Basicity vs Nucleophilicity - Steric Hindrance

The Organic Chemistry Tutor
27 Dec 201615:50
EducationalLearning
32 Likes 10 Comments

TLDRThe transcript delves into the distinction between bases and nucleophiles, using the reaction mechanisms of S1 and E1 as examples. It explains how water can act as either, depending on the situation. The difference is defined by their actions: nucleophiles attack electron-deficient atoms, while bases accept protons. The strength of a base or nucleophile is influenced by the charge of the species and the solvent environment, with periodic trends and steric effects playing a significant role. The use of dbn as an example illustrates how extreme steric hindrance can lead to E2 reactions with high yields.

Takeaways
  • 📌 The distinction between a base and a nucleophile is that a base abstracts a hydrogen atom, while a nucleophile attacks an electrophile, which is an atom that is electron deficient.
  • 🔄 In the context of reactions involving chlorobutane and water, water can act as either a nucleophile or a base, leading to two possible mechanisms: S1 and E1 reactions.
  • 💧 When water acts as a nucleophile, it leads to an S1 reaction, resulting in the formation of an alcohol, whereas acting as a base leads to an elimination reaction, producing an alkene.
  • ⚖️ Comparing water and hydroxide, hydroxide is a stronger base and a better nucleophile due to its negative charge, highlighting that more negatively charged species tend to be stronger bases and nucleophiles.
  • 📈 Periodic trends play a significant role in determining the strength of bases and nucleophiles, with base strength increasing up the group and nucleophilic strength varying based on the solvent type.
  • 🥃 In protic solvents, nucleophilic strength increases down the group, while in aprotic solvents, it increases up the group, with hydroxide consistently being a stronger base across different solvents.
  • 🔆 Steric effects influence the behavior of a molecule as a base or a nucleophile, with bulky bases favoring E2 reactions and unhindered, non-bulky bases favoring SN2 reactions.
  • 🍸 The reaction between a primary alcohol and a strong, unhindered base like methoxide predominantly follows the SN2 mechanism, resulting in the formation of an ether.
  • 🔌 DBN (1,5-diazabicyclo[4.3.0]non-5-ene) is an example of a highly bulky base that almost exclusively promotes E2 reactions with a near 100% yield due to its steric hindrance.
  • 🌟 Steric factors are crucial in determining the reactivity of molecules, where increased bulkiness enhances basicity but can reduce nucleophilicity, affecting the overall outcome of reactions.
Q & A

  • What are the two mechanisms that can occur in the reaction between chlorobutane and water?

    -The two mechanisms that can occur are the S1 (Substitution 1) reaction and the E1 (Elimination 1) reaction.

  • What is the role of water in the S1 reaction?

    -In the S1 reaction, water acts as a nucleophile, attacking the carbocation intermediate formed after the leaving group departs.

  • How does water behave in the E1 reaction?

    -In the E1 reaction, water acts as a base, abstracting a hydrogen atom from the carbon adjacent to the leaving group, leading to the formation of an alkene.

  • What is the fundamental difference between a base and a nucleophile?

    -A base abstracts a hydrogen atom, making it a proton acceptor, while a nucleophile attacks an electrophile, which is an atom or group that is electron deficient.

  • How can you determine which is a stronger base or nucleophile, water or hydroxide?

    -Hydroxide is more basic and a better nucleophile than water because it has an additional negative charge, making it more reactive.

  • What is the trend for base strength in the context of the periodic table?

    -Base strength generally increases as you move up and towards the left on the periodic table, as the ability to accept protons becomes stronger.

  • How does the solvent type affect nucleophilic strength?

    -In a protic solvent, nucleophilic strength increases as you move down the group, while in a polar aprotic solvent, nucleophilic strength increases as you move up the group.

  • Why is fluoride a weaker nucleophile in a protic solvent compared to iodide?

    -In a protic solvent, fluoride's nucleophilicity is reduced because it is solvated and shielded by the solvent's hydrogen atoms, making it less accessible for reactions compared to iodide.

  • What is the impact of steric effects on the behavior of a base or nucleophile?

    -Sterically hindered bases, like tert-butoxide, are less effective as nucleophiles due to their bulkiness, which can prevent them from approaching and reacting with carbon atoms effectively.

