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Questions and Answers

What type of bond formation occurs in an SN2 mechanism?

Formation of a stable carbocation

Formation of a covalent bond with stereoinversion (correct)

Formation of an anionic species

Formation of a racemic mixture

How many steps are involved in an SN1 reaction mechanism?

Two steps (correct)

Three steps

One step

Four steps

In the SN1 reaction, what is the rate-determining step?

Proton transfer to methanol

Formation of the carbocation (correct)

Ionization of the halide ion

Formation of the C-Nu bond

What is a characteristic of the products formed in an SN1 reaction involving a stereocenter?

A racemic mixture is produced Signup and view all the answers

What happens during the rearrangement of a carbocation in an SN1 mechanism?

It can rearrange to a more stable carbocation Signup and view all the answers

Which of the following is NOT true regarding the SN2 mechanism?

There is a reactive intermediate Signup and view all the answers

What is the role of the nucleophile in the SN1 mechanism?

It reacts with the carbocation to form a covalent bond Signup and view all the answers

What factor most directly influences the rate of an SN1 reaction?

Stability of the carbocation Signup and view all the answers

What condition favors an SN2 reaction in secondary alkyl halides?

Good nucleophile Signup and view all the answers

What is the major product of a β-elimination reaction according to Zaitsev's rule?

The more stable alkene Signup and view all the answers

Which step is rate-determining in the E1 mechanism?

Ionization of the C-X bond Signup and view all the answers

What is the general structure of a haloalkane?

Contains a halogen atom covalently bonded to an sp3 hybridized carbon Signup and view all the answers

Which of the following is NOT a halogen substituent name in nomenclature?

Chlor- Signup and view all the answers

Which mechanism is favored by strong bases during β-elimination reactions?

E2 mechanism Signup and view all the answers

What type of solvent favors SN1 reactions?

Protic solvents with weak nucleophiles Signup and view all the answers

What type of reaction occurs in nucleophilic substitution?

A nucleophile replaces a halogen substituent Signup and view all the answers

What distinguishes the SN2 mechanism from other nucleophilic substitution mechanisms?

The C-X and C-Nu bonds break and form simultaneously Signup and view all the answers

What role does the carbocation play in the E1 mechanism?

It is an intermediate that facilitates proton transfer Signup and view all the answers

In terms of stereochemistry, what is a characteristic outcome of an SN2 reaction?

Inversion of configuration Signup and view all the answers

Why are halides considered good leaving groups in nucleophilic substitution reactions?

They form stable anions Signup and view all the answers

Which of the following statements about the E2 mechanism is true?

It occurs in a concerted fashion Signup and view all the answers

Which of the following is an alternative to chlorofluorocarbons (CFCs) that is less ozone-depleting?

Hydrochlorofluorocarbons (HCFCs) Signup and view all the answers

In the context of haloalkane reactivity, what is $eta$-elimination?

Loss of a halogen and an adjacent hydrogen to form a $ u$-bond Signup and view all the answers

Which statement about nucleophiles is true?

Nucleophiles can possess a lone pair or a negative charge Signup and view all the answers

Which of the following is a characteristic of a good leaving group?

High stability of the resulting species Signup and view all the answers

In which situation would hydroxide ion, OH–, be a good leaving group?

When it is protonated to form OH2+ Signup and view all the answers

Which solvent type is most favorable for SN1 reactions?

Protic solvents containing –OH groups Signup and view all the answers

What is the main effect of aprotic solvents on nucleophiles in SN2 reactions?

They solvate nucleophiles which decreases their effectiveness Signup and view all the answers

Which of the following statements is true regarding carbocation formation?

Carbocations form easily in protic solvents Signup and view all the answers

What kind of nucleophile is favored in aprotic solvents for SN2 reactions?

Good nucleophiles Signup and view all the answers

What effect does the solvation of nucleophiles have in protic solvents on SN2 reactions?

It inhibits nucleophilic attack Signup and view all the answers

Under which condition wouldSN1 be the more likely mechanism?

Protic solvents with poor nucleophiles Signup and view all the answers

What primarily determines the rate of SN2 reactions?

