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Arrow pushing (curly arrows) in organic chemistry

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Intros
Lessons
  1. Drawing reaction mechanisms.
  2. Rules for drawing curly arrows.
  3. A note on curly arrow placement.
  4. Example: Curly arrows in nucleophilic addition.
  5. Example: Double bonds as nucleophiles.
Topic Notes
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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.

Fundamentals of Arrow Pushing

Arrow pushing is a fundamental concept in organic chemistry that helps visualize the movement of electrons during chemical reactions. This technique, also known as electron pushing, uses curly arrows to represent the flow of electrons from one atom or bond to another. Understanding arrow pushing is crucial for predicting reaction mechanisms and outcomes.

What Are Curly Arrows?

Curly arrows are symbolic representations used to show the movement of electrons in chemical reactions. These arrows indicate the source and destination of electron pairs, helping chemists track how bonds are formed, broken, or rearranged during a reaction. Curly arrows always point from an electron-rich area (the source of electrons) to an electron-poor area (where the electrons are going).

Types of Curly Arrows

There are two main types of curly arrows used in arrow pushing: full-headed arrows and half-headed arrows. Each type serves a specific purpose in depicting electron movement.

1. Full-Headed Arrows

Full-headed arrows () represent the movement of an electron pair. These arrows are used to show the formation of new bonds or the movement of lone pairs. For example, in a nucleophilic addition reaction, a full-headed arrow would show the lone pair of electrons from a nucleophile attacking an electrophilic carbon.

2. Half-Headed Arrows

Half-headed arrows () represent the movement of a single electron. These arrows are typically used in radical reactions or when depicting the homolytic cleavage of bonds. For instance, in the homolytic cleavage of a halogen molecule, two half-headed arrows would show each atom receiving one electron from the shared pair.

Guidelines for Arrow Pushing

To effectively use arrow pushing, it's essential to follow these key guidelines:

  1. Electron-rich to Electron-poor: Always draw arrows from electron-rich areas (sources of electrons) to electron-poor areas (destinations for electrons). This principle is fundamental to understanding the direction of electron flow in reactions.
  2. Identifying nucleophiles and electrophiles: In many reactions, arrows will start from nucleophiles (electron-rich species) and point towards electrophiles (electron-poor species).
  3. Consider Electronegativity: Electrons generally move from less electronegative atoms to more electronegative atoms.
  4. Account for Charges: Pay attention to formal charges on atoms, as they can indicate electron-rich or electron-poor sites.
  5. Use the Correct Arrow Type: Choose between full-headed and half-headed arrows based on whether you're showing the movement of an electron pair or a single electron.

Examples of Arrow Pushing

Let's consider a few examples to illustrate the use of curly arrows:

Example 1: Nucleophilic Addition (Full-Headed Arrow)

In the addition of HCN to a carbonyl compound, a full-headed arrow would show the lone pair from the CN- ion attacking the carbonyl carbon. Another full-headed arrow would show the pi bond electrons moving to form a lone pair on the oxygen.

Example 2: Radical Reaction (Half-Headed Arrow)

In the homolytic cleavage of Cl2 by light, two half-headed arrows would show each chlorine atom receiving one electron from the shared pair, resulting in two chlorine radicals.

Importance of Arrow Pushing

Mastering arrow pushing is crucial for several reasons:

  • It helps predict reaction mechanisms and products.
  • It aids in understanding complex organic reactions.
  • It provides a visual tool for communicating reaction pathways.

Conclusion

Understanding and effectively using arrow pushing is essential for anyone studying organic chemistry. By following the guidelines and practicing with various examples, you can gain a deeper insight into the mechanisms of chemical reactions. Remember, always start from electron-rich areas and move towards electron-poor areas, and make sure to correctly identify nucleophiles and electrophiles in your reactions.

Electron-Rich Species and Nucleophiles

Electron-rich species play a crucial role in organic chemistry as nucleophiles, driving many important reactions. These species are characterized by their ability to donate electrons to form new chemical bonds. Understanding the sources of electrons and how to represent them in reaction mechanisms is essential for mastering organic chemistry.

Sources of Electrons for Arrow Pushing

There are several key sources of electrons that can act as nucleophiles in organic reactions:

1. Formal Charges

Formal charges represent the difference between the number of valence electrons in a free atom and the number of electrons assigned to that atom in a molecule. Negative formal charges indicate an excess of electrons, making these sites electron-rich and potentially nucleophilic. For example, the hydroxide ion (OH-) has a negative formal charge on oxygen, making it a strong nucleophile. When drawing curly arrows from formal charges, start the arrow at the negative charge and point it towards the electrophilic center.

