Reactions of Grignard Reagents with Carbonyls|
This experiment explores the reactivity pattern for the addition of Grignard reagents to three different carbonyl groups: a ketone, an ester, and a carbonate. Grignard reagents are organometallic compounds that have a carbon-metal bond, such as carbon-magnesium. Grignard reagents are formed from the reaction of an alkyl, cycloalkyl, or aryl halide and magnesium metal in dry ether. The reaction is shown below in figure 1.
Figure 1: Formation of a Grignard reagent
The Carbon bonded to the metal is both a strong nucleophile and base. The Carbon with the carbanion character can participate in nucleophile reactions such as nucleophile substitution or carbonyl addition. The experiment completed is an example of a carbonyl addition using a Grignard reagent.
An imperative aspect of the Grignard reagent is that is must be performed under dry or aprotic conditions. The carbanion is an extremely strong base and can abstract protons from water; this will allow less carbanions to undergo the reaction.
The mechanism for the reaction in the experiment is shown below:
Firgure 2: Reaction mechanism of Diethyl Carbonate with Phenylmagnesium Bromide to form Triphenylmethanol
* Benzophenone white powder
* Dry Ether Clear Liquid
* Grignard (Phenylmagnesium Bromide) apple juice color
* Addition of Benzophenone and dry Ether to Grignard color change: red to brown to orange * Placing test tube in ice bath and adding HCl solid started to form, used stirring rod to mix solution; color change: creamy yellow color * Addition of dry ether to test tube had two layers, top layer was clear but still a bit cloudy, the bottom layer was clear * Addition of anhydrous Sodium Sulfate anhydrous Sodium Sulfate was small, white rocks, looked like salt on a pretzel; clumped up when poured into test tube, kept adding until did not clump up anymore * Decanted dry ether layer to test tube and placed in hot water bath bubbled around the rim and boiling chip * Test tube in ice bath and addition of 2 mL Petroleum Ether precipitate started to form * Addition of hot 2 mL 2-Propanol to test tube and placing in hot bath color of solution was a deep orange-yellow color * Table 2: GC Settings
Appearance of final productwhite, flakey crystals
Linear Velocity:23.0 cm/s Split:400
Table 1 Amount used in Experiment
Benzophenone: 0.364 grams
Dry Ether: 3.0 mL (total)
HCl:5.0 mL (total)
Table 3: GC 1 Standards
GC 1 Standards
Peak #Ret. TimeCorrected Ret. Time
Petroleum Ether:3 mL
Table 4: GC 2 Standards
GC 2 Standards
Peak #Ret. Time Corrected Ret. Time
Bromide: 3.0 mL
The mechanism that formed Trimethylphenol is the carbonyl addition mechanism, and that mechanism is seen in figure 2. The carbonyl carbon on the diethyl carbonate is weakly electrophilic and is attacked by the carbanionic carbon of the Grignard reagent, phenylmagnesium bromide. The attack causes the carbonyl oxygen to have a -1 charge as a sp3 bonded oxygen. The lone pair on the oxygen then formed a double bond between oxygen and carbon, making the once sp3 oxygen now sp2. The single bonded oxygen breaks away from the compound leaving methylbenzoate. The carbonyl carbon on the methylbenzoate is still weakly electrophilic and is once again attacked by the carbanionic carbon of the Grignard reagent. The reaction is the same however this time leaving a...