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Chapter 6 - Alkyl Halides
Chapter 6 - Alkyl Halides: Nucleophilic Substitution and Elimination
125
Organic Chemistry
Undergraduate 3
10/26/2011

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Term
An __________ __________ simply has a halogen atom bonded to one of the sp3 hybrid carbon atoms of an alkyl group.
Definition
1) Alkyl halide
Term
A __________ __________ has a halogen atom bonded to one of the sp2 hybrid carbon atoms of an alkene.
Definition
1) Vinyl halide
Term
An __________ __________ has a halogen atom bonded to one of the sp2 hybrid carbon atoms of an aromatic ring.
Definition
1) Aryl halide
Term
The carbon-halogen bond in an alkyl halide is __________ because halogen atoms are more __________ than carbon atoms.
Definition
1) Polar
2) Electronegative
Term
A carbon atom has a partial __________ charge, making it __________; a nucleophile can attack this __________ carbon, and the __________ atom can leave as a halide ion, taking the bonding pair of electrons with it.
Definition
1) Positive
2) Electrophile
3) Electrophilic
4) Halogen
Term
A compound of formula CH2X2 (a methylene group with two halogens) is called a __________ __________; a compound of formula CHX3 is called a __________; and a compound of formula CX4 is called a __________ __________.
Definition
1) Methylene halide
2) Haloform
3) Carbon tetrahalide
Term
__________ __________ are classified according to the nature of the carbon atom bonded to the halogen.
-If the halogen-bearing carbon is bonded to one carbon atom, it is primary (1°) and the alkyl halide is a __________ __________.
-If two carbon atoms are bonded to the halogen-bearing carbon, it is secondary (2°) and the compound is a __________ __________.
-A __________ __________ (3°) has three other carbon atoms bonded to the halogen-bearing carbon atom.
-If the halogen-bearing carbon atom is a methyl group (bonded to no other carbon atoms), the compound is a __________ __________.
Definition
1) Alkyl halide
2) Primary halide
3) Secondary halide
4) Tertiary halide
5) Methyl halide
Term
A __________ __________ (Latin, geminus "twin") has two halogens bonded to the same carbon.
A __________ __________ (Latin, vicinus, "neighboring") has the two halogens bonded to adjacent carbon atoms.
Definition
1) Geminal dihalide
2) Vicinal dihalde
Term
__________ (also called chloroflourocarbons, or CFCs) are flourinated haloalkanes that were developed to replace ammonia as a refrigerant gas.
Definition
1) Freons
Term
In an alkyl halide, the halogen atom is bonded to an __________ hybrid carbon atom.
The halogen is more __________ than carbon, and the C-X bond is __________ with a partial __________ charge on carbon and a partial __________ charge on the halogen.
Definition
1) Sp3
2) Electronegative
3) Polarized
4) Positive
5) Negative
Term
The __________ of the halogens increase in the order:
I < Br < Cl < F
Definition
1) Electronegativities
Term
The carbon-halogen __________ __________ increase as the halogen atoms become __________ in the order:
C-F < C-Cl < C-Br < C-I
Definition
1) Bond lengths
2) Larger
Term
Bond __________ __________ increase in the general order:
C-I < C-Br < C-F < C-Cl
Definition
2) Dipole moments
Term
A __________ __________ __________ is the vector sum of the individual bond dipole moments.
Definition
1) Molecular dipole moments
Term
The __________ __________ __________ is the strongest intermolecular attraction in alkyl halides.
Definition
1) London dispersion force
Term
Molecules with large __________ __________ have higher London attractions, resulting in higher boiling points.
Molecules with higher __________ __________ also generally have higher boiling points because they are heavier and have greater __________ __________.
Compounds that are __________ and have __________ __________ also have lower boiling points as a result of their smaller surface areas.
Definition
1) Surface areas
2) Molecular weights
3) Surface areas
4) Branched
5) Spherical shapes
Term
Alkyl __________ and __________ are less dense than water; alkyl __________ and __________ are more dense than water.
Definition
1) Flourides
2) Chlorides
3) Bromides
4) Iodides
Term
An __________ position is a carbon atom next to a carbon-carbon double bond. __________ intermediates are stabilized by __________ with the double bond, allowing the charge of the radical to be delocalized.
