Organic Chemistry- Chapter 9

Nucleophilic Substitution and β-Elimination

Any reaction in which one nucleophile is substituted for another at a tetravalent carbon; an electron-rich nucleophile replaces a leaving group

Nucleophilic substitution

All nucleophiles are electron _______ and can be considered Lewis _____

Sources; bases

True or False:

All nucleophiles and leaving groups have a (-) charge

False

A strong nucleophile removes _______ and attacks ______ centers

Protons; carbon

True or False:

Nucleophilic substitution and beta-elimination are competing reactions

True

If a nucleophile is negatively charged in a substitution reaction, the atom donating the pair of electrons becomes _______ in the product

Neutral

If a nucleophile is uncharged in a substitution reaction, the atom donating the pair of electrons becomes ________ in the initial product

Positive

In the Sn2 reaction mechanism, bond forming and bond breaking occur ______________

Simultaneously

Sn2 reactions are ___________

Bimolecular

In an Sn2 reaction, what makes up the reaction rate?

The concentration of the haloalkane and the base

In the Sn1 reaction mechanism, bond breaking and bond forming occur __________

Separately

An Sn1 reaction occurs in how many steps?

Two

An Sn2 reaction occurs in how many steps?

One

What is the rate determining step of an Sn2 reaction mechanism?

The only step

What is the rate determining step in an Sn1 reaction mechanism?

The first step, when the leaving group leaves

In the Sn1 reaction mechanism, the leaving group departs _____

First

True or False:

A carbocation intermediate is formed in Sn2 reaction mechanisms

False

Sn1 reactions are ____________

Unimolecular

In an Sn1 reaction, what makes up the reaction rate?

The concentration of the haloalkane

In an ___ reaction mechanism, departure of the leaving group is assisted by the incoming nucleophile

Sn2

In an ___ reaction, both the nucleophile and the leaving group are involved in the transition state

Sn2

Which reaction mechanism, Sn1 or Sn2, results in the inversion of configuration if the reaction takes place at a chiral center?

Sn2

Which reaction mechanism, Sn1 or Sn2, prefers polar aprotic solvents?

Sn2

True or False:

Backside attack occurs only in the Sn2 reaction mechanism

True

Which reaction mechanism, Sn1 or Sn2, gives racemization if the reaction occurs at a chiral center?

Sn1

In an ___ reaction mechanism, carbocation intermediates occur

Sn1

Which reaction mechanism, Sn1 or Sn2, prefers polar protic solvents?

Sn1

In an ___ reaction mechanism, the C-Lv bond is completely broken before Nu-C bond formation begins

Sn1

A nucleophilic substitution in which the solvent is also the nucleophile

Solvolysis

If an Sn1 reaction mechanism occurs in two steps, how many transition states exist?

Two

In an Sn2 reaction mechanism, doubling the concentration of either the haloalkane or the nucleophile _______ the rate of the reaction

Doubles

For Sn1 reaction mechanisms, doubling the concentration of the __________ can double the rate of the reaction

Haloalkane

For Sn1 reactions, a planar and achiral ___________ is formed

Carbocation

True or False:

A carbocation intermediate can be attacked with roughly equal probability from either face

True

__________ structure influences the reaction rate and the mechanism

Haloalkane

Haloalkanes that can form more stable carbocations react ______ if an Sn1 mechanism occurs

Faster

1,2 shifts can occur during ___ reaction mechanisms

Sn1

The more stable the anion produced upon reaction, the ______ the leaving group’s ability

Better

List the type of solvent:

Hydrogen bond donors; often contain on -OH group

Protic

List the type of solvent:

Can not hydrogen bond; commonly acetone, diethyl ether, dimethyl sulfoxide, and N, N-dimethylformamide

Aprotic

What solvents interact strongly with ions and polar molecules?

Polar solvents

What solvents do not interact strongly with ions and polar molecules?

Nonpolar solvents

What solvent type accelerates Sn1 reactions?

Polar protic solvents

What solvent type accelerates Sn2 reactions?

Polar aprotic solvents

Why do polar protic solvents accelerate Sn1 reactions?

Polar protic solvents stabilize the carbocation intermediate and the leaving group by preventing their reconnecting

Why do polar aprotic solvents accelerate Sn2 reactions?

