organic chapter 7
Which of the following nucleophiles favor the SN2 mechanism? Select all that apply.
-CN CH3O- -SH
Which of the following statements correctly describe the IUPAC method for naming alkyl halides? (Select all that apply.)
.The parent chain is the longest continuous chain of C atoms that is directly connected to the halogen. To name a halogen substituent, change the -ine ending of the halogen to the suffix -o.
In the transition state of an SN2 reaction, the incoming nucleophile and outgoing nucleophile are---
180 degrees apart
Arrange the following in order of increasing reaction rate for an SN2 reaction. Place the least reactive species at the top of the list.
3 alkyl halide 2 alkyl halide1 alkyl halidemethyl halide
Select all the statements that correctly describe the mechanism of the SN1 reaction.
A carbocation intermediate is formed in an SN1 reaction. In an SN1 reaction both retention and inversion of stereochemistry are observed for reaction at a stereogenic center. The reaction rate is not affected by changing the concentration of the nucleophile.
Select the statement that correctly describes the effect of the leaving group on nucleophilic substitution reactions.
A good leaving group increases the rate of both SN1 and SN2 reactions.
Select all of the statements that correctly describe nucleophilic substitution of an alkyl halide.
A nucleophile donates an electron pair to an electrophilic C atom. As the C-X bond is cleaved, the electrons of the C-X σ bond are accepted by the leaving group.
Which of the following statements correctly describe nucleophiles and bases? (Select all that apply.)
A nucleophile is a Lewis base that attacks electrophiles other than protons. Nucleophiles and bases have a lone pair or π bond. structurally similar
Consider the following nucleophilic substitution reaction: Nu:- + R-X → R-Nu + X:- Which of the following statements correctly describes the possible mechanisms of this reaction? (Select all that apply.)
A two-step mechanism is possible where bond breaking occurs before bond making. Bond breaking and bond formation may be simultaneous.
How does a weak nucleophile influence the mechanism of nucleophilic substitution that occurs?
A weak nucleophile decreases the rate of competing SN2 reactions, thus favoring the SN1 mechanism.
Select the statements that correctly describe the characteristics of a polar aprotic solvent.
Acetone is a polar aprotic solvent because it cannot act as a hydrogen-bond donor. A polar aprotic solvent solvates cations by ion-dipole interactions.
Select all the statements that correctly reflect the relationship between base strength and nucleophilicity.
Across a period (left to right) nucleophilicity decreases as base strength decreases.c) For two nucleophiles with the same nucleophilic atoms, the stronger base is the stronger nucleophile. Across a period (left to right) nucleophilicity decreases as base strength decreases.c) For two nucleophiles with the same nucleophilic atoms, the stronger base is the stronger nucleophile.
Select all the statements that correctly describe inductive effects.
Alkyl groups can stabilize a positive charge by induction. Inductive effects operate through σ bonds only.
Select all statements that correctly describe the trends in the physical properties of alkyl halides.
Alkyl halides are insoluble in water as they are weakly polar and the alkyl chain is hydrophobic. Boiling points increase with the size of the halogen due to the increasing polarizability.
Select all of the statements that correctly describe the properties of alkyl halides.
Alkyl halides are weakly polar molecules. Alkyl halides exhibit dipole-dipole interactions. Alkyl halides are soluble in organic solvents.
Select all statements that correctly describe organic halides (X represents the halogen).
Allylic and benzylic halides have X bonded to an sp3 hybridized C atom. Aryl halides have X bonded to a benzene ring. Vinyl and aryl halides do not undergo reactions that are typical of alkyl halides.
How is nucleophilicity (nucleophile strength) related to basicity?
Although it is generally true that a strong base is a strong nucleophile, nucleophile size and steric factors can sometimes change this relationship.
Alkyl halides undergo nucleophilic substitution reactions. Select all components that are required for a nucleophilic substitution reaction to take place.
An sp3 hybridized C bonded to the leaving group A leaving group A nucleophile
Arrange the following compounds in order of increasing reactivity in an SN2 reaction (from least reactive at the top of the list to most reactive at the bottom).
