Lab E: Diels-Alder Reaction Flashcards
[4 + 2] cycloaddition reaction
[4+2] Cycloaddition reaction - of a 1,3-diene (conjugated diene) and an dienophile – to form a new six membered ring
5 characteristics of a Diels-Alder Reaction
- Thermal cycloaddition reaction
- Concerted reaction
- Three π bonds break
- Two new C-C σ bonds and One new π bond are formed in the product
- Forms new six-membered rings
describe diene reactivity (2): cis and trans conformations
- Can only react in the s-cis conformation (diene becomes unusually reactive)
- Unreactive in the s-trans conformation.
reactivity of dienophiles: what makes dienophiles more reactive?
Electron withdrawing groups make the dienophile more
electrophilic, and thus more reactive
rank in increasing reactivity order: H2C=CHZ, ZCH=CHZ, H2C=CH2 (if Z is an electron-withdrawing group)
most reactive
ZCH=CHZ
H2C=CHZ
H2C=CH2
describe the stereospecificity in the dienophile
a cis-dienophile forms a cis-substituted cyclohexene
a trans-dienophile forms a trans-substituted cyclohexene
CHWR acronym
CHWS acronym
CHWR = CHilled Water Return
- water flows OUT of the condenser
CHWS = CHilled Water Supply
- water flows INTO the condenser
for CHWR and CHWS, does the water flow INTO or OUT of condenser?
CHWR (return) - water flows OUT of condenser
CHWS (supply) - water flows INTO condenser
what is the purpose of a reflux?
to heat a reaction mixture at its boiling temperature (the boiling point of the solvent used in the reaction) to form the product without losing any material inside the reaction flask
what is/determines the boiling temperature of a reaction mixture?
the boiling temp of a reaction mixture = boiling point of the SOLVENT used in the reaction
what are 2 advantages of a reflux?
- maintains a constant temperature in the reaction flask
- reaction is able to reach equilibrium with minimal evaporation of material inside reaction flask
draw reaction scheme 1 for diels-alder reaction
draw reaction scheme 2 for diels-alder reaction (stereospecificity)
what are 3 advantages of using 3-sulfolene?
- non-hygroscopic solid
- not a flammability hazard
- excess 1,3-butadiene and SO2 are gases at room temp and are distilled out during reflex
scheme 1 is an example of what type of reaction?
cheletropic elimination
cheletropic elimination
reaction where a small, stable molecule is given off in the reaction (in our case it is SO2)
Why were we not able to directly use 1,3-butadiene in the reaction with maleic anhydride?
we cannot use 1,3-butadiene directly because it is a gas at room temperature since its boiling point is -44 degrees Celcius
–> to solve this, it is necessary to first form in situ (in the place) from the cheletropic elimination of buta-diene sulfone and consumed immediately afterwards
describe the overall reaction scheme in 2 steps
- cheletropic elimination
- diels-alder cycloaddition
what is the limiting reagent between s-cis-1,3-butadiene and maleic anhydride?
maleic anhydride is the limiting reagent
health hazards:
maleic anhydride
butadiene sulfone
petroleum ether
xylenes
sulfur dioxide
maleic anhydride - corrosive (can cause burns to the skin and eyes)
- if inhaled, can cause an allergic reaction or irritation
butadiene sulfone - corrosive, respiratory irritant and has foul smell
petroleum ether - toxic, flammable
xylenes - toxic, flammable
sulfur dioxide - toxic foul-smelling gas
when running the reaction, the hot plate cord and heating mantle cord must be placed where?
hot plate cord and heating mantle cord must be under white bar and securely wrapped around metal bar
why must the heating mantle be plugged into the outlet underneath the hood and not a regular outlet?
if you plug it into the regular outlet, it is a serious safety violation (expand on this!!)
what is the purpose of the aluminum foil in reflux?
to insulate the round bottom flask for the first 3 minutes and bring the solution up to a boil
what is the boiling point range for the diels-alder product (4-cyclohexene-cis-1,2-dicarboxylic anhydride)
97-103 degrees C
what is the molecular formula and name of the diels alder product?
