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Level 193

NucleopHilic Substitution & Elimination


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acid *
A species that can donate a proton.
alkyl halide (haloalkane)
A derivative of an alkane in which one (or more) of the hydrogen atoms has been replaced by a halogen.
alkyl shift (symbolized ~R)
Movement of an alkyl group with a pair of electrons from one atom (usually carbon) to another. Alkyl shifts are examples of rearrangements that convert carbocations into more stable carbocations.
allylic
The saturated position adjacent to a carbon-carbon double bond.
allylic halogenation
Substitution of a halogen for a hydrogen at the allylic position
allylic shift
A rearrangement that results from reaction at either end of a resonance-stabilized allylic intermediate.
Anti
180 ° apart
aprotic solvent
A solvent that has no acidic protons; a solvent with no O--H or N--H groups.
aryl halide
An aromatic compound (benzene derivative) in which a halogen is bonded to one of the carbon atoms of the aromatic ring.
base *
An electron-rich species that can abstract a proton.
concerted reaction
A reaction in which the breaking of bonds and the formation of new bonds occur at the same time (in one step).
dehydrohalogenation
An elimination in which the two atoms lost are a hydrogen atom and a halogen atom.
electrophile (Lewis acid)
A species that can accept an electron pair from a nucleophile, forming a bond.
electrophilicity (electrophile strength)
The kinetic reactivity of an electrophile.
elimination
A reaction that involves the loss of two atoms or groups from the substrate, usually resulting in the formation of a pi bond.
freons *
A generic name for a group of chlorofluorocarbons used as refrigerants, propellants, and solvents. Freon-12® is CF2Cl2, and Freon-22® is CHClF2.
geminal dihalide
A dihalide with both halogens on the same carbon atom.
haloalkane (alkyl halide)
A derivative of an alkane in which one (or more) of the hydrogen atoms has been replaced by a halogen.
halogen exchange reaction
A substitution where one halogen atom replaces another; commonly used to form fluorides and iodides.
hydride shift (symbolized ~H)
Movement of a hydrogen atom with a pair of electrons from one atom (usually carbon) to another. Hydride shifts are examples of rearrangements that convert carbocations into more stable carbocations.
hydroxylic solvent
A solvent containing OH groups (the most common type of protic solvents).
inversion of configuration (see also Walden inversion) *
A process in which the groups around an asymmetric carbon atom are changed to the opposite spatial configuration, usually as a result of back-side attack. The SN2 reaction goes with inversion of configuration.
leaving group
The atom or group of atoms that departs during a substitution or elimination. The leaving group can be charged or uncharged, but it leaves with the pair of electrons that originally bonded the group…
methyl shift (symbolized ~CH3)
Rearrangement of a methyl group with a pair of electrons from one atom (usually carbon) to another. A methyl shift (or any alkyl shift) in a carbocation generally results in a more stable carbocation.
nucleophile (Lewis base)
An electron-rich species that can donate a pair of electrons to form a bond.
nucleophilic substitution
A reaction where a nucleophile replaces another group or atom (the leaving group) in a molecule.
organic synthesis
The preparation of desired organic compounds from readily available starting materials.
polarizable
Having electrons that are easily displaced toward a positive charge. Polarizable atoms can begin to form a bond at a relatively long distance.
primary halide, secondary halide, tertiary halide
These terms specify the substitution of the halogen-bearing carbon atom (sometimes called the head carbon). If the head carbon is bonded to one other carbon, it is primary; it is bonded to two carbons…
protic solvent
A solvent containing acidic protons, usually O-H or N-H groups.
racemization
The loss of optical activity that occurs when a reaction shows neither clean retention of configuration nor clean inversion of configuration.
reagent
The compound that serves as the attacking species in a reaction.
rearrangement *
A reaction involving a change in the bonding sequence within a molecule. Rearrangements are common in reactions such as the SN1 and E1 involving carbocation intermediates.
retention of configuration
Formation of a product with the same configuration as the reactant. In a nucleophilic substitution, retention of configuration occurs when the nucleophile assumes the same stereochemical position in the product as the leaving group occupied in the reactant.
solvolysis
A nucleophilic substitution or elimination where the solvent serves as the attacking reagent. Solvolysis literally means "cleavage by the solvent."
stereocenter
An atom that gives rise to stereoisomers when its groups are interchanged. Asymmetric carbon atoms and double-bonded carbons in cis-trans alkenes are the most common stereocenters.
stereospecific reaction
A reaction in which different stereoisomers react to give different stereoisomers of the product.
steric hindrance
Interference by bulky groups that slow a reaction or prevent it from occurring.
Substrate
Contains aliphatic carbon
syn
Adding to (or eliminating from) the same face of a molecule.
transition state
In each individual step of a reaction, the state of highest energy between reactants and products. The transition state is a relative maximum (high point) on the reaction-energy diagram.
vicinal dihalide
A dihalide with the halogens on adjacent carbon atoms.
vinyl halide
A derivative of an alkene in which one (or more) of the hydrogen atoms on the double-bonded carbon atoms has been replaced by a halogen.
