Lab E: Diels Alder Reaction Flashcards
Draw out the reaction schematic for the Diels Alder reaction scheme. Label all products and reagents.
Refer to notes
Why does the product(4-cyclohexene-cis-1,2-dicarboxylic anhydride) need to be soluble in the initial solvent(i.e. xylenes)?
You need product to be soluble in solvent because otherwise it will crash out before you isolate.
Diel-Alder is a __________(1) reaction in which two ____________(2) molecules react to form a ring.
(1) cycloaddition
(2) unsaturated
Why are Diels-Alder reaction among the most useful reactions available?
It produces functionalized six-membered rings, which are ubiquitous in important organic molecules including a wide range of pharmaceuticals.
The Diels-Alder reaction works for an enormous variety of dienes and dienophiles. however, it works best if the dienophile contains
an electron withdrawing group directly attached to the alkene. This is because the dienophile is the electrophile of this reaction, and so the more electron-deifinicent the alkene is, the more reactive it is.
What is the electrophile of this reaction?
the dienophile which is malice anhydride
What is the diene? What is the dienophile?
Refer to notes
What is the dienophile in this experiment?
maleic anhydride
Why is malice anhydride extremely reactive?
It has two electron withdrawing substituents making it an extremely reactive dienophile
The Diels-Alder reaction is a ____________(1) reaction; bond breaking and bond forming occur simultaneously.
concerted
What is a direct benefit of the concerted nature of the Diels-Alder reaction? Apply this to our specific experiment.
It is stereospecific; one can reliably predict the stereochemistry of the product based on the stereochemistry of the starting materials. For example, if the substituents on the dienophile are cis, they will be cis in the product, and likewise for a trans-dienophile. In our case, Maleic anhydride is a cis-alkene(it is cis because it is cyclic) and so the fused bicyclic ring system in our Diels-Alder product is cis.
Make a drawing to demonstrate the stereospecificity of the Diels-Alder reaction.
Refer to notes
Why is 1,3-Butadiene not used directly in this experiment?
1,3-butadiene is a gas at room temperature. The use of gases as reagents is common in synthetic organic chemistry, however, it requires the use of special equipment that is not amenable to this lab course. We will instead use Butadiene sulfone, which is an easily manipulated solid, to generate 1,3-butadiene within the reaction flask or in-situ.
Who discovered the diets-alder reaction?
Otto Fiels and Kurt Alder
What are the hazards associated with Maleic anhydride adn Butadiene sulfone?
Both are corrosive solids that can cause burns to the skin and eyes. If inhaled, malice anhydride can cause an allergic reaction or irritation. Butadiene sulfone is also a respiratory irritant and has a foul smell.
What are the hazards associated with petroleum ether and xylenes?
both are toxic and flammable organic solvent. You should avoid inhalation adn skin contact with both solvents.
What are the hazards associated with sulphur dioxide?
Sulfur dioxide is a toxic foul-smelling gs that will be evolved during the course o this reaction.
How can you avoid all of the hazards associated with the chemicals in this lab?
Keep you hood down to prevent the gas from escaping from the hood
What is the waste disposal procedure for xylenes, petroleum ether, pipettes, and waste acetone?
The filtrate, which contains xylenes and petroleum ehter should be disposed of in the specially labelled “diels-alder waste container”
All pipettes should go into the sharps contianer
waste acetone should go into the
“diels-alder waste container”
There are some important safety considerations in this lab. Firstly, the RBF should NOT be set directly on the _____(1), without the heating mantle. If you do this, ________(2). Secondly, the heating mantle is made up of _________(3) that supports the base of the RBF. If this is cracked or in very brittle condition, _____________(4). The heating mantle should be secured firmly by _______(5) and plugged into the ________(6). The heating mantle should never be plugged into _____(7). This is because ___(8)
(1) surface of the hotplate
(2) you will end up boiling away all of the organic solvent in the flask and it would not classify as a reflux reaction. Moreover, the organic solve is flammable.
(3) insulate fiberglass
(4) do not use the heating mantle
(5) looping the cord around the cylindrical bar in front of your fume hood
(6) Variac
(7) wall outlet
(8)The heating mantle is capable of producing enough heat to completely evaporate any or all of the organic material if used on full power, which is why it is plugged into a varian.
Draw a reflux schematic
refer to notes(print out) and the back of the DA checklist sheet
Summarize this reaction
- In a RBF, obtain maleic anhydride, butadiene sulfone, xylenes, and stir bar
- Reflux set-up: place heating mantle on hot-plate/stirrer. Rest the clamped RBF flush inside the heating mantle. Attach a water-cooled reflux condenser to the top of the RBF. Ask TA to plug the heating mantle into the variac below the fume hood. Only turn on stirring on the hot-plate/stirrer and NO HEAT.
- Use a piece of foil to insulate the RBF until it comes to a boil. This is the start of the reflux time period. Reflux for 30 mins with gentle stirring.
- At the end of the reflux, allow the reaction to cool for 3 minutes
- Add xylenes to the RBF
- Place clamped RBF inside a warm water bath. Then decant the supernatant solution into a beaker(leaving behind any brown sludge that may have formed)
- add 10 mL of petroleum ether to the beaker and you should observe the dials-alder product precipitate out.
