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Questions and Answers
What is the name of the process where the spreading out of charge occurs via the overlap of an empty p orbital with an adjacent sigma bond?
What is the name of the process where the spreading out of charge occurs via the overlap of an empty p orbital with an adjacent sigma bond?
Hyperconjugation
What is the general term used to describe the systematic preparation of a compound from a readily available starting material?
What is the general term used to describe the systematic preparation of a compound from a readily available starting material?
Organic synthesis
Which type of alkyl halide undergoes SN1 reactions rapidly?
Which type of alkyl halide undergoes SN1 reactions rapidly?
- Primary
- Secondary
- Tertiary (correct)
Which of the following factors INCREASES the rate of SN1 and SN2 reactions?
Which of the following factors INCREASES the rate of SN1 and SN2 reactions?
A negatively charged nucleophile is always a stronger nucleophile than its conjugate acid?
A negatively charged nucleophile is always a stronger nucleophile than its conjugate acid?
Match the following factors with how they affect the SN1 or SN2 reaction.
Match the following factors with how they affect the SN1 or SN2 reaction.
What type of alkyl halide undergoes SN2 reactions with ease?
What type of alkyl halide undergoes SN2 reactions with ease?
What does 'R' represent in the general formula for alkyl halides: R-X?
What does 'R' represent in the general formula for alkyl halides: R-X?
The Hammond postulate states that the transition state of a reaction resembles the structure of the species (reactant or product) to which it is closer in energy.
The Hammond postulate states that the transition state of a reaction resembles the structure of the species (reactant or product) to which it is closer in energy.
What type of alkyl halide undergoes SN1 reactions only?
What type of alkyl halide undergoes SN1 reactions only?
During which step of an SN1 reaction is a carbocation formed?
During which step of an SN1 reaction is a carbocation formed?
Which of the following types of solvents would most likely favor an SN2 reaction?
Which of the following types of solvents would most likely favor an SN2 reaction?
What type of alkyl halide undergoes SN2 reactions, but can sometimes also undergo SN1 reactions?
What type of alkyl halide undergoes SN2 reactions, but can sometimes also undergo SN1 reactions?
Flashcards
Alkyl halides
Alkyl halides
A halogen atom attached to an sp3 hybridized carbon atom.
Primary (1°), Secondary (2°), Tertiary (3°) Alkyl Halides
Primary (1°), Secondary (2°), Tertiary (3°) Alkyl Halides
A system for classifying alkyl halides based on the number of carbon atoms directly bonded to the carbon bearing the halogen.
Naming Alkyl Halides
Naming Alkyl Halides
The process of naming an alkyl halide by identifying the longest carbon chain, numbering the substituents, and alphabetizing their names.
Common Names of Alkyl Halides
Common Names of Alkyl Halides
Common names are often used for simple alkyl halides, replacing the 'ane' suffix with 'ide'.
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Polarity of Alkyl Halides
Polarity of Alkyl Halides
Alkyl halides exhibit weak polarity due to the unequal sharing of electrons between the carbon and halogen. These differences generate dipole-dipole interactions.
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Polar Carbon-Halogen Bond
Polar Carbon-Halogen Bond
The carbon-halogen bond possesses a dipole moment due to the electronegativity difference between carbon and the halogen, making the carbon atom electron deficient.
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Nucleophilic Substitution
Nucleophilic Substitution
A reaction involving the replacement of a leaving group (usually a halide) on a molecule by a nucleophile.
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Nucleophile
Nucleophile
A species that donates an electron pair to another molecule, often attacking electron-deficient atoms (like carbon). They can be negatively charged or neutral.
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Leaving Group
Leaving Group
The atom or group leaving a molecule in a nucleophilic substitution reaction. Good leaving groups are weak bases, while poor leaving groups are strong bases.
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Leaving Group Ability
Leaving Group Ability
The weaker the base, the better a species is as a leaving group. Strong bases are poor leaving groups.
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Bases
Bases
Species that readily donate electron pairs, particularly to protons.
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Basicity
Basicity
A measure of how readily a species donates its electron pair to a proton, determined by the pKa of its conjugate acid.
