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Electron-withdrawing and donating effects

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Lessons
  1. Electron withdrawing and donating effects
  2. Intro to 'electronic effects'
  3. Electron-donating groups (EDGs)
  4. Electron-withdrawing groups (EWGs)
  5. Mesomeric effects (M)
  6. Inductive effects (I)
  7. Mesomeric vs inductive effect
Topic Notes
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Introduction to Electron-Withdrawing and Donating Effects

In organic chemistry, electron-withdrawing groups (EWGs) and electron donating groups (EDGs) play a crucial role in determining molecular reactivity. These effects influence the distribution of electrons within a molecule, significantly impacting its chemical behavior. The introduction video provides a comprehensive overview of these concepts, serving as an essential foundation for understanding more complex organic reactions. EWGs, such as halogens and nitro groups, pull electrons away from the molecule's core, while EDGs, like alkyl groups and amines, push electrons towards it. This electron movement affects bond strengths, acidity, basicity, and overall reactivity. By grasping these fundamental principles, students can better predict and explain various organic reactions. The video emphasizes the importance of these effects in areas such as drug design, materials science, and synthetic organic chemistry. Mastering electron-withdrawing and donating effects is crucial for anyone pursuing a deeper understanding of organic chemistry and its applications in real-world scenarios.

Understanding Electron-Withdrawing and Donating Groups

In organic chemistry, electron-withdrawing groups (EWGs) and electron-donating groups (EDGs) play crucial roles in determining the reactivity and properties of molecules. These functional groups significantly influence the electron distribution within a molecule, affecting its behavior in chemical reactions and its overall characteristics.

Electron-withdrawing groups (EWGs) are substituents that pull electrons away from a molecule or a specific region of a molecule. They decrease electron density in their vicinity, making the attached atom or group more electrophilic. Common examples of EWGs include -NO2 (nitro), -CN (cyano), -COOH (carboxyl), -CHO (aldehyde), -COR (ketone), and halogens (-F, -Cl, -Br, -I). These groups typically contain electronegative atoms or unsaturated bonds that can accept electrons through resonance or inductive effects.

On the other hand, electron-donating groups (EDGs) are substituents that push electrons towards a molecule or a specific region of a molecule. They increase electron density in their vicinity, making the attached atom or group more nucleophilic. Common examples of EDGs include -OH (hydroxyl), -NH2 (amino), -NR2 (alkyl-substituted amino), -OR (alkoxy), and alkyl groups (-CH3, -C2H5, etc.). These groups often contain atoms with lone pairs of electrons or are capable of releasing electrons through inductive or resonance effects.

The effects of EWGs and EDGs on nucleophiles and electrophiles are significant in organic reactions. Nucleophiles, which are electron-rich species seeking to donate electrons, are strengthened by the presence of EDGs. The increased electron density makes them more reactive towards electrophiles. Conversely, EWGs weaken nucleophiles by reducing their electron density. Electrophiles, which are electron-deficient species seeking to accept electrons, are strengthened by the presence of EWGs. The decreased electron density makes them more susceptible to attack by nucleophiles. EDGs, in contrast, weaken electrophiles by increasing their electron density.

When comparing EWG vs EDG, several key differences emerge. EWGs decrease the electron density of the attached atom or group, making it more positively charged or less negatively charged. This effect can stabilize negative charges or destabilize positive charges in reaction intermediates. EDGs, conversely, increase the electron density of the attached atom or group, making it more negatively charged or less positively charged. This effect can stabilize positive charges or destabilize negative charges in reaction intermediates.

The impact of EWGs and EDGs on molecular properties is profound. They influence various aspects such as acidity, basicity, reactivity, and spectroscopic properties. For instance, EWGs increase the acidity of adjacent protons by stabilizing the conjugate base, while EDGs decrease acidity. In aromatic systems, EWGs activate the ring towards electrophilic aromatic substitution at the meta position, while EDGs activate the ring at the ortho and para positions. These groups also affect the polarity of molecules, with EWGs generally increasing polarity and EDGs decreasing it.

In spectroscopic analysis, EWGs and EDGs influence chemical shifts in NMR spectroscopy. EWGs typically cause downfield shifts (higher ppm values) for nearby protons or carbon atoms, while EDGs cause upfield shifts (lower ppm values). In IR spectroscopy, the presence of these groups can affect the frequency of stretching vibrations for adjacent bonds. UV-Vis spectroscopy is also impacted, with EWGs often causing bathochromic shifts (red shifts) and EDGs causing hypsochromic shifts (blue shifts) in conjugated systems.

