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Grignard Reaction

Lab #5: Grignard Reaction – Synthesis of Triphenylmethanol John Kang Chem 152L Performed: 7/20/04 Date submitted: ________________ Lab Partners: Sang Lee, Vicky Lai TA: John Stanko Abstract: This experiment explored the synthesis of triphenylmethanol through the use of Grignard reagents. The percent yield of the product was 10% on a relatively humid day. The melting point was calculate to be 127. 2oC with a literature value of 162oC. An IR spectrum of the product was taken and used for positive identification of the product. The mechanism of the formation of triphenylmethanol was explored.

Byproducts were also considered and their mechanisms duly noted. Introduction: This experiment explores the use of organometallic reagents in addition reactions. Organometallic compounds have a carbon-metal bond, such as a carbon-magnesium or carbon-lithium bond. Transition metals can also be used such as rhodium or iridium. Organometallic compounds have a wide range of uses, including being consumed as a reagent and acting as a catalyst. Grignard reagents are a very common organometallic compound. Grignard reagents are formed from the reaction of an alkyl, cycloalkyl, or aryl halide and magnesium metal.

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This reaction is shown below as Figure 1. [pic] Fig. 1: Formation of a Grignard reagent The carbon bonded to the metal is an excellent nucleophile and base. This carbon with carbanion character can partake in typical nucleophilic reactions such as nucleophilic substitution or carbonyl addition. The experiment performed is an example of carbonyl addition using a Grignard reagent. On critical aspect of the a reaction involving a Grignard reagent is that it must be performed under dry conditions. The carbanion is a very strong base and can abstract protons from water, allowing less carboanions to undergo the desired reaction.

In this experiment, phenylmagnesium bromide is the prepared Grignard reagent. Its synthesis is detailed below as Figure 2. [pic] Fig. 2: Synthesis of phenylmagnesium bromide The electrophile that the Grignard reagent attacks is the carbonyl group in benzophenone. The reaction is a carbonyl addition reaction and is detailed below as Figure 3. [pic] Fig. 3: Phenylmagnesium bromide carbonyl addition to benzophenone to form triphenylmethanol There are some safety issues to consider. Bromobenzene may be toxic if inhaled and should be worked with under the hood.

Diethyl ether and magnesium are very flammable and should be kept away from heat. This report will detail the percent yield and melting point of the synthesized triphenylmethanol and their implications. An IR spectrum of the product is also included and analyzed. The mechanism of triphenylmethanol formation will be discussed, along with any byproducts that were formed in the reaction. Experimental: The lab procedure was followed closely. 1 The experiments were formed at standard conditions: 25oC and 1 atmosphere. Results: The mass of triphenylmethanol synthesized was 0. 103g.

From Figure 3, the molar mass of triphenylmethanol can be calculated to be 260g/mol. The moles of product formed can be calculated as [pic]. To calculate the percent yield, the expected yield must first be calculated. The mass of benzophenone used was 0. 7g. From Figure 3, the mass of benzophenone can be calculated to be 158g/mol. Thus the expected molar yield can be calculated to be [pic]. The percent yield can be calculated as [pic]. It should be noted that if the limiting reagent is assumed to be the bromobenzene solution, the results are drastically different. 400? L of pure bromobenzene was used.

At a density of 1. 5g/mL and a molar mass of 157g/mol, the moles of bromobenzene used can be calculated to be [pic]. 2 Since bromobenzene is assumed to be limiting and all reactions occur in a 1:1 ratio, the expected yield is also [pic]. Thus the percent yield is calculated to be [pic] This result will be discarded because it defies logic. The experimental melting point was measured to be between 122. 3oC and 132. 1oC for an average of 127. 2oC. The literature melting point is from 160-163oC for an average of 162oC. An IR spectrum of the triphenylmethanol product has been attached separately as Figure 4.

Discussion: The mechanism by which trimethylphenol is formed is a carbonyl addition mechanism. The overall reaction is summarized in Figure 3. The carbonyl carbon on the benzphenon is weakly electrophilic and is attacked by carbanionic carbon of the Grignard reagent, phenylmagnesium bromide. This attack causes the carbonyl oxygen to assume a -1 charge as a sp3 bonded oxygen. The magnesium from the Grignard reagent then forms a complex with the oxygen and a mild acidic workup is added to protonate the oxygen to release it from the magnesium complex.

The percent yield of the trimethylphenol product was rather low at 10%. However, due to the humid conditions during the day of the experiment, this low yield was to be expected. Grignard reagents react quickly with water as bases, forming OH- ions. Since the Grignard reagent was phenylmagnesium bromide, abstracting a proton would have formed benzene. This benzene would have been removed in the various separation processes in the experiment and would not have been taken into account for the final yield.

One way to avoid the formation of benzene is to make absolutely sure that DRY, DRY, DRY glassware was used. 1 Another possible byproduct that would have lowered yield would be biphenyl, whose structure looks like two benzene rings connected by a single bond. This byproduct would have formed from a nucleophilic attack of phenylmagnesium bromide on benzene, which was mentioned previously as another byproduct. Since the benzene has no electron-withdrawing groups such as nitro groups, a nucleophilic attack would not be favored and would thus occur slowly.

Consequently the main contaminating byproduct was probably benzene and only a little biphenyl was formed. The melting point of the experimental product was 127. 2oC, approximately 35oC below the literature value of 162oC. This is evidence of moderate to heavy impurity contamination. It is likely that some benzene and biphenyl product was also isolated along with the triphenylmethanol product. The IR spectrum provides strong evidence supporting the formation of triphenylmethanol. Most significant is the O-H stretching absorption from 2800-3000cm-1, corresponding to the –OH group on the product.

Secondary identifiers are the two C=C stretching absorptions at 1500cm-1 and 1600cm-1, which are used to identify benzene derivatives. 3 The experiment can be considered a success considering the humid conditions during the day of the experiment that caused relatively low yields. Positive identification was made using the IR spectrum of the product. References: 1. Chem 152 lab manual. Blackboard. http://courses. duke. edu. 2. ICSC 1016 BROMOBENZENE. http://www. ilo. org/public/english/protection/safework/cis/products/icsc/dtasht/_icsc10/icsc1016. htm. Date visited: 7/27/2004. 3. Organic Chemistry, 4th Ed. Loudon.


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