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20 Cards in this Set

  • Front
  • Back

What is a conjugated system?

Alternating single and double bonds

Why are conjugated systems more stable than isolated double bonds?

• Double bonds are sp2 hybridized -> there is 1 unhybridized p orbital per carbon

• These p orbitals overlap one another, sharing electron density

• The more spread out the electrons, the more stable the molecule

What is the relationship between heat of hydrogenation and stability?

The lower the heat of hydrogenation, the more stable the molecule

What 3 factors determine the order of increasing heat of hydrogenation?

1) The less double bonds, the lower the ∆H

2) The more conjugated, the lower the ∆H

3) The more substituted the Cs, the lower the ∆H

2 and 3 create more stable molecules:

Basically, the more stable, the lower the ∆H

True or false:

The heat of hydrogenation for a conjugated system is equal to the sum of its individual double bonds.


The heat of hydrogenation for conjugated double bonds is lower than the sum of its parts

Because it's more stable!

When electrophilic addition (ex. HBr) is being conducted upon a diene, but there is only 1 equivalent of the electrophile, how do you know which double bond will be attacked?

FIRST: What not to do

SECOND: What to do

What not to do:

DO NOT just look at which DB is on the highest degree carbon -> the cation will not go to the highest degree C if this is not the one which allows resonance.

What to do:

Assess which double bond would give the highest degree PAIR of carbocations

A) Before resonance

B) After resonance

What do the 1,2 and 1,4 products mean?

The two major products of 1 eq. electrophilic addition to dienes

1,2 = when halogen is added to the 2nd carbon; product of pre-resonance carbocation

1, 4 = when halogen is added to the 4th carbon; product of post-resonance carbocation

Of the two major products from a 1 eq. electrophilic addition to dienes, how is it determined which of those predominates?

What are the controlled products called?

Controlled by the conditions:

Kinetically controlled = always 1,2 product

Thermodynamically controlled = can be either 1,2 or 1,4 or NEITHER

Kinetically controlled product




• Always the 1,2 product

• Predominates under low temp

• Results from the proximity effect = halogen is simply closer to the 2nd C carbocation, and so this addition happens first and fastest

Thermodynamically controlled product



How forms?

How to know which is the thermo product?

• Can be either the 1,2 or 1,4 product or NEITHER

• Predominates under high temp

• The 1,2 product forms first, but there is enough heat energy to reverse the reaction and form the 1,4 -> an equilibrium is established

• Product which contains the remaining DB between the MOST substituted carbons -> highest pair wins

When is there no thermodynamically controlled product?

When there is a tie between the degrees of the carbons containing the remaining double bond in each product

Ex. when both 1,2 and 1,4 products have a double bond between two 2nd degree carbons

What are the reagents of a Dies-Alder reaction?

What is required for the arrow?

Draw mechanism

1) A conjugated 4 carbon diene
2) Either a 2 carbon alkene or alkyne dienophile
Occurs in one concerted step!


1) A conjugated 4 carbon diene

2) Either a 2 carbon alkene or alkyne dienophile

Occurs in one concerted step!

When the dienophile is an alkene, what is the product?

When it is an alkyne, what is the product?

Alkene: cyclohex-1-ene

Alkyne: cyclohex-1,4-diene

What increases the reactivity of a diene?


Donating groups that have lone pairs to resonate into the system





-R doesn't resonate, but has inductive effects

What increases the reactivity of a dienophile?


Withdrawing groups that pull electron density away from the pi bond


-CN (C≡N)


Is stereochemistry retained or inverted from a Diels-Alder reaction?

Is it syn or anti?

Stereochemistry is retained

Use dashes and wedges to represent

This is a syn addition

What is s-cis and s-trans conformation?

Which one undergoes the reaction?

What happens if you have the other?

s-cis = when diene is in the C conformation, both double bonds are facing the same direction

s-trans = when diene is in a zig-zag conformation, double bonds are going in opposite directions

Only s-cis can undergo the Diels-Alder reaction, but if s-trans is present, it will simply rotate around the single bond into the s-cis conformation

What are the products when both the diene and dienophile are unsymmetrically substituted?

What is the major product?

Two products are possible which are constitutional isomers of one another

The major product will always have either a 1,2 or 1,4 substituted configuration

1,3 is negligibly minor product

How is the 1,2 or 1,4 product successfully obtained?

The most negative carbon on the diene and the most positive carbon on the dienophile must line up with one another

How can you tell which carbon is the most negative or positive?

Through resonance from either the electron donating or withdrawing groups