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Nucleophilicity
Nucleophilicity
A measure of how readily a species donates its electron pair to other atoms, determined by the rate constant of reaction.
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Nucleophilicity Parallels Basicity
Nucleophilicity Parallels Basicity
The stronger the base, the stronger the nucleophile for species with the same nucleophilic atom. Negatively charged nucleophiles are stronger than their conjugate acids.
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Steric Hindrance
Steric Hindrance
The reactivity of a nucleophile is decreased by the presence of bulky groups near the reaction site, making nucleophilic attack more difficult.
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Protic Solvents
Protic Solvents
Solvents that contain a hydrogen atom bound to oxygen or nitrogen, capable of hydrogen bonding with solutes.
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Aprotic Solvents
Aprotic Solvents
Solvents that lack an acidic hydrogen atom and cannot participate in hydrogen bonding.
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Nucleophilicity in Polar Protic Solvents
Nucleophilicity in Polar Protic Solvents
In protic solvents, nucleophilicity increases down the periodic table due to weaker solvation of larger anions.
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Nucleophilicity in Polar Aprotic Solvents
Nucleophilicity in Polar Aprotic Solvents
In aprotic solvents, anions are not well solvated and are therefore more reactive. Nucleophilicity generally parallels basicity.
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SN2 (Substitution Nucleophilic Bimolecular) Mechanism
SN2 (Substitution Nucleophilic Bimolecular) Mechanism
A reaction mechanism where bond breaking and bond making occur simultaneously in a single step.
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SN1 (Substitution Nucleophilic Unimolecular) Mechanism
SN1 (Substitution Nucleophilic Unimolecular) Mechanism
A reaction mechanism where bond breaking precedes bond making, involving a carbocation intermediate. The rate of reaction depends only on the alkyl halide.
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Carbocation
Carbocation
A reactive intermediate formed in SN1 reactions where the carbon atom has a positive charge. Stability increases with the number of alkyl groups attached.
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Hyperconjugation
Hyperconjugation
The spreading out of electron density through overlap of an empty p orbital with adjacent sigma bonds, leading to stabilization of carbocations.
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Hammond Postulate
Hammond Postulate
The principle stating that the transition state of a reaction will resemble the species (reactants or products) to which it is closer in energy.
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Transition States in Endothermic Reactions
Transition States in Endothermic Reactions
In endothermic reactions, the transition state resembles the products more. Stabilizing the products lowers the transition state energy, increasing the reaction rate.
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Transition States in Exothermic Reactions
Transition States in Exothermic Reactions
In exothermic reactions, the transition state resembles the reactants more. Product stability has little effect on the transition state energy.
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Substrate Reactivity in SN2 Reactions
Substrate Reactivity in SN2 Reactions
The rate of an SN2 reaction is influenced by the type of alkyl halide. Methyl and primary halides react quickly, while tertiary halides do not react due to steric hindrance.
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Substrate Reactivity in SN1 Reactions
Substrate Reactivity in SN1 Reactions
The rate of an SN1 reaction is affected by the stability of the carbocation intermediate. Tertiary halides react quickly due to stable tertiary carbocations.
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Predicting SN1 or SN2
Predicting SN1 or SN2
A process of predicting the preferred mechanism (SN1 or SN2) for a nucleophilic substitution reaction based on the alkyl halide, nucleophile, leaving group, and solvent.
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Organic Synthesis
Organic Synthesis
A step-by-step process for creating a desired organic compound from readily available starting materials.
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Retrosynthesis
Retrosynthesis
A method of analyzing a target molecule and determining the necessary starting materials and reactions required to synthesize it.
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Alkyl Halides
- Alkyl halides are halogen atoms bonded to an sp³ hybridized carbon atom.
- The halogen atom in halides is often denoted by the symbol "X".
- Alkyl halides are classified as primary (1°), secondary (2°), or tertiary (3°) depending on the number of carbons bonded to the carbon with the halogen atom.
- C is sp³ hybridized.
Naming Alkyl Halides
- Follow the same rules as naming alkanes.
- Find the parent chain.
