Expt 4 Alkene Syn Wk1 Proc

7/25/2019 Expt 4 Alkene Syn Wk1 Proc

1/8

CHEM 212-2008

Experiment 4Procedure for Weeks 1 & 2

How Can a Complex

Alkene be Synthesized?

R' OH

R Br?

?

R Mg Br?

A. How can a Grignard Reaction be Successfully Accomplished?

Pre-lab Assignment:

1. Read:This Experiment handout

2. In Your Notebook:a. Prepare a Table of Compounds including the names, mol. wts., bps, densities and

solubilities in water and organic solvents for bromobutane, 2-methylpropanal, 2-methyl-3-heptanol, diethyl ether and magnesium metal (name and mol wt. only).

b. Write the Waste Disposal Instructions.

Category 3: Bromobutane.

Category 4: 2-Methylpropanal.

Category 5: Conc. H2SO4.

Category 6: All aqueous extracts.

Category 7: Mg, sodium sulfate.Special:

Excess ether and ether distillate -> Recovered Ether container in the hood.

The forerun of the fractional distillation -> Forerun waste container.

c. Using complete structural formulas of ALL organic compounds used in this

experiment (Rs are not acceptable as symbols in the structures), write balanced

equations for:

The preparation of butylmagnesium bromide.

The reaction of butylmagnesium bromide with

o 2-methylpropanal (Note: -- This is an aldehyde)

o watero molecular oxygen

o carbon dioxide

o unreacted 1-bromobutane in the dry ether solution.

The reactions of aqueous sulfuric acid with:

o the major organic product

o the side products from the previous bullet .

d. Starting on a new set of facing pages, write a complete stepwise procedure for the

synthesis of 2-methyl-3-heptanol through STEP 4. # 7.


2/8

CHEM 212 Experiment 4 2 Alkene Synthesis

e. Prepare a matching Results section including your calculated values for the volume of

1-bromobutane and 2-methylpropanal, as well as, the grams of magnesium needed for the

synthesis. Include all of your calculations.

3. On separate sheets of paper complete the flow diagram for the preparation and

purification of 2-methyl-3-heptanol. A partial flow diagram framework is attached at the end

of this activity. Completion of the diagram requires filling in blanks and addition of boxes

for some of the final purification steps. Place your flow diagram in your data binder underthe Experiment 4 tab.


3/8

Alkene Synthesis 3 CHEM 212 Experiment 4

Introduction:

For this synthesis we have selected one of the four Grignard reactions that resulted from our

analysis. Thus, our synthesis will begin with 1-bromobutane and 2-methylpropanal.

The preparation of the Grignard reagent, RMgX, is represented in equation (1), where RX

(1)solvent

dry ether

R-MgXR-X + Mg

represents 1-bromobutane. The exact nature of the Grignard reagent in solution is not known. It

is believed to be a mixture of numerous species. These species are highly solvated by ether and

are complexed with one another. It is customary to represent the Grignard reagent by the

formula RMgX when writing chemical equations, but it should be kept in mind that the species

in solution are of a much more complex nature.

The ether solvent is an essential part of the Grignard reagent, for ether is known to form a

complex with the magnesium that is present in the reagent. Several cases are known where

Grignard reagents have been prepared in the absence of ether, but the yields are not good.

Satisfactory yields are usually obtained when ether is present. The most common ether solvent is

diethyl ether, (C2H5)2O, due to its low cost and ease of removal (its boiling point is 36 C). Theorganic halide may, in general, be of any organic substituent (alkyl or aryl) and the halide may

be bromide, chloride, or iodide.

The preparation of the Grignard reagent must be carried out under anhydrousconditions and, if

possible, in the absence of oxygen. It is exceedingly important to maintain completely dry

conditions throughout, for the presence of water inhibits the initiation of the reaction and

destroys the reagent once it is formed.

The acid-base reaction that occurs when the Grignard reagent comes in contact with water is

shown in equation (2).

RH + HOMgX (2)R-MgX + H2O

The Grignard reagent is a strong base, since one of the carbon atoms bears substantial negative

charge (R-Mg++X-). As a base, the Grignard reagent removes a proton from water. The overall

effect is the hydrolysis of the reagent, with the formation of a hydrocarbon (RH) and a basic

magnesium salt which coats the unreacted magnesium and inhibits further formation of the

Grignard reagent. Thus it is critical to exclude water from the reaction mixture. Other weakly

acidic compounds such as alcohols and carboxylic acids also destroy the Grignard reagent and

inhibit the reaction by analogous acid-base reactions.