Walden inversion (see also inversion of configuration)
A step in a reaction sequence in which an asymmetric carbon atom undergoes inversion of configuration.
Zaitsev's rule (Saytzeff's rule)
An elimination usually gives the most substituted alkene product. Zaitsev's rule does not always apply, but when it does, the reaction is said to give Zaitsev orientation.
Akyl Halide-
simply has a halogen atom bonded to one of the sp3 hybrid carbon atoms of an akyl group
Vinyl Halide-
has a halogen bonded to one of the sp2 hybrid carbon atoms of an alkene
Aryl Halide-
has a halogen atom bonded to one of the sp2 hybrid carbon atoms of an aromatic ring.
The carbon-halogen bond in an alkyl halide is....
polar because halogen atoms are more electronegative than carbon atoms.
1-chlorobutane
Haloalkane Names-
CH2X2
methylene halide
CHX3
haloform
CX4
carbon tetrahalide
Geminal Dihalide-
has the two halogen atoms bonded to the same carbon atom.
Vicinal Dihalide-
has the two halogens bonded to adjacent carbon atoms.
Solvents-
cleaning and degreasing work
Reagents-
the conversion of alkyl halides to organometallic reagents
Anesthetics-
causes unconsciousness and relaxation.
Freons-
has caused a whole in the ozone layer.
Pesticides-
lindane is used in shampoos to kill lice.
up the column
Electonegativity increases when you go....
down the column
Bond length increases when you go....
With CX4.....
the polar bonds cancel making the molecular dipole moment 0.
The London Force....
is the strongest intermolecular attraction in alkyl hallides.
surface attractions.
London Forces are....
higher boiling points.
Molecules with high molecular weights have....
Van de Waals Radi gives you a.....
general idea of what the surface area is.
water.
Alkyl Halides are densor than...
Free-radical halogenation is....
sometimes useful because the reagents are cheap.
increases.
as ?H increases, the energy needed....
decreases.
as ?H decreases, the energy needed....
Halides are....
good leaving groups.
donor.
Nucleophile means...
Carbon-iodine bonds must begin...
to break when carbon oxygen bonds begins to form.
kr depends on....
energy of the transition state and the temperature.
Transition state-
is a point of maximum energy.
SN2 mechanism-
substitution, nucleophilic biomolecular.
Halogen Exchange-
when one halogen replaces another.
a stronger nucleophile.
A species with a (-) charge is always....
CH3OH
CH3O- >
its conjugate acid.
A base is always a stronger nucleophile than...
Basicity=
equilibrium constant
Nucleophilicity=
rate of attack
the column
Bonding increases as you move down...
Tertiary=
hindered, stronger base, yet weaker nucleophile
Primary=
unhindered, weaker base, yet stronger nucleophile
Protic Solvent-
has acidic protons
Aprotic Solvent-
solvents w/O O-H or n-H groups
Polar Aprotic Solvent-
O-H or n-H groups are not PRESENT; so hydrogen bonds with anions cannot be formed
Good leaving groups are.....
weak bases; conjugate bases of strong acids
hydroxide, alkoxide, and amide
Ions that are strong bases and poor leaving groups:
the most
Tertiary halides are hindered...
the least
Primary halides are hindered.....
Walden Inversion-
Back-side attack inverts the configuration of the carbon atom (umbrella effect)
Stereospecific Reaction-
one in which different steroisomers react to give different stereoisomers of the product.
Solvolysis-
solvent acts as a nucleophile.
SN1 Reaction-
Subsitution, Nucleophilic, Unimolecular
Ionization-
the rate limiting step (1st step)
Nucleophilic Attack-
-2nd Step
as soon as it forms.
Nucleophile reacts with the carbocation almost ....
Sn1
Both mechanisms are sensitive to the nature of the leaving group but the _______ mechanism is more sensitive
SN2
?:CN
Inductive Effect
stabilization of carbocations by donating electrons through sigma bonds.
Hyperconjugation-
stabilization of carbocations through overlap of filled orbitals with the empty P orbital of the carbocation.
3°>2°>1°>CH3X
SN1 reactivity:
undergo SN1 or SN2 reactions.
Vinyl and Aryl Halides do not....
SN1 reactions require...
ionization to form vinyl or aryl cations.
SN2 reaction require .....
the nucleophile to attack from the back side. The double bonds and ring electron repulsion make this impossible.
The leaving group....
breaks bonds with carbon.
weak base.
The leaving group should be a....
negative charge.
The leaving group takes on a ....
in polar solvents.
SN1 reactions go much more readily....
two ions.
The rate limiting step forms...
Ionization takes...
place in the transition state.
Protic solvents are...
more effective because anions form hydrogren bonds with the OH oxygen group.
Dielectric Constant (e)-
a measure of the solvents polarity.
ionization occurs a lot faster..
in highly polar solvents (ex. water & alcohols)
SN1 are not..
stereospecific, because the carbcation intermediate is sp2 hybridized and planar.
