SYNTHESIS OF 2-METHYLBUTANOIC ACID

• Fit a 1 -litre three-necked flask with a mechanical stirrer, a double surface condenser and a separatory funnel and provide both the condenser and funnel with calcium chloride guard-tubes. 

• Prepare an ethereal solution of but-2-ylmagnesium chloride from 12.5g (0.51 mol) of dry magnesium turnings, 46 g (52.5 ml, 0.5 mol) of dry 2-chlorobutane (1) and 400 ml of anhydrous ether, cognate preparation. 

• When the spontaneous reaction has subsided reflux the reaction mixture for a further 1 hour. 

• Cool the flask in a mixture of ice and salt to — 12°C and add a further 100 ml of anhydrous ether. 

• Weigh out (rough balance) 125g of Cardice (2) on a piece of stiff paper: wrap the Cardice in a stout cloth, and, by means of a pestle, break it into small lumps. 

• Empty the Cardice into a dry 1500-ml beaker and at once pour in the Grignard reagent in a slow steady stream; any unreacted magnesium will adhere to the sides of the flask. 

• A vigorous reaction occurs. Stir the mass well, and allow it to stand untilall the Cardice has evaporated. 

• Then add slowly a mixture of 300 g of crushed ice and 75 ml of concentrated hydrochloric acid. 

• Stir until the gelatinous compound is decomposed and there is a clean separation into two layers. 

• Pour the mixture into a separatory funnel; rinse the beaker with 50 ml of ether and transfer this to the funnel. 

• Separate the upper layer and extract the aqueous layer with three 40 ml portions of ether. 

• Cool the combined ether extracts by the addition of ice, and add cautiously 100 ml of 25 per cent sodium hydroxide solution; run off and keep the aqueous layer and repeat the extraction with a further 50 ml of alkali solution of the same strength. 

• The organic acid is thus converted into the sodium salt and passes into the aqueous layer; test the extracts with phenolphthalein to make certain that all the acid has been removed. 

• Distil the alkaline extract until its volume is reduced by about 10 per cent; this removes ether and other volatile impurities. 

• Allow to cool, and cautiously acidify with concentrated hydrochloric acid; it is advisable to stir the mixture during the acidification process. Separate the upper layer of acid. 

• Distil the water layer from a 1-litre flask until no more oily drops pass over; saturate the distillate with salt, remove the acid layer and combine it with the main product. 

• Dry the combined acid fractions with anhydrous calcium sulphate, and distil. 

• Collect the 2-methylbutanoic acid at 173-174 °C. The yield is 40g (79%).

Notes to keep in mind:

1.  2-Chlorobutane is employed in preference to the bromide because it is cheaper and the yield of acid is slightly higher.

2. Cardice should be handled with gloves or with a dry towel; if Cardice is held for a long time in the hand it may cause frost bite. The crushed Cardice should be used immediately otherwise it may absorb water which would react with some of the Grignard reagent.