In addition to the reaction with water, there are other side reactions that may occur during

formation of the Grignard reagent, as shown in equations (3-5).

Reaction with Oxygen:

ROO-MgX RO-MgX (3)R-MgX + O2

Reaction with Carbon Dioxide:

O C O R C

O

O MgXR-MgX + (4)

Coupling with the organic halide:

R-MgX + R-X (5)R-R + MgX2

It is possible to minimize these reactions by taking certain precautions when carrying out the

experimental work. The reactions with oxygen and carbon dioxide may be avoided by carrying


4/8


out the reaction under an inert atmosphere (such as nitrogen or helium gas). Also, when diethyl

ether is used as a solvent, an inert gas is not essential since ethers very high vapor pressure

excludes a certain amount of air from the reaction vessel.

The coupling reaction [equation (5)] is an example of a Wurtz reaction. It is not possible to

eliminate this coupling reaction completely, but it may be minimized by using dilute solutions to

avoid localized high concentrations of halide. This is done by using very efficient stirring and by

slowly adding the halide to the magnesium in ether. Normally the rate of addition of halide(dissolved in ether) and the rate of reflux (when diethyl ether is used) should be adjusted so that

they are about equal. Alkyl iodides are much more prone to coupling reactions than are the

bromides and chlorides, so that the latter are preferable for preparing Grignard reagents even

though they are less reactive. If water has been carefully excluded, the most important side

reaction is the coupling process, but it is not a significant problem. Once the Grignard reagent is

prepared, it is used directly in subsequent reactions. The small amount of by-products need not

be separated from the reagent before adding the carbonyl compound.

In our synthesis, butylmagnesium bromide is prepared from 1-bromobutane. It is then allowed to

react with 2-methylpropanal to give the salt of 2-methyl-3-heptanol as shown in equation (6),

and the salt is protonated with dilute sulfuric acid, equation (7).

3 3 --ROMgBr + H2O + Mg

+2+ HSO4-+ Br- + H2O (7)H2SO4

The addition of 2-methylpropanal to the solution of butylmagnesium bromide is done slowly

with stirring and, if necessary, cooling, since the reaction is highly exothermic and ether is

volatile. The reaction is completed by adding dilute aqueous sulfuric acid to hydrolyze the

magnesium salt and form the final organic product. In addition, the acid dissolves the insoluble

basic magnesium salt that is formed. Without the acid, an unworkable emulsion may result.

After the ether solution of organic products has been separated from the aqueous solution of

magnesium salts, it is extracted successively with solutions of sodium bisulfite, sodium

bicarbonate and sodium chloride to remove some unreacted starting material, some products of

the side reactions and excess water, respectively, from the ether. (You should be able to

determine the specific reactions involved in each extraction.) After further drying, the ether is

removed and the product is purified by fractional distillation.

Experimental Procedure Weeks 1 & 2.

Synthesis.

STEP 1: Calculation of reagent volumes and masses:

0.21 mole of 1-bromobutane (measure by volume) 0.20 mole of Mg. (measure by mass)

0.21 mole of 2-methylpropanal (measure by volume)

STEP 2: Preparation of the Grignard Reagent:

1. Obtain the following equipment required for this step but do not assemble it until after 2.

A 250 mL round bottom flask

A Claisen adapter

A 125 mL separatory funnel

A reflux condenser


5/8


Two drying tubes filled with CaCl2.

A magnetic stirring bar.

Two blue plastic clips

BE CERTAIN THAT ALL OF THE GLASSWARE IS SCRUPULOUSLY DRY!!

2. To eliminate any possible film of moisture in the reaction flask, add the required amount of

magnesium turnings and a magnetic stirring bar to the round bottom flask and attach adrying tube to the neck. Then heat the flask thoroughly by placing it in a heating well

(regulator set on 5) and rotating it frequently until the entire glass surface is hot to the touch.

Cool the flask to room temperature before proceeding. As the flask cools, it draws in dry air

through the calcium chloride drying tube. NOTE: Cooling can be hastened by holding the

flask under a stream of tap water. However, take care that no water enters the flask.