Answer Structure C Structure B Structure A
Select the statement that correctly explains why backside attack by the nucleophile is preferred in SN2 reactions.
Backside attack minimizes repulsion between the nucleophile and the leaving group, which are both electron rich.
Alkyl halides have _________ boiling points than alkanes that contain the same number of carbon atoms. The melting and boiling points of alkyl halides ________ as the size of the halogen increases
Blank 1: higher or greater Blank 2: increase
A 2° substrate can react with a nucleophile via both an SN1 and SN2 mechanism. Which of the following solvents would favor the SN2 mechanism in the presence of a nucleophile?
DMSO Acetone CH3CH2OCH2CH3
Which of the following molecules do not contain a good leaving group? (Select all that apply.)
CH3CH2CH2NH2 CH3CH2OCH3 CH3CH2CH2CH3
Select all of the molecules that contain good leaving groups.
CH3CH2I CH3CH2OH2
Place the following nucleophiles in order of increasing nucleophilic strength (weakest at the top to strongest at the bottom) for a nucleophilic substitution reaction carried out in ethanol (CH3CH2OH).
CH3CH2O- CH3CH2S- CH3CH2Se-
Select all the statements that correctly describe the behavior of the methoxide anion CH3O- as a base and as a nucleophile.
CH3O- is a strong nucleophile in dimethyl sulfoxide (DMSO). CH3O- forms H-bonds to polar protic solvents. CH3O- is a strong base.
Place the following nucleophiles in order of increasing nucleophilic strength (weakest at the top to strongest at the bottom) for a nucleophilic substitution reaction carried out in acetone.
CH3S- CH3O- CH3HN-
CHCl3 CCl4 CH2Cl2
CHLOROFORM Carbon tetrachloride dichloromethane
Select all the statements that describe the mechanism of the SN2 reaction.
Complete inversion of stereochemistry is observed at the reacting center. The reaction follows second-order kinetics. The bond to the nucleophile is formed in the same step as the bond to the leaving group is broken.
Consider the SN2 reaction shown in the figure. Which of the following correctly describe the kinetics of this reaction? (Select all that apply.)
Doubling the concentration of CH3CH2I will result in a doubling of the reaction rate. This reaction exhibits second-order kinetics.
Select all of the statements that correctly describe the interactions between the given species and water.
F- is solvated by hydrogen bonding with water. K+ is solvated by ion-dipole interactions with wate
True or false: In the SN2 reaction shown the rate equation is: Rate = k[CH3Br] True false question.
FALSE
Which of the following should be used to draw any nucleophilic substitution product? (Select all that apply.)
Find the sp3 hybridized carbon that is connected to the leaving group. Substitute the nucleophile for the leaving group and assign charges (if necessary) to any atom that is involved in bond breaking/formation.
The following steps are needed to draw the product of any nucleophilic substitution reaction. Place the steps in the correct order with the first step at the top of the list.
Find the sp3 hybridized carbon with the leaving group. Identify the nucleophile, the species with a lone pair or π bond. Substitute the nucleophile for the leaving group and assign charges (if necessary) to any atom involved in bond breaking/formation.
Select all statements that correctly describe nucleophiles that are used in substitution reactions.
For negatively charged nucleophiles, all atoms originally bound to the nucleophilic center stay bonded to it after substitution occurs. Negatively charged nucleophiles are used as salts with Li+, Na+, or K+ counterions to balance charge.
Select all the statements that correctly describe the characteristics of a good leaving group.
Good leaving groups accept an electron pair readily. Good leaving groups have strong conjugate acids. Good leaving groups are weak bases.
Select all the statements that correctly describe the characteristics of a good leaving group.
Good leaving groups have strong conjugate acids. Good leaving groups are weak bases. Good leaving groups accept an electron pair readily.
Which statement correctly defines hyperconjugation?
Hyperconjugation is the overlap of an empty p orbital with the electrons of an adjacent σ bond.
Place the following groups in order of decreasing leaving group ability. Place the best leaving group at the top of the list.
I-, Br-, HO-, H2N-
Select all statements that correctly describe substitution and elimination reactions of alkyl halides.
