Name: Chevaunne Sinclair

Id Number: 18085139

Course: Organic Chemistry II

Lab Session: Tuesday- Thursday 8-11

Lab #: 1

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Title: Grignard Preparation and Reaction

Aim:

To prepare a Grignard reagent for use in a reaction.

To form triphenyl methanol using methyl benzoate and phenylmagnesium bromide.

Abstract:

This lab was a dry lab, and so was conducted theoretically. It has as its aims to prepare a

Grignard reagent and to use this reagent along with an ester to prepare an alcohol.

The Grignard reaction is an organometallic chemical reaction in which alkyl- or aryl-

magnesium halides (Grignard reagents) act as nucleophiles andattack electrophilic carbon

atoms that are present within polar bonds to yield a carbon-carbon bond, thus

altering hybridization about the reaction center. When one mole of Grignard reagent is

reacted with an ester, an intermediate ketone, which cannot be isolated, is formed.

Another mole of Grignard reagent will form the alcohol. In this lab, the Grignard reagent

is prepared using magnesium chips, ether, bromobenzene and a setup of reflux apparatus.

This Grignard reagent will then react with methyl benzoate to form triphenyl methanol.

This lab was a dry lab, and so was conducted theoretically.

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Introduction: The French chemist François Auguste Victor Grignard discovered an

organometallic chemical reactionreaction in alkyl- or aryl-magnesium halides (Grignard

reagents), act as nucleophiles, attack electrophilic carbon atoms that are present within

polar bonds like a carbonyl group to yield a carbon-carbon bond, which also results in an

altering of hybridization about the reaction center. This reaction is integral in the forma-

tion of carbon-carbon bonds and other carbon-heteroatom bonds. Reaction mechanisms

are outlined below:

This lab would utilize various reagents. Their basic information and structures are

outlined below.

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Phenylmagnesium bromide is a magnesium-containing organometallic compound. Its

molecular formula is C6H5MgBr, It is a Grignard reagent and is commercially available as

solutions of diethyl ether or THF. It may be prepared in the laboratory by

treating bromobenzene with magnesium metal, usually in the form of ribbon. A small

amount of iodine may be used to activate the magnesium to initiate the reaction.

Bromobenzenes are a group of halobenzenes formed in a substitution

reaction between bromineand benzene with a hydrogen bromide by-product. The name

strictly refers to monobromobenzene, a clear pale yellow liquid with a molecular formula

of C6H5Br, however it can be used to refer to a benzene containing any number of

bromine molecules.

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Methyl benzoate is an ester with the chemical formula C6H5COOCH3. It is formed by the

condensation of methanol and benzoic acid. It is a colorless to slightly yellow liquid that

is insoluble with water, but miscible with most organic solvents. It has a pleasant smell,

strongly reminiscent of the fruit of the feijoa tree, and it is used in perfumery. It also finds

use as a solvent and as apesticide used to attract insects.

Triphenylmethanol (also known as triphenylcarbinol) is an aromatic organic compound.

Its molecular formula is C19H16O. It is a white crystalline solid that is insoluble in water

and petroleum ether, but well soluble in ethanol, diethyl ether and benzene. In strongly

acidic solutions, it produces an intensely yellow color, due to the formation of a stable

carbocation. It can be prepared rom methylbenzoate or benzophenone and bromobenzene.

Ether is a class of organic compounds that contain an ether group —

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an oxygen atom connected to two alkyl or aryl groups — of general formula R–O–R. A

typical example is the solvent and anesthetic diethyl ether, commonly referred to simply

as "ether" (CH3-CH2-O-CH2-CH3). Ethers are slightly polar as the COC bond angle in the

functional group is about 110 degrees, and the C - O dipoles do not cancel out. Ethers are

more polar than alkenes but not as polar as alcohols, esters or amides of comparable

structure. However, the presence of two lone pairs of electrons on the oxygen atoms

makes hydrogen bonding with water molecules possible, causing the solubility of

alcohols. The general formula is shown below:

Magnesium is a chemical element with the symbol Mg, atomic number 12, atomic

weight 24.3050 and common oxidation number +2. Magnesium, an alkaline earth metal,

is the ninth most abundant element in the universe by mass.

Hydrochloric acid is the solution of hydrogen chloride (H Cl ) in water. It is a

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highly corrosive,strong mineral acid and has major industrial uses. It is found naturally

in gastric acid.

Petroleum ether, also known as benzine, VM&P Naphta, Petroleum Naphta, Naptha

ASTM, Petroleum Spirits, X4 or Ligroin, is a group of various volatile, highly

flammable, liquid hydrocarbon mixtures used chiefly as nonpolar solvents.

Water covers 71% of the Earth's surface. It has a liquid form or state, a solid state, ice,

and a gaseous state, water vapor or steam. Water has a density 0.998 g/cm³ of, a boiling

point of 99.974 °C and a melting point of 0 °C (273.15 K) (32 °F).