Polar aprotic solvents do not interact strongly with the nucleophile, which is preferred

Good nucleophiles are generally ______

Anions

Moderate nucleophiles are generally _______ with one or more available ____ _____

Neutral; lone pairs

Poor nucleophiles are generally _____ ______ ________

Polar protic solvents

Small nucleophiles with very little steric hindrance are better nucleophiles for ___ reactions

Sn2

Sn1 reaction rate is dependent on the concentration of the…

Haloalkane

Sn2 reaction rate is dependent on the concentration of the…

Haloalkane and nucleophile

True or False:

Sn1 reaction mechanisms typically result in racemization, but partial racemization is also common

True

True or False:

While common, not every Sn2 reaction mechanism proceeds with backside attack by the nucleophile

False

A carbocation in which the allylic carbon bears the (+) charge

Allylic carbocation

A (+) charged carbon bonded to one or more alkyl groups

Alkyl carbocations

Allylic carbocations are ____ stable than comparably substituted alkyl carbocations

More

True or False:

A primary allylic cation has the same stability as a secondary alkane/ene/yne cation

True

A carbocation in which a carbon attached to a benzene ring bears the (+) charge

Benzylic carbocation

True or False:

Allylic and benzylic carbocations are not specially stabilized

False

Allylic and benzylic carbocations are specially stabilized because of…

Resonance delocalization

Do Sn1 or Sn2 reactions ever occur on sp2 or sp carbons?

No

Sn1 mechanisms should be considered for _______ and ________ haloalkanes, even if the haloalkanes are primary

Allylic; benzylic

________ haloalkanes are protected from backside attack

Tertiary

Do primary halides with three β branches undergo Sn2 reactions?

No

The most stable anions are the best _______ ______

Leaving groups

The most stable anions are the ____ conjugate bases of ______ acids

Weak; strong

List three good leaving groups

I-, Br-, Cl-

List three poor leaving groups

OH-, CH3O-, NH2-

Dielectric constant > 15 =

Polar solvent

Dielectric constant <5 =

Nonpolar solvent

Dielectric constant in between 5 and 15 =

Borderline polar solvent

Water, formic acid, methanol, and ethanol are all common polar ______ solvents

Protic

DMSO, acetonitrile, acetone, and DMF are all common polar _______ solvents

Aprotic

Diethyl ether, toluene, and hexane are all common ________ _______ solvents

Nonpolar aprotic

For ___ reactions, the solvent should stabilize (+) and (-) sites

Sn1

__________ the polarity of solvents _________ the rate of an Sn1 reaction

Increasing; increases

__________ the polarity of solvents _________ the rate of an Sn2 reaction

Decreasing; increases

___ reactions are viable in all solvents that dissolve the nucleophile and electrophile

Sn2

A kinetic property measured by the rate at which a nucleophile causes nucleophilic substitution on a reference compound under a standardized set of experimental conditions

Nucleophilicity

True or False:

All nucleophiles are also Bronsted-Lowry acids

False

In general, sterically unhindered strong bases are ____ nucleophiles

Good

Nucleophiles that have a conjugate acid with a pKa above 11 are ______

Strong

Nucleophiles that have a conjugate acid with a pKa around 11 are ________

Moderate

Nucleophiles that have a conjugate acid with a pKa below 11 are ____

Weak

The less a nucleophile interacts with a solvent, the _______ its nucleophilicity

Greater

The stronger the interaction of the nucleophile with the solvent, the _____ its nucleophilicity

Lower

The following shows increasing nucleophilicity for what type of solvent?

I- < Br- < Cl- < F-

Polar aprotic

The following shows increasing nucleophilicity for what type of solvent?

F- < Cl- < Br- < I-

Polar protic

In polar protic solvents, ______ ion has the greatest nucleophilicity

Iodide

In polar aprotic solvents, ________ dictates nucleophilicity

Basicity

In polar protic solvents, ______________ dictates nucleophilicity

Polarizability

In polar aprotic solvents, anions are ______ solvated and are ____ to participate in nucleophilic substitution reactions

Weakly; free

In polar protic solvents, anions are highly ________ by hydrogen bonding with solvent molecules, so they are ____ free to participate in nucleophilic substitution reactions

Solvated; less

Iodide is the ____ nucleophile in a polar protic solvent because it is the most ___________

Best; polarizable

True or False:

Small nucleophiles are preferred and sometimes required for a backside attack on a carbon atom

True

Methyl or primary haloalkanes react through ___ reaction mechanisms

Sn2

Why do methyl or primary haloalkanes always react via Sn2 mechanism?

There is an absence of steric hindrance; good conditions for backside attack

Secondary haloalkanes react via ___ mechanism in aprotic solvents with ____ nucleophiles

Sn2; good

Secondary haloalkanes react via ___ mechanism in protic solvents with ____ nucleophiles

Sn1; poor