In a substitution reaction the halogen X is replaced by a nucleophile. Alkyl halides undergo elimination reactions with Bronsted-Lowry bases. Elimination reactions lead to the formation of a new π bond
Which of the following correctly describe the general relationships between nucleophilicity and basicity? (Select all that apply.)
It is generally true that the stronger base is the stronger nucleophile. In polar protic solvents, nucleophilicity increases with increasing size of the nucleophile.
Select all the statements that correctly describe why increasing the substitution of the alkyl halide decreases the rate of the SN2 reaction.
More alkyl groups at the reaction site raise the energy of the transition state, slowing the reaction. More alkyl groups near the reaction site hinder the approach of the nucleophile, slowing the reaction.
Select all the statements that correctly describe why increasing the substitution of the alkyl halide decreases the rate of the SN2 reaction.
More alkyl groups near the reaction site hinder the approach of the nucleophile, slowing the reaction. More alkyl groups at the reaction site raise the energy of the transition state, slowing the reaction.
Select all of the statements that correctly describe the interactions between NaOCH3 (sodium methoxide) and dimethyl ether (CH3OCH3).
Na+ is solvated by ion-dipole interactions with dimethyl ether. CH3O- is solvated by ion-dipole interactions with dimethyl ether.
Which of the following statements correctly describe nucleophiles and bases? (Select all that apply.)
Nucleophiles and bases have a lone pair or π bond. A nucleophile is a Lewis base that attacks electrophiles other than protons.
Which of the following statements correctly describes the trend for nucleophilicity in a polar protic solvent?
Nucleophilicity increases down a group of the periodic table as basicity decreases.
Select all statements that describe an SN1 reaction of an alkyl halide.
Only 1 reactant appears in the rate equation. The rate follows first-order kinetics.
The mechanism of a nucleophilic substitution reaction depends on the identity of the alkyl halide. The substitution mechanism for 3° alkyl halides is _____, whereas 1° and methyl halides react only by the _____ mechanism. For 2° alkyl halides other factors influence the mechanism of the reaction.
SN1, SN2
Select all statements that correctly describe nucleophilic strength in a polar protic solvent.
Smaller anions are more strongly solvated than larger anions and are less nucleophilic. HS- is a more powerful nucleophile than HO-.
Which type of alkyl halide is most reactive in an SN1 reaction?
Tertiary (3°)
Which of the following statements is/are correct about a vinyl halide?
The C-X bond is shorter and stronger than the C-X bond of an alkyl halide. The carbon atom of the C-X bond is sp2 hydbridized.
Select all the statements that correctly reflect the Hammond postulate.
The Hammond postulate provides an estimate of the energy of the transition state for a reaction. The rate of a reaction depends on Ea, and the stability of a product depends on ΔG°.
Select all the statements that correctly describe the reaction coordinate vs. energy diagram for an SN1 reaction.
The energy of the first transition state is higher than the energy of the second transition state. The first step of the reaction is endothermic. The energy diagram shows two energy maxima corresponding to the two transition states.
Which of the following factors need to be considered when attempting to determine the mechanism of a substitution reaction? (Select all that apply.)
The leaving group (good or poor) The nucleophile (strong or weak) The alkyl halide (methyl, 1°, 2°, or 3°) The solvent (protic or aprotic)
Which of the following statements correctly identify the errors in the following r an SN2 reaction? Select all that apply.
The nucleophile should be attacking from the backside, not the frontside. There should be a δ- symbol on the Br in the transition state. There should not be a δ+ symbol on the C atom in the transition state.
Select all the statements that correctly describe the structure of a carbocation.
The positively charged C atom of a carbocation has an empty p orbital available for bonding. The positively charged C atom of a carbocation is sp2 hybridized.
SN1 reaction
The rate is doubled when the concentration of the alkyl halide is doubled.
SN2 reaction
The rate is quadrupled when the concentration of both the alkyl halide and nucleophile are doubled.
Consider the nucleophilic substitution reaction shown, and select the statements that correctly describe the outcome (the pKa values of CH3SH and HI are 10.4 and -10 respectively).
The reaction will occur as written because I- is a weaker base than -SCH3. The reaction will occur as written because I- is a better leaving group than -SCH3.