Understanding the nature and effects of electron-withdrawing and electron-donating groups is essential for predicting and explaining the behavior of organic compounds in various chemical reactions. This knowledge is fundamental in fields such as drug design, materials science, and synthetic organic chemistry. By manipulating the presence and position of EWGs and EDGs, chemists can fine-tune the reactivity and properties of molecules to achieve desired outcomes in chemical synthesis and molecular design.

The Mesomeric Effect in Organic Chemistry

The mesomeric effect, also known as the resonance effect, is a fundamental concept in organic chemistry that plays a crucial role in determining the electron-withdrawing and electron-donating properties of molecules. This effect arises from the delocalization of electrons within a molecule, particularly in systems with conjugated double bonds or lone pairs of electrons. Understanding the mesomeric effect is essential for predicting molecular reactivity, stability, and various chemical properties.

At its core, the mesomeric effect is closely related to the concept of resonance structures. Resonance structures are alternative representations of a molecule's electron distribution that cannot be adequately described by a single Lewis structure. These structures contribute to the overall electronic configuration of the molecule, providing a more accurate picture of its behavior. The mesomeric effect is a consequence of this electron delocalization and can significantly influence a molecule's properties.

There are two types of mesomeric effects: positive mesomeric effect (+M) and negative mesomeric effect (-M). The positive mesomeric effect occurs when an atom or group donates electrons to the conjugated system, increasing electron density in certain parts of the molecule. Conversely, the negative mesomeric effect involves the withdrawal of electrons from the conjugated system, resulting in decreased electron density in specific regions.

Examples of groups exhibiting a positive mesomeric effect include -OH, -OR, -NH2, and -NR2. These groups possess lone pairs of electrons that can be donated to the conjugated system, enhancing electron density. For instance, in phenol (C6H5OH), the -OH group donates electrons to the benzene ring, increasing the electron density at the ortho and para positions. This effect makes these positions more reactive towards electrophilic substitution reactions.

On the other hand, groups demonstrating a negative mesomeric effect include -NO2, -CN, -COOH, and -CHO. These groups withdraw electrons from the conjugated system, decreasing electron density in certain areas. For example, in nitrobenzene (C6H5NO2), the -NO2 group pulls electrons away from the benzene ring, reducing electron density at the ortho and para positions. This effect makes these positions less reactive towards electrophilic substitution reactions but more susceptible to nucleophilic attacks.

The mesomeric effect has a profound impact on molecular reactivity. It influences the distribution of charge within a molecule, affecting its polarity and the strength of its bonds. This, in turn, determines how the molecule will interact with other chemical species. For instance, molecules with strong electron-donating groups (positive mesomeric effect) are more likely to undergo electrophilic reactions, while those with strong electron-withdrawing groups (negative mesomeric effect) are more prone to nucleophilic attacks.

Furthermore, the mesomeric effect can stabilize or destabilize certain molecular structures. Resonance structures that distribute charge more evenly throughout the molecule generally contribute to greater stability. This principle is crucial in understanding the stability of carbocations, carbanions, and free radicals in organic reactions. For example, the stability of the phenoxide ion (C6H5O-) is enhanced by the delocalization of the negative charge throughout the benzene ring, a direct result of the mesomeric effect.

In addition to influencing reactivity and stability, the mesomeric effect also impacts physical properties such as dipole moments, acidity, and basicity. Molecules with strong electron-withdrawing groups tend to be more acidic, as they can better stabilize the conjugate base. Conversely, molecules with strong electron-donating groups are generally more basic, as they can more readily accept a proton.

Understanding the mesomeric effect is crucial for predicting the outcome of organic reactions and designing new molecules with specific properties. In drug design, for instance, chemists can manipulate the electron distribution in molecules to enhance their binding affinity to target proteins or improve their solubility and bioavailability. Similarly, in materials science, the mesomeric effect is exploited to create molecules with desired electronic or optical properties for applications in solar cells, organic light-emitting diodes (OLEDs), and other advanced materials.