3. Set up the flask with a Claisen adapter containing a reflux condenser on the straight neck and

the separatory funnel on the bent neck (see the demonstration apparatus). Leave space

under the flask for your magnetic stirrer, a heating well or and ice bath, if needed.

4. Pour 10 mL of anhydrousether slowly down the condenser into the reaction flask.

5. Then combine the 1-bromobutane with ~30 mL of anhydrousether, mix well and pour thesolution into the separatory funneland connect a drying tube to the top of the funnel.

6. Obtain an additional 20 mL of anhydrous ether and keep it ready to add when the

reaction begins. (See 8.below)

Read the following overview of the reaction initiation process. Use this overview to add

cautions to the reaction steps rather than copying the overview directly into your procedure.

Overview: The reaction is initiated by mixing a small portion of the alkyl halide solution

with the magnesium metal. This is usually the most difficult part of the

procedure. Once initiated, the reaction is strongly exothermic, so the solution

must be immediately diluted with the extra 20 mL of anhydrous etherto slowthe reaction down and to limit the possibilities of side reactions. When the

initial burst of reaction begins to subside, additional alkyl halide solution is

added slowly to maintain a controlled rate of reaction [As monitored by the

boiling rate of the ether solution].

7. Initiation of the reaction: Turn on the magnetic stirrer and admit a portion (~5 mL) of the

1-bromobutane solution into the flask. Evidence that the reaction has started will be the

appearance of cloudiness and bubbling. It may take up to 5 min. for the reaction to start.

If stirring is not sufficient to initiate the reaction, a second attempt, as described here, should

be made. Disconnect the flask and holding it in the palm of one hand, crush a piece of

magnesium against the bottom with the flattened end of a long stirring rod. Watch forbubbles indicating local boiling at that spot. You may also feel the warmth of reaction on

your hand. If the reaction still fails, reassemble the apparatus, and heat the flask to boiling

with your heating well and boil the solution for ~5 min. If heating also fails to initiate the

reaction, ask your instructor for help.

8. As soon as the reaction starts,slowlypour the additional 20 mL of ANHYDROUS ether

(See step 6. above)down the condenser to dilute the reaction mixture and thus limit the

possibility of the unwanted coupling reaction.


6/8


9. When the initial reaction burst subsides, gradually add the remaining contents of the funnel

by allowing the solution to drip into the reaction flask at a rate of ~1 drop per sec. (addition

should take about 15 minutes) If the condenser floods badly with ether, cool the flask in an

ice water bath for a few seconds onlyand continue.

Note: When a condenser floods, vapor rises into it faster than the condensing liquid can

flow down. The result is escape of vapor, sometimes noticeable as turbidity in the air above

the condenser, and the filling of the innermost section of the condenser with roiling or

turbulent liquid.

10. When the addition is complete and the reaction begins to subside, heat the mixture to

continue active boiling until practically all of the Mg has dissolved (~20 min. more) or

until the amount of unreacted Mg does not decrease in 5 min.

STEP 3:Preparation of 2-Methyl-3-heptanol

1. While the Grignard solution is refluxing (step 10. above), measure into two different dry

containers and cover to keep dry:

0.21 mole of 2-methylpropanal (measure by volume)

25 mL anhydrous ether2. When the formation of the butyl magnesium bromide is complete, remove the heating mantle

from the system and cool the flask to room temperature with an ice bath.

3. When the reaction flask is at room temperature, combine the 2-methylpropanal with the

25 mL of anhydrous ether, mix well and pour this solution into the separatory funnel that

held the 1-bromobutane solution in STEP 2.

4. Turn on your magnetic stirrer and begin slow addition of the 2-methylpropanal solution to

the butylmagnesium bromide solution. Be prepared to cool the flask with an ice bath if the

addition leads to uncontrolled refluxing. The addition may require as much as 30 min. After

the addition is complete, allow the reaction mixture to stir for an additional 10 min.

5. Place about 150 mL of crushed ice into a 500 mL beaker and add 9 mL of concentrated

sulfuric acid. In a hood, pour the reaction mixture slowly and with stirring into the ice-acid

mixture. Continue stirring the mixture until all remaining magnesium has reacted and

dissolved. After the addition is complete, transfer the cold mixture, which may contain some

precipitate, to a separatory funnel and shake it gently. The precipitate should dissolve.