C8H8O3
4-cyclohexene-cis-1,2-dicarboxylic anhydride
why do we use a heating mantle to heat up the compounds instead of a hot plate like in Lab A and B
we use a heating mantle (aka INDIRECT heat) to warm up FLAMMABLE compounds
also we don’t use the hot plate because it would boil off all the organic solvent
waste disposal:
- filtrate (containing xylenes and petroleum ether) –>
- unused xylenes or petroleum ether –>
- waste acetone –>
- filtrate (containing xylenes and petroleum ether) –> “Diels Alder Waste Container”
- unused xylenes or petroleum ether –> “Diels Alder Waste Container
- waste acetone –> “Diels Alder Waste Container
–> basically everything for this lab is disposed into the DIELS ALDER WASTE CONTAINER!
boiling point for petroleum ether
36-50C
boiling point for zylenes
137-140C
the gas we see rising into the water condenser during reflux is which gas?
sulfur dioxide SO2
- theres another one??? (1,3-butadiene?) - CHECK!!
why do we use petroleum ether?
Pet-ether is a low boiling solvent, so it helps crash out the diels alder product out of mixture with xylenes since the product has a greater solubility in xylenes
Product has greater solubility in xylenes (in solution)
describe the physical appearance of butadiene sulfone and maleic anhydride
Butadiene sulfone-white powdery solid
Maleic Anhydride -white chunky solid
describe what the solution looks like before and after reflux
clear solution –> yellow solution after refluxing
physical appearance of the diels alder product?
white, shiny (lustrous) crystals
describe the physical appearance of the precipitate after adding petroleum ether
whitish, cloudy precipitation floating around in solution
“crash out” meaning
crashing out - a rapid crystallization or precipitation of a substance from a solution
what are the 2 reactants in this reaction
butadiene sulfone and maleic anhydride
diels-alder reaction is an example of what type of reaction?
cycloaddition reaction
why do we use butadiene sulfone and not s-cis-1,3-butadiene as our reactant even thought s-cis-1,3-butadiene is what undergoes the actual diels-alder reaction?
s-cis-1,3-butadiene exists as a gas at room temperature since its boiling point is -44C. ==> thus we need to first generate s-cis-1,3-butadiene in situ form (during reflux) from butadiene sulfone through cheletropic elimination and then use s-cis-1,3-butadiene immediately
how do we add the solids into the round bottom reaction flask?
by twisting the weigh boats and pouring out the solids
what is the purpose of using xylenes?
xylenes is the SOLVENT for this reaction
what are the 2 side arms of the condenser?
at the BOTTOM of condenser: CHWS = chilled water supply
at the TOP of the condenser: CHWR = chilled water return
describe the flow of the cold water in the condenser (what direction does it flow in)
chilled water supply comes in from the bottom of the condenser and flows UPWARD and finally exits at the top of the condenser
BOTTOM –> TOP
why does the RB flask need to sit flush on the heating mantle?
to ensure an efficient heat transfer from heating mantle to RB flask
why is the heating mantle set on TOP of the hot plate for this reaction but NOT for LAB F fischer esterification reflux?
this diels-alder reaction requires STIRRING, which is why the hot plate is used (the heat setting on the hot plate is NOT turned on)
for lab F, fischer esterification, this reaction does NOT require STIRRING, so the heating mantle is just set on the ring stand instead
why is the heating mantle cord wrapped around and under the white metal frame?
as a safety precaution - to make sure the cord isn’t flopping around and to avoid the cord from being frayed
why can the RB flask NOT be placed directly on the hot plate stirrer?
- you will end up boiling away your solvent
- it is a safety hazard since the organic solvent is flammable
what is the heating mantle made up of?
insulated fiberglass that supports the base of the RB flask
why is the heating mantle never plugged into a regular outlet?
because the heating mantle is capable of producing enough heat to completely evaporate any or all organic material if it was ever used on full power
–> instead, heating mantle is plugged into the variac since the variac can CONTROL the voltage and thus the temperature of the heating mantle
full name of variac
Variable Voltage Transformer
when should you not use a heating mantle? (safety precautions)
- if the fiber glass fibric appears to be cracked or in brittle condition
- if the heating mantle electric cord appears to be frayed
what is a reflux reaction?
a reaction that is carried out at the boiling point of the organic solvent that is used in the reaction
where is the hot plate stirrer plugged into?
a regular outlet on the side of the fume hood
describe the components of a condenser
inner hollow tube
outer tube (outer jacket)
2 side arms
bottom - CHWS
top - CHWR
what flows in/through the inner hollow tube of the condenser?
the solvent of the reaction vaporizing and condensing during reflux
WHY does the condenser water flow UPWARDS?
in a reflux reaction, once it reaches the boiling point of the organic solvent inside the reaction RB flask, the solvent will vaporize and the vapors of the solvent will come up the INNER HOLLOW TUBE of the condenser
what occurs in the outer jacket of the condenser?
the process of water flowing into and out of the condenser
what happens when the vapors of the organic solvent rise up through the inner hollow tube of the condenser?