Which of the following statements about the SN1 reaction of (R)-2-bromobutane with water are correct?
The starting material is a chiral alkyl halide. The product of the reaction will be a mixture of two enantiomers. The reaction proceeds through a planar carbocation intermediate, which is achiral.
Select all the statements that correctly describe the following SN1 reaction mechanism.
There should be two products with opposite absolute stereochemistry. In the first step, the arrow should end on the Br, not on the C at the reaction center.
Consider the SN2 reaction shown in the figure. Which of the following correctly describe the kinetics of this reaction? (Select all that apply.)
This reaction exhibits second-order kinetics. Doubling the concentration of CH3CH2I will result in a doubling of the reaction rate. Need help? Review these concept resources.
Which of the following factors favor SN2 reactions? (Select all that apply.)
Use of a polar aprotic solvent Use of primary and methyl halides
Select all the solvents from the following list that are both polar and protic.
acetic acid, CH3COOH ethanol, CH3CH2OH water, H2O
Consider the compound shown. In an SN1 reaction of this substrate, loss of the leaving group generates a carbocation that is _____. A nucleophile can attack this carbocation from either face, leading to
achiral; enantiomeric; racemic
For any reaction, the energy of the transition state determines the ___________ energy of the reaction and therefore reaction rate. In an endothermic reaction, the energy of the transition state is closer to the energy of the __________ of the reaction. A more stable product will therefore form at a ________ rate since anything that stabilizes the product also stabilizes the transition state.
activation product greater
In a nucleophilic substitution reaction the nature of the solvent plays an important role, particularly for 2° substrates, which can react via either mechanism. The rate of any SN2 reaction is increased by the use of a polar _____ solvent, whereas a polar _____ solvent promotes the SN1 mechanism.
aprotic protic
Select all statements that complete the following sentence. SN2 reactions _____.
are bimolecular reactions follow second-order kinetics
The SN1 reaction involves loss of the leaving group form a ________ intermediate, which then reacts with a ___________. Since the intermediate is planar, attack can occur from either face and the product will be an equal mixture of both ___________ if the reaction site is the only stereogenic center
carbocation, nucleophile, enantiomers
As the degree of substitution of an alkyl halide increases, the rate of the SN2 reaction it undergoes will _____. A 1° alkyl halide will therefore have a _____ rate of reaction than a 2° alkyl halide.
decrease, faster
The steric bulk of the tert-butoxide anion __________ its reactivity as a nucleophile without affecting its ability to act as a base. A sterically hindered base that is a poor nucleophile is called a ______ base.
decreases, nonnucleophilic
When comparing atoms in the same group of the periodic table, basicity generally ______ and leaving group ability ______ down the group.
decreases; increases
The nucleophile the carbon atom at which substitution occurs The Lewis base in this reaction is the The Lewis acid in this reaction is the
donates an electron pair. accepts an electron pair from the nucleophile. nucleophile. carbon atom at which substitution occurs.
Alkyl groups are electron-_____ by induction, which allows the stabilization of an adjacent positive charge. The greater the number of alkyl groups attached to the positively charged C of a carbocation, the _____ the inductive effect and the _____ stable the carbocation.
donating, greater, more
Select the reaction types that are characteristic of alkyl halides.
elimination substitution
Alkyl halides typically undergo ______ -type reactions with Bronsted-Lowry bases and ______ -type reactions with nucleophiles.
elimination, substitution
Racemization is the formation of equal amounts of two products that are ____ of each other from a single chiral starting material.
enantiomers
Transition state resembles the products.
endothermic
Transition state resembles the reactants.
exothermic
True or false: A substitution reaction has two steps, and doubling the concentration of the alkyl halide is found to double the reaction rate. This is a bimolecular SN2 reaction.
false
Describe a polar protic solvent
form intermolecular hydrogen bondsContain an OH or NH bond.Solvate both cations and anions well.
