[AMEDIO]

http://img823.imageshack.us/i/32895350.png/

The correct order is:

(A) x>y

(B) x<y

(C) x=y

(D) can't predict

i think its x<y coz i have read that electron withdrawing substituents in the dienophile such as CHO , CN , NO2 etc. promote the reac.
and also by presence of electron releasing groups in dienes...but the answer given is x>y
can anyone explain why??and also i'll be glad to know why these  electron withdrawing substituents in the dienophile such as CHO , CN , NO2 etc. promote the reac.
and also by presence of electron releasing groups in dienes this happens??

[Polytriazole]

I think I see where you went wrong...you've got the reactant names backwards.

The diene is the compound with the two alkenes (carbon-carbon double bonds) in it, the compound on the left in both of your drawings.  This compound reacts better when it is functionalized with electron-donating groups like alkyl or alkoxy groups.

The dienophlie is the compound with a single double bond, so called because it attracts the diene.  This compound reacts better when it has electron-withdrawing groups attached like nitro, carbonyl, and alcohol groups. 

In your diagram, the second equation contains a diene with an electron-donating group (a methyl group) and a dienophile with an electron-withdrawing group (a nitro group), thus making each of the reactants less likely to react.

[AMEDIO]

oops sorry for that...
actually its the other way round only
but why is this case an exception....

[AMEDIO]

http://img823.imageshack.us/i/32895350.png/

The correct order is:

(A) x>y

(B) x<y

(C) x=y

(D) can't predict

i think its x<y coz i have read that electron withdrawing substituents in the dienesuch as CHO , CN , NO2 etc. promote the reac.
and also by presence of electron releasing groups in dienophiles...but the answer given is x>y
can anyone explain why??and also i'll be glad to know why these  electron withdrawing substituents in the diene such as CHO , CN , NO2 etc. promote the reac.
and also by presence of electron releasing groups in dienophile this happens??

[AMEDIO]

i have corrected it here

http://img823.imageshack.us/i/32895350.png/

The correct order is:

(A) x>y

(B) x<y

(C) x=y

(D) can't predict

i think its x<y coz i have read that electron withdrawing substituents in the dienesuch as CHO , CN , NO2 etc. promote the reac.
and also by presence of electron releasing groups in dienophiles...but the answer given is x>y
can anyone explain why??and also i'll be glad to know why these  electron withdrawing substituents in the diene such as CHO , CN , NO2 etc. promote the reac.
and also by presence of electron releasing groups in dienophile this happens??

[medist]

I think the basic principle in DA reacn is the interaction btwn HOMO of the diene and LUMO of the dienophile. EWG on the dienophile reduces the energy of the LUMO and EDG on the diene increase the energy of the diene. So that the energy gap between the HOMO and LUMO decreases and the  rate of reaction increases.

I'm sorry if its not correct.

 

[AMEDIO]

its actually the other way round
ewg is on diene and edg on dienophile
i had typed it wrongly initially
moreover , u can also check in the fig.
so plzz can anyone explain it now??

[Arctic-Nation]

Polytriazole and surya are correct in their explanations, but you since you're still confused I'll try to summarize it.

In a normal Diels-Alder reaction, a diene (a compound with two double bonds in a cis configuration) reacts with a dienophile (a compound with a double bond). The diene reacts through its HOMO and thus benefits from electron-donating substituents, which increase its energy. The dienophile reacts through its LUMO, and benefits from electron-withdrawing substituents which lower its energy.

In your case, you have system without any substituents, which can be assumed to be at 'zero-level' in substituent influence. Your other system has a dienophile with an EWG (thus lower energy, instead of increased), and a diene with an EDG (thus increased energy, instead of lowered). This creates a kind of energy-mismatch, with reacting orbitals being pushed away from each other rather than being brought closer together. Thus, the reaction rate will be decreased compared to the 'zero-level'. In other words, x>y.

Hope this clears things up.

[medist]

nice explanation by Arctic