2. Lone Pairs

Lone pairs are non-bonding pairs of electrons associated with a single atom. These unshared electrons can readily participate in reactions as nucleophiles. Common examples include the lone pairs on oxygen in water or alcohols, nitrogen in amines, and halogens. When representing lone pair nucleophiles in arrow pushing, begin the curly arrow at the lone pair and direct it to the electrophilic site.

3. Pi Bonds

Pi bonds, found in alkenes, alkynes, and aromatic compounds, contain electrons that can act as nucleophiles. The electron density in pi bonds is more loosely held compared to sigma bonds, making them available for reactions. When drawing arrows from pi bonds, start the arrow at the center of the pi bond and curve it towards the electrophile.

4. Carbon-Metal Sigma Bonds

In organometallic compounds, the carbon-metal sigma bond can be polarized, with the carbon atom bearing a partial negative charge. This makes the carbon a potential nucleophile. Examples include Grignard reagents (RMgX) and organolithium compounds (RLi). When representing these in arrow pushing, begin the arrow at the carbon-metal bond and direct it to the electrophilic center.

Common Nucleophiles and Their Representation

Several nucleophiles are frequently encountered in organic chemistry:

  • Halide ions (Cl-, Br-, I-): Represented by their negative charge
  • Hydroxide (OH-) and alkoxide (RO-) ions: Arrows start from the negative charge on oxygen
  • Amines (R3N): Arrows originate from the lone pair on nitrogen
  • Cyanide ion (CN-): Arrow begins at the negative charge or the carbon atom
  • Enolates: Arrows start from the negatively charged oxygen or the alpha carbon
  • Grignard reagents (RMgX): Arrows begin at the carbon-magnesium bond

When drawing mechanisms involving these nucleophiles, it's crucial to accurately represent the electron movement. For charged species, the arrow typically starts at the negative charge. For neutral species with lone pairs, the arrow originates from the lone pair. In the case of carbon nucleophiles like enolates or Grignard reagents, the arrow may start from the carbon atom or the bond, depending on the specific representation.

Understanding electron-rich species and their behavior as nucleophiles is fundamental to predicting and explaining organic reactions. By mastering the concepts of formal charges, lone pairs, pi bonds, and carbon-metal sigma bonds as sources of electrons, chemists can effectively use arrow pushing to illustrate reaction mechanisms. This skill is invaluable for analyzing existing reactions and designing new synthetic pathways in organic chemistry.

Practice identifying electron-rich species in various molecules and drawing cur

Electron-Poor Species and Electrophiles

Electron-poor species play a crucial role in organic chemistry as electrophiles, driving many important reactions. These species are characterized by their ability to accept electrons, making them key players in a wide range of organic transformations. Understanding the nature of electrophiles and their behavior is essential for comprehending reaction mechanisms and predicting chemical outcomes.

Electrophiles are molecules or ions that are electron-deficient, meaning they have a lower electron density compared to their surroundings. This electron deficiency makes them highly reactive towards electron-rich species, known as nucleophiles. The term "electrophile" literally means "electron-loving," reflecting their tendency to seek out and accept electrons from other sources.

There are several ways in which a species can become electron-poor and act as an electrophile:

  1. Positive Charges: Ions with a full positive charge are strong electrophiles. Examples include carbocations (R3C+) and protons (H+). These species have a complete absence of electrons in their outer shell, making them highly reactive.
  2. Partial Positive Charges: Molecules with polarized bonds can have areas of partial positive charge, which can act as electrophilic sites. For instance, in carbonyl compounds (C=O), the carbon atom bears a partial positive charge due to the electronegativity difference with oxygen.
  3. Vacant p Orbitals: Some molecules have empty p orbitals that can accept electrons. Boron trifluoride (BF3) is a classic example, where boron has an empty p orbital, making it a strong Lewis acid and electrophile.

