Chapter 9 Flashcards
Nucleophilic substitution
Any reaction in which a nucleophile replaces another electron-rich group called a leaving group
Nucleophile
An electron-rich molecule or ion that donates a pair of electrons to another atom or ion to form a new covalent bond
If the nucleophile is negatively charged in a substitution reaction, the atom donating the pair of electrons becomes ______ in the product.
neutral
If the nucleophile is uncharged in the substitution reaction, the atom donating the electrons becomes _________ in the product.
positively charged
What are the two limiting mechanisms for nucleophilic substitution?
SN2 and SN1
In the SN2 reaction mechanism, bond forming and bond breaking occur _________.
simultaneously
SN2 reactions are bimolecular because:
both the nucleophile and haloalkane concentrations influence reaction rate
The nucleophile must approach from the ________ in SN2 reactions because:
backside, this has to occur in order for it to populat the C-Lv antibonding orbital and allow the reaction
In the SN1 mechanism, the leaving group departs first in the _________, leaving a _______ intermediate that reacts with the nucleophile in a second step.
rate-determining step, carbocation
SN1 reactions are unimolecular because:
only the haloalkane concentration influences the reaction rate
In SN2 reactions, both the ______________ are involved in the transition state.
nucleophile and leaving group
SN2 reactions result in ________ of configuration at the reaction center.
inversion
SN2 reactions are accelerated more in ________ solvents.
polar aprotic
The relative rates of SN2 reactions are governed by _______ factors, namely:
steric, the degree of crowding around the site of the reaction
In SN2 reactions, departure of the leaving group is assisted by:
the incoming nucleophile
Backside attack by the nucleophile is facilitated in two ways:
Fist, because of the polarization of the C-Lv bond, the carbon atom has a partial positive charge and therefore attracts the electron-rich nucleophile. Second, the electron density of the nucleophile entering from the backside assists in breaking the C-Lv bond, thereby helping the leaving group leave.
An SN1 reaction occurs in two steps. Step 1 is a slow, _________ ionization of the C-Lv bond to form a carbocation intermediate.
rate-determining
In SN1 reactions, the reaction at a chiral center gives:
largely racemization, often accompanied with a slight excess of inversion of configuration
SN1 reactions often involve carbocation ________ and are accelerated by ________ solvents.
rearrangements, polar protic
SN1 reactions are governed by ______ factors, namely:
electronic, the relative stabilities of carbocation intermediates
Solvolysis
A nucleophilic substitution in which the solvent is also the nucleophile
Because an SN2 reaction is bimolecular, doubling the concentration of either the haloalkane or nucleophile:
doubles the rate of the reaction
Haloalkanes that can form more stable carbocations react faster if an:
SN1 mechanism occurs
Steric hindrance on the backside of the C-Lv bond of a haloalkane:
slows down or possibly prevents an SN2 mechanism
The more stable the anion produced upon reaction, the:
better the leaving group ability
Protic solvents
Hydrogen-bond donors
Aprotic solvents
Cannot serve as hydrogen-bond donors
Common protic solvents
those containing -OH groups; water, formic acid, methanol, ethanol, acetic acid
Most common aprotic solvents:
acetone, diethyl ether, dimethyl sulfoxide (DMSO)
Polar solvents
interact strongly with ions and polar molecules
Nonpolar solvents
Do not interact strongly with ions and polar molecules
The dielectric constant is the most commonly used measure of:
solvent polarity
Polar aprotic solvents accelerate SN1 reactions by:
stabilizing the charged carbocation intermediate
Polar protic solvents accelerate SN2 reactions because:
they do not interact strongly with the nucleophile
Good nucleophiles are generally:
anions
Moderate nucleophiles are generally:
neutral, with one or more available lone pairs
Poor nucleophiles are generally:
polar portic solvents
All things being equal, the stronger the interaction of a nucleophile with the solvent:
the lower the nucleophilicity
Small nucleophiles with very little steric hindrance are better nucleophiles for:
SN2 reactions
Good leaving groups:
I->Br->Cl- (approx. equal to) H2O
Poor leaving groups:
OH-, CH3O-, NH2-
Moderate leaving groups:
F->CH3COO-
The greater the value of the dielectric constant, the:
better it solvates and thus the smaller the interaction between ions of opposite charge dissolved in it.
A solvent is a polar solvent if it has a dielectric constant of:
15 or greater
A solvent is a nonpolar solvent if it has a dielectric constant of:
less than 5
In polar protic solvents, the least basic halide ion has the ________ nucleophilicity.
greatest
Bulky bases do not work well in:
SN2 reactions
Methyl or primary haloalkanes rect with SN2 mechanisms because:
of an absence of steric hindrance and lack of carbocation stability
Secondary haloalkanes react with an ____ mechanism in aprotic solvents with good nucleophiles, but through the ____ mechanism in protic solvents with poor nucleophiles.