Definition
1) Allylic
2) Allylic
3) Resonance
Term
A bromine radical abstracts an __________ __________ atom to give a resonance-stabilized allylic radical.
Definition
1) Allylic hydrogen
Term
The second product of an allylic bromination results from a reaction at the carbon atom that bears the radical in the second resonance form of the __________ __________; this second compound is said to be the product of an __________ __________.
Definition
1) Allylic radical
2) Allylic shift
Term
N-Bromosuccinimide (__________) is often used as the __________ source in free-radical brominations because it combines with the HBr side product to regenerate a constant low concentration of bromine.
Definition
1) NBR
2) Bromine
Term
When a halogen atom can leave with its bonding pair of electrons to form a stable halide ion we categorize that halide as a good __________ __________.
-When another atom replaces a halide ion, the reaction is known as a __________.
-When a halide ion leaves with another atom or ion (often H+) the reaction is an __________.
-In many eliminations a molecule of H-X is lost from the alkyl halide to produce an alkene; an __________ reaction known as a __________ as a hydrogen has been removed from the alkyl halide.
Definition
1) Leaving group
2) Substitution
3) Elimination
4) Elimination
5) Dehalogenation
Term
In a __________ __________, a nulceophile replaces a leaving group from a carbon atom, using its lone pair of electrons to forma new bond to the carbon atom.
Definition
1) Nucleophilic substitution
Term
In an __________ both the halide ion and another substituent are lost; the result if the formation of a __________ bond.
Definition
1) Elimination
2) Pi
Term
In the reaction of hydroxide with iodomethane to produce methanol, the hydroxide ion is a strong __________ (donor of an electron pair) because the oxygen atom has unshared pairs of __________ and a __________ charge.
In this scenario, iodomethane is the __________, or the compound attacked by the reagent.
The carbon atom of iodometan is __________ because it is bonded to an electronegative iodine atom.
__________ __________ is generally drawn away from the carbon by the halogen atom, giving the carbon atom a partial positive charge.
Definition
1) Nucleophile
2) Electrons
3) Negative
4) Substrate
5) Electrophilic
6) Electron density
Term
In SN2 reactions, the rate is found to __________ when the concentration of __________ reactant is doubled.
Definition
1) Doubled
2) Either
Term
The SN3 abbreviation stands for __________, __________, __________. The term __________ means that the transition state of the rate-limiting step (the ONLY step in this reaction) involves the collision of __________ molecules.
Definition
1) Substitution, Nucleophilic, bimolecular
2) Bimolecular
3) Two
Term
The SN2 reaction of methyl iodide with hydroxide ion is a __________ __________, taking place in __________ step with bonds breaking and forming at the same time.
The middle form is a __________ __________, a point of __________ energy rather than an intermediate.
Definition
1) Concerted reaction
2) One
3) Transition state
4) Maximum
Term
In SN2 reactions, the __________ __________ is much higher in energy than any reactant or product because it involves a five-coordinate carbon atom with two partial bonds.
Definition
1) Transition state
Term
The order of reactivity for substrates in an __________ reaction is:
CH3X > 1° > 2° (3° alkyl halides cannot occur)
Definition
1) SN2
Term
Halides can be converted to other halides by __________ __________ __________, in which one halide displaces another.
Definition
1) Halogen exchange reactions
Term
For SN2 reactions, a __________ nucleophile is much more effective than a __________ one in attacking an electrophilic carbon atom; a species with a __________ charge is a stronger nucleophile than a species with a __________ charge.
Definition
1) Strong
2) Weak
3) Negative
4) No or zero
Term
*A __________ is always a stronger nucleophile than its __________ __________.*
Definition
1) Base
2) Conjugate acid
Term
__________ is defined by the equilibrium constant for abstracting a proton, while __________ is defined by the rate of attack on an electrophilic carbon atom. If a new bond is formed to a proton it has reacted as a __________; if a new bond is to carbon, it has reacted as a __________.
Definition
1) Basicity
2) Nucleophilicity
3) Base
4) Nucleophile
Term
*Trends in Nucleophilicity:
1. A species with a __________ charge is stronger than a similar species with a __________ charge. In particular, a __________ is a stronger nucleophile than its __________ __________.
2) Nucleophilicity __________ from left to right in the periodic table, following the __________ in electronegativity from left to right. The more __________ elements have more tightly held nonbonding electrons that are __________ reactive toward forming new bonds.