6. Separate the layers transferring the ether layer to an Erlenmeyer flask.

7. Extract the aqueous layer with two 25-mL portions of ether and add these ether extracts to

the original ether layer.

8. Rinse the Claisen head and reaction flask with 6 M HCl. Allow the flask to sit with the HCl

until all remaining magnesium has reacted and dissolved. Then rinse the Claisen head andreaction flask with water and acetone. Do NOTrinse the condenser or separatory funnel.

Return all glassware to the appropriate bins.

9. The procedure may be stopped at this point if necessary. If so, transfer the combined ether

layers to an Erlenmeyer flask and stopper it tightly with a cork.

STEP 4: Preliminary Purification

1. Extract the combined ether solutions (in the separatory funnel) with 30 mL of saturated

sodium bisulfite solution venting the funnel to relieve pressure.


7/8


2. Extract the ether layer with two30-mL portions of 10% sodium bicarbonate solution, again

venting the funnel frequently.

3. Test the pH of the aqueous phase after the second sodium bicarbonate extraction. The

solution should be as basic as the original sodium bicarbonate solution used for the

extraction.

4. If the second extract is not basic enough, extract the ether solution with another 30 mL

portion of sodium bicarbonate and continue these extractions until the aqueous phase tests

appropriately basic after the extraction.

5. Extract the ether solution with 30 mL of saturated sodium chloride solution.

6. Remove the remainder of the water by drying the ether solution over anhydrous sodium

sulfate.

7. If necessary, stopper the flask snugly with a cork, mark the solvent level and allow the

solution to stand over the drying agent until the next lab period.

STEP 5: Final Purification.

1. Filter the solution from the drying agent through a fluted filter into a 250 mL round bottom

flask (If the volume of the solution has decreased significantly, you may need to add some

ether so that the filtration can be accomplished).

2. Remove most of the ether from the product by simple distillation (remember the boiling

chip).

3. When the vapor temperature rises about 20 C above the bp of ether, stop the simple

distillation. The volume of solution left in the pot should be about 25 mL. The distillate,

recovered ether, should be placed in the bottles marked for it.

4. Transfer the contents of the simple distillation pot to a 50 mL round bottom flask, rinse theoriginal flask with 3 mL of your simple distillation distillate (Recovered Ether), andconvert your simple distillation apparatus to a fractional one by adding a column packed with

glass beads.5. Be sure that the thermometer bulb is properly adjusted in the still head. Insulate the flask and

still head with glass wool and Al foil, and continue distilling.

6. Collect any forerun boiling more than 3 C below the products bp. separately from theproduct, record the boiling range and approximate volume of the forerun then collect aproduct fraction with a boiling range of ~5 C or less.

7. Record the boiling range and approximate volume of the product. If your yield of2-methyl-3-heptanol is less than 8 mL, check with your instructor. Your thermometer maybe incorrectly placed.

8. Weigh the product and calculate the % yield.

9. Place the product in a labeled vial or flask and keep it tightly stoppered for the Week 3 & 4

procedure. Discard the forerun in forerun waste container.

Partial Flow Diagram Framework for the

Synthesis and Purification of 2-Methyl-3-Heptanol

Abbreviations

MH-OH = 2-methyl-3-heptanol E = etherBu = C4H9 2-mp = 2-methylpropanal

= missing entry

This diagram assumes that a trace of water is present in the original reaction.


8/8


Bu-H

Complete the flow diagram by adding other "boxes" for extraction and distillation steps

E, H2O, O2(trace)

Ether

H2O, E(trace)

Na+, HSO3-

Aqueous

NaHSO3extraction

1. H2O, H2SO42. Ether extraction3. Combine extracts

Aqueous

Ether

H2O, E(trace)

H3O+, HSO

4

-

Mg2+

Add 2-mp

E, Bu-Br, 2-mp, H2O, O2(trace)

SecondReactionSolution

First

ReactionSolution

E, Mg, Bu-Br, BuOMgBr, Bu-Bu, BuCO

2MgBr,

O2(trace))

reflux

E, Bu-Br, Mg,O2(trace), H2O(trace)Gas Phase

E, Mg, Bu-Br, 2-mp,

O2(trace)

H2, Bu-H

Gas Phase

Documents

Expt 4 Alkene Syn Wk1 Proc