In conclusion, the mesomeric effect is a powerful concept in organic chemistry that provides insights into the electronic behavior of molecules. By understanding resonance structures

The Inductive Effect and Its Influence on Electron Distribution

The inductive effect is a fundamental concept in organic chemistry that plays a crucial role in understanding the behavior of molecules. This effect refers to the transmission of charge through a chain of atoms in a molecule, resulting in a permanent dipole. The inductive effect significantly influences electron distribution within molecules, affecting their reactivity, stability, and overall chemical properties.

At its core, the inductive effect arises from the difference in electronegativity between atoms in a molecule. Electronegativity is the ability of an atom to attract electrons in a chemical bond. When atoms with different electronegativities are bonded together, the more electronegative atom pulls the electron density towards itself, creating a slight negative charge (δ-) on that atom and a slight positive charge (δ+) on the less electronegative atom.

There are two types of inductive effects: positive inductive effect (+I) and negative inductive effect (-I). A positive inductive effect occurs when a substituent group donates electrons to the carbon chain, increasing the electron density. Conversely, a negative inductive effect happens when a substituent group withdraws electrons from the carbon chain, decreasing the electron density.

Examples of groups that exhibit a positive inductive effect include alkyl groups such as methyl (-CH3), ethyl (-CH2CH3), and tert-butyl (-C(CH3)3). These groups are electron-donating and push electrons towards the carbon chain. On the other hand, groups that show a negative inductive effect include halogens (F, Cl, Br, I), nitro (-NO2), and cyano (-CN) groups. These electron-withdrawing groups pull electrons away from the carbon chain.

The strength of the inductive effect depends on several factors, including the electronegativity difference between atoms, the size of the substituent group, and the distance from the reactive site. Generally, the inductive effect decreases as the distance from the substituent increases, following the order: 1 > 2 > 3 > 4, where the numbers represent the position of carbon atoms in the chain.

It's important to distinguish the inductive effect from the mesomeric effect, also known as the resonance effect. While both influence electron distribution in molecules, they operate through different mechanisms. The inductive effect involves the polarization of sigma (σ) bonds and occurs through single bonds. In contrast, the mesomeric effect involves the delocalization of pi (π) electrons and occurs in systems with multiple bonds or lone pairs of electrons.

The mesomeric effect can be either positive (+M) or negative (-M), similar to the inductive effect. However, the mesomeric effect is typically stronger than the inductive effect when both are present in a molecule. This is because the mesomeric effect involves the direct overlap of p-orbitals, allowing for more efficient electron delocalization.

Understanding the inductive effect is crucial for predicting and explaining various chemical phenomena. For instance, it helps explain the acidity or basicity of organic compounds. Carboxylic acids with electron-withdrawing groups adjacent to the -COOH group tend to be stronger acids due to the negative inductive effect stabilizing the conjugate base. Conversely, carboxylic acids with electron-donating groups are generally weaker acids.

The inductive effect also influences reaction rates and mechanisms. For example, in nucleophilic substitution reactions, the presence of electron-withdrawing groups can accelerate the reaction by making the carbon center more electrophilic. In contrast, electron-donating groups can slow down such reactions by increasing electron density at the reaction site.

Electronegativity plays a central role in determining the direction and strength of inductive effects. The greater the electronegativity difference between atoms, the stronger the inductive effect. This is why highly electronegative atoms like fluorine and oxygen exert strong negative inductive effects, while less electronegative atoms like carbon in alkyl groups show positive inductive effects.

In conclusion, the inductive effect is a vital concept in organic chemistry that significantly impacts electron distribution in molecules. By understanding this effect and its interplay with other factors like the mesomeric effect and electronegativity, chemists can better predict and explain molecular behavior, reactivity, and properties. This knowledge is essential for designing

Common Electron-Withdrawing and Donating Groups in Organic Chemistry

In organic chemistry, understanding electron-withdrawing groups (EWGs) and electron-donating groups (EDGs) is crucial for predicting molecular properties and reactivity. These functional groups play a significant role in determining the electronic distribution within molecules, affecting their behavior in various chemical reactions.