because of the cold water that is circulating in the outer jacket of the condenser, those solvent vapors that are climbing up the inside of the condenser are CONDENSED and DRIVEN BACK INTO the reaction RB flask!
describe how a reflux reaction works and how it prevents any solvent from being lost
the condenser has an inner hollow tube where the solvent vapors climb up and an outer jacket where the chilled water supply circulates throughout the condenser. when the reaction has reached the boiling point of the solvent, the solvent will start to vaporize and its vapors will climb up the inner hollow tube of the condenser. however, because there is the cold water circulating on the outside (in the outer jacket), this will cause the solvent vapors to CONDENSE and get DRIVEN BACK DOWN and INTO the reaction flask. thus, no solvent is lost during the reaction despite the reaction being conducted at its boiling point
what is the key idea of how a reflux reaction works?
makes sure that the solvent of the reaction is not lost (aka maintain the volume of solvent) through the process of VAPORIZATION and CONDENSATION
where do any gases that are formed in the reaction escape from?
any gases formed in the reaction are able to easily escape from the inner hollow tube of the condenser and out from the TOP junction –> these gases are then vented out through the exhaust lines in the fume hood
why is it important that the fume hood is lowered/covered during a reflux reaction?
to prevent any gases formed from the reaction to escape and come out of the fume hood
–> there are exhaust lines that vent out the gases in the fume hood
what happens if you attach the supply hose to the top of the condenser instead of the bottom?
you will not have a cool water condenser that is necessary to carry out a reflux reaction –> could end up boiling away all your solvent which is a flammability hazard
how do you check that your solution has come up to a boil? why do you turn off the stir to check?
need to turn off the stirring first and look for bubbles in the solution
–> need to make sure that any bubbles you see are from the BOILING and not from the stirring of the solution
what happens if the solution hasn’t come up to a boil yet?
need to cover the solution with the tin foil again and then check again
what gas is vaporing and condensing up (and down) the condenser?
the solvent - in this reaction XYLENES
what is the white smoke you see climbing up the condenser?
SULFER DIOXIDE GAS –> which is the gas byproduct from the cheletropic elimination reaction
–> it is eliminated by escaping through the top of the condenser
why do we use a reflux reaction for the cheletropic elimination of butadiene sulfone
so that when the reaction occurs, the s-cis-1,3-butadiene product can immediately be generated and react with the available maleic anhydride inside the reaction flask to form the diels-alder product
why do you need to check periodically if your condenser is cool to the touch?
because if the condenser feels warm, it indicates that your solution is not refluxing but is actually BOILING, which is BAD –> also this is a safety hazard
where do you place the heating mantle to cool after reflux?
let it cool by hanging it on the OUTSIDE of the fume hood
why do you add additional xylene after reflux is done?
to just dilute the solution inside RB flask
why do you decant the solution after reflux?
to avoid any impurities –> typically but rarely a brown sludge will form in the diels-alder reaction
–> to allow only the SUPERNATENT SOLUTION that contains your product to pass through
what is the process of decanting?
just pouring the SUPERNATENT solution from the RB flask into a 50mL beaker and making sure any brown sludge that has formed is left behind
should you wet the buchner funnel filter paper with water during vacuum filtration for the diels-alder product?
NO because this reaction is solely ORGANIC –> do not want any aqueous solution involved
why do we use a spatula to scrap out the diels-alder product for vacuum filtration?
the diels-alder product will most likely be stuck to the walls of the beaker, so need to use a spatula to scrap it out and onto the buchner funnel filter paper
cycloaddition definition
a reaction in which two unsaturated molecules react to form a ring
diels alder reaction definition
the cycloaddition of dienes and alkenes (dienophiles) to form cyclohexenes
- also concerted reaction
- also stereospecific reaction
what is the electrophile in the diels alder reaction?
dienophile
what increases the reactivity of a dienophile?
if the dienophile contains an ELECTRON WITHDRAWING GROUP directly attached to the alkene –> since the dienophile is the electrophile of the reaction, the more electron-deficient the alkene is, the more reactive it is
what does generated in-situ mean?
generated within the reaction flask
describe the stereospecificity of our reaction
maleic anhydride is a cis-alkene because it is cyclic –> our diels-alder product is also cis
describe the IR spectroscopy of our diels-alder product
- absorbance bands around 1840cm^-1 and 1761cm^-1 due to in-phase and out-of-phase stretch of 2 C=O groups
- sp3 C-H stretch around 2960cm^-1
- very mini sp2 C-H stretch peak slightly to the left (downfield) of sp3 C-H stretch