In a vinyl or aryl halide, the carbon involved in the C-X bond is sp2 hybridized and has a ______% s character than the carbon atom of an alkyl halide C-X bond. Vinyl or aryl C-X bonds are therefore shorter and ______ than alkyl C-X bonds, therefore requiring more energy to break.
higher, stronger
A polar protic solvent solvates anions very well by the formation of _____ bonds. This shields the anion from reaction and causes it to become a _____ effective nucleophile. The stronger the base the more strongly it is solvated and the _____ a nucleophile it becomes.
hydrogen less weaker
A polar organic solvent that contains no OH or NH bonds cannot donate _________ bonds and is known as a polar __________ solvent. This type of solvent solvates __________ well but interacts much less strongly with _________, although it does dissolve both types of ions
hydrogen, aprotic, cations, anions
In a polar protic solvent, nucleophilicity _____ down a group of the periodic table as the size of the anion _____ and basicity _____.
increases increases decreases
Leaving group ability ---from left to right across a row of the periodic table as basicity ---
increases decreases
As the substitution of the C bearing the leaving group increases, the rate of the SN1 reaction for the substrate _____. A secondary alkyl halide will therefore react ____ readily than a tertiary alkyl halide in this type of reaction.
increases less
Alkyl groups donate electron density via ----effects. Because of this donation, R3C+ is ---stable than H3C+.
inductive more
Nucleophiles and bases are structurally similar since both have either a(n) _____ pair of electrons or a ____ bond. They differ in their site of reaction. A base attacks a _____, whereas a nucleophile reacts at a(n) ____-philic center, often a carbon atom.
lone pi proton electro
The SN1 reaction of t-Butylchloride [(CH3)3CCl] proceeds through a ______-energy of activation than isopropylchloride [(CH3)2CHCl]. This is because the t-butyl cation [(CH3)3C+] is ______ stable than the isopropyl cation [(CH3)2CH+], making it a ______-energy intermediate.
lower; more; lower
hexane
non polar
In any nucleophilic substitution reaction, a ________-phile attacks an ________ hybridized C atom and displaces a ________ group.
nucelo, sp3, leaving
In the SN2 reaction a(n) ___________ attacks an electrophilic carbon center at the same time as the bond to the _______ group breaks. This attacks is decribed as a _________-side attack, which results in complete ________ of sterochemistry at a sterogenic center.
nucleophile, leaving, back, inversion
n SN2 reaction always results in ________ of stereochemistry if the leaving group is bonded to a stereogenic center. This indicates that the nucleophile always attacks from the ________ with respect to the leaving group.
nversion Blank 2: back, opposite, backside, opposite side, or rear
A reaction coordinate diagram for the SN2 reaction shown would have ________ transition state(s) and ________ reactive intermediates.
one zero
Hyperconjugation describes the overlap of the empty ___________ orbital on the positively charged C atom of a carbocation with an adjacent sigma bond. A more substituted carbocation has more adjacent sigma bonds with more possibility for hyperconjugation. The greater the substitution of a carbocation, the _______ its stability
p greater
acetone
polar aprotic
do not solvate nucleophiles well and nucleophiles are therefore more nucleophilic.
polar aprotic solvents
ethanol
polar protic
stabilize ionic intermediates by solvation
polar protic
The C-X bond of an alkyl halide is _____ since the halogen (X) is _____ electronegative than carbon. The carbon atom of the C-X bond is _____ and as a result alkyl halides react readily with _____.
polar; more; electrophilic; nucleophiles
Equilibrium favors the --- of nucleophilic substitution when the leaving group is a weaker base than the nucleophile.
products
The rate of any reaction depends on the activation energy for the reaction, and hence the energy of the transition state. In an endothermic reaction, the transition state resembles the ______. In this type of reaction, if the stability of the reaction product decreases, the reaction rate will ______ because the value of Ea for a less stable product is ______.
products decrease higher
In an endothermic reaction,anything that stabilizes the ______ also stabilizes the transition state. Lowering the energy of the transition state ______ the energy of activation, which ______ the rate of reaction.
products, decreases, increases
A polar organic solvent that is capable of forming intermolecular hydrogen bonds is called a polar _________ solvent. This type of solvent forms ion-dipole interactions with __________, and solvates _________ by hydrogen bonding.
protic cations anions
The rate of any reaction depends on the activation energy for the reaction, and hence the energy of the transition state. In an exothermic reaction, the transition state resembles the ______ most closely. In this type of reaction, if the stability of the reaction product increases, the reaction rate will ______ because the value of Ea for a more stable product is ______.
reactants, not change, the same
A nucleophilic substitution reaction takes place where nucleophile A replaces leaving group B. The pKa's of H-A and H-B are 6.2 and 18.0 respectively. The ____ are favored at equilibrium.
reactantsH-B is a weaker acid then H-A. Therefore, B is a stronger base than A. Equilibrium favors the reactants when the leaving group is a stronger base than the nucleophilic.
