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Mechanism

The Diels-Alder reaction mechanism still remains the topic of much debate because of the uncertainty as to whether the mechanism is stepwise, concerted, or free-radical. For the time being arrow-pushing concerted mechanism is preferred on order to better understand the beauty of this reaction.

Some important facts

1. The Diels-Alder reaction (a + size 12{"4π"+"2π"} {} cycloaddition) is a very important synthetic method for the preparation of cyclohexanes, mainly because the stereochemistries of the products are so well controlled.

2. Normal Diels-Alder reaction is favoured by electron withdrawing groups (EWG) on the electrophilic dienophile and by electron donating groups on the nucleophilic diene.

Some common examples of dienes and dienophiles:

Dienes

Dienophiles

3. The diene must be cisoid (this refers to the conformation about the single bond). Example: trans-butene does not undergo DA reaction where cis-butene does.

4. The reaction is usually thermodynamically favored due to the conversion of bonds size 12{"2π" - "bonds"} {} into 2 new stronger σ bonds size 12{σ- "bonds"} {} .

5. The Diels-Alder reaction is stereospecific with respect to both the diene and the dienophile.

6. Under the reaction conditions, the product can be broken down to its starting materials known as retro Diels-Alder reaction.

7. To predict the relative rates of reaction of dienes, use these rules.

(a) Only conjugated dienes react as dienes in Diels-Alder processes. Isolated and

cumulated dienes may react as dienophiles, but they cannot react as dienes.

(b) Dienes in the s-trans conformation do not react in Diels-Alder reactions. Acyclic

dienes may rotate around a single bond to become s-cis, but some dienes are locked in the

s-trans conformation in cyclic structures and cannot react.

(c) Dienes containing electron-releasing groups (usually alkyl groups) react faster,

especially with dienophiles that contain electron-withdrawing groups (almost always groups that contain electronegative atoms). The opposite combination also gives relatively fast reactions, but the starting materials are unusual.

Stereochemistry of d.a. reaction

1. Regioselectivity: In absence of solvent and catalytic effect, regioselectivity depends on presence of partial positive or negative character on the terminal carbon centers of dienes or dienophile.

In this case para will be major product. (Convince yourself of this before you continue)

2. Stereoselectivity: Addition of two molecules is syn on both components (bonds form from same species at the same time). This is shown by the examples below:

cis-dienophile gives cis-substituents in the product.

trans-dienophile gives trans-substituents in the product.

If both substituents on the diene are Z, then both end up on the same face of the product.

If substituents on the diene are E and Z, then they end up on opposite faces of the product.

A study of the HOMO and LUMO orbitals for the reactions would probably prove very useful in determining (a) The preference for the endo product (b) Substituent effects on the rate of reaction (c) Substituent effects on the regiochemistry of the reaction. Nucleophilic (diene) characterized by the HOMO and Electrophilic (dienophile) characterized by the LUMO.

Questions & Answers

find the 15th term of the geometric sequince whose first is 18 and last term of 387
Jerwin Reply
I know this work
salma
The given of f(x=x-2. then what is the value of this f(3) 5f(x+1)
virgelyn Reply
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Abhi
how do they get the third part x = (32)5/4
kinnecy Reply
can someone help me with some logarithmic and exponential equations.
Jeffrey Reply
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ninjadapaul
20/(×-6^2)
Salomon
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ninjadapaul
I don't understand what the A with approx sign and the boxed x mean
ninjadapaul
it think it's written 20/(X-6)^2 so it's 20 divided by X-6 squared
Salomon
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I got X =-6
Salomon
ok. so take the square root of both sides, now you have plus or minus the square root of 20= x-6
ninjadapaul
oops. ignore that.
ninjadapaul
so you not have an equal sign anywhere in the original equation?
ninjadapaul
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salma
Commplementary angles
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what is a good calculator for all algebra; would a Casio fx 260 work with all algebra equations? please name the cheapest, thanks.
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a perfect square v²+2v+_
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Abdirahman Reply
algebra 2 Inequalities:If equation 2 = 0 it is an open set?
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or infinite solutions?
Kim
The answer is neither. The function, 2 = 0 cannot exist. Hence, the function is undefined.
Al
y=10×
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rolling four fair dice and getting an even number an all four dice
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Kristine 2*2*2=8
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Differences Between Laspeyres and Paasche Indices
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No. 7x -4y is simplified from 4x + (3y + 3x) -7y
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Need to simplify the expresin. 3/7 (x+y)-1/7 (x-1)=
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. After 3 months on a diet, Lisa had lost 12% of her original weight. She lost 21 pounds. What was Lisa's original weight?
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what's the easiest and fastest way to the synthesize AgNP?
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I start with an easy one. carbon nanotubes woven into a long filament like a string
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Porter
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Yasmin
what is the function of carbon nanotubes?
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I'm interested in nanotube
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what is nano technology
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what is system testing?
AMJAD
preparation of nanomaterial
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Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
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good afternoon madam
AMJAD
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AMJAD
what is the application of nanotechnology?
Stotaw
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
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anybody can imagine what will be happen after 100 years from now in nano tech world
Prasenjit
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
Azam
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
Prasenjit
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
Damian
silver nanoparticles could handle the job?
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Azam
Hello
Uday
I'm interested in Nanotube
Uday
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Prasenjit Reply
At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
Ali Reply
the Beer law works very well for dilute solutions but fails for very high concentrations. why?
bamidele Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Chem217labsfall07. OpenStax CNX. Oct 16, 2007 Download for free at http://cnx.org/content/col10463/1.4
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