3) Nucleophilicity __________ down the periodic table, following the __________ in size and polarizability, and the __________ in electronegativity.
Definition
1A) Negative
1B) No or zero
1C) Base
1D) Conjugate acid
2A) Decreases
2B) Increase
2C) Electronegative
2D) Less
3A) Increases
3B) Increase
3C) Decrease
Term
As we go down a column in the periodic table, the atoms become __________, with more electrons at a greater distance from the nucleus. The electrons are more loosely held, and the atom is more __________: it's electrons can move more freely toward a positive charge, resulting in stronger bonding in the __________ __________.
Definition
1) Larger
2) Polarizable
3) Transition state
Term
The __________ the bonding between carbon and the halogen in the transition state, the __________ the energy of the transition state.
Definition
1) Greater
2) Lower
Term
When bulky groups with interfere with a reaction by virtue of their size, we call the effect __________ __________.
Definition
1) Steric hindrance
Term
Steric hindrance has little effect on __________ because __________ involves attack on an unhindered proton. When a __________ attacks a carbon atom, however, a __________ __________ cannot approach the carbon atom so easily.
-If we want a species to act as a __________, we use a __________ reagent like ter-butoxide ion.
-If we want it to react as a __________, we use a __________ __________ reagent, like ethoxide.
Definition
1) Basicity
2) Basicity
3) Nucleophile
4) Bulky nucleophile
5) Base
6) Bulky
7) Nucleophile
8) Less hindered
Term
A __________ __________ is one that has acidic protons, usually in the form of O-H or N-H groups. These groups form hydrogen bonds to negatively charged __________.
Definition
1) Protic solvent
2) Nucleophiles
Term
__________ anions are solvated more strongly than __________ anions in a protic solvent because the solvent approaches a small anion more closely and forms stronger __________ __________. More energy is required to strip off solvent from a __________, strongly solvated ion than from a __________ diffuse, less strongly solvated ion.
Definition
1) Small
2) Large
3) Hydrogen bonds
4) Small
5) Larger
Term
In protic solvents, polarizability __________ with increasing atomic number, and the solvation energy __________ with increasing atomic number. Therefore, nucleophilicity generally __________ down a column in the periodic table.
Definition
1) Increases
2) Decreases
3) Increases
Term
__________ __________ enhance the nucleophilicity of anions as an anion is more reactive in an __________ __________ because it is not so strongly solvated (does not require large amounts of energy to break hydrogen bonds).
Definition
1) Aprotic solvents
2) Aprotic solvents
Term
__________ __________ __________ have strong dipole moments to enhance solubility, yet they have no O-H or N-H groups to form __________ __________ with anions.
Specific solvating reagents are added to enhance the __________ of a reaction without affecting the reactivity of the __________.
Definition
1) Polar aprotic solvents
2) Hydrogen bonds
3) Solubility
4) Nucleophile
Term
The alkyl halide is often referred to as the __________ or the compound being attacked by the reagent.
Definition
1) Substrate
Term
A leaving group serves two purposes in the SN2 reaction:
1. It __________ the C-X bond, making the carbon atom __________.
2. It leaves with the pair of __________ that once bonded it to the __________ carbon atom.
To fill these roles, a good leaving group should be:
1. __________ withdrawing, to polarize the carbon atom,
2. Stable (not a strong __________) once it has left, and
3. __________ to stabilize the transition state.
Definition
1A) Polarizes
1B) Electrophilic
2A) Electrons
2B) Electrophilic
1AA) Electron
2AA) Base
3AA) Polarizable
Term
__________ __________ are strongly electronegative, making common substrates for SN2 reactions along with __________, __________, and __________ which form strongly polarized bonds with carbon.
Definition
1) Halogen atoms
2) Nitrogen
3) Oxygen
4) Sulfur
Term
In SN2 reactions (and generally), a reactive leaving group raises the energy of the __________ __________ and the __________, driving the equilibrium forward.
Definition
1) Transition state
2) Products
Term
Good leaving groups should be __________ bases; they are therefore the __________ __________ of strong acids. The hydrohalic acids (HCl, HBr, and HI) are strong acids and their __________ __________ (Cl-, Br-, I-) are all __________ bases. __________, __________, and __________ ions also serve as good leaving groups.