The SN2 reaction proceeds through backside attack to minimize the ______ between the incoming electron-______ nucleophile and electron-______ leaving group.
repulsion, rich, rich
Alkyl halides react readily with electron-_____ reagents such as nucleophiles and bases because the carbon atom of the carbon-halogen bond is electron ______.
rich, deficient
Reaction with a weak nucleophile will favor the _____ mechanism for nucleophilic substitution, whereas reaction with a strong nucleophile in high concentration will favor the _____ mechanism.
sn1 sn2
A carbocation has a positively charged C atom that is ________ hybridized and has a vacant ________ orbital. Overlap of this empty orbital with an adjacent C-H σ bond is called ________. This overlap delocalizes the positive charge, leading to increased stability.
sp2, p, hyperconjugation
_____ results in a decrease in reactivity due to the presence of bulky groups at the site of a reaction.
steric hinderance
Lithium diisopropylamide (LDA) is shown in the diagram. Select all of the statements that correctly describe this species.
sterically hindered non
Potassium tert-butoxide [KOC(CH3)3] is a ______ base, but a ______ nucleophile due to ______.
strong; weak; steric hinderance
In a polar aprotic solvent such as acetone, the _____ the base the stronger the nucleophile. In this type of solvent, nucleophilicity _____ down a group on the periodic table as basicity _____.
stronger decreases decreases
In a polar aprotic solvent such as acetone, Cl- is a _____ nucleophile than Br- but in a polar protic solvent Cl- is the _____ nucleophile of the pair.
stronger weaker
The hydroxide anion -OH is a _____ base than the chloride anion Cl- and therefore in a polar aprotic solvent -OH will be a _____ nucleophile.
stronger, stronger
For two nucleophiles with the same nucleophilic atom, the _________ base is the stronger nucleophile. For this reason, the acetate anion CH3COO- is a _________ nucleophile than the ethoxide anion CH3CH2O-
stronger, weaker
True or false: Negatively charged nucleophiles are often used as salts. The identity of the counterion used to balance the charge is often inconsequential, and therefore it is omitted from the chemical equation.
t
True or false: The nucleophile and solvent must be examined when attempting to determine the mechanism of a substitution reaction.
t
A carbocation has _____ groups bonded to the positively charged C atom, which is _____ hybridized and therefore has a trigonal ____ geometry. A nucleophile can attack a carbocation from either face due to its structure.
three sp2 planar
The Hammond postulate provides an estimate of the energy of the ______ of a reaction.
transition state
True or false: Aryl and vinyl halides have the halogen atom bonded directly to an sp2 hybridized C atom.
true
In a nucleophilic substitution reaction---- step mechanism involves first bond breakage to form -----intermediate, followed by bond formation.
two carbocation
An SN1 reaction is a _____-step process whose energy diagram contains _____ transition state(s) and _____ reactive intermediate(s).
two, two, one
Phosphate, diphosphate, and triphosphate make good leaving groups in biological substitutions because they are ______.
weak, resonance-stabilized bases
All good leaving groups are _____ bases with relatively _____ conjugate acids that have _____ pKa values.
weak; strong; low
NH3 is a ____ nucleophile than -NH2 because a negatively charged nucleophile is always a ___ nucleophile than its conjugate acid.
weaker stronger
Alkyl halides are ______ polar molecules that exhibit ______ interactions between molecules.
weakly, dipole-dipole
nduction refers to the withdrawal or donation (release) of electron density that operates through _____ bonds. Alkyl groups _____ electron density by induction and are therefore able to stabilize a positive charge.
σ, donate
