Grignard Reaction

Synthesis of 2-methyl-2-heaxanol Abstract: The preparation of 2-methyl-2-hexanol (40%) was carried out via the Grignard reaction between butyl magnesium bromide and acetone in diethyl ether solvent media. Product was isolated from reaction mixture two phase partitioning and fractional distillation, respectively.

Introduction: The Grignard reactions dates as far back as 1900,where it was discovered by a French chemist Victor Grignard2.The reaction initially entails making the Grignard reagent by reacting an alkyl halide(1) with magnesium(2) to yield a organometallic compound(4) with a nucleophilic carbon. Reaction is carried out under anhydrous conditions and in a ether solvent media which serves to stabilise the Grignard reagent2, (scheme 1). The organometallic compound is then reacted with a carbonyl compound (5) of choice to produce a alcohol(6),scheme 2.

The importance of the Grignard reaction lies in that it allows one to easily synthesise primary(7),secondary(8)and tertiary (9) alcohols from formyl aldehyde(10) ,aldehydes(11) and ketones(12) in good yields(85-95%)2,(scheme 3) 1

It virtually impossible to prepare Grignard reagents with compounds containing a acidic proton, (i.e. compounds containing hydroxyl, amino, thiol and carboxyl groups) 2, as the Grignard reagent is formed it then simply attacks the remaining acidic reagents. Grignard reagents are strong necrophiles and Lewis bases so easily loose integrity in the presence of water (16) to form an alkane (17), (scheme 4).

Grignard reagents have a wide variety of industrial applications which include synthesis of drugs such as Naproxen® which is a antiinflammatory drug used for treatment of Gout, pain and fever3. , The aim of this experiment was to synthesise 2-methyl-2-hexanol (18) from reacting butyl magnesium bromide (19) with acetone (18) under anhydrous conditions in a diethyl ether solvent. The butyl magnesium bromide was prepared from magnesium (2) and 2-bromobutane (21),scheme 5.

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Preparation of 2-methl-2-hexanol (19) is a two step process,(scheme 6).First step entails preparing the Grignard reagent, by reacting magnesium with 2-bromobutane leading to a polarity inversion,(UPOLONG), and hence the production of a nucleophilic terminus carbon. Once the Grignard has been prepare it is then reacted with its molar equivalent of acetone (20) to yield 2methyl-hexanol (19). Results: Preparation of butyl magnesium bromide

Reagents Used: 2-Bromo butane Volume (ml) : 10, 8 Density (g/ml) : 1,263 Molar mass (g/mol) :137, 02 Amount (mmol) : 100 *Boiling temperature (oC) :101, 4

Magnesium Mass (g) : 2,431 Molar mass (g/mol):24, 31 Amount (mmol) :100

*correct at 760 torr 3

Scheme 7 shows that 2-bromobutane reacts with magnesium in a 1:1 stoichiometric ratio, and hence 2-bromobutane is the limiting reagent in this reaction. The maximum amount of butyl magnesium bromide that can be made is 100mmol. % Yield was assumed to be 100% as all the magnesium was used up in the reaction. Grignard reaction between butyl magnesium bromide and acetone

Butyl magnesium bromide

Acetone

Molar mass (g/mol): 161, 33 Amount (mmol) : 100 Mass (g) : 16, 1

Molar mass(g/mol): 58,08 Volume (ml) : 7,4 Density (g/mol) : 0,79 Amount (mmol) : 100

Butyl magnesium bromide and acetone reacted in 1:1 stoichiometric ratio and so theoretical yield of 2-methyl-2-hexanol is 100mmol. Yield = Actual amount of product synthesised / Theoretical amount expected x 100 = 40mmol/100mmol = 40%

Discussion: The preparation of 2-methyl-2-hexanol is a two step process using reagents 2-bromobutane, 4

magensium and acetone. The initial step involved preparation of a nucleophilic butyl magnesium bromide (19) from 2-bromobutane (21) and magnesium(2),scheme 9.The product (2-methyl-2hexanol) was obtained in a 40% yield and was found to be 145-155 C at a pressure of 57 torr, this compares favourably with the literature1 value of 135-143 C.

Acetone (20) was added to the newly formed butyl magnesium bromide (19), the electrophonic magnesium chelates with the electronegative oxygen whilst simultaneously the nucleophillic carbon bonded to the magnesium attack the slightly positive acetone carbonyl carbon.This attack leads to the formation of a unstable enol inermediate which is stabilised by the now positive MgBr (21). Once the reaction has reached completion the reaction mixture is treated with (25% m/v) solution of ammonium chloride (22). This serves two purposes,firstly it disturbs the enol-MgBr complex and then it protonates the enol converying it into 2-methyl-2-hexanol (19). Isolation of 2-methyl-2-hexanol Once the product had been produced it was isolated from the reaction mixture by two phase partionng and fractional distallation,respectively. Two phase partioning was carried out using a seperatory funnel and extracting the organic componets of the mixture by washing with the reaction solvent,(diethyl ether).After this the inorganic components of the ether mixture were removed by washing the mixture with distilled water,10% (m/v) sodium carbonate solution and a saturated brine 5

solution. Any remaining water in the resulting oraganic mixture was then removed by addition of anhydrous magnesium sulfate. Rotary vaporation removed excess diethyl ether solvent and other organic impurities.Fractional distallation was then done at a pressure of 57 torres to isolate the 2-methyl-2-hexanol which boiled off at a temeprature of 64-68 C. The isolated product was at a very yield of 40%,which is not like the expected yield,this may be as a result of the power failure that occurred whilst the experiment was in progress and so had to be aborted due to lack of ventilation.This could have somehow affected the highly sensetive Grignard reagent. References: 1. L.M. Harwood and C.J Moody Instructors Manual to Accompany Experimental Organic Chemistry, Principals and Practices. Blackwell scientific Publications.,1989.,447-449. 2. T.W. Solomons and C.B. Fryhle ,Organic Chemistry 8th Edition, John Wiley and Sons,Danvers,2004,736-740 3. http://en.wikipedia.org/wiki/Ethylene _glycol 4. http://en.wikipedia.org

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