Ch 10 : Answers

Chapter 10 : Conjugation in Alkadienes and Allylic Systems

Conjugation in Alkadienes and Allylic Systems Answers

Qu1: (a) The diene is cyclopentadiene and the dieonphile is furan.The sole formation of 1 (and not 2) at 20^oC is an example of a reaction under kinetic control.

(b) Product 1 is endo and product 2 is exo. Look at the relative position of the furan unit with respect to the bridghead CH₂ unit.

(c) The sole formation of 1 (and not 2) at 20^oC is an example of a reaction under kinetic control.

(d) The mixture of 1 and 2 formed at 80^oC from cyclopentadiene and furan is an example of a reaction under thermodynamic control since heating 1 also provides the same equilibrium mixture.

(e) No since this would require an inversion of sp³ hybridised C atoms..

(f) We can not convert the remaining 10% of 1 into 2 since the 9:1 ratio represents the equilibrium mixture.

(g) The Hammond Postulate indicates that the transition state will be most like the species it is closest to in energy, so if the reaction is exothermic it should be the starting materials, cyclopentadiene and furan.

(h)

Qu 2: (a)

In this Diels-Alder reaction, since the concerted additon reaction is stereospecific, the cis dienophile gives cis products, but since both faces can be attacked enantiomeric products are produced. Compare the position of the methyl group from the diene with the following example.

(b)

In this Diels-Alder reaction, since the concerted additon reaction is stereospecific, the cis dienophile gives cis products, but since both faces can be attacked enantiomeric products are produced. Note how the position of the methyl group has changed in this example.

Qu 3:

First looking for the diene of the Diels-Alder reaction. If you draw the curly arrows for the retro-Diels-Alder reaction, by starting from the C=C in the product then the furan diene is readily revealed and the dienophile must be MeO₂CCH=CHCO₂Me. Given that the product has the two ester groups trans-, the dienophile must have also been trans.

Qu 4:

Qu 5: (a)

In the Diels-Alder reaction the dienophile is usually the electrophile, so electron withdrawing groups will enhance their reactivity. Since the -C=O group is electron withdrawing by resonance, then we get giving iii > ii > i.

(b)

This Diels-Alder reaction requires that the diene be able to adopt an s-cis conformation. Since cyclopentadiene is locked in this conformation, butadiene can achieve it by rotation about the C2-C3 bond. BUT, (i) is locked in s-trans and so it is therefore un-reactive. Thus, ii > iii > i.

(c)

We are looking at the diene component of a Diels-Alder reaction. Benzene does not react like a diene very easily as is it would result in the loss of aromaticity. Between the 2 substituted butadienes, the cis methyl groups in iii sterically destabilise the reactive s-cis conformation making it less favorable, so ii > iii. Overall, ii > iii > i.

Qu1:	(a) The diene is cyclopentadiene and the dieonphile is furan.The sole formation of 1 (and not 2) at 20^oC is an example of a reaction under kinetic control.
	(b) Product 1 is endo and product 2 is exo. Look at the relative position of the furan unit with respect to the bridghead CH₂ unit.
	(c) The sole formation of 1 (and not 2) at 20^oC is an example of a reaction under kinetic control.
	(d) The mixture of 1 and 2 formed at 80^oC from cyclopentadiene and furan is an example of a reaction under thermodynamic control since heating 1 also provides the same equilibrium mixture.
	(e) No since this would require an inversion of sp³ hybridised C atoms..
	(f) We can not convert the remaining 10% of 1 into 2 since the 9:1 ratio represents the equilibrium mixture.
	(g) The Hammond Postulate indicates that the transition state will be most like the species it is closest to in energy, so if the reaction is exothermic it should be the starting materials, cyclopentadiene and furan.
	(h)

Qu 2:	(a)
	In this Diels-Alder reaction, since the concerted additon reaction is stereospecific, the cis dienophile gives cis products, but since both faces can be attacked enantiomeric products are produced. Compare the position of the methyl group from the diene with the following example.
	(b)
	In this Diels-Alder reaction, since the concerted additon reaction is stereospecific, the cis dienophile gives cis products, but since both faces can be attacked enantiomeric products are produced. Note how the position of the methyl group has changed in this example.


Qu 3:
	First looking for the diene of the Diels-Alder reaction. If you draw the curly arrows for the retro-Diels-Alder reaction, by starting from the C=C in the product then the furan diene is readily revealed and the dienophile must be MeO₂CCH=CHCO₂Me. Given that the product has the two ester groups trans-, the dienophile must have also been trans.

Qu 4:

Qu 5:	(a)
	In the Diels-Alder reaction the dienophile is usually the electrophile, so electron withdrawing groups will enhance their reactivity. Since the -C=O group is electron withdrawing by resonance, then we get giving iii > ii > i.
	(b)
	This Diels-Alder reaction requires that the diene be able to adopt an s-cis conformation. Since cyclopentadiene is locked in this conformation, butadiene can achieve it by rotation about the C2-C3 bond. BUT, (i) is locked in s-trans and so it is therefore un-reactive. Thus, ii > iii > i.
	(c)
	We are looking at the diene component of a Diels-Alder reaction. Benzene does not react like a diene very easily as is it would result in the loss of aromaticity. Between the 2 substituted butadienes, the cis methyl groups in iii sterically destabilise the reactive s-cis conformation making it less favorable, so ii > iii. Overall, ii > iii > i.