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Arrow Pushing: The Key to Organic Chemistry Mechanisms
Unlock the power of arrow pushing in organic chemistry. Learn to track electron flow, predict reactions, and master complex mechanisms. Elevate your understanding of molecular transformations today!

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Now Playing:Arrow pushing (curly rrows) in organic chemistry – Example 0a
Intros
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  1. Drawing reaction mechanisms.
  2. Drawing reaction mechanisms.
    Rules for drawing curly arrows.
  3. Drawing reaction mechanisms.
    A note on curly arrow placement.
Reaction mechanisms
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Notes

In this lesson, we will learn:

  • To recall the guidelines of arrow-pushing in organic reaction mechanisms.
  • To apply arrow-pushing to suggest organic reaction mechanisms.

Notes:

  • In kinetics, we introduced the idea of a reaction mechanism, talked about what is required for a chemical reaction and why they happen:
    • Particles need to collide with enough energy (the activation energy) and the correct arrangement in order to react. One collision meeting both conditions is unlikely, so most molecular collisions don’t actually lead to a reaction.
    • Reactions are often driven by charge interaction between particles – attraction of opposite charge, either ions or partial charges (like δ\delta+ / δ\delta-).

  • Curly arrows or ‘arrow-pushing’ is widely used in organic chemistry to show the reaction mechanism.
    There are some key ideas behind arrow-pushing that must be followed when arrow pushing in reaction mechanisms.
    • Curly arrows show the movement of electrons. This happens in two ways:
      • Full-headed arrows show a pair of electrons moving. The majority of chemical reactions involve pairs of electrons moving.
      • Half-headed arrows show one electron moving. These are very common in free-radical chemistry but is otherwise rare.
    • Curly arrows are drawn from an area of negative charge. This includes:
      • Formal negative charges (negative ions) such as Cl- or CN-
      • Lone pairs, such as H2O: or : NH3
      • Pi bonds, such as H2C=CH2
      • Sigma bonds between carbon and electropositive atoms, such as R-Li (called organolithium reagents) or R-Mg (Grignard reagents).
      Remember that compounds where curly arrows come from are donating electrons – the examples here are all possible nucleophiles!
    • Curly arrows are drawn going toward an area of positive charge or electron deficiency. This includes:
      • Formal positive charges (positive ions) such as H+ carbocations (R3C+).
      • Partial positive charges (δ\delta+) such as a carbonyl carbon atom C=O, or carbon bonded to a halogen such as C-Cl.
      • Vacant p orbitals, such as boron in BH3
    • Curly arrows must show charge being conserved.
      • When a nucleophile donates an electron pair to form a covalent bond, it effectively loses one electron; the extra electron it had now ‘belongs to’ the electrophile in the covalent bond.
      • Depending on the nucleophile, the outcome can look slightly different.
        See the image below for two examples

  • Worked example:
    The reaction of a general nucleophile and a carbonyl group, with two variations, is shown below.
    The chemistry going on is not important here; the two reactions are just to show more examples of curly arrows being used from electron-rich to electron-poor species.
Concept

Introduction to Arrow Pushing in Organic Chemistry

Arrow pushing, also known as using curly arrows, is a fundamental concept in organic chemistry that plays a crucial role in understanding reaction mechanisms. The introduction video provides a comprehensive overview of this technique, highlighting its significance in visualizing electron movement during chemical reactions. Arrow pushing is an essential tool that allows chemists to track the flow of electrons as bonds break and form. By using curved arrows to represent electron pairs, this method illustrates how electrons move from areas of high electron density to areas of low electron density. This technique is particularly useful in predicting reaction outcomes, understanding intermediates, and explaining the formation of products. Mastering arrow pushing enables students and researchers to interpret complex organic reactions, propose reasonable mechanisms, and design new synthetic pathways. As you delve deeper into organic chemistry, proficiency in arrow pushing becomes invaluable for solving problems and understanding the intricacies of molecular transformations.

Example

Drawing reaction mechanisms. Rules for drawing curly arrows.

Step 1: Introduction to Curly Arrows

Curly arrows, also known as arrow pushing, are a fundamental concept in organic chemistry used to illustrate the movement of electrons during chemical reactions. These arrows help in visualizing how bonds break and form, transforming reactants into products. Understanding the rules and guidelines for drawing curly arrows is essential for accurately depicting reaction mechanisms.

Step 2: What is a Curly Arrow?

A curly arrow is used to show the movement of electrons. There are two main types of curly arrows:

  • Full-headed arrow: This arrow indicates the movement of a pair of electrons. For example, if an atom X is bonded to an atom Y, and X is more electronegative, the electrons in the covalent bond will move towards X, breaking the bond and resulting in a negative charge on X and a positive charge on Y.
  • Half-headed arrow: This arrow shows the movement of a single electron. It is commonly used in free radical chemistry, where bonds between two identical atoms break, and each atom retains one electron from the bond.

Step 3: Origin and Destination of Curly Arrows

Curly arrows are drawn from electron-rich areas to electron-poor areas. Electron-rich areas are typically nucleophiles, which are species that donate electrons. These include:

  • Formal charges: Species with a negative charge, such as Cl- or CN-, are good nucleophiles because they have an extra electron to donate.
  • Lone pairs: Atoms with lone pairs of electrons, such as nitrogen in ammonia (NH3) or oxygen in water (H2O), can act as nucleophiles.
  • Pi bonds: Double bonds, such as those in ethene (C2H4), can also act as nucleophiles. The pi bond can break, and the electrons can be used to form a new bond.
  • Carbon-metal sigma bonds: In organometallic compounds, such as organolithium or Grignard reagents, the carbon is more electron-rich due to the metal's lower electronegativity, making it a good nucleophile.