Definition
1) Weak
2) Conjugate bases
3) Conjugate bases
4) Weak
5) Sulfate
6) Sulfonate
7) Phosphate
Term
__________ and __________ ions and other strong bases are poor leaving groups for SN2 reactions.
Definition
1) Hydroxide
2) Alkoxide
Term
A neutral molecule often serves as the leaving group from a __________ __________ species.
Definition
1) Positively charged
Term
In SN2 reactions, a good leaving group should be __________ to maintain partial bonding with the carbon atom in the transitions state.
This bonding helps stabilize the __________ __________ and reduce the __________ __________.
Definition
1) Polarizable
2) Transition State
3) Activation energy
Term
For simple alkyl halides, the relative rates for __________ displacement are:
CH3X > 1° > 2° >> 3° (does not occur)
Definition
1) SN2
Term
In SN2 reactions, even a bulky __________ __________ (like neopentyl bromide) undergoes SN2 reactions at a rate similar to that of a __________ __________
Definition
1) Primary halide
2) Tertiary halide
Term
The SN2 reaction requires attack by a nucleophile on the __________ __________ of the __________ carbon atom.
Definition
1) Back side
2) Electrophilic
Term
During an SN2 reaction, __________-__________ __________ literally turns the tetrahedron of the carbon atom inside out, like an umbrella caught by the wind. In the product, the nucleophile assumes a stereochemical position __________ the position the leaving group originally occupied; this is known as __________ __________ __________ at the carbon atom.
Definition
1) Back-side attack
2) Opposite
3) Inversion of configuration
Term
The SN2 displacement (back-side attack) is the most common example of a __________ __________, a step (in a reaction sequence) where an __________ carbon atom undergoes inversion of configuration.
Definition
1) Walden inversion
2) Asymmetric
Term
SN2 displacement is a good example of a __________ __________ in which different stereoisomers react to give different stereoisomers of the product.
Definition
1) Stereospecific reaction
Term
*To study the mechanism of a nucleophilic substitution, we often look at the product to see if the reaction is stereospecific.
-If it is, the __________ mechanism is a good possibility, especially if the reaction kinetics are __________ order.
Definition
1) SN2
2) Second
Term
A reaction which characterizes the SN1 mechanism in which a solvent acts as a nucleophile is known as a __________.
Definition
1) Solvolysis
Term
In SN1 reactions, the rate depends only on the concentration of the __________.
Definition
1) Substrate
Term
The SN1 reaction stands for __________, __________, and__________, where the term __________ means there is only one molecule involved in the transition state of the rate-limiting step.
Definition
1) Substitution
2) Nucleophilic
3) Unimolecular
4) Unimolecular
Term
The SN1 mechanism is a multistep process: The first step is a slow __________ to form a __________; the second step is a __________ attack on the carbocation by a __________; the final step is the loss of a __________ to the original solvent.
Definition
1) Ionization
2) Carbocation
3) Fast
4) Nucleophile
5) Proton
5)
Term
The nucleophiles in an SN1 reaction are usually __________, as a strong nucleophile would be more likely to attack the __________ and force some kind of second-order reaction.
Definition
1) Weak
2) substrate
Term
*The SN1 Reaction*
The SN1 reaction involves a __________-step mechanism. A __________ ionization gives a carbocation that reacts quickly with a (usually __________) nucleophile. The reactivity of __________ reactions is:
3° > 2° > 1°
-Step 1: Formation of the __________ (rate-limiting)
-Step 2: Nucleophilic attack on the __________ (fast).
IF THE NUCLEOPHILE IS WATER OR AN ALCOHOL, A THIRD STEP IS NEEDED TO DEPROTONATE THE PRODUCT.
-Step 3: __________ to form the product.
Definition
1) Two
2) Slow
3) Weak
4) SN1
S1) Carbocation
S2) Carbocation
S3) Deprotonation
Term
In an SN1 reaction, the __________ step is highly (endothermic/exothermic) and its large activation energy determines the overall reaction rate. The __________ step is strongly (endothermic/exothermic) with a lower-energy transition state.
Definition
1) First
2) Endothermic
3) Second
4) Exothermic
Term
The __________-__________ step of the __________ reaction is ionization to form a carbocation, a strongly (endothermic/exothermic) process.
Definition
1) Rate-determining
2) SN1
Term
Rates of SN1 reactions depend strongly on __________ stability.
