An alkyl halide is often referred to as the __: literally, the compound that is being attacked by the reagent. Besides alkyl halides, a variety of other types of compounds serve as _- in __ reactions. To be a good __ for __ attack by a __, a molecule must have an __ with a good __, adn that carbon atom must not be too __ for a __ to attack.
solvent
solvents
SN2
solvent
Sn2
nucleophile
electrophilc C atom
leaving group
sterically hindered
nucleophile
A leaving group serves two purposes in the Sn2 reaction?
It polarizes the C-X bond, making the carbon atom electrophilic
It leaves with the pair of electrons that once bonded it to the electrophilc carbon atom.
To fill these roles, a good leaving gorup should be what?
electron withdrawing, to polarize the carbon atom
stable (not a strong base) once it has left, and
polarizable, to stabilize the transition state
The leaving group must be __ to create a __ on the carbon atom, making the carbon __. An __ leaving gorup also stabilizes the negatively charged transition state.
electron withdrawing
partial positive charge
electrophilic
electron-withdrawing leaving group
__ are strongly electronegative, so alkyl halides are common __ for __ reactions. O, N, and S also form strongly __ with carbon; given the right substituents, they can form the basis for excellent __.
halogens
substrates
Sn2
polarized bonds
leaving groups
The leaving group must be __ once it has left with the pair of electrons that bonded it to carbon. A stable leaving group is needed for __. The leaving group is leaving in the __; a reactive leaving group would do what?
stable
favorable energetics
transition state
raise the energy of the transition state, slowing down the reaction
Also, the energy of the leaving group is reflected in the energy of what?
A reactive leaving group would raise the energy of the __, driving the equilibrium towards what?
the products
products
reactants
Good leaving groups should be __; therefore, they are the __ of __. The __ are strong, and their conjugates are weak. Other weak bases, such as sulfate ions, sulfonate ions, and phosphate ions, can also serve as good __.
weak bases
conjugate bases
strong acids
hydrohalic acids (HCl, HBr, HI)
leaving groups
__, __ and other strong bases are poor leaving groups for Sn2 reactions
When can a neutral group be a good leaving group? An example.
The need to protonate alcohols limits the choice of nucleophiles to those few that are ?
hydroxide ion
alkoxide ions
when it leaves from a positively charged species
water
weak bases
Finally, a good leaving group should be __. Why?
polarizable
to maintain partial bonding with the carbon atom in the transition state
This bonding helps __ the __ and reduce the __.
The departure of a leavving group is like the attack of a nucleophile, except hta tthe bond is __, not __.
stabliize
transition
Ea
breaking
forming
Polrizing nucleophiles and polarizable leaving groups both stabilize the __ by engagin gin more bonding at a longer distance. __, one of the most polarizable ions, is both a good nucleophile and a good leaving group. __, however, is a hard ion.
transition state
iodide ion
fluoride
True or False:
All alkyl groups undergo Sn2 reactions at the same rate.
What is the most important factor in its reactivity towards Sn2 displacement?
False: differnet rates
the structure of the substrate
The reaction goes rapidly with __ and with most __. It is slower with __. __ fail to react at all by the Sn2 mechanism.
methyl halides
primary halides
secondary halides
tertiary halides
All slow-reacting compounds have one property in common: what
the back side of the electrophilic carbon atom is crowded by the presence of bulky groups.
tertiary halides are more hindered than secondary halides, which are more hindered than primary halides
Even a bulky primary halide undergoes __ reaction at a rate similar to that of a tertiary halide. The relative rates show that it is the bulk of the __, rather than an electronic effect, that hinders the reactivity of __ in the __ displacement.
Sn2
alkyl groups
bulky alkyl halides
Sn2
This effect on the rate is another example of __. When the nucleophile approaches the back of the __, it must come within bonding distance of hte __ of the C-X sp3 orbital. If there are two alkyl groups bonded to the carbon attom, this process is difficult. __ make it impossible. Just one can produce a large amount of __ if it is unusually bulky.
electrophilic carbon atom
back lobe
three alkyl groups
steric hindrance
The Sn2 reaction requires what?
attack by a nucleophile on the back side of an electrophilic carbon atom
A carbon atom can have only __, so the leaving group must leave as the __ bonds ot the carbon atom. The nucleophile's electrons intsert into the __ of carbon's sp3 hybrid orbital in its __ combo with the orbital of the leaving gorup because the bonding MO is already filled.
four filled bonding orbitals
nucleophile
back lobe
anticbonding combination
The transition state shows what?
partial bonding to both the nucleophile and the leaving group
__ literally turns the tetrahedron on the carbon atom inside out. In the product, the nucleophile assumes a __ position opposite the position the leaving gorup originally occupied. We call this an __ at the C atom.
back-side attack
stereochemical position
inversion of configuration
In the case of an __, __ gives the opposite configuration of the carbon atom. The __ is the most common example of a __, a step where an asymmetric carbon atom undergoes inversion of cionfiguration.
asymmetric C atom
back-side attack
Sn2 displacement
Walden inversion
In some cases, __ of configuration is readily apparent.
The Sn2 dsplacement is a good example of a __: one in which different stereoisomers reaect to give different stereoisomers of the product.
