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Grignard reagents
- Subset of organometallic reagents
- Can be separated from spectator ions
- May be simplified to carboanions
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Hydride reagents
- May be simplified to hydride (H-)
- Common reducing agents
- Example: NaBH4 or LiAlH4
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SN2
Concerted, involves two species in one step, there is always a LG and nucleophile
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Nucleophile
Has negative charge that can be used to form new bond
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Substrate
The molecule containing the LG
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Coordination
When a bond forms between two species (nucleophile and electrophile)
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Heterolysis-
When a bond breaks and the electrons involved end up on one of the atoms that was originally involved
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Nucleophilic addition
- Nucleophile adds to polar pi bond
- Breaks pi bond so that the nucleophile can attach to the molecule
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Nucleophile elimination
A pi bond is formed when the nucleophile is kicked off
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E2
- Concerted, uses a base, there is nothing replacing the LG
- Will always form a pi bond at the end
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Electrophile addition
When a non-polar pi bond breaks to accomodate an electrophile
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Electrophile elimination
When an electrophile is kicked off to make a new non-polar pi bond
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Carbocation rearrangements
- Can be 1,2-hydride or methyl shift
- When a H or CH3 are shifted from one carbon to the next to provide more stabilization
- The higher the number of C the more stabilized it will be with + charge, and the more likely it will be to give up the CH3 or H
- Opposite of coordination
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Driving force
It's better to have charge stability than bond energy
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Keo-enol tautomerization
- Keto- the H is on the alpha carbon
- Enol- the H is on the O (makes an alcohol)
Normally keto is favored
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Intermediates:
- Highest in energy
- Occur between two transition states
- Correspond with:
- n elementary steps, n transition states, n-1 intermediates
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Are there any intermediates for SN2/E2
No, because they are concerted
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What does the rate of SN2 depend on?
Both the substrate/LG and the nucleophile
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What does the rate of SN1 depend on?
Only the rate of the substrate/LG
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What does the rate of E2 depend on?
The concentration of the substrate/LG and the base
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What does the rate of E1 depend on?
Just the concentration of the substrate/LG
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Why is SN1 stereoselective?
The leaving and substitution do not happen in the same step, so the nucleophile does not run into steric hinderance
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Why is SN2 stereospecific?
The LG and substitution are happening at the same time so the nucleophile can run into steric hinderance or charge repulsion if it goes to the same plane as the LG
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Which reaction is more sensitive to leaving group ability?
SN1
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Better leaving groups tend to be ____ bases
Weaker
The more stable the LG is (in the form in which it has left), the better it's leaving group ability is
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What are the three things to consider when looking at a C-LG?
- Hybridization of the carbon
- # of alkyl groups bonded to the carbon
- Proximity of double and triple bonds
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What type of solvent weakens the attacking group?
Protic, because the ion-dipole forces are stronger and can solvate the attacking group, making it weaker
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What type of solvent has no affect on the strength of the attacking group?
Aprotic, because the ion-dipole forces are not as strong and do not solvate the attacking group, so it's strength is not affected
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Is there a reversal of the nucleophilicites in protic vs aprotic solvents?
Yes
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The more solvated a nucleophile is, the more ___ it is
- Stable
- Meaning it becomes less nucleophilic
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Polar protic can solvate:
Both cations and anions
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Polar aprotic solvents can solvate:
Only cations
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When is more than one diastereomer produced during an E2 reaction?
When there is more than one H that can be removed from the same carbon. But the H that is anticoplanar to the LG will produce the most stable diastereomer
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Solvent mediated proton transfer
When the solvent helps in moving an H from one of the groups that is already bonded to the molecule to another functional group on the same molecule
Usually it isn't feasible for there to be an intramolecular proton transfer, so this happens instead
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What is the order of disubstituted carbons ranking from least stable to most stable
Cis<Trans<Geminal
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Why does heat favor elimination reactions over substitution?
Bc entropy is increased when there is an elimination reaction (3 products instead of 2)
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