Definition
1) Carbocation
Term
Alkyl groups stabilize carbocations by donating electrons through sigma bonds (the __________ ___________) and through overlap of filled orbitals with the empty p orbital of the carbocation (__________). Highly substituted __________ are therefore the most stable:
3° > 2° > 1° > CH3X
Definition
1) Inductive effect
2) Hyperconjugation
3) Carbocations
Term
Alkyl groups hinder the __________ by blocking attack of the strong nucleophile, but alkyl groups enhance the __________ by stabilizing the carbocation intermediate.
Definition
1) SN2
2) SN1
Term
__________ and __________ halides generally do not undergo SN1 or SN2 reactions; a __________ reaction would require ionization to form a __________ or __________ cation, either of which is less stable than most __________ carbocations.
Definition
1) Vinyl
2) Aryl
3) SN2
4) Vinyl
5) Aryl
6) Alkyl
Term
In SN1 reactions, a highly __________ leaving group helps stabilize the __________-__________ transition state through partial bonding as it leaves.
The leaving group should be a __________ base, very stable after it leaves with the pair of electrons that bonded it to the __________ atom.
Definition
1) Polarizble
2) Rate-limiting
3) Weak
4) Carbon
Term
The SN1 reaction goes much more readily in __________ solvents that stabilize ions. The rate-limiting step forms __________ ions, and ionization takes place in the __________ __________. Polar solvents solvate these ions by interaction of the solvent's __________ __________ with the __________ of the ion. __________ solvents such as alcohols and water are even more effective solvents due to the hydrogen bonds that can be formed with the anions.
Definition
1) Polar
2) Two
3) Transition state
4) Dipole moment
5) Charge
6) Protic
Term
SN1 reactions require highly __________ solvents that strongly solvate ions.
Definition
1) Polar
Term
One measure of a solvent's ability to solvate ion is its __________ __________, a measure of the solvent's polarity.
Definition
1) Dielectric constant
Term
In the SN1 mechanism the reaction is not __________: the carbocation intermediate is __________ hybridized and __________; a nucleophile is able to attack the carbocation from either face.
Definition
1) Stereospecific
2) Sp2
3) Planar
Term
A process in which both enantiomers of a product are possible (whether or not the two enantiomers are produced in equal amounts) is called __________.
Definition
1) Racemization
Term
In SN1 reactions, if a nucleophile attacks a carbocation from the front side, the product molecule shows __________ __________ __________; attack from the back side gives a product molecule showing __________ __________ __________. __________ is simply a combination of retention and inversion.
Definition
1) Retention of configuration
2) Inversion of configuration
3) Racemization
Term
In SN1 reactions, when racemization occurs, there is often more __________ than __________ of configuration; this is because the __________ __________ often blocks the front side of the carbocatiion, leaving the back side open and thus __________ more likely.
Definition
1) Inversion
2) Retension
3) Leaving group
4) Inversion
Term
Carbocations frequently undergo structural changes, called __________, to form more stable ions.
A __________ may occur after a __________ has formed or it may occur as the __________ __________ is leaving. __________ do not occur in SN2 reactions because no carbocation is formed, making the process impossible.
Definition
1) Rearrangements
2) Rearrangement
3) Carbocation
4) Leaving group
5) Rearrangements
Term
A __________ __________ is the movement of a hydrogen atom with its bonding pair of electrons. The __________ __________ converts the initially formed __________ (level) carbocation to a more stable __________ (level) carbocation).
Definition
1) Hydride shift
2) Hydride shift
3) Secondary
4) Tertiary
Term
*Hydride Shift in an SN1 Reaction:
-Step 1: Unimolecular ionization gives a __________.
-Step 2: A __________ __________ forms a more stable carbocation
-Step 3: A solvent (__________ nucleophile) attacks the rearranged carbocation.
-Step 4: __________ gives the rearranged product.
Definition
1) Carbocation
2) Hydride shift
3) Weak
4) Deprotonation
Term
A __________ __________ is another rearrangement characterized by the migration of a methyl group together with its pair of electrons.
Definition
1) Methyl shift
Term
*Methyl Shift in an SN1 Reaction:*
-Step 1: Ionization occurs with a __________ __________.
NOTE: THE SHIFT OCCURS WHILE THE BROMIDE ION IS LEAVING, UNLIKE THE HYDRIDE SHIFT WHICH OCCURS IN TWO SEPARATE STEPS.