Step 4: Identifying Electrophiles

Electrophiles are electron-poor species that accept electrons. They include:

  • Formal positive charges: Species like H+ are strong electrophiles because they can readily accept electrons.
  • Partial positive charges: Atoms bonded to more electronegative atoms, such as carbon bonded to oxygen, have a partial positive charge and can act as electrophiles.
  • Vacant p orbitals: Atoms like boron, which have vacant p orbitals, can accept electrons, making them electrophiles.

Step 5: Conserving Charge

When drawing curly arrows, it is crucial to conserve charge. Electrons are not created or destroyed; they are merely moved from one place to another. For example, if a negatively charged nucleophile (Nu-) donates electrons to a positively charged electrophile (E+), the resulting product will have a neutral charge. The nucleophile loses its negative charge, and the electrophile gains a negative charge, balancing the overall charge.

Step 6: Drawing Curly Arrows Correctly

When drawing curly arrows, ensure they have a curl to indicate the movement of electrons. A straight arrow is used to show a phase of the reaction, while a double-headed arrow indicates resonance structures or equilibrium. Properly drawn curly arrows help in accurately depicting the flow of electrons from electron-rich to electron-poor areas, ensuring a clear understanding of the reaction mechanism.

FAQs

  1. What is arrow pushing in organic chemistry?

    Arrow pushing, also known as electron pushing, is a technique used in organic chemistry to visualize the movement of electrons during chemical reactions. It uses curved arrows to show how electrons move from electron-rich areas to electron-poor areas, helping to explain bond formation, breaking, and rearrangement.

  2. What's the difference between full-headed and half-headed arrows?

    Full-headed arrows () represent the movement of an electron pair, typically used for showing bond formation or lone pair movement. Half-headed arrows () represent the movement of a single electron, commonly used in radical reactions or homolytic bond cleavage.

  3. How do you identify nucleophiles and electrophiles in arrow pushing?

    Nucleophiles are electron-rich species that donate electrons, while electrophiles are electron-poor species that accept electrons. In arrow pushing, arrows typically start from nucleophiles (e.g., lone pairs, negative charges) and point towards electrophiles (e.g., positive charges, electron-deficient atoms).

  4. Why is charge conservation important in arrow pushing mechanisms?

    Charge conservation ensures that the total charge of reactants equals the total charge of products in any chemical reaction. It helps in predicting charges on intermediates and products, identifying errors in proposed mechanisms, and understanding the overall electron flow in complex reactions.

  5. How do you represent electron movement from double bonds in arrow pushing?

    When using double bonds as nucleophiles, draw the curly arrow starting from the center of the pi bond towards the electrophile. This represents the movement of the pi electrons attacking the electrophile. It's important to draw the arrow from the bond itself, not from a specific carbon atom, to accurately reflect the delocalized nature of the pi electrons.

Prerequisites

Understanding arrow pushing (curly arrows) in organic chemistry is crucial for mastering complex organic reactions. However, to fully grasp this concept, it's essential to have a solid foundation in several prerequisite topics. These fundamental areas provide the necessary context and knowledge to effectively utilize arrow pushing techniques.

One of the most critical prerequisites is reaction mechanisms. Familiarity with reaction mechanisms is vital because arrow pushing is essentially a visual representation of these processes. By understanding how reactions occur step-by-step, students can more easily interpret and apply curly arrows to illustrate electron movement during chemical transformations.

Another key prerequisite is an introduction to chemical reactions. This foundational knowledge helps students recognize different types of reactions and their general patterns. When students have a solid grasp of chemical reactions, they can more readily predict and visualize electron flow, which is the essence of arrow pushing in organic chemistry.

Equally important is the understanding of nucleophiles and electrophiles. These concepts are fundamental to organic chemistry and play a crucial role in arrow pushing. Nucleophiles, being electron-rich species, and electrophiles, being electron-poor, determine the direction of electron flow in reactions. Mastering these concepts allows students to accurately predict and represent electron movement using curly arrows.

By thoroughly understanding these prerequisite topics, students build a strong foundation for learning arrow pushing techniques. Reaction mechanisms provide the overall framework, while knowledge of chemical reactions offers context for various transformations. The concept of nucleophiles and electrophiles guides students in determining the direction of electron flow. Together, these prerequisites enable students to approach arrow pushing with confidence and clarity.

As students delve into arrow pushing, they'll find that their grasp of these fundamental concepts significantly enhances their ability to visualize and interpret organic reactions. The interconnectedness of these topics becomes apparent, with each prerequisite contributing to a more comprehensive understanding of electron movement in organic chemistry. This holistic approach not only facilitates learning arrow pushing but also deepens overall comprehension of organic chemistry principles.

In conclusion, mastering these prerequisite topics is not just beneficial but essential for success in understanding and applying arrow pushing techniques in organic chemistry. By building a strong foundation in reaction mechanisms, chemical reactions, and the behavior of nucleophiles and electrophiles, students set themselves up for a more intuitive and thorough grasp of curly arrow notation and its applications in complex organic reactions.