- Name and number the substituents.
- Alphabetize the substituents.
Common Names of Alkyl Halides
- Common names are often used for simple alkyl halides.
- For example, "ine" becomes "ide" (e.g. iodine becomes iodide).
- tert-butyl iodide is an example
Polar Carbon-Halogen Bond
- Halogens create polar bonds in alkyl halides, making the carbon atom electron deficient.
- This slightly positive carbon becomes a reactive site for alkyl halides.
- Allows for substitution and elimination reactions.
Nucleophilic Substitution
- A nucleophile substitutes for the halide in a reaction.
- Nucleophiles can be negatively charged or neutral and donate an electron pair.
- The halide leaves, and the bond is heterolytically cleaved.
- The more stable the halide, the better it is at accepting the electron pair.
- The halide is known as a leaving group.
Nucleophiles and Bases
- Nucleophiles and bases are structurally similar, both having a lone pair or a bond.
- They differ in what they attack.
- Bases attack protons.
- Nucleophiles attack other electron-deficient atoms (usually carbons).
Nucleophilicity vs. Basicity
- Basicity measures how easily an atom donates its electron pair to a proton.
- Nucleophilicity measures how easily an atom donates its electron pair to other atoms.
Nucleophilicity Parallels Basicity
- For nucleophiles with the same atom, stronger bases are stronger nucleophiles.
- The relative nucleophilicity of different nucleophiles can be determined by comparing the pKa values of their conjugate acids.
Nucleophilicity in Polar Protic Solvents
- Smaller, more electronegative anions are solvated more strongly in polar protic solvents, effectively shielding them from reaction.
- Nucleophilicity increases down a column of the periodic table as the size of the anion increases (opposite to basicity).
Nucleophilicity in Polar Aprotic Solvents
- Polar aprotic solvents solvate cations by ion-dipole interactions.
- Anions are not well solvated in polar aprotic solvents, making them more reactive.
- Generally, nucleophilicity decreases as the structure becomes bulkier.
Common Nucleophiles
- A table listing common negatively charged and neutral nucleophiles.
Bond Breaking and Making in Nucleophilic Substitution Mechanisms
- Bond making and breaking can occur simultaneously.
- Bond breaking can occur first.
- Bond making can occur first.
SN2 Reactions
- Substitution Nucleophilic bimolecular reactions.
- Involves the inversion of configuration
- A second order reaction (rate depends on both reactants).
- A bimolecular reaction (one step mechanism).
SN1 Reactions
- A substitution nucleophilic unimolecular reaction.
- Has two steps (bond breaking occurs first, followed by bond forming), and a carbocation intermediate.
- A first order reaction (rate depends on the concentration of only the alkyl halide).
Carbocation Stability
- Carbocation stability increases with increasing alkyl substitution.
- Alkyl groups are electron-donating (better than hydrogen) and stabilize the positive charge.
- Stability is due to the inductive effect and hyperconjugation.
Hyperconjugation
- Hyperconjugation is the spreading out of charge by the overlap of an empty p orbital with an adjacent σ bond, stabilizing the carbocation intermediate.
The Hammond Postulate
- The transition state structure resembles the species (reactant or product) to which it is closer in energy.
- The closer the transition state is in energy to the product, the faster the reaction.
Transition State, Endothermic Reactions
- Transition states in endothermic reactions resemble the products.
- Lowering the product's energy helps to lower the transition state's energy which increases the reaction rate.
Transition State, Exothermic Reactions
- Transition states in exothermic reactions resemble the reactants.
- Lowering the product energy has little effect on transition state energy, so reaction rate is not affected.
Application of the Hammond postulate to SN1 reactions
- The rate-determining step in SN1 reactions is carbocation formation, an endothermic process.
- More stable carbocations (3° > 2° > 1°) form faster because their formation is less energetically demanding.
Predicting the Mechanism
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Factors to consider: Alkyl halide type, nucleophile strength, leaving group ability, and solvent type (protic vs. aprotic).
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The alkyl halide type, nucleophile, leaving group, and solvent influence the choice of the reaction mechanism.
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