-Step 2: Attack by ethanol gives a __________ version of the rearranged product.
-Step 3: __________ gives the rearranged product.
Definition
1) Methyl shift
2) Protonated
3) Deprotonation
Term
*Effect of the Nucleophile on SN1/SN2:*
SN1: Nucleophile strength is unimportant (usually __________).
SN2: __________ nucleophiles are required
Definition
1) Weak
2) Strong
Term
*Effect of the Substrate on SN1/SN2:*
__________ Substrates: 3° > 2° (1° and CH3X are unlikely)
__________ Substrates: CH3X > 1° > 2° (3° is impossible).
Definition
1) SN1
2) SN2
Term
*Effect of the Solvent on SN1/SN2:*
SN1: Strong __________ and __________ __________ ionizing solvents such as alcohols and waters are needed.
SN2: Reactions go faster in less __________ solvents and in __________ __________ solvents.
Definition
1) Polar
2) Polar protic
3) Polar
4) Polar aprotic
Term
*Kinetics of SN1/SN2:*
SN1: Rate is proportional to the concentration of the __________ __________ but not the concentration of the __________; follows first-order rate equation.
SN2: Rate is proportional to the concentrations of both the __________ __________ and the __________.
Definition
1) Alkyl halide
2) Nucleophile
3) Alkyl halide
4) Nucleophile
Term
*Stereochemistry of SN1/SN2:*
SN1: Reactions involve flat __________ intermediate that can be attacked from either face; gives mix of __________ and __________ (__________) products.
SN2: Reactions take place through back-side attack only, leading to complete __________.
Definition
1) Carbocation
2) Retension
3) Inversion
4) Racemization
5) Inversion
Term
*Rearrangements of SN1/SN2:*
SN1: Carbocation intermediate can rearrange by __________ or __________ __________ to produce a more stable carbocation.
SN2: There is no carbocation intermediate so rearrangements are not possible.
Definition
1) Hydride shift
2) Alkyl shift
Term
An __________ involves the loss of two atoms or groups from the substrate, usually with the formation of a __________ bond.
Definition
1) Elimination
2) Pi
Term
The abbreviation E1 stands for __________ __________. The mechanism is termed __________ because the rate-limiting transition state involves a __________ molecule rather than a collision between __________ molecules.
Definition
1) Elimination
2) Unimolecular
3) Unimolecular
4) Single
5) Two
Term
*The E1 Reaction:*
The E1 reaction requires ionization to a carbocation intermediate like the __________ reaction, so it follows the same order of reactivity:
3° > 2° >> 1°. However, in an E1 reaction a base (usually __________) deprotonates the carbocation to produce an __________.
-Step 1: Unimolecular ionization to produce a __________ (rate-limiting).
-Step 2: __________ by a __________ base produces an alkene (fast).
Definition
1) SN1
2) Weak
3) Alkene
4) Carbocation
5) Deprotonation
6) Weak
Term
The E1 mechanism is different than the SN1 mechanism in that the __________ base deprotonates the carbocation to produce an __________.
Definition
1) Weak
2) Alkene
Term
In E1 reactions, the rate depends only on the concentration of the __________ __________ and not on the strength or concentration of the base.
Definition
1) Alkyl halide
Term
In E1 reactions, a __________ reaction is an elimination of a hydrogen and halogen atom.
Definition
1) Dehydrohalogneation
Term
In the second step of the E1 mechanism, the carbon atom next to the C+ must rehybridize to __________ as the __________ attacks the protons and electrons flow into the new __________ orbital.
The potential-energy diagram for E1 mechanisms is similar to that of SN2: The ionization (first) step is strongly (endothermic/exothermic), with a rate-limiting __________ __________. The second step is a fast (endothermic/exothermic) deprotonation by a __________.
Definition
1) Sp2
2) Base
3) Pi
4) Exothermic
5) Transition state
6) Endothermic
7) Base
Term
*Rearrangement in the E1 Reaction:*
-Step 1: __________ to form a carbocation (slow).
-Step 2: A __________ __________ forms a more stable carbocation (fast).
-Step 3: The __________ basic solvent removes either adjacent proton (__________).
