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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
If the rate does not depend on the concentration of the nucleophile, what is it?
When does the nucleophile react?
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
- unimolecular x2
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
- second order rx
- lose a proton
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_
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
Compare and contrast Sn1 and Sn2 reaction energy diagrams?
the Sn1 has a true intermediate, the carbocation, which appears as a relative min in the diagram
Reagents and conditions that favor formation of the __ do what to the Sn1 reaction?; reagents and conditions that hinder its formationi __.
- retard the reaction
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
- the stability of the carbocations
- highly substituted carbocations
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
__ 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
- aryl cation
- back-side attack
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
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
__ 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.
__ 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.
- formation and separation
- highly polar solvents
One measure of a solvent's ability to solvate ions is its __, a measure of the solvent's polarity.
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.
- carbocation intermediate
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
- optically active
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 __.
- retention of configuration
- inversion of configuration
__ 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
- 1: ionization of a tetrahedral carbon gives a flat carbocation
- 2: a nucleophile may attack either side of the carbocation.
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
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.
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
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 __.
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
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 __.
- hydride shift
- alkyl shift
- secondary carbocations
- tertiary or resonance-stabilized carbocations