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When a solvent is the nucleophile, it is called __.
How can you know when something isnt Sn2?
- a solvolysis
- because Sn2 requires a strong nucleophile and a substrate that isn't hindered
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If the rate does not depend on the concentration of the nucleophile, what is it?
When does the nucleophile react?
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This type of substitution is called an __, for __, __, __. The term __ means there is only one molecule involved int he transition state of the rate limiting step.
- Sn1 reaction
- sub.
- nucleophilc
- unimolecular x2
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The Sn1 mechanism is what? The first step is a __. The second is a __.
The carbocation is a strong __, reacting very fast with nucleophiles, including weak nucleophiles. The __ in Sn1 is usualyl weak ebcause a strong one would be more likely to attack the substrate and force some kind of __. If the __ is an uncharged molecule, the positively charged product msut __ to give the final uncharged product.
- multistep process
- slow ionization to form a carbocation
- fast attack on the carbocation by a nucleophile
- electrophile
- nucleophile
- second order rx
- nucleophile
- lose a proton
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Key mechanism of Sn1 reactions
- 1: form carbocation (rate-limiting)
- 2: nucleophilic attack on the carbocation (fast) (if the nucleophile is water or an alcohol, a third step is needed to deprotonate the product_
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The reaction energy diagram of hte Sn1 reaction shows what?
The ionization (first step) is __, and its large EA determines what?
The nucleophilic attack (2nd step) is __, with a __.
In effect, a nucleophile reacts with the __ when?
- why the rate does not depend on the strength or concentration of the nucleophile
- highly endothermic
- the overall reaction rate
- strongly exothermic
- lower-energy transition state
- the carbocation
- almost as soon as it forms
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Compare and contrast Sn1 and Sn2 reaction energy diagrams?
- intermediates?
the Sn1 has a true intermediate, the carbocation, which appears as a relative min in the diagram
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Reagents and conditions that favor formation of the __ do what to the Sn1 reaction?; reagents and conditions that hinder its formationi __.
- accelerate
- retard the reaction
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The rate-limiting step of the Sn1 reaction is what?
The first TS resembles the __ adn the rates of teh Sn1 reaction depend strongly on __.
__ are more stable.
ionization to form a carbocation, a strongly endothermic process
- carbocation
- the stability of the carbocations
- highly substituted carbocations
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Reactivity toward Sn1 sub mechanisms follows the stability of carbocations in what order?
The order is __ the Sn2 reaction? Why?
- tertiary > secondary > primary > methyl
- opposite because alkyl groups enhance the Sn1 by stabilizing the carbocation intermediate but hinder the Sn2 by blocking attack of teh strong nucleophile
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__ of the carbocation can also promote the Sn1 reaction.
__ and __ generally do not undergo Sn1 or Sn2 reactions. An Sn1 reaction would require __ to form a __ or __, either of which is less stable than most alkyl carbocations. An Sn2 reaction would require __ by the nucleophile, which is made impossibly by the repulsion of the electrons in the double or aromatic rign.
- resonance stabilization
- vinyl and aryl halides
- ionization
- vinyl
- aryl cation
- back-side attack
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The leaving group is breaking its bond to carbon in the __ of the Sn1 mechanism. A __ helps stabilize the __ through partial bonding as it leaves. The leaving group should be a __, very stable after it leaves with the pair of electrons that bonded it to carbon.
- rate-limiting ionization step
- highly polarizable leaving group
- rate-limiting transition state
- weak base
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The Sn1 reaction goes much more readily in __ that stabilize ions. The __ forms two ions, and ionization is taking place in the __.
__ solvate these ions by an interaction of the solvent's __ with the charge of the ion.
- polar solvents
- rate-limiting step
- transition state
- polar solvents
- dipole moment
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__ such as alcohols and water are even more effictive solvents because __ form H bonds with the -OH hydrogen atom, and cations complex with the nonbonding electrons of the -OH oxygen atom.
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__ of an alkyl halide requires __ and __ of positive and negative charges, similar to what happens when NaCl dissolves in water. Therefore, Sn1 reactions require __ that strongly solvate ions.
- ionization
- formation and separation
- highly polar solvents
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One measure of a solvent's ability to solvate ions is its __, a measure of the solvent's polarity.
dielectric constant
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The Sn2 reaction is __: the nucleophile attacks from the back side of the electrophilic carbon atom, giving __ of configuration.
In contrast, the Sn1 reaction is not __. In the Sn1 mechanism, the __ is sp2 hybridized and planar.
- stereospecific
- inversion
- stereospecific
- carbocation intermediate
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A nucleophile can attack the __ from either face.
Attack from both faces gives __ of the product. Such a process, giving both __ of the product, is called __. The product is either __ or at least less __ than the starting material.
- the carbocation
- both enantionmers
- enantiomers
- racemization
- racemic
- optically active
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If a __ attacks the carbocation from the front side (the side the leaving group left), the product molecule shows __. Attack from the back side gives a product molecule showing __.
- nucleophile
- retention of configuration
- inversion of configuration
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__ is simply a combo of retention and inversion. When __ occurs, the product is rarely completely __, however; there is often more __ than __ of configuration. As the leaving group leaves, it partially blocks the front side of the carbocation. The back side is unhindered, so attack is more likely there.
- racemization x2
- racemic
- inversion
- retention
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Racemization techniques.
- 1: ionization of a tetrahedral carbon gives a flat carbocation
- 2: a nucleophile may attack either side of the carbocation.
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Carbocations frequently undergo structural changes, called __, to form more stable ions. A __ may occur after a carbocation has formed or it may occur as the leaving group is leaving. __ are not seen in Sn2 reactions, where no __ is formed and the one-step mechanism allows no opportunity for __.
- rearragements x3
- carbocation
- rearrangement
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The rearranged product results from a __, the movement of a hydrogen atom with its bonding pair of electrons. A __ is represented by the symbol ~H. In this case, the __ converts the initially formed secondary carbocation to a more stable tertiary carbocation. Attack by the __ gives the rearranged product.
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Hydride shift mechanism
- 1: unimolecular ionization gives a carbocation
- 2: a hydride shift forms a more stable carbocation (this rearrangment involves movement of a H atom with its bonding pair of electrons over to the empty p orbital of the carbocation.
- 3: Solvent (a weak nucleophile) attacks the rearranged carbocation
- 4: Deprotonation gives the rearranged product
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When neopentyl bromide is boiled in ethanol, it gives only a rearranged substitution product. This products results from a __, the migration of amethyl group together with its pair of electrons. Without rearrangement, ionization of neopentyl bromide wiould give a very unstable __.
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Steps of methyl shift in an Sn1 reaction.
- 1: ionization occurs with a methyl shift
- 2: Attack by ethanol gives a protonated version of the rearranged product
- 3: deprotonation gives the rearranged product
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Because rearrangement is required for __, only rearranged products are observed. In general, we should expect __ inr eactions involving carbocations whenever a __ or an __ can form a more stable carbocation. Most rearrangements convert __ to __ or __.
- ionizaition
- rearrangements
- hydride shift
- alkyl shift
- secondary carbocations
- tertiary or resonance-stabilized carbocations
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