Common examples of electrophiles include:

  • Carbocations (R3C+)
  • Protons (H+)
  • Carbonyl compounds (aldehydes and ketones)
  • Alkyl halides (R-X, where X is a halogen)
  • Lewis acids (BF3, AlCl3)
  • Electron-deficient π systems (alkenes with electron-withdrawing groups)

In arrow pushing diagrams, which are used to represent reaction mechanisms, electrophiles are typically shown as the species receiving the arrow. The arrow represents the movement of electrons from the nucleophile to the electrophile. For example:

    Nu: + E+  Nu-E

Here, "Nu:" represents a nucleophile with a lone pair of electrons, and "E+" represents an electrophile. The arrow shows the electrons moving from the nucleophile to form a bond with the electrophile.

Charge Conservation in Arrow Pushing Mechanisms

An essential principle in arrow pushing mechanisms is the conservation of charge. This concept states that the total charge of the reactants must equal the total charge of the products in any chemical reaction. When drawing arrow pushing diagrams, it's crucial to keep track of charges and ensure they are balanced throughout the mechanism.

For example, consider the reaction of a neutral nucleophile with a positively charged electrophile:

    R-OH + H+  R-OH2+

In this case, the overall charge starts at +1 (neutral + positive) and ends at +1, maintaining charge conservation. When pushing arrows, each arrow represents the movement of two electrons. If an arrow is drawn from a neutral species to a positive one, the positive charge is effectively "moved" to the species that donated the electrons.

Understanding charge conservation helps chemists:

  • Predict the charges on intermediates and products
  • Identify errors in proposed mechanisms
  • Ensure the overall reaction is balanced
  • Understand the flow of electrons in complex reactions

In conclusion, electron-poor species and electrophiles are

Worked Examples of Arrow Pushing

Arrow pushing is a fundamental technique in organic chemistry used to illustrate reaction mechanisms. This method visually represents the movement of electrons during chemical reactions, helping chemists understand how bonds are formed and broken. Let's explore some detailed worked examples of arrow pushing, focusing on reactions involving strong and weak nucleophiles.

Example 1: Strong Nucleophile - Nucleophilic Substitution (SN2)

Consider the reaction of methyl bromide (CH3Br) with hydroxide ion (OH-), a strong nucleophile:

CH3Br + OH- CH3OH + Br-

Step 1: The nucleophile (OH-) approaches the electrophilic carbon of CH3Br from the opposite side of the leaving group (Br).

Step 2: Draw a curved arrow from the lone pair of electrons on the oxygen of OH- to the carbon atom.

Step 3: Simultaneously, draw another curved arrow from the C-Br bond to the bromine atom.

Step 4: The result is the formation of a new C-O bond and the breaking of the C-Br bond.

In this SN2 reaction, the strong nucleophile attacks in a single step, inverting the stereochemistry at the carbon center. Note how charge is conserved throughout the reaction: we start with one negative charge (on OH-) and end with one negative charge (on Br-).

Example 2: Weak Nucleophile - Addition-Elimination Reaction

Let's examine the reaction of acetyl chloride (CH3COCl) with water (H2O), a weak nucleophile:

CH3COCl + H2O CH3COOH + HCl

Step 1: The weak nucleophile (H2O) approaches the electrophilic carbonyl carbon.

Step 2: Draw a curved arrow from one of the lone pairs on oxygen of H2O to the carbonyl carbon.

Step 3: Draw another curved arrow from the C=O π bond to the oxygen, forming a temporary negative charge on oxygen.

Step 4: The chloride ion leaves, taking the electrons from its bond with carbon. Draw an arrow from this bond to the chlorine.

Step 5: The negatively charged oxygen abstracts a proton from another water molecule. Draw an arrow from the O-H bond of water to the oxygen of the intermediate.

Step 6: Finally, draw an arrow from the remaining lone pair on the negatively charged oxygen to the hydrogen it's abstracting.

This mechanism demonstrates how a weak nucleophile reacts in multiple steps, first adding to the carbonyl to form a tetrahedral intermediate, then eliminating the leaving group. Charge is conserved at each step, with the final products being neutral.

Example 3: Resonance-Stabilized Carbocation Formation

Consider the formation of a resonance-stabilized carbocation from 2-methylpropene:

(CH3)2C=CH2 + H+ (CH3)2C+-CH3

Step 1: Draw the initial alkene structure, showing the π bond.

Step 2: Draw a curved arrow from the π bond to one of the carbon atoms (preferably the more substituted one) to show electron movement.

Step 3: Draw another curved arrow from the other carbon of the former π bond to the approaching proton (H+).