Electron-Withdrawing Groups (EWGs)

EWGs are functional groups that pull electrons away from a molecule or substituent. Common electron-withdrawing groups include:

  • Halogens (F, Cl, Br, I)
  • Nitro group (-NO2)
  • Cyano group (-CN)
  • Carbonyl groups (C=O): aldehydes, ketones, carboxylic acids, esters
  • Sulfoxide group (-SO-)
  • Sulfonyl group (-SO2-)
  • Trifluoromethyl group (-CF3)

Among these, the nitro group (-NO2) is considered one of the strongest electron-withdrawing groups. Halogens, while electron-withdrawing through induction, can also act as weak electron donors through resonance.

Electron-Donating Groups (EDGs)

EDGs, on the other hand, push electrons towards a molecule or substituent. Common electron-donating groups include:

  • Alkyl groups (-CH3, -C2H5, etc.)
  • Amino group (-NH2)
  • Hydroxyl group (-OH)
  • Alkoxy groups (-OR)
  • Phenyl group (-C6H5)
  • Thiol group (-SH)

The amino group (-NH2) is one of the strongest electron-donating groups due to its ability to donate electrons through both inductive and resonance effects.

Electronic Effects of Functional Groups

Halogens: Halogens exhibit a dual nature. They are electron-withdrawing through induction due to their high electronegativity, but can also act as weak electron donors through resonance. The inductive effect decreases in the order F > Cl > Br > I, while the resonance effect increases in the same order.

Carbonyl groups: These groups (C=O) are strongly electron-withdrawing due to the high electronegativity of oxygen. They can participate in resonance, which affects the reactivity of adjacent atoms. Aldehydes and ketones are particularly important in many organic reactions.

Aromatic rings: Benzene and other aromatic rings can act as both electron-withdrawing and electron-donating groups, depending on the substituents attached to them. The delocalized π-electron system of aromatic rings can stabilize charged intermediates in reactions.

Relative Strength of EWGs and EDGs

The relative strength of EDGs, from strongest to weakest, generally follows this order:

-NO2 > -CN > -SO2R > -CHO > -COR > -COOH > -COOR > -Cl > -Br > -I > -Ph > -H

For EDGs, the order of strength (strongest to weakest) is typically:

-O- > -NH2 > -NHR > -NR2 > -OH > -OR > -NHCOR > -Ph > -CH3 > -H

Impact on Molecular Properties and Reactivity

The presence of EWGs and EDGs significantly affects molecular properties and reactivity:

  • Acidity and Basicity: EWGs increase the acidity

Applications of Electron-Withdrawing and Donating Effects in Organic Reactions

Electron-withdrawing and electron-donating effects play a crucial role in organic reactions, significantly influencing the reactivity and behavior of molecules. These effects are fundamental in determining nucleophile strength and electrophile strength, which in turn affect the outcome and rate of various organic reactions. Understanding these concepts is essential for predicting reaction outcomes and designing synthetic strategies in organic chemistry.

Electron-withdrawing groups (EWGs) are substituents that pull electron density away from a molecule or reaction center. Common EWGs include -NO2, -CN, -COOH, and halogens. On the other hand, electron-donating groups (EDGs) increase electron density in a molecule or at a specific site. Examples of EDGs include -OH, -NH2, -OR, and alkyl groups. These electronic effects can occur through inductive effects (through sigma bonds) or resonance effects (through pi bonds).

The strength of nucleophiles and electrophiles is directly influenced by these electronic effects. Nucleophiles are electron-rich species that donate electrons in a reaction, while electrophiles are electron-poor species that accept electrons. Generally, EDGs enhance nucleophilicity by increasing electron density, making the molecule more prone to donate electrons. Conversely, EWGs increase electrophilicity by decreasing electron density, making the molecule more likely to accept electrons.

One classic example where these effects are crucial is in electrophilic aromatic substitution reactions. In these reactions, the presence of EDGs or EWGs on the aromatic ring significantly affects the rate and regioselectivity of the substitution. For instance, consider the nitration of toluene:

Toluene, with its electron-donating methyl group, is more reactive towards electrophilic attack than benzene. The methyl group activates the ring, directing the incoming electrophile (NO2+) primarily to the ortho and para positions. This is because the methyl group increases electron density at these positions through resonance, making them more nucleophilic.

Conversely, if we consider the nitration of nitrobenzene:

The nitro group is strongly electron-withdrawing, deactivating the ring towards electrophilic attack. It directs incoming electrophiles to the meta position, as the ortho and para positions are electron-deficient due to resonance effects. This makes nitrobenzene much less reactive than benzene or toluene in electrophilic aromatic substitution reactions.