Calcium carbonate is a chemical compound with the chemical formula Ca C O 3. It is a

common substance found in rock in all parts of the world, and is the main component

of shells of marine organisms, snails, pearls, and eggshells. Calcium carbonate is the

active ingredient in agricultural lime, and is usually the principal cause of hard water.

Materials/Apparatus:

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Materials Apparatus

Bromobenzene Claisen adapter

Mg metal Hotplate

Phenylmagnesium bromide Condenser

Methyl Benzoate Separting funnel

Hydrochloric acid Round bottom flask

Ether Stirring rod

Calcium chloride stopcock

ice 100-mL reaction flask

water Magnetic stir bar

Ligroin 50-mL Erlenmeyer flasks,

100-mL graduated cylinder

Diagram showing apparatus used in experiment:

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Method:

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Place a 250-mL beaker half-full with water to warm (45-50 oC) on the hot plate.

Generate the Grignard Reagent

Have an ice-water bath on hand. Formation of the Grignard reagent is exothermic.

The Grignard reagent is prepared in a dry 100-mL reaction flask fitted with a long reflux

condenser. A calcium chloride drying tube inserted in a cork that will fit either the flask

or the top of the condenser is also made ready. Assemble the reaction flask containing the

magnetic stir bar with the condenser and drying tube. Allow the set-up to cool to room

temperature. The flask on cooling pulls dry air through the calcium chloride. Weigh out 2

g of magnesium. Crush with a pestle in a mortar to break up some of the oxide coating

and expose some fresh metal surface. Add to the 100-mL reaction flask. Follow with 10

mL of THF. Prepare in a 50-mL Erlenmeyer flask a solutionco ntaining 8 mL of bro-

mobenzene in 15 mL of THF. Now, add 5 mL of the bromobenzene solution dropwise

(via syringe) down the condenser into the 100-mL reaction flask containing the magne-

sium in THF with warming in the water bath. The THF will start to boil. The solution

should soon turn colorless but cloudy. When it is evident that the reaction has started add

dropwise (via syringe) another 5 mL of the bromobenzene solution. Maintain moderate

reflux with intermediate heating and cooling (raise the reaction flask out of the warm wa-

ter bath and, if needed, into the ice-water bath). If the reaction is occurring too vigorous,

raise the set-up out of the warm water bath, allow the reactionto cool and subside, and

add 5 mL of THF. Add the remainder of the bromobenzene solution dropwise (via sy-

ringe) down the condenser. Again, maintain a moderate reflux rate of the reaction mix-

ture. Raise the reaction flask out of the water bath if the reflux looks to be too vigorous

and cool the reaction with the ice-water bath but do not cool the reaction so much that it

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stops. When all the bromobenzene has been added and the reaction stops refluxing spon-

taneously, heat the reaction for 20-30 minutes to ensure complete formation of the Grig-

nard reagent.

Add the Carbonyl Compound (Methyl benzoate)

While the Grignard reagent is processing, prepare the solution of carbonyl compound in

the same 50-mL Erlenmeyer flask with 15 mL of dry ether. Cool the reaction mixture for

several minutes. Add the solution of Methyl Benzoate dropwise (via the same syringe)

down the condenser. After addition of 5 mL of solution, place back in the water bath to

begin warming again. Continue adding the methyl Benzoate. When addition is complete,

rinse the 50-mL Erlenmeyer flask with 5 mL of THF and add. Reflux for another 15-20

minutes.

Quench the Grignard Reaction

Cool the reaction mixture. Place 25 mL of 1 M Hydrochloric acid and 25 g of ice in a

250-mL beaker. Set aside 10-20 mL of this cold aqueous acid solution. Pour the reaction

mixture into the 250-mL beaker. Rinse the reaction flask with the reserve aqueous acid

solution and pour into the beaker. Stir the mixture until the salts and bits of unreacted

magnesium dissolve.

Isolation the Crude Triphenylmethanol

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While the mixture is allowed to stir, set up for an extraction using a 125-mL separatory

funnel. Filter the solution through a cotton-plugged funnel into the separatory funnel.

Shake, vent, and separate the layers. Then proceed as directed:

1. Return the aqueous layer and extract with 50 mL of ether;

2. Wash the combined organic layer in succession with 25 mL of water 25 mL saturated

aqueous sodium bicarbonate solution, and 25 mL of saturated aqueous sodium chlo-

ride solution;

3. Dry the organic layer over a drying agent.

Filter the solution through another cotton-plugged funnel into a 125-mL Erlenmeyer

flask. Add 20 mL of ligroin or hexanes and set on a hot plate to slowly concentrate. Con-

centration allows for the crystallization of triphenylmethanol from the mixed solvents.

