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20 Cards in this Set
- Front
- Back
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What is a conjugated system? |
Alternating single and double bonds |
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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 |
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What is the relationship between heat of hydrogenation and stability? |
The lower the heat of hydrogenation, the more stable the molecule |
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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 |
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True or false:
The heat of hydrogenation for a conjugated system is equal to the sum of its individual double bonds. |
False!
The heat of hydrogenation for conjugated double bonds is lower than the sum of its parts
Because it's more stable! |
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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 |
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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 |
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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 |
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Kinetically controlled product Which? When? How? |
• 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 |
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Thermodynamically controlled product Which? When? 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 |
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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 |
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What are the reagents of a Dies-Alder reaction? What is required for the arrow?
Draw mechanism |
Reagents: 1) A conjugated 4 carbon diene 2) Either a 2 carbon alkene or alkyne dienophile
Occurs in one concerted step! |
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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 |
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What increases the reactivity of a diene? 5 |
Donating groups that have lone pairs to resonate into the system
-OH -OR -NH -S -R doesn't resonate, but has inductive effects |
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What increases the reactivity of a dienophile? 3 |
Withdrawing groups that pull electron density away from the pi bond
carbonyls -CN (C≡N) -NO₂‾ |
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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 |
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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 |
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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 |
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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 |
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How can you tell which carbon is the most negative or positive? |
Through resonance from either the electron donating or withdrawing groups |