Definition
1) Ionization
2) Hydride shift
3) Weakly
4) Fast
Term
*Summary: Carbocation Reactions:*
A carbocation can:
1. React with its own __________ __________ to return to the reactant.
2. React with a __________ to form a substitution product (SN1).
3. Lose a __________ to form an elimination product (an __________; E1).
4. __________ to a more stable carbocation to react further.
Definition
1) Leaving group
2) Nucleophile
3) Proton
4) Alkene
5) Rearrange
Term
In most E1 and E2 eliminations where there are two or more possible elimination products; the product with the most substituted __________ __________ will predominate; this principle is called __________ __________ and reactions that give this desired alkene are said to follow __________ __________.
Definition
1) Double bond
2) Zaitsev's rule
3) Zaitsev orientation
Term
*__________ __________ states that in elimination reactions, the most substituted alkene usually predominates:
R2C=CR2 > R2C=CHR > RHC=CHR or R2C=CH2 > RHC=CH2
Definition
1) Zaitsev's Rule
Term
The abbreviation E2 stands for __________ __________; in this process the rate is proportional to both the concentrations of the __________ __________ and the __________, giving a second-order orientation.
Definition
1) Elimination
2) Bimolecular
3) Alkyl halide
4) Base
Term
In the general mechanism of the E2 reaction, a strong __________ abstracts a proton on a carbon atom adjacent to the one with the __________ __________; as this occurs a __________ __________ is formed and the leaving group leaves.
Definition
1) Base
2) Leaving group
3) Double bond
Term
Like the SN2 reaction, the E2 elimination is a __________ reaction in which bonds break and form at the same time in a __________ step.
Definition
1) Concerted
2) Single
Term
*Reactivity of the Substrate in the E2:*
The order of reactivity of alkyl halides toward E2 dehydrohalogenation is found to be:
__________ > __________ > __________
This reactivity order reflects the greater stability of highly substituted __________ __________.
Definition
1) 3°
2) 2°
3) 1°
4) Double bonds
Term
*Mixtures of Products in the E2:*
The E2 reaction requires abstraction of a proton on a __________ atom NEXT to the carbon bearing the halogen. In most cases, __________ __________ predicts that the major product will be the most substituted alkene.
Definition
1) Carbon
2) Zaitsev's Rule
Term
Like the SN2 reaction, the E2 reaction follows a __________ __________ in which bond breaking and forming occur simultaneously, with partial formation of new bonds lowering the energy of the __________ __________. For the reaction to be successful a specific geometric arrangement is required: for SN2 reactions it is a __________ -__________ __________ attack; for E2 reactions it is a __________ arrangement of orbitals.
Definition
1) Concerted mechanism
2) Transition state
3) Back-side attack
4) Coplanar
Term
The E2 elimination requires partial formation of a new __________ bond with its parallel __________ orbitals in the transition state. The electrons that once formed a C-H bond must begin to overlap with the orbital that the __________ __________ is vacating. Formation of this new __________ bond implies that two __________ orbitals must be parallel so that __________ overlap is possible as the hydrogen and halogen leave and the orbitals rehybridize to the __________ orbitals of the new pi bond.
Definition
1) Pi
2) P
3) Leaving group
4) Pi
5) P
6) Pi
7) P
Term
In an E2 elimination reaction, when the hydrogen and halogen are __________ to one another (0 = 180°) their orbitals are aligned; this is known as __________-__________ conformation.
When the hydrogen and halogen eclipse one another (0 = 0°), their orbitals are once again aligned; this is the __________-__________ conformation.
The __________-__________ (0 = 180) is staggered and has the __________ energy while the __________-__________ is eclipsed and has the __________ energy.
Definition
1) Anti
2) Anti-coplanar
3) Syn-coplanar
4) Anti-coplanar
5) Lowest
6) Syn-coplanar
7) Highest
Term
In E2 reactions, the __________-__________ conformation is the most stable and the __________-__________ conformation is the least stable.
Definition
1) Anti-coplanar
2) Syn-coplanar
Term
Molecules that are rigidly held in eclipsed (__________-__________) conformations with a hydrogen and leaving group are most likely to leave through a __________-__________ concerted mechanism.
Definition
1) Syn-coplanar
2) Syn-coplanar
Term
The E2 elimination reaction is a __________ reaction because different stereoisomers of the starting material react to produce different stereoisomers of the product.
Definition
1) Stereospecific.