These effects also play a significant role in other organic reactions. For example, in nucleophilic addition reactions to carbonyl compounds, the presence of EWGs on the carbonyl carbon increases its electrophilicity, making it more reactive towards nucleophilic attack. Consider the following worked example:

Comparing the reactivity of acetone (CH3COCH3) and chloroacetone (CH2ClCOCH3) towards nucleophilic addition:

The chlorine atom in chloroacetone is electron-withdrawing, making the carbonyl carbon more electrophilic compared to acetone. As a result, chloroacetone will react faster with nucleophiles like NaCN in a nucleophilic addition reaction.

In predicting reaction outcomes, it's essential to consider the cumulative effects of multiple substituents. For instance, in the case of disubstituted benzenes undergoing electrophilic aromatic substitution:

- For p-nitrotoluene, the electron-donating methyl group and electron-withdrawing nitro group have opposing effects. The stronger -NO2 group dominates, directing electrophiles to the meta position relative to itself.

- In p-aminophenol, both -NH2 and -OH are strong EDGs, working cooperatively to highly activate the ring. Electrophilic substitution will occur rapidly at the ortho positions relative to both groups.

Understanding these electronic effects is crucial for organic chemists in designing synthetic routes and predicting reaction outcomes. By carefully considering the electron-withdrawing and donating effects of substituents, chemists can manipulate reactivity, control regioselectivity, and optimize reaction conditions in various

Conclusion: The Importance of Understanding Electron-Withdrawing and Donating Effects

In this article, we've explored the crucial concepts of electron-withdrawing and donating effects in organic chemistry. These fundamental principles play a pivotal role in determining molecular reactivity and are essential for predicting chemical behavior. Understanding these effects is key to mastering organic chemistry and developing a deeper insight into molecular interactions. We encourage you to rewatch the introductory video for a comprehensive overview of these concepts. To further enhance your knowledge, practice solving problems in organic chemistry related to electron-withdrawing and donating effects, and explore more advanced topics in organic chemistry. By grasping these principles, you'll be better equipped to tackle complex organic reactions and mechanisms. Remember, a solid foundation in these concepts will serve you well in your future studies and applications of organic chemistry. Don't hesitate to delve deeper into this fascinating subject and continue your journey of discovery in the world of molecular reactivity.

To further enhance your knowledge, practice solving problems in organic chemistry related to electron-withdrawing and donating effects, and explore more advanced topics in organic chemistry. By grasping these principles, you'll be better equipped to tackle complex organic reactions and mechanisms. Remember, a solid foundation in these concepts will serve you well in your future studies and applications of organic chemistry. Don't hesitate to delve deeper into this fascinating subject and continue your journey of discovery in the world of molecular reactivity.

Electron Withdrawing and Donating Effects

Electron withdrawing and donating effects Intro to 'electronic effects'

Step 1: Introduction to Electronic Effects

In this section, we will explore the concept of electronic effects in molecules, focusing specifically on electron withdrawing and donating effects. These effects play a crucial role in determining the behavior of electrophiles and nucleophiles in chemical reactions. Understanding these effects is essential for predicting the reactivity and outcome of organic reactions.

Step 2: Definitions of Electron Withdrawing and Donating Groups

We will start by recalling the definitions of electron withdrawing groups (EWG) and electron donating groups (EDG). EWGs are groups that pull electron density away from the rest of the molecule, making the molecule more electron deficient. On the other hand, EDGs donate electron density to the molecule, making it more electron rich. These groups significantly influence the reactivity of nucleophiles and electrophiles.

Step 3: Effects on Nucleophiles and Electrophiles

Next, we will delve into how EWGs and EDGs affect nucleophiles and electrophiles. Nucleophiles are electron-rich species that seek out electron-deficient sites (electrophiles) to react with. The presence of EWGs or EDGs can either enhance or diminish the reactivity of these species. For instance, an EWG on an electrophile makes it more reactive towards nucleophiles, while an EDG on a nucleophile increases its reactivity towards electrophiles.

Step 4: Understanding Underlying Electronic Effects

We will then explore the underlying electronic effects that drive the properties of EWGs and EDGs. These effects are primarily due to two processes: inductive effects and resonance effects. Inductive effects involve the transmission of electron density through sigma bonds, while resonance effects involve the delocalization of electrons through pi bonds. Both processes contribute to the overall electron withdrawing or donating nature of a group.