  • What is DBN and how does it illustrate the steric effects between a base and a nucleophile?

    -DBN (1,5-diazabicyclo[4.3.0]non-5-ene) is a highly bulky base that favors E2 (Elimination 2) reactions almost exclusively due to its steric hindrance, which prevents it from acting as a nucleophile.

  • What is the reaction outcome when DBN reacts with a secondary alcohol and a halide?

    -When DBN reacts with a secondary alcohol and a halide, it undergoes an E2 reaction, yielding an alkene with nearly 100% yield, due to its high basicity and inability to act as a nucleophile.

Outlines
00:00
📚 Base and Nucleophile: Understanding the Difference

This paragraph introduces the concepts of bases and nucleophiles, using the reaction between chlorobutane and water to illustrate their roles. It explains that water can act as either a nucleophile, attacking the carbocation, or as a base, abstracting a hydrogen atom. The distinction is clarified: nucleophiles attack electron-deficient atoms (electrophiles), while bases accept protons. The paragraph also discusses how to determine the strength of a base or nucleophile, comparing water and hydroxide, and NH2- with NH3. It introduces periodic trends, showing that base strength increases up the group and nucleophilic strength varies with the solvent and position in the periodic table.

05:01
🧪 Solvent Effects on Nucleophile and Base Strength

This paragraph delves into the influence of solvents on the strength of nucleophiles and bases. It differentiates between protic solvents, which have hydrogen bonded to oxygen or nitrogen, and aprotic solvents, which lack such hydrogen bonds. The summary explains how nucleophilic strength changes depending on the solvent type, with examples of each. It addresses the original question of SH- versus OH-, clarifying that in protic solvents like ethanol, sulfur is more nucleophilic than oxygen, while in aprotic solvents, hydroxide is a better nucleophile. The paragraph also explores the concept of steric effects, using methoxide and tert-butoxide as examples to demonstrate how bulkiness can impact a molecule's ability to act as a nucleophile or base.

10:03
🔬 Steric Factors and Reaction Mechanisms

This paragraph focuses on the impact of steric factors on the basicity and nucleophilicity of molecules, particularly in the context of alcoholate ions. It contrasts methoxide and tert-butoxide, highlighting how the bulkiness of tert-butoxide makes it a better base but a less effective nucleophile. The discussion includes the favored reaction mechanisms for strong, unhindered bases (SN2) and bulky bases (E2). The paragraph introduces DBN (1,5-diazabicyclo[4.3.0]non-5-ene) as an example of a highly bulky base that promotes E2 reactions with high yields. The summary concludes with a specific reaction between DBN and a secondary alcohol, resulting in a 100% yield of propene and illustrating the steric effects between bases and nucleophiles.