Steric factors related to nucleophile approach Signup and view all the answers

Which haloalkane is least likely to participate in an SN2 reaction?

2-bromo-2-methylpropane Signup and view all the answers

Which statement regarding nucleophilicity is accurate?

Structure and charge affect the nucleophilicity of the atom. Signup and view all the answers

What is the correct relative rate order for SN1 reactions based on carbon structure?

3° > 2° > 1° > methyl Signup and view all the answers

Which factor does NOT influence nucleophilicity in SN2 reactions?

Strength of the leaving group Signup and view all the answers

Which situation best describes a condition where SN2 mechanisms are favored?

A strong nucleophile and a methyl haloalkane Signup and view all the answers

Which type of haloalkanes can react via both SN1 and SN2 mechanisms under simple conditions?

Secondary alkyl halides Signup and view all the answers

What characteristic is typically observed in the structure of a haloalkane that leads to SN2 reaction failure?

Steric hindrance around the reactive site Signup and view all the answers

What type of reaction is favored for 2° and 3° haloalkanes in polar protic solvents?

A mixture of substitution and elimination reactions Signup and view all the answers

Which condition is likely to favor an E2 reaction over an SN2 reaction?

Using a strong base Signup and view all the answers

For methyl and primary haloalkanes, which mechanism is favored when a good nucleophile is present?

Both SN2 and E2 can occur Signup and view all the answers

What is a common result when a weak base and poor nucleophile are used in a reaction involving haloalkanes?

E1/SN1 reactions are likely Signup and view all the answers

Which factor principally determines the ratio of SN2 to E2 products?

The base/nucleophile used Signup and view all the answers

What happens to the reaction pathway when a strong base is paired with a good nucleophile in haloalkanes?

Both SN2 and E2 reactions can occur Signup and view all the answers

Which product formation is more unpredictable when poor nucleophiles are involved in reactions?

Product ratios of SN1 and E1 Signup and view all the answers

In what scenario would E2 reaction be predominantly favored in the presence of steric hindrance?

Using a large, strong base Signup and view all the answers

Study Notes

Haloalkanes (Structure and Reactivity)

Haloalkanes, also known as alkyl halides, are organic compounds containing a halogen atom (F, Cl, Br, or I) covalently bonded to an sp³ hybridized carbon. The general formula is R-X, where R represents the organic framework and X represents the halogen.

Structure

Haloalkanes are named as substituents, akin to alkyl sidechains.
Halogen substituents are named as fluoro-, chloro-, bromo-, or iodo-.
The numbering system should prioritize the lowest possible number for the first substituent.
Named in alphabetical order with other substituents.
Reactivity is driven by the nature of the C-X bond.

Nomenclature

Halogen groups are named as substituents, similar to alkyl sidechains.
Examples of substituent names include fluoro-, chloro-, bromo-, and iodo-.
Numbering should give the first substituent the lowest possible number.
Listing in alphabetical order with other substituents.

Common Haloalkanes

Many polyhaloalkanes are used as solvents.
They are often referred to by their common/trivial names within the brackets.
- Examples include dichloromethane (methylene chloride), trichloromethane (chloroform), 1,1,1-trichloroethane (methyl chloroform), and trichloroethylene (trichlor).

Freons and Alternatives

Freons are chlorofluorocarbons (CFCs) that were widely used.
Less ozone-depleting alternative compounds are available such as hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs).

Reactivity: Substitution & Elimination

Two key reaction types are nucleophilic substitution (replacing a halogen with a nucleophile) and β-elimination (losing a halogen and an adjacent hydrogen to form a C-C π-bond).

Nucleophilic Substitutions

Substitution happens on the sp³ hybridized carbon atom.
The halogen atom acts as the leaving group.
Halides are good leaving groups forming stable anions .
The carbon atom is electrophilic due to the polarity of the C-X bond.
Nucleophiles, often anions, will attack this carbon.
Many types of functional groups can be produced from different nucleophiles.

SN2 Mechanism

At one extreme, C-X and C-Nu bond breaking and forming are simultaneous in one step.
Designated SN2 (substitution, nucleophilic, bimolecular).
Rate is directly proportional to the concentration of both haloalkane and nucleophile ([R-X][Nu:]).
Inversion will occur in stereochemistry.