Step 5: Predicting Reactivity in Organic Reactions

With a solid understanding of electronic effects, we can now apply this knowledge to predict the reactivity of nucleophiles and electrophiles in organic reactions. By analyzing the presence of EWGs and EDGs in the reactants, we can determine the strength and reactivity of these species. This predictive ability is crucial for designing efficient and successful chemical reactions.

Step 6: Practical Application and Worked Examples

Finally, we will apply our understanding of electronic effects to practical examples. We will work through several examples to predict the strength of nucleophiles and electrophiles based on the presence of EWGs and EDGs. These examples will help solidify our understanding and demonstrate the practical importance of electronic effects in organic chemistry.

Conclusion

In conclusion, understanding electron withdrawing and donating effects is essential for predicting the reactivity and outcome of organic reactions. By analyzing the presence of EWGs and EDGs, we can determine the strength and reactivity of nucleophiles and electrophiles. This knowledge is crucial for designing efficient and successful chemical reactions.

FAQs

Here are some frequently asked questions about electron-withdrawing and donating effects:

1. What is the difference between electron-withdrawing groups (EWGs) and electron-donating groups (EDGs)?

Electron-withdrawing groups (EWGs) pull electrons away from a molecule or reaction center, decreasing electron density. Examples include -NO2, -CN, and -COOH. Electron-donating groups (EDGs) push electrons towards a molecule or reaction center, increasing electron density. Examples include -OH, -NH2, and alkyl groups.

2. How do you determine if a group is electron-withdrawing or electron-donating?

To determine if a group is electron-withdrawing or electron-donating, consider its electronegativity and resonance effects. Groups with highly electronegative atoms (e.g., halogens) or those that can stabilize negative charges through resonance are typically electron-withdrawing. Groups with less electronegative atoms or those that can donate electrons through resonance are usually electron-donating.

3. What is the inductive effect in organic chemistry?

The inductive effect is the transmission of charge through a chain of atoms in a molecule, resulting from differences in electronegativity. It can be either electron-withdrawing (-I effect) or electron-donating (+I effect). The inductive effect decreases with distance from the source and primarily affects sigma bonds.

4. How do electron-withdrawing and donating groups affect molecular reactivity?

Electron-withdrawing groups increase the electrophilicity of a molecule, making it more susceptible to nucleophilic attack. They also stabilize negative charges and destabilize positive charges. Electron-donating groups increase nucleophilicity, stabilize positive charges, and destabilize negative charges. These effects influence reaction rates, regioselectivity, and overall molecular behavior.

5. What is the mesomeric effect, and how does it differ from the inductive effect?

The mesomeric effect, also known as the resonance effect, involves the delocalization of electrons through conjugated systems. Unlike the inductive effect, which occurs through sigma bonds, the mesomeric effect operates through pi bonds and lone pairs. It can be either electron-withdrawing (-M effect) or electron-donating (+M effect) and is generally stronger than the inductive effect when both are present in a molecule.

Prerequisite Topics

Understanding electron-withdrawing and donating effects in organic chemistry is crucial for grasping the behavior of molecules and predicting reaction outcomes. However, to fully comprehend this concept, it's essential to have a solid foundation in several prerequisite topics. These fundamental concepts provide the necessary context and tools to analyze and interpret electron-withdrawing and donating effects effectively.

One of the most critical prerequisites is drawing structures in organic chemistry. This skill is fundamental because it allows you to accurately represent molecules and their electron distributions. When studying electron-withdrawing and donating effects, you'll need to visualize how electrons move within a molecule, which is impossible without proper structural representations.

Another crucial concept is arrow pushing (curly arrows) in organic chemistry. This technique is invaluable for tracking electron movement and understanding how electron-withdrawing and donating groups influence reactivity. By mastering arrow pushing, you'll be able to predict and explain the effects of various substituents on molecular properties and reaction mechanisms.

Understanding nucleophiles and electrophiles is also essential when studying electron-withdrawing and donating effects. These concepts are closely related, as electron-withdrawing groups tend to create electrophilic centers, while electron-donating groups often enhance nucleophilicity. Recognizing these relationships will help you predict reactivity patterns and understand why certain reactions occur.