Mindmap
Keywords
💡Nucleophile
A nucleophile is a chemical species that donates an electron pair to an electrophile in a chemical reaction. In the context of the video, a nucleophile attacks an electron-deficient atom, such as a carbon atom with a positive charge (carbocation), in reactions like SN1 and SN2. An example from the script is water acting as a nucleophile in the reaction with chlorobutane, leading to the formation of an alcohol in an SN1 reaction.
💡Base
A base is a substance that can accept a proton (H+) or donate a pair of electrons. In the video, bases are described as proton acceptors, and their ability to abstract a hydrogen atom from a molecule is highlighted. The difference between a base and a nucleophile is clarified by their respective roles in reactions; bases promote elimination reactions leading to the formation of alkenes, while nucleophiles promote substitution reactions.
💡Electrophile
An electrophile is a chemical species that is deficient in electrons and seeks to gain electrons in a chemical reaction. In the video, electrophiles are atoms or molecules that have a positive charge or are electron deficient, and they are attracted to nucleophiles. The video uses the example of a carbocation, which is an electron-deficient carbon atom, as an electrophile.
💡SN1 Reaction
An SN1 reaction, or Substitution Nucleophilic Unimolecular reaction, is a two-step reaction in which a nucleophile replaces a leaving group in a molecule. In the video, the SN1 reaction is described as the process where water, acting as a nucleophile, attacks a carbocation intermediate formed by the departure of a leaving group in chlorobutane.
💡SN2 Reaction
An SN2 reaction, or Substitution Nucleophilic Bimolecular reaction, is a single concerted step where a nucleophile replaces a leaving group in a molecule. In the video, the SN2 reaction is contrasted with the SN1 reaction, with the emphasis on the simultaneous bond breaking and forming, leading to the formation of a product without a carbocation intermediate.
💡E1 Reaction
An E1 reaction, or Elimination Unimolecular reaction, is a two-step reaction where a base abstracts a hydrogen atom from a molecule, leading to the formation of a carbocation intermediate, which then loses a leaving group to form an alkene. In the video, the E1 reaction is described as the process where water, acting as a base, abstracts a hydrogen atom from chlorobutane, leading to an elimination reaction.
💡Steric Effects
Steric effects refer to the influence of the size and shape of molecules on the rate and outcome of chemical reactions. In the video, steric effects are discussed in the context of how bulky bases like tert-butoxide (t-BuO-) favor E2 reactions over SN2 reactions due to their difficulty in approaching the carbon atom to be attacked.
💡Protic Solvent
A protic solvent is a solvent that contains hydrogen atoms bonded to electronegative atoms such as oxygen or nitrogen, which can form hydrogen bonds. In the video, protic solvents are described as increasing nucleophilic strength as you move down the group in the periodic table, and they affect the reactivity of nucleophiles like fluoride and iodide differently.
💡Aprotic Solvent
An aprotic solvent is a solvent that does not contain hydrogen atoms bonded to electronegative atoms and therefore cannot form hydrogen bonds. In the video, aprotic solvents are described as affecting nucleophilic strength differently than protic solvents, with nucleophilic strength increasing up the group in the periodic table in aprotic environments.
💡DBN
DBN, or 1,5-diazabicyclo[4.3.0]non-5-ene, is a strong, bulky base that is used in organic chemistry to promote E2 reactions with high yields. In the video, DBN is described as being so bulky that it does not act as a nucleophile at all, focusing solely on its basic properties.
💡Leaving Group
A leaving group is a functional group or atom that can be removed from a molecule during a reaction, often because it has a tendency to accept an electron pair. In the video, the leaving group's role is crucial in both SN1 and E1 reactions, where it departs from the molecule, leading to the formation of a carbocation intermediate or directly participating in the elimination process.
Highlights

Explaining the difference between a base and a nucleophile using the reaction of chlorobutane with water as an example.

Water can act as both a nucleophile and a base in reactions with chlorobutane.

Two possible mechanisms for the reaction: S1 and E1.

The role of the leaving group in both S1 and E1 reactions.

Nucleophiles attack electrophiles, while bases abstract hydrogen atoms.

The distinction between nucleophilic and basic behavior of water in the reaction.

Comparison of water and hydroxide in terms of basicity and nucleophilicity.

The influence of negative charge on the strength of a base and nucleophile.

Understanding periodic trends to determine the strength of bases and nucleophiles.

Base strength increases up and towards the left on the periodic table.

Nucleophilic strength varies depending on the solvent type.

Protic solvents increase nucleophilic strength down the group, while aprotic solvents increase it up the group.

Fluoride is weaker in a protic solvent compared to iodide due to solvation effects.

Iodide is a stronger nucleophile in a protic solvent because it retains its negative charge.

Steric effects influence the basicity and nucleophilicity of a reaction.

Methoxide and tert-butoxide illustrate the impact of steric factors on nucleophilicity and basicity.

DBN (1,5-diazabicyclo[4.3.0]non-5-ene) is a bulky base that produces almost 100% E2 reaction yield.

The reaction between DBN and a secondary alcohol yields propene with a 100% yield, demonstrating steric effects.

Transcripts

Browse More Related Video

7.7 How to Distinguish Between Substitution and Elimination Reactions (SN2 SN1 E2 E1) | OChem

Nucleophilic Strength

SN2 SN1 E1 E2 Reaction Mechanisms Made Easy!

Chem 51A 11/30/09 Ch. 8. Elimination Reactions. Introduction to E2 Reactions

SN1/SN2/E1/E2 - working through problems!

Nucleophiles and Electrophiles

Rate This

5.0 / 5 (0 votes)

Thanks for rating:

Related Tags
Chemical ReactionsOrganic ChemistryBase and NucleophileS1 and E1 ReactionsNucleophilic StrengthBasicityPeriodic TrendsSolvent EffectsSteric FactorsDBN Reagent