Grignard Reagent.

This was then left undisturbed for the reaction to commence.  After the ether had boiled, the reaction began to subside.  At this point the flask is heated using a heating mantle for fifteen minutes to reflux, therefor ensuring completion of the reaction.

After the fifteen-minute reflux, the silica gel drying tube was removed and 1g of benzophenone was added gradually down the condenser.  The silica drying tube was then replaced and the reaction allowed to reflux for a further fifteen minutes.

After this the contents were allowed to cool and transferred to a 250cm3 conical flask, which also contained 25cm3 of cold dilute HCl.  At this point more boiling occurred, this was more ether evaporating.  The resulting crystals were then filtered using a Buchner funnel and flask.  The crystals were then dissolved in pet ether, Boiling point 80 - 100°C and recrystallised to give a higher yield.

Results:

Calculations:

1. Measure the yield of product, calculate % yield!

182g of Benzophenone (Mol Wt) should give 260g of (C6H5)3 COH (Mol Wt),

∴ 1g of C6H5COC6H5 should give (260 / 182) = 1.43g of (C6H5)3 COH,

Actual yield (result obtained) = 0.27g,

∴ % yield = (Actual / Theoretical) x 100 ⇒ (0.27 / 1.43) x 100 = 18.88%

        2.  Find the melting point of your sample and the literature value!

Melting point of sample = 154°C

Literature melting point of tertiary alcohol =  162 ºC

3. Write the mechanism for the reaction!

C6H5 – MgBr        +    (C6H5)2 C = O                      (C6H5)3 – C – O –   +   MgBr+

(C6H5)3 – C – O – MgBr       HOH        (C6H5)3 – C – OH

                         

4. From the following wavenumber (cm –1), identify the following type of vibration!

Wavenumber (cm –1)                   Type of vibration

700. Bending Vibration

765. Bending Vibration

1015. Stretching Vibration

1150. Stretching Vibration

3450. Stretching Vibration

5. Moisture is rigorously excluded during the preparation of a Grignard reagent because;

The carbon atom attached to the metal has a partial negative charge, making it susceptible to attack by electrophilic reagents such as water.

6. (C6H5)3COH may also be obtained from C6H5MgBr by reacting with;

                                               Oet                                                      O -

2(C6H5 – MgBr)  +  C6H5.C = O                C6H5 – MgBr  +  (C6H5)2 .C = O

                                        O -

C6H5 – MgBr  +  (C6H5)2 .C = O  +  MgBr +                (C6H5)3.C – O – MgBr

(C6H5)3.C – O – MgBr     HOH        (C6H5)3 – C – OH

Conclusion:

        1g of benzophenone should have given 1.43g of a tertiary alcohol.  However from my experiment and calculations, 0.27g of the tertiary alcohol was obtained from 1g of benzophenone giving an 18.88% yield.  

        Although refluxed twice and recrystallised, this low yield could have occurred for numerous reasons including:

                ∙  Out of date chemicals,

                ∙  Buchner filtration, resulting in a loss of product: e.g: Within filter paper.

                ∙  Inaccurate weight and measurements of reactants