When crystallization begins, remove from the hot plate and allow the solution to cool to

room temperature then to 0oC. Collect the first crop of triphenylmethanol by vacuum fil-

tration using a Hirsch or Büchner funnel. Weigh the dry product. Check the melting point

to determine whether a recrystallization is needed. A second crop could be obtained by

concentrating the mother liquor.

Recrystallization (Purification) of the Crude Triphenylmethanol

After checking the melting points of all crops, determine if recrystallization is needed. If

any crop is deemed pure, use that for the carbocation formation. Recrystallization of

triphenylmethanol can be accomplished by dissolve the crude material in hot dichloro-

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methane and adding four times the volume of hexanes. Heat to dissolve and allow to

cool. Collect by vacuum filtration and determine yields.

Expected results: When this Grignard reagent is added to the methyl benzoate, the for-

mation of crystals after crystallization will indicate that the reaction has produced triph-

enylmethanol. The major product is triphenylmethanol and the minor product is produced

in a side reaction which is biphenyl.

Discussion: The purpose of this experiment was to synthesize the tertiary alcohol triph-

enylmethanol from a Grignard reagent, phenyl magnesium bromide. In a reaction be-

tween an ester and a Grignard reagent, two moles of Grignard reagent are used. The first

mole reacts with the ester to form an intermediate ketone (however, this is not a synthesis

reaction for ketones as they cannot be isolated in this reaction) and the other mole reacts

with the ketone to give a tertiary alcohol. The Grignard reagent is easily

formed by reaction of an alkyl halide, in particular a bromide, with magnesium metal in

anhydrous diethyl ether or tetrahydrofuran (THF).

The Grignard reagent is a strong base and a strong nucleophile. As a base, it will react

with all protons that are more acidic than those found on alkenes and alkanes. Thus Grig-

nard reagents react readily with water, alcohols, amines, thiols, etc., to generate the

alkane:

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The reaction proceeds satisfactorily only if the reagent and the apparatus are thoroughly

dry. If a wet solvent/solvent that contains traces of water is used, the Grignard reagent is

destroyed as fast as it is formed. The magnesium metal, in the form of a course powder,

has a coat of oxide on the outside. A fresh surface can be exposed by crushing a piece un-

der the absolutely dry ether in the presence of the organic halide. Reaction will begin at

the exposed surface, as evidenced by a slight turbidity in the solution and evolution of

bubbles. Once the exothermic reaction starts, it proceeds easily, the magnesium dissolves,

and a solution of the Grignard reagent is formed. The solution is often turbid and gray

due to impurities in the magnesium.

The Grignard reagent was synthesized from Bromobenzene and magnesium and then re-

acted with methyl benzoate to produce triphenylmethanol. The reaction of phenyl magne-

sium bromide and benzophenone was quenched with hydrochloric acid, and an extraction

was performed in order to separate the organic phase containing the triphenylmethanol

from the aqueous phase. The triphenlmethanol was then isolated and purified by crystal-

lization and vacuum filtration. 

Mechanism:

Formation of the Grignard reagent form the insertion of magnesium between the aryl and

the halo groups.

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The primary impurity in the present experiment is biphenyl, formed by the reaction of

phenylmagnesium bromide with unreacted bromobenzene. The most effective way to

lessen this side reaction is said to be to add the bromobenzene slowly to the reaction mix-

ture so that it will react with the magnesium and not be present in high concentration to

react with previously formed Grignard reagent. The impurity is easily eliminated, since it

is much more soluble in hydrocarbon solvents than triphenylmethanol.

One equivalent of the Grignard reagent is added to the electrophilic center of the carbonyl

compound, methyl benzoate, to form benzophenone. However, benzophenone is more re-

active than methyl benzoate and immediately reacts with a second equivalent of Grignard

reagent. In order to produce triphenylmethanol, two equivalents of the Grignard reagent

is required to react with the ester.

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Limitations:

1. Apparatus must be free of water, or water will react with the reagent to produce

benzene.

2. The ether compound evaporate easily at RTP, this could affect the results.

3. The side reaction that occurs, if not removed could also affect your results.

4. Accuracy of amount of reagent used could affect expected percentage yield.

Precautions:

1. Diethyl ether is extremely flammable. Use only in a fume hood and keep

away from flames and heat sources, such as hot plates.

2. be sure to use dry glass wear.

References: : Organic chemistry by John Mcmurry the 7th edition, Organic chemisty by

L.G Wade the 6th edition, Chemistry by Raymond Chang, Wikipedia the free

encyclopedia, sparknotes.com, Websters dictionary, Msnencartaonline.com, Perry's

Chemical Engineers' Handbook (7th Edition ed.). McGraw-Hill.

http://infohost.nmt.edu/~jaltig/Chem333LGrignard.pdf

Hornback, Joseph M. Organic Chemistry 2nd Edition, Cengage Learning, 2005

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