Term
*Effect of the Base in E1/E2:*
__________: Base strength is unimportant (usually __________)
__________: Requires __________ base.
Definition
1) E1
2) Weak
3) E2
4) Strong
Term
*Effect of the Solvent in E1/E2:*
E1: Requires a good __________ solvent or __________ __________ solvent such as water or alcohol.
E2: Solvent __________ is not very important; strong base still reacts in very __________ solvent.
Definition
1) Polar
2) Polar protic
3) Polarity
4) Nonpolar
Term
*Effect of the Substrate on E1/E2:*
For both E1 and E2 reactions, the order of reactivity is:
__________ > __________ > __________ (1° will usually not go __________).
E1: Rate-limiting step is formation of __________ and reactivity reflects stability of __________.
E2: More substituted halides generally form more stable __________.
Definition
1) 3°
2) 2°
3) 1°
4) E1
5) Carbocation
6) Carbocation
7) Alkene
Term
*Kinetics of E1/E2:*
E1: Rate is proportional to the concentration of __________ __________; follows first-order rate equation.
E2: Rate is proportional to both concentrations of __________ __________ and __________; follow second-order rate equation.
Definition
1) Alkyl halide
2) Alkyl halide
3) Base
Term
*Orientation of Elimination for E1/E2:*
In most E1 and E2 eliminations with two or more products; the product with most substituted __________ __________ (most stable) predominates; principle known as __________ __________ and the most highly substituted product is __________ __________.
Definition
1) Double bond
2) Zitsev's Rule
3) Zitsev product
Term
*Stereochemistry of E1/E2:*
E1: No particular geometry required.
E2: Takes places through __________ mechanism which requires a __________ arrangement of the bonds to the atoms being eliminated. Transition state is usually __________-__________ although it may be __________-__________ in rigid systems (eclipsed).
Definition
1) Concerted
2) Coplanar
3) Anti-coplanar
4) Syn-coplanar
Term
*Rearrangements in E1/E2:*
E1: Carbocation intermediate can perform rearrangement through __________ or __________ __________ to produce a more stable carbocation.
E2: No rearrangements possible (concerted mechanism).
Definition
1) Hydride shift
2) Alkyl shift
Term
*Predicting Substitutions and Eliminations*
1. The strength of the __________ or __________ determines the order of the reaction.
-If a strong __________ (or ___________) is present, it will force second-order kinetics, either SN2 or E2. A strong nucleophile attacks the electrophilic __________ atom or abstracts a __________ faster than the molecule can ionize for first-order reactions.
Definition
1) Nucleophile
2) Base
3) Nucleophile
4) Base
5) Carbon
6) Proton
Term
*Predicting Substitutions and Eliminations*
2. Primary halides usually undergo the __________ reaction, occasionally the __________ reaction.
-Primary halides rarely undergo first-order reactions, since primary carbocations are rarely formed. With good nucleophiles __________ is usually observed; with a strong base __________ may be observed.
Definition
1) SN2
2) E2
3) SN2
4) E2
Term
Sometimes __________ __________ or __________ temperatures are used to force a PRIMARY halide to ionize usually with rearrangement to form a more stable carbocation. In these instances,rearranged __________ and __________ products are observed.
Definition
1) Silver salts
2) High
3) SN1
4) E1
Term
*Predicting Substitutions and Eliminations*
3. __________ halides usually undergo the E2 reaction (__________ base) or a mixture of SN1 and E1 (__________ base).
-Tertiary halides cannot undergo the __________ reaction. A __________ base forces second-order kinetics by the E2 mechanism; in the absence of the base tertiary halides react by first-order processes.
Definition
1) Tertiary
2) Strong
3) Weak
4) SN2
5) Strong
Term
*Predicting Substitutions and Eliminations*
4. The reactions of __________ halides are the most difficult to predict.
Definition
1) Secondary
Term
*Predicting Substitutions and Eliminations*
5. Some nucleophiles and bases favor substitution or elimination.
-To promote elimination, the __________ should readily abstract a proton but not readily attack a carbon atom. A bulky strong __________, such as tert-butoxide enhances elimination. __________ temperatures also favor elimination in most cases as more molecules are formed.
To promote substitution, we need a good __________ with limited basicity: a highly __________ species that is the conjugate base of a strong acid.
Definition
1) Base
2) Base
3) High
4) Nucelophile
5) Polarizable
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