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Grignard Reagent Lab Report | Carboxylic Acid | …

A carboxylic acid is produced with one more carbon than the original Grignard reagent.

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Reactions and applications of Grignard reagents include asymmetric syntheses using Grignard reagents, Grignard reactions with inorganic chlorides, Grignard reagents as bases, metal-assisted modified Grignard reactions, intramolecular Grignard reactions, Grignards as methacrylate polymerization catalysts, and Grignard reagents as supports for the Ziegler-Natta process.

To prepare a carboxylic acid, the Grignard reagent is carboxylated in a reaction with dry ice.

After a reaction is completed, the solution often times does not only contain the desired product, but also undesired byproducts of the reaction, unreacted starting material(s) and the catalyst (if it was used). These compounds have to be removed in the process of isolating the pure product. A standard method used for this task is an extraction or often also referred to as . Strictly speaking, the two operations are targeting different parts in the mixture: while the extraction removes the target compound from an impure matrix, the washing removes impurities from the target compound i.e., water by extraction with saturated sodium chloride solution. Washing is also used as a step in the recrystallization procedure to remove the impurity containing mother liquor adhering to the crystal surface.

Many liquid-liquid extractions are based on acid-base chemistry. The liquids involved have to be immiscible in order to form two layers upon contact. Since most of the extractions are performed using aqueous solutions (i.e., 5 % NaOH, 5 % HCl), the miscibility of the solvent with water is a crucial point as well as the compatibility of the reagent with the compounds and the solvent of the solution to be extracted. Solvents like dichloromethane (=methylene chloride in older literature), chloroform, diethyl ether, or ethyl ester will form two layers in contact with aqueous solutions if they are used in sufficient quantities. Ethanol, methanol, tetrahydrofuran (THF) and acetone are usually not suitable for extraction because they are completely miscible with most aqueous solutions. However, in some cases it is possible to accomplish a phase separation by the addition of large amounts of a salt (“salting out”). Commonly used solvents like ethyl acetate (8.1 %), diethyl ether (6.9 %), dichloromethane (1.3 %) and chloroform (0.8 %) dissolved up to 10 % in water. Water also dissolves in organic solvents: ethyl acetate (3 %), diethyl ether (1.4 %), dichloromethane (0.25 %) and chloroform (0.056 %). Oxygen containing solvents are usually more soluble in water (and vice versa) because of their ability to act as hydrogen bond donor and hydrogen bond acceptor. The higher water solubility lowers the solubility of weakly polar or non-polar compounds in these solvents i.e., wet Jacobsen ligand in ethyl acetate. Other solvents such as alcohols increase the solubility of water in organic layers significantly because they are miscible with both phases and act as a mediator. This often leads to the formation of emulsions.

The most important point to keep in mind throughout the entire extraction process is which layer contains the product. For an organic compound, it is relatively safe to assume that it will dissolve better in the organic layer than in most aqueous solutions unless it has been converted to an ionic specie, which makes it more water-soluble. If a carboxylic acid (i.e., benzoic acid) was deprotonated using a base or an amine (i.e., lidocaine) was protonated using an acid, it would become more water-soluble because the resulting specie carries a charge. Chlorinated solvents (i.e., dichloromethane, chloroform) exhibit a higher density than water, while ethers, hydrocarbons and many esters possess a lower density than water (see solvent table), thus form the top layer . One rule that should always be followed when performing a work-up process:

Never dispose of any layer away until you are absolutely sure (=100 %) that you will never need it again. The only time that you can really be sure about it is if you isolated the final product in a reasonable yield, and it has been identified as the correct compound by melting point, infrared spectrum, etc. Keep in mind that it is always easier to recover the product from a different layer in a beaker than from the waste container or the sink. In this context it would be wise to label all layers properly in order to be able to identify them correctly later if necessary.

In order to separate compounds from each other, they are often chemically modified to make them more ionic i.e., convert a carboxylic acid into a carboxylate by adding a base. Standard solutions that are used for extraction are: 5 % hydrochloric acid, 5 % sodium hydroxide solution, saturated sodium bicarbonate solution (~6 %) and water. All of these solutions help to modify the (organic) compound and make it more water-soluble and therefore remove it from the organic layer. More concentrated solutions are rarely used for extraction because of the increased evolution of heat during the extraction, and potential side reactions with the solvent.





Any reactions using the Grignard reagent are carried out with ..

Grignard reagents react with water to produce alkanes. This is the reason that everything has to be very dry during the preparation above.

After separation of the organic and the aqueous layer, the amine can be recovered by addition of a strong base like NaOH or KOH to the acidic extract i.e., lidocaine synthesis. Note that amides are usually not basic enough to undergo the same protonation (pKa of conjugate acid: ~ -0.5).




Most neutral compounds cannot be converted into salts without changing their chemical nature. Many of these neutral compounds tend to react in undesired ways i.e., esters undergo hydrolysis upon contact with strong bases or strong acids. One has to keep this in mind as well when other compounds are removed. For instance, epoxides hydrolyze to form diols catalyzed by acids and bases. Ketones and aldehydes undergo condensation reactions catalyzed by both, acids and bases. Esters also hydrolyze to form carboxylic acids (or their salts) and the corresponding alcohol. In order to separate these compounds from each other, chromatographic techniques are often used, where the compounds are separated based on their different polarities (see Chromatography chapter).




Based on the discussion above the following overall separation scheme can be outlined. Which sequence is the most efficient highly depends on the target molecule. There is obviously no reason to go through the entire procedure if the compound sought after can be isolated in the first step already. Note that many of these steps are interchangeable in simple separation problems.

The mechanisms for these reactions aren't required by any UK A level syllabuses, but you might need to know a little about the nature of Grignard reagents.

Grignard Synthesis of Triphenylmethanol ..

Which of the two reagents should be used depends on the other compounds present in the mixture. Sodium hydroxide is usually easier to handle because it does not evolve carbon dioxide as a byproduct. In addition, the concentration can be increased significantly if is needed. However, if compounds were present that are sensitive towards strong bases or nucleophiles (i.e., esters, ketones, aldehydes, etc.), sodium bicarbonate should be used. It does not react with these compounds because it is a weaker base and a weak nucleophile (due to its resonance stabilization). Note that the formation of carbon dioxide as a byproduct causes a pressure build-up in the separatory funnel, the centrifuge tube or the conical vial. Thus, additional precautions (i.e., frequent venting) have to be taken to prevent any accidents resulting from the pressure build up in the extraction vessel. The target compound can subsequently be recovered by adding a mineral acid to the basic extract i.e., benzoic acid in the Grignard experiment in Chem 30CL.



The great value of the Grignard reaction to the synthetic chemist is its general applicability as a building block for an impressive range of structures and functional groups.

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    1) The addition of Grignard reagents to formaldehyde furnishes primary alcohols

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    Any reactions using the Grignard reagent are carried out with the mixture produced from this reaction

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Synthesis of triphenylmethanol using benzoic acid? | …

Despite all the limitations in preparing Grignard Reagent, it is still the most convenient way to synthesize certain alcohols and large molecules.

One Part of Chemistry: Grignard Synthesis of Triphenylmethanol

Grignard reagents are made by adding the halogenoalkane to small bits of magnesium in a flask containing ethoxyethane (commonly called diethyl ether or just "ether"). The flask is fitted with a reflux condenser, and the mixture is warmed over a water bath for 20 - 30 minutes.

EXPERIMENT 22 (1) SYNTHEISIS OF A GRIGNARD REAGENT (2) ..

This Grignard Reagent will then be carbonated by reaction with dry ice (solid carbon dioxide) to produce, upon acidification, benzoic acid.

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