In SN1 reactions the nucleophile....
can attack the carbon from either face.
gives both enatiomers of the product.
In SN1 reactions the attack from both faces...
Ionization is..
the formation/seperation of postive and negative charges.
Racemization-
the process that gives both enatiomers of the product; combo of retention and inversion.
Retention of Configuration-
nucleophile attack from the front
Inversion of Configuration-
nucleophile attack from the back
When the leaving group leaves,
it partially blocks the front side, making a nucleophilic attack from the back more favorable.
Rearrangements-
what carbocations undergo to form more stable ions.
The hydride shift transforms...
a secondary carbocation to a tertiary carbocation (~H)
SN1 Stereochemistry:
Mixture of retention and inversion; racemization.
SN2 Stereochemistry:
Complete Inversion (umbrella effect)
SN1:
Rearrangements are common.
SN2:
Rearragements are impossible.
Elimination-
involves the loss of two atoms or groups from the substrate, usually with formation of a pi bond.
In a E1 rate....
the rate only depends on the concentration of the alkyl.
Dehydrohalogenation-
elimination of a hydrogen and halogen atom.
In a E1 reaction-
the base attacks the proton and electrons flow into the new pi bond.
a base
In E1 reactions, solvents act as...
a nucleophile
In SN1 reactions, solvents act as...
Zaitsev's Rule-
in elimination reactions, the most substituted alkene usually predominates.
Concerted Mechanism-
bonds break and form at the same time.
Anti Coplanar-
(?=180°) orbitals are aligned
Syn-Coplanar:
(?=0°) hydrogen and halogen eclipse each other
E1 base strength...
is unimportant (usually weak).
E2 requires...
strong bases.
ionizing solvent.
E1 requires a good....
not so important.
E2 solvent polarity is...
3°>2°>1°
E1, E2:
E1, E2:
Usually Zaitsev orientation.
E2 reactions require..
coplanar arrangement for the transition state.
common
E1: Rearrangements are..
rearrangents.
E2: No..
Acid
A compound containing Hydrogen which it will donate in a reaction.
Acidity (acid strength)
The thermodynamic reactivity of an acid, express quantitatively by the acid dissociation constant (K_a_)
Lewis acid (electrophile)
A species that can accept the electron pair from a nucleophile, forming a bond.
allylic hologenation
Substitution of a halogen for a hydrogen at the allylic position.
anti-coplanar
Having a dihedral angle of 180°.
syn-coplanar
Having a dihedral angle of 0°.
Base
A cleaning product is usually a ACID/BASE?
basicity (base strength)
The thermodynamic reactivity of a base, expressed quantitatively by the base-dissociation constant K_b_.
Lewis base (nucleophile)
An electron-rich species that can donate a pair of electrons to form a bond.
E1 reaction (elimination, unimolecular)
A multistep elimination where the leaving group is lost in a slow ionization step, then a proton is lost in a second step. Zaitsev orientation is generally preferred.
E2 reaction (elimination, bimolecular)
A concerted elimination involving a transition state where the base is abstracting a proton at the same time that the leaving group is leaving. The anti-coplanar transition state is generally preferred. Zaitsev orientation is usu…
freons
A generic name for a group of chlorofluorocarbons used as refrigerants, propellants, and solvents. Freon-12® is CF_2_Cl_2_, and Freon-22® is CHCIF_2_.
inversion of configuration (also see Walden inversion)
A process in which the groups around an asymmetric carbon atom are changed to the opposite spatial configuration, usually as a resultant of back-side attack. The SN_2_ reaction goes with (this term).
Lewis acid
accepts lone electron pair
Lewis Base
A substance that can donate a pair of electrons to form a covalent bond.
methyl shift (symbolized ~CH_3_)
Rearrangement of a methyl group will be pair of electrons from one atom (usually carbon) to another. This (this term) (or any alkyl shift) and a carbocation generally results in a more stable carbocation.
nucleophilicity (nucleophile strength)
The kinetic reactivity of the nucleophile; a measure of the rate of substitution in their reaction with a standard substrate.
rearrangement
In reaction involving a change in the bonding sequence within a molecule. These reactions (this term) are common in reactions such as the S_N_1 and E1 involving carbocation intermediates.
substitution (displacement)
A reaction in which an attacking species (nucleophile, electrophile, or free radical) replaces another group.
S_N_2 reaction (substitution, nucleophilic, bimolecular)
The concerted displacement of one nucleophile by another on an sp^3 hybrid carbon atom.
S_N_1 reaction (substitution, nucleophlic, unimolecular)
A two-step interchange of nucleophiles, with bond breaking proceeding bond formation. The first step is ionization to form a carbocation. The second step is the reaction of the carbocation with the nucleophile.
Zaitsev's rule (Saytzeff's rule), Zaitsev orientation
An elimination usually gives the most substituted alkene product. This (this term) does not always apply, but when it does, the reaction is said to give _______.