Step 4: Show the resulting carbocation, with a positive charge on the central carbon.

Step 5: Illustrate resonance by drawing curved arrows from the C-H bonds of the methyl groups to the adjacent carbocation center, forming additional resonance structures.

This example demonstrates how arrow pushing can be used to show the formation of a carbocation and its

Arrow Pushing with Double Bonds

Understanding the nuances of using double bonds as nucleophiles in arrow pushing mechanisms is crucial for mastering organic chemistry. Double bonds, also known as pi bonds, play a vital role in many chemical reactions, particularly in electrophilic addition reactions. When dealing with double bonds in arrow pushing, it's essential to grasp the concept of covering one atom with the curly arrow, which represents the movement of electrons.

In arrow pushing mechanisms involving double bonds, the curly arrow typically starts from the center of the pi bond and points towards the electrophile. This representation signifies that the electrons from the pi bond are attacking the electrophile. It's important to note that we don't draw the arrow starting from one specific carbon atom of the double bond, but rather from the bond itself.

To illustrate this technique, let's consider a classic example: the reaction of bromine with ethene. This reaction demonstrates the electrophilic addition of bromine to a carbon-carbon double bond. The mechanism proceeds as follows:

1. The pi electrons from the ethene double bond attack one of the bromine atoms in the Br2 molecule.

2. This attack causes the Br-Br bond to break heterolytically, with both electrons moving to the second bromine atom.

3. The result is the formation of a brominium ion intermediate and a bromide ion.

4. In the next step, the bromide ion acts as a nucleophile and attacks one of the carbon atoms in the brominium ion, forming the final dibrominated product.

This example highlights the importance of induced dipoles in these reactions. The approaching bromine molecule induces a temporary dipole in the pi bond of ethene, making one end slightly positive and the other slightly negative. This polarization facilitates the initial attack of the pi electrons on the bromine molecule.

The formation of carbocations is another crucial aspect of these mechanisms. In some reactions involving double bonds, a carbocation intermediate may form instead of a bridged ion like the brominium ion. Carbocations are highly reactive species with a positive charge on a carbon atom. They play a significant role in determining the regiochemistry and stereochemistry of addition reactions.

When drawing arrow pushing mechanisms with double bonds, it's essential to remember that the pi electrons are delocalized between the two carbon atoms. This delocalization is why we represent the curly arrow as coming from the center of the bond rather than from a specific atom. This approach accurately reflects the nature of the pi bond and helps in understanding the reactivity of alkenes.

The concept of covering one atom with the curly arrow is particularly important when dealing with conjugated systems or resonance structures. In these cases, the movement of electrons through multiple pi bonds can be represented by a series of curly arrows, each covering one atom as it moves through the system.

Understanding these nuances of arrow pushing with double bonds is crucial for predicting reaction outcomes, explaining observed products, and designing synthetic routes. It allows chemists to visualize the flow of electrons and understand how bonds are broken and formed during a reaction.

In conclusion, mastering the use of double bonds as nucleophiles in arrow pushing mechanisms requires a deep understanding of pi bond reactivity, induced dipoles, and the formation of reactive intermediates like carbocations. By practicing with examples like the bromine addition to ethene, students can develop the skills needed to tackle more complex organic reactions and mechanisms. This knowledge forms the foundation for understanding a wide range of organic transformations, from simple addition reactions to complex rearrangements and substitutions.

Conclusion and Further Applications

Arrow pushing is a fundamental skill in understanding organic reaction mechanisms. It visually represents electron movement, allowing chemists to predict and explain chemical transformations. Key points include identifying nucleophiles and electrophiles, recognizing electron-rich and electron-poor areas, and following the flow of electrons. Mastering arrow pushing requires consistent practice and application to various reaction types. This introduction video provides a solid foundation, but viewers are encouraged to apply these concepts to more complex organic reactions. As you delve deeper into organic chemistry, you'll find that arrow pushing becomes an invaluable tool for deciphering intricate mechanisms. Remember, proficiency in arrow pushing is essential for success in organic reaction mechanisms courses and research. By continually practicing and exploring diverse reactions, you'll enhance your ability to predict outcomes and design synthetic routes. Embrace the challenge of applying these principles to increasingly complex organic reactions, and watch your understanding of chemical processes flourish.

Drawing Reaction Mechanisms: Rules for Drawing Curly Arrows

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.

Prerequisite Topics

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.

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.