Perhaps one of the most directly relevant prerequisites is conjugation and resonance structures. Electron-withdrawing and donating effects are often explained through resonance, which describes the delocalization of electrons in a molecule. Understanding how to draw and interpret resonance structures is crucial for explaining the stability and reactivity of molecules affected by these electronic effects.

By mastering these prerequisite topics, you'll be well-equipped to tackle the complexities of electron-withdrawing and donating effects. You'll be able to draw accurate structures, visualize electron movement, understand the interplay between nucleophiles and electrophiles, and use resonance to explain molecular behavior. This foundation will not only help you grasp the concept at hand but also enable you to apply this knowledge to more advanced topics in organic chemistry, such as predicting reaction outcomes, understanding molecular stability, and designing synthetic strategies.

Remember, organic chemistry is a cumulative subject where each new concept builds upon previous ones. By ensuring a strong grasp of these prerequisites, you'll find that understanding electron-withdrawing and donating effects becomes much more intuitive and manageable. This comprehensive understanding will serve you well throughout your studies in organic chemistry and beyond.

In this lesson, we will learn:

  • To recall the definitions of electron-withdrawing group (EWG) and electron-donating groups (EDG).
  • To understand the effects of EWGs and EDGs on nucleophile and electrophile strength.
  • To understand the underlying electronic effects that produce these properties.
  • To apply mesomeric and inductive effects to predict nucleophile and electrophile strength.

Notes:

  • When running an organic reaction to make a desired product, the reactivity of both the electrophile and nucleophile need to be thought about.
    The more electron rich a nucleophile, or electron poor the electrophile, the better. This is because of the smaller HOMO-LUMO gap discussed in Nucleophiles and electrophiles . These conditions will lead to greater reactivity and a higher product yield; the smaller the HOMO-LUMO gap, the greater tendency for reactant bonds to break and product bonds to form.
    This can be predicted in reactions because substituents in organic molecules have electron withdrawing \, or electron donating \, effects.

  • An electron donating group \, (EDG) has the net effect of increasing electron density in a molecule through the carbon atom it is bonded to. By increasing electron density on adjacent carbon atoms, EDGs change the reactivity of a molecule:
    • EDGs make nucleophiles stronger. With EDGs attached, a nucleophilic center is even more electron rich and ready to attack electrophilic sites.
    • EDGs make carbon centers weaker electrophiles and less reactive to nucleophiles, because any (partial) positive charge it has will be minimized or nullified if the EDG is strong enough.
    Examples of good electron donating groups are groups with lone pairs to donate, such as:
    • The oxygen anion, -O-
    • Alcohol groups, -OH
    • Amine groups, -NH2 or -NR2
    • Ethers, -OR
    • Alkyl groups are also weakly electron-donating.

  • An electron withdrawing group \, (EWG) is a group that reduces electron density in a molecule through the carbon atom it is bonded to. By reducing electron density on adjacent carbon atoms, EWGs change the reactivity of a molecule:
    • EWGs make electrophiles stronger, because the electron-withdrawing effect makes any carbon center even more electron deficient than before.
    • EWGs make any nucleophilic species less reactive, for the same reason as they strengthen electrophiles. Nucleophiles need electron density to react with electrophiles; if an EWG is ‘withdrawing’ electrons, this is taking away the source of the nucleophile’s strength!
    The strongest EWGs are groups with pi bonds to electronegative atoms:
    • Nitro groups (-NO2)
    • Aldehydes (-CHO)
    • Ketones (-C=OR)
    • Cyano groups (-CN)
    • Carboxylic acid (-COOH)
    • Esters (-COOR)
    Halogens are also electron-withdrawing; the effect gets weaker going down the group.

  • Electron-withdrawing and donating properties come from two different electronic effects that we need to understand:
    • The mesomeric effect \, (M) is a group’s ability to delocalize electrons through resonance structures.
      Resonance \, is a state where a chemical compound has multiple ‘forms’ that are readily interconverting, due to the movement of delocalized electrons through the structure. This is what benzene does with its double bonds.
      Molecules with resonance forms \, that readily interconvert can stabilize points of localized charge.
      By taking up another structure, the charge is ‘shared’ across other atoms. This is the basis for the stability of aromatic rings.
      • A group with a positive mