilst attaching other end of rubber tubing to Büchner flask.
2. Fit with Büchner funnel to Büchner flask ensuring there is a good tight fit. This is usually obtained using a Büchner ring or a rubber ring.
3. Switch on vacuum pump, or the tap to which your filter pump is attached
4. Check for good suction by placing your hand across the top of the funnel
5. Place a piece of filter paper inside Büchner funnel and wet this with same solvent used in preparing solid. Should see the paper being sucked down against holes in funnel.
6. To filter sample slowly pour reaction mixture from a beaker into centre of filter paper.
7. Rinse out the beaker with the solvent so that all of solid crystals collect in Büchner funnel.
8. Rinse crystals in Büchner funnel with more solvent and leave them under suction for a few mins so crystal starts to dry
See card 7

Question 9

Recrystallisation

Answer 9

Solid product obtained after filtration will contain impurities which can be removed by carrying out recrystallisation. Purification by recrystallisation depends on desired product and impurities having different solubilities in chosen solvent.

1. Pour a quantity of chosen solvent into conical flask. If solvent is flammable warm solvent over a water bath. If solvent is water place conical flask on tripod and gauze over Bunsen and warm solvent.
2. Tip impure sample into second conical flask or beaker.
3. Slowly add solvent to impure sample until it dissolves in solvent, should add minimum vol of solvent needed to dissolve solid.
4. Once solid has dissolved allow solution to cool. Crystals of desired product should form in conical false or beaker. When no more crystals form, filter crystals under reduced pressure to obtain dry crystalline solid.

Question 10

Melting point determination

Answer 10

Test purity of compound.
Pure substance usually has very sharp melting range of one or two degrees. Melting range is difference between temperature at which sample starts to melt and temp at which melting is complete. If compound contains impurities, solid melts over range over temperatures. Impure substance has lower melting pt. than pure sample.
1. Ensure sample completely dry and free flowing
2. Take glass capillary tube or melting pt tube. Hold one end of capillary tube in hot flame of Bunsen burner. Rotate tube in flame until end of tube is sealed.
3. Capillary tube is allowed to cool and is then filled with crystals to Baku 3 mm depth. Usually carried out by pushing the open end of capillary into solid sample to force some of the solid into the tube.
4. Once u prepared you sample you will need to take it’s melting pt. (either using electrically heated melting point apparatus or using an oil bath or Thiele tube method)

Question 11

Using electrically heated melting point apparatus

Answer 11

1. Place capillary tube containing sample into sample hole and 0-300 thermometer in thermometer hole of melting point apparatus.
2. Using rapid heating setting, heat up sample whilst observing sample thru magnifying window.
3. Once solid is seen to melt, record melting pt. Allow melting pt apparatus to cool.
4. Prepare a second sample in a new capillary tube and place in new capillary tube and place in melting point apparatus and again heat up sample.
5. As melting point is approached, set to low and raise temp slowly whilst observing the sample. An accurate determination of melting pt can be obtained.

Question 12

Using an oil bath or Thiele tube method

Answer 12

1. Set up Thiele tube or oil bath
2. Attach capillary tube containing sample to thermometer using rubber band.
3. Insert thermometer thru hole in cork is using a Thiele tube or clamp thermom if using an oil bath. The end of the thermometer and end of the capillary tube should dip into oil.
4. Using micro burner, slowly heat side arm of Thiele tube or oil bath whilst observing the solid. When solid starts to melt remove heat and record temp at which all solid has melted. Important to heat oil slowly when approaching m.p. and repeat determination of m.p. to ensure accuracy

Question 13

How to get from starting molecule to target molecule

Answer 13

1. Identify functional groups in starting and target molecules
2. Identify sequence of chemical reactions that could convert functional groups in starting molecule