SN1 Mechanism

Two-step mechanism:
- Step 1 (rate-determining): C-X bond breaks forming a carbocation intermediate plus a halide ion.
- Step 2: Nucleophile reacts with the carbocation to form a new C-Nu bond.
Designated SN1 (substitution, nucleophilic, unimolecular).
Rate depends only on the concentration of haloalkane ([R-X]).
Racemization will occur in stereochemistry.
Carbocation rearrangements are possible.

SN1 Carbocation Rearrangements

Carbocation rearrangements are possible, often to more stable 3° carbocations before attack by nucleophile.

Stereochemistry

SN1 reactions at stereocenters lead to racemic mixtures.
SN2 reactions at stereocenters lead to inverted stereochemistry.

SN1 vs. SN2 Mechanisms

Factors that affect the rate of the reaction including the structure of the nucleophile, the haloalkane, the leaving group, and the solvent.

Nucleophilicity

A kinetic property related to how nucleophiles influence the rate of an SN2 reaction with a reference haloalkane in standard conditions .
Size, charge, and electronegativity of the nucleophilic atom influence nucleophilicity.

Structure of the Haloalkane

The substitution of the carbon atom attached to the halogen has a big impact on the reaction rate via SN1 or SN2.
SN1 reactions are determined by electronic factors (3° > 2° > 1° > methyl).
SN2 reactions are determined by steric factors (methyl > 1° > 2° > 3°).

Structure of the Leaving Group

The best leaving groups form stable species (I-, Br-, Cl-, and H₂O).

The Solvent: Protic Solvents

Protic solvents are hydrogen bond donors containing -OH groups (e.g., water, methanol, ethanol).
They favor SN1 reactions because they greatly stabilize both carbocations and leaving groups via solvation.

The Solvent: Aprotic Solvents

Aprotic solvents are not hydrogen bond donors.
They usually favor SN2 reactions (e.g., DMSO, acetone, acetonitrile).
Formation of carbocations is considerably more difficult in aprotic solvents relative to protic solvents.

SN1 vs. SN2 in Haloalkanes

Methyl/primary haloalkanes proceed best via SN2.
Secondary haloalkanes react by various reactions, often mixture of SN2 and SN1 depending on the bases/nucleophiles present and solvent/conditions.
Tertiary haloalkanes are more apt to follow SN1 mechanisms, but E1 mechanism is just as likely

β -Elimination (Formation of Alkenes)

Beta-elimination is a reaction where atoms/groups are removed from adjacent carbons.
- A common reaction type involves dehydrohalogenation (loss of H-X).
- Losing a halogen atom and a hydrogen adjacent to it form a π-bond which creates an alkene.

β-Elimination Selectivity

Zaitsev's rule indicates formation of the more stable alkene (more substituted) as the major product. Trans alkenes are favored in cis-trans situations.

Beta-Elimination Mechanisms

Two mechanisms for β-elimination:
- E1 mechanism is a two-step process: first, a carbocation forms; then, a hydrogen is removed
- E2 mechanism is a concerted one-step process in which both the leaving group and a hydrogen are removed simultaneously.

Elimination Reactions

Strong bases favor E2, while weak bases favor E1.
E1 is easier with 3° or 2° alkyl halides.
E2 is possible with 3° alkyl halides, but easier with 1°, 2°

Substitution versus Elimination

SN and E reactions are competing pathways. Ratio dependent on product relative rates.

SN1 versus E1

Reactions of 2°, 3° haloalkanes in polar protic solvents yield mixtures of substitution and elimination products.
Poor base/nucleophile, and presence of the solvent, favour carbocation formation. Product ratios are hard to predict.

SN2 versus E2

Predicting the ratio of SN2 to E2 is easier, the base/nucleophile is a key factor.
Large strong bases favor E2 reactions due to steric reasons.
Small good nucleophiles favor SN2

SN versus E for Methyl and 1° Haloalkanes

SN versus E for 2°,3° Haloalkanes

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