- Cool the beaker in an ice-water bath
- Do a vacuum filration for 5 minutes. record wet mass
transfer a small amount of the ample into a vial for IR analysis.
What is the solvent in this reaction?
xylenes
Why is it important that the RBF sits flush inside the heating mantle?
Ensures efficient heat transfer from heating mantle to RBF
How should the water connections be attached to the water condenser?
The bottom of the water condenses should be attached to the chilled water supply(CHWS) and the top should be connected to the chilled water return(CHWR)
What is something that should be checked regularly when working with a water condenser
check if the condenser feels cool to the touch every 5 mins
What should the aviary dial be set to?
55%
What is a variac?
a voltage transformer that you are able to adjust the voltage for to achieve a desrived temperature.
Why do we only do 1 vacuum filtration for 5 minutes?
we are dealing with organic solvent and they evaporate really fast.
To accurately check for boiling you should
turn the stirring off and check for bubbles
What is the white gas observed in the reflux?
sulfur dioxide and some butadiene
What should you observe during a reflux?
the solvent vapours climb up into th water-cooled condenser adn are then recondensed back into the RBG
What is the physical appearance of the final Diels-Alder product at the very end. How about before
Final product: white powder/flaky appearance
before it was vacuum filtered: gel-like consistency, clear crystal formation
What is an important consideration when doing vacuum filtration in this experiment?
Do not wet the filter paper with water since the reaction is solely done using organic solvent. Hence, you do not want to introduce an aqueous solvent into the system.
Why do we add petroleum ether at the end of the reaction?
At the end of the reflux, the Diels-lder product is sitting in xylenes. The Diels-Alder product has a greater solubility in xylenes and that is why it stays in solution in xylenes. In order to help it crash out, we add this other solvent(petroleum ether). This solvent is a low boiling solvent and it helps the DA product crash out.
You should not pull out the heating mantle until
you actually have to set up the reflux
Should solid or liquid chemicals be added to the reaction flask first?
Solid
Does the water condenser need ot be clamped?
no
All organics(i.e. pet ether and xylenes) should be kept _______(1) inside the fume hood
(1) covered
How does the condenser work?
the condenser has an inner hollow tube and an outer jacket. Water flows in from the bottom and exits from the top, thus moving in an upward direction inside the condenser and forming a coiled circular loop of cold water that circulates through the condenser. In a reflux reaction, once the solution reaches the boiling point of the organic solvent u=inside the RBF, then the solvent will vaporize and the vapours of the solvent will climb up th inner hollow tube of the condenser. The process of water flowing into and out of the condenser occurs in the outer jacket. The cold water circulating in the outer jacket(upward direction) causes the solvent valors(climbing through the inside of the hollow tube) to condense and are driven back into th reaction RBF. No solvent is ever lost because through this reflux process we maintain the solvent cvolume through vaporization and condensation. Any uses formed in the rection can escape from the inner hollow tube and are vented through the exhaust lines inside th fume hood, thus why the fume hood should stay closed.
What are the five characteristics of a Diels-Alder reaction?
- Thermal cycloaddition reaaction
- Concerted reaction
3 3 pi bonds break - Two new C-C sigma bonds and one new pi bond are formed in the product
- forms new six-membered rings
When are dienes reactive/unreactive?
- can only react in the s-cis conformation(diene becomes unusually reactive)
- unreactive in the s-trans conformation
Remember that the E barrier between cis and trans is small –> we can interconvert between he two conformations if the formation is not locked.
Draw the general mechanism for a DA reaction.
Refer to notesx
How many pi bonds and pi electrons in the diene? how about the dienophile?
Diene: 4 pi electrons, 2 pi bonds
Dienophile: 2 pi electrons, 1 pi bond
This is why the DA reaction is known as a 4+2 reaction
What does it mean for the DA reaction to be a pericicular reaction?
Reaction proceeds in a circular reaction. Curved arrows represent the cyclic movement of electrons
In a DA reaction, you are breaking ___(1) pi bonds between the diene and the dienophile. It is forming ____(2) new sigma bonds and ____(3) new pi bond.
3
2
1
DA reactions can occur at room T if they are especially reactive but usually need
heat
Should the hotplate be on the ring stand?
no
The reflux reaction is the go-to reaction for
flammable materials
The direction of water flow in a water condenser is up because
it allows for condensation and prevents things from escaping the condenser
What are some examples of EWGs
CHO
COCH3
COOCH3
O with a double bond
an O part of the ring itself or attached to a C in the ring
What is the purpose of a reflux reaction?
To heat a reaction mixture at its boiling temperature(the boiling point of the solvent using in the rection) to form the product, without losing any material inside the reaction flask
What are the advantages of a reflux?
- Maintains a constant temperature in the reaction flask
- Reaction is able to reach equilibrium with minimal evaporation of material inside the reaction flask
What are the three advantaged of using 3-sulfolene?
- non-hygroscopic solid
- not a flammability hazards
- excess 1,3-butadiene and SO2 are gases at room temp. and are distilled out during reflux
What is the physical appearance of each of the reactants? What about xylenes?
Butadiene sulfone: white powder
Maleic anhydride: white power
xylene: clear, colourless liquid
What was the color of the solution after the reflux?
yellow-orange, clear
