Grignard Synthesis of Triphenylmethanol
The aim of this experiment was to prepared Grignard reagent (phenylmagnesium bromide) from bromobenzene and magnesium, to use the reagent prepared to synthesise a tertiary alcohol (triphenylmethanol) by reacting reagent and ester (methyl benzoate), and the product formed is analyse by infrared spectroscopy (IR), melting point, thin layer chromatography (TLC), gas chromatography mass spectrum (GCMS) and finally Nuclear Magnetic Resonance spectroscopy. Introduction
Formation of long hydrocarbon chain compound as becomes very useful in organic chemistry synthesis. In 1912, Victor Grignard a French chemist was awarded Nobel Prize in chemistry for discovery of organometallic compound known as Grignard reagent (Anon., 2009). Grignard reagent consists of carbons chain and a metal bonded together, the metal is usually magnesium. Due to electronegativity between magnesium and carbon, Mg is less electronegative than carbon. Therefore it partial positively charges and the carbon is partially negatively charge. Carbon is not stable therefore it is very reactive as it more nucleophilic. Nucleophiles are compounds negatively charged in other word positive charge loving that donate their pair of electrons to form a new bond. They can also be called Lewis bases as they accept proton from electrophilic species. Grignard reagent can be simply prepared by redox reaction of magnesium metal and alkyl halide in dry diethyl ether solvent. In this experiment the alkyl halide use is bromobenzene, to form the reagent phenylmagnesium bromide.
Figure 1: reaction of Mg and bromobenzene.
Phenylmagnesium bromide was expected product of this reaction however there are some by-product such as biphenyl which is formed from the reagent prepared reacting with unreacted bromobenzene.
Figure 2: Biphenyl as by-product of the reaction
The reagent formed is very reactive capable of performing as a strong base and extremely strong nucleophile. Anhydride diethyl ether will be used as the reaction solvent for the synthesis of both the phenylmagnesium bromide and triphenylmethanol as Grignard reagent is very reactive and can easily be protonated in present of acidic proton (H+). For example if water or similar solvent is use during the reaction, the alkyl group will pick up the proton from water and the HO- will be added to the magnesium bromide as a counter ion.
Figure 3: Grignard reagent reacts with water
Grignard reaction result in production of secondary or tertiary alcohol when reacted with carbonyl compound. Aldehyde reaction with Grignard reagent results in formation of secondary alcohol while ketone and other carboxylic acid derivatives results into formation of tertiary alcohol. Carbonyl (C=O) compounds are very reactive due to electronegativity between oxygen and carbon. Oxygen is more electronegative than carbon, there for it pull most of the electron from the carbon making it partial positive. The carbon act as electrophile, in other words Lewis acid which accepts lone pair of electrons from nucleophile. The desired product of this experiment is triphenylmethanol, it can be synthesise easily by reaction of one equivalent of phenylmagnesium bromide with Benzophenone which involve only nucleophilic addition, but in this experiment methyl benzoate will be use instead of Benzophenone. Both nucleophilic addition and substitution is involves in this reaction. Two equivalent of phenylmagnesium bromide is required, one for substitution and one for addition. Nucleophilic substitution is process by which nucleophile is added to added to a compound containing a good leaving group. The nucleophile attack the carbonyl and break the C=O. The carbonyl group reform and one group will be lost from the structure. For this process to proceed, the product form must have lower energy require for it stability than the starting material otherwise the reaction will reverse back to starting material. The...
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