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Comparison of Sn1 and Sn2:
Effect of nucleophile
- Sn1: nucleophile strength is unimportant (usually weak)
- Sn2: strong nucleophiles are required
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The nucleophile takes part in the __ of the Sn2 reaction but in the __ of the Sn1. Therefore, a strong __promotes the Sn2 but not the Sn1.
Weak __ fail to promote the Sn2 reaction; therefore, the reactions with weak __ often go by the Sn1 mechanism if the substrate is __ or __.
- slow step x2
- nucleophile x4
- Sn1
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The structure of the substrate is an important ifactor in determining the sub mechanism.
Most __ and __ are poor substrates for Sn1 becuase they cannot do what?
However, what makes them good for Sn2?
- methly halides and primary halides
- easily ionize to high-energy methyl and primary carbocations
- they are unhindered
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Tertiary halides are too __ to undergo Sn2 reactions, but they can ionize to form __. Tertiary halides undergo substiution through the __ mechanism. Secondary halides can undergo substitution by __.
- hindered
- tertiary carbocations
- Sn1
- either mechanism
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The slow step of the Sn1 reaction involves formation of __. Solvation of these ions is crucial to __ and __ for their formation. Very __ such as water adn alcohol are needed for the Sn1. The solvent may be heated to reflux to provide the energy needed for ionization.
- two ions
- stabilizing them
- lowering the activation energy
- polar ionizing solvents
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Less __ is generated in the __ of the Sn2 reaction. Strong solvation may weaken the strength of the __ because of the energy needed to strip off the solvent molecules .Thus, the Sn2 reaction goes faster in __ if the nucleophile will dissolve. __ may enhance the strength of weak nucleophiles.
- charge separation
- transition state
- nucleophile
- less polar solvents
- polar aprotic solvents
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Summarize effect of solvent for both mechanisms
- Sn1: good ionizing solvent required
- Sn2: may go faster in less polar solvent
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The rate of the sn1 is proportional to hat?
The rate of Sn2 is proportional to what?
- the concentration of the alkyl halide but not hte concentration of the nucleophile (first order rate equation)
- concentrations of both the alkyl halide adn the nucleophile (second order)
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The Sn1 reaction involves a __ that cab be attacked from either face, giving a mix of __ and _.
The Sn2 takes place through a __, which __ the stereochem of hte carbon atom. __ is the result
- flat carbocation intermediate
- inversion and retention of configuration
- back-side attack
- complete inversion of configuraiton
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The Sn1 reaction involves a __, which can __, usually by a __ or an __, to give a more __.
- carbocation intermediate
- rearrange
- hydride or aklyl shift
- stable carbocation
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The Sn2 reaction takes place in __ with __. No __ is possible.
Generalize rearrangement in both.
- one step
- no intermediates
- no rearragenemt
- Sn1: common
- Sn2: impossible
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An __ involves the loss of two atoms or groups from the substrate, usually with the formation of a __. Elimination reactions frequently accompany and compete with __.
- elimination
- pi bond
- substituion reactions
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Depending on teh reagents and conditions, elims can be __ or __.
first or second order
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The abbreviation E1 stands for __ , __. The mechanism is __ because the __ involves a single molecule rather than a collision between two molecules.
- elimination, unimolecular
- unimoleuclar
- rate-limiting TS
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The slow step of an E1 reaction is the same as in the Sn1 reaction: __. In a fast second step, a base abstracts a proton from the carbon atom adjacent the __. The electrons that once formed the c-h bond now form a __ between two carbon atoms.
- unimolecular ionization to form a carbocation
- C+
- pi bond
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The E1 reaction requires __, so it follows the same order of reactivity: __
Mechanism of E1
- ionization to a carbocation intermediate
- tertiary> secondary> prim> methyl
- 1: unimolecular ionization to give a carbocation (rate-limiting)
- 2: deprotonation by a weak base (often the solvent) gives the alkene (fast)
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Because the __ involves __ of the __, the rate equation is __. The rate depends on __ and not on __.
The weak base taeks part in the __.
- rate limiting step
- unimolecular iionization
- alkyl halide
- first order
- concentration of the alkyl halide only
- the strength or concentration of hte base
- fast second step of the reaction
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The E1 reaction almost always competes witht eh __. Whenever a carbocation is formed, it can undergo either __ or __, and __ often result.
__ is an elimination of a hydrogen and a halogen atom. Under the first order conditions (absence of a strong base), __ takes place by the E1 mechanism: Ionization of hte alkyl halide gives a __, which loses a proton to give the alkene. __ results from __ on the carbocation.
- Sn1 reaction
- sub or elim
- mixtures of products
- dehydrohalogenation x2
- carbocation intermediate
- sub
- nucleophilic attack
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Why arent Sn1 and E1 not really used?
they produce mixture of products
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In the second step of the E1 mechanism, the carbon atom next to the C+ must __ to __ as the base attacks the protons and electrons flow intot he new __.
Ionization step is __, with a __. Teh second step is a __by a base. The base is not invoveld int he reaction until __, so the rate depends only on the __. Weak bases are __ in E1 reactions.
- rehybridize
- sp2
- pi bond
- strongly endothermic
- rate limiting step
- fast exothermic deproonation
- after the RLS
- concentration of the alkyl halide
- common
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Like other carbocation reactions, teh E1 may be accompanied by __.
Difference of rearrangement in Sn1 and E1
- rearrangement
- the solvent acts like a base in E1 and a nucleophile in Sn1
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A carbocation can ?
- 1) react with its own leaving group to return to the reactant
- 2) react with a nucleophile to form a substitution product
- 3) lose a proton to form an elim product
- 4) rearrange to a more stable carbocation, then react further
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