Chapter 6 Flashcards
What features do all alkene reactions gave in common?
What happens if the alkene does not have the same substituents on both sp2 carbons? Which sp2 carbon gets the hydrogen?
we need to determine which cation forms at a faster rate by evaluating their stabilities
In a addition of alkene with a hydrogen halide, where is the positive charge on the intermediate cation?
the sp2 carbon that does not become attached to the proton (its electrophile that will take the anion halogen)
How are carbocations classified?
based on the carbon that carries the positive charge:
■ A primary carbocation has a positive charge on a primary carbon.
■ A secondary carbocation has a positive charge on a secondary carbon.
■ A tertiary carbocation has a positive charge on a tertiary carbon.
What determines the stability of carbocations?
stability increases as the number of alkyl substituents attached to the positively charged carbon increases
Why do alkyl groups stabilize carbocations?
because the decrease the concentration of positive charge on the carbon
How do alkyl groups decrease the concentration of positive charge on the carbon?
the orbital of an adjacent C-H sigma bond can overlap an empty p orbital of the positively charged sp2 carbon and decreases its positive charge (gets “shared”) which makes it more stable.
What does the positive charge on a carbocation signifies?
an empty p orbital
What is hyperconjugation?
delocalization of electrons by the overlap of a s bond orbital with an empty orbital on an adjacent carbon
Which sigma bond orbitals can interract with the empty p orbital of the positively charge carbon in a carbocation?
the sigma bond orbitals attached to the positively charge carbons (the ones with red dots)
Why does the most stable carbocation form the more rapidly?
because of the relative stabilities of the transitional states
the same reasons that stabilize the most stable carbocation also stabilizes its transition state, which makes it lower in energy, which makes the activation energy smaller than the less stable carbocation.
On the reaction coordinate diagram, two points that have similar energies mean they also have similar _______.
structure
the closer their energies the more similar their structures
How is the structure of the transition state depending if its a endergonic or exergonic reaction?
Is formation of a carbocation an exergonic or endergonic reaction?
endergonic reaction
In the formation of a carbocation, does the transition state resemble the products or reactants more? Why?
products
because it is an endergonic reaction
Why is the difference in the stabilities of the transition states less than the difference in the stabilities of the carbocations?
because the amount of positive charge in the transition state is less than the amount of positive charge in the product
What is the rate-limiting step in addition reactions?
the formation of the carbocation
What determines if the more rapidly formed carbocation will be the only product of a reaction or both the more rapid and the slower carbocations will be prduced?
depends on the difference in rates of reactions
if the rates are similar, both will be produced with amounts relative to the rates
if rates difference is large, the major product will be the only product (the one that forms the most rapidly)
Which would result in a more stable carbocation?
more hyperconjugation
How are the major and minor product formed from the different carbocations related?
they are constitutional isomers
What are the degrees of regioselectivity?
moderately regioselective, highly regioselective, or completely regioselective
example forming of tertiary and primary carbocations is a more regioselective reaction than one that makes a secondary and a tertiary
How would you qualify this reaction?
not regioselective
To which carbon will the electrophile add itself to?
the electrophile (in this case, H + ) adds preferentially to C-1 because it is the sp2 car- bon bonded to the most hydrogens.
Or we can say that H + adds to C-1 to form a secondary carbocation, which is more stable than the primary carbocation that would be formed if H + added to C-2.
Explain what is a 1,2-hydride shift
a hydride ion shifts from one carbon to an adjacent carbon to achieve a more stable arrangement (eg positive charge on a tertiary carbon instead of secondary or primary)
What is a hydride ion?
Hydrogen with a lone pair (has a negative charge)
What is a 1,2-methyl shift?
What is the major product from reactions with the shift?
rearrangement where the methyl group moves with its electrons from one carbon to an adjacent carbon
the one formed by adding the nucleophile to the rearranged carbocation
What can you observe from the same alkene transformed to alcohol through acid-catalyzed addition of water versus hydroboration-oxydation?
they have their H and OH groups switched
Why is boron in borane regioselective?
- it goes on the most hydrogen sp2 carbon since it is the less substituted and have the most space
- if comparing the transition states, the positive charge is on a secondary carbon in opposed to a primary carbon
What is a vicinal halohydrin?
the halogen and the OH group are bonded to adjacent carbons