SN2, SN1
Tertiary haloalkanes react through an SN1 mechanism because:
the steric hindrance disfavors SN2 backside attack, and the attached alkyl groups stabilize a carbocation.
beta-elimination reaction
involves removal of atoms or groups of atoms from adjacent carbon atoms
Dehydrohalogenation
A beta-elimination reaction that involves the loss of an H and a halogen atom from adjacent carbons to create an alkene from a haloalkane.
Zaitzev’s rule
Predicts that beta-elimination reactions give primarily the more substituted alkene. Such reactions are called Zaitsev eliminations
All nucleophiles are:
bases
Strong bases promote:
beta-elimination reactions
Strong bases that serve effectively in beta-elimination of haloalkanes:
OH-, OR-, NH2-, and acetylide anions
The two limiting mechanisms for beta-elimination reactions:
E1 and E2
In the E1 mechanism:
The leaving group departs to give a carbocations, then a proton is take off an adjacent carbon atom by the base to create the product alkene.
E1 reactions are unimolecular because:
only haloalkane concentration influences the rate of the reaction
In the E2 mechanism:
the halogen departs at the same time that an H atoms is removed by the base from an adjacent carbon atom to create the product alkene.
E2 reactions are bimolecular because:
both the haloalkane and base concentrations influence the rate of the reaction.
How do the E1 and E2 mechanisms differ?
By the timing of the bond-breaking and bond-forming steps
An E1 reaction occurs in two steps: a slow, __________ breaking of the C-Lv bond to form a carbocation intermediate followed by a _____ proton transfer to the solvent to form an alkene.
rate-determining, rapid
E1 reaction
Breaking of the C-Lv bond to give a carbocation completes before any reaction occurs with the base to lose a hydrogen and form the carbon-carbon double bond.
Mechanism steps for the E1 reaction:
Step 1: break a bond to give stable molecules or ions
Step 2: take a proton away
An energy diagram for an E1 reaction shows ___ transition states and ___ carbocation intermediate.
two, one
An E2 reaction occurs in one step:
simultaneous reaction with the base to remove a hydrogen, formation of an alkene, and departure of the leaving group
E2 reactions are stereoselective in that:
the lowest energy transition state is the state in which the leaving group and H atoms that depart are oriented anti and coplanar (anti-periplanar)
The anti and coplanar requirement for E2 reactions determines:
whether E or Z alkenes are produced.
For E2 reactions involving cyclohexane derivatives:
both the leaving group and departing H atom must be axial
Although in principle any base can be made to indue an E2 reaction under appropriate experimental conditions, chemists commonly employ:
Particularly strong bases such as OH-, alkoxides, and amide anions (NR2-) (conjugate acid pKa’s above 11)
Involving E2 reactions, when using bases whose conjugate acid pKa’s are near or below 11 (such as carboxylates, thiolates, and cyanide), the intention is:
Therefore:
to effect a substitution reaction by using these reactants as nucleophiles
one simplifying aspect of the competition between substituition and elimination is to consider an E2 pathway ony when hydroxide, alkoxides, acetylides, and amide anions are used.
For E2 reactions, double bond character is so highly developed in the transition state:
that the relative stability of possible alkenes commonly determines which regioisomer is the major product.
For E2 reactions, the transition state with the lowest energy is that:
leading to the most highly substituted alkene
For E2 reactions, ____ double bonds predominate over ____ double bonds in the products when either is possible.
trans, cis
For E2 reactions, sterically hindered bases where isomeric alkenes are possible, the major product is often:
the less substituted alkene because the reaction occurs primarily at t he most accessible H atom
The lowest energy transition state of an E2 reaction is commonly the one in which:
The -Lv and -H are oriented anti and coplanar to each other
Anti and coplanar
At a dihedral angle of 180 degrees
Why do E2 reactions prefer anti and coplanar geometry?
It allows the proper orbital overlap between the bas, the proton being removes, and the departing leaving group.
Methyl or primary haloalkanes do not react through:
E1 or SN1 mechanisms
For methyl or primary haloalkanes, ___ is favored for all nucleophiles except for exceptionally strong bases or sterically hindered ones, which cause ___ to predominate.
SN2, E2
Secondary haloalkanes can react through:
any of the mechanisms
For secondary haloalkanes, if the nucleophile is a strong base, ___ predominates.
E2
For secondary haloalkanes, weak bases react predominantly by an ___ mechanism.
SN2
For secondary haloalkanes, poor nucleophiles react through:
a combination of SN1/E1 pathways
Tertiary haloalkanes cannot react by:
an SN2 mechanism
For tertiary haloalkanes, if the nucleophile is a strong base, ___ predominates.
E2
For tertiary haloalkanes, other nucleophiles that are not a strong base in a polar protic solvent, the reaction is through:
a combination of SN1/E1 pathways
Certain nucleophilic displacements that have the kinetic characteristic of SN1 reactions involve:
two successive SN2 reactions
Many reactions involving two successive SN2 reactions involve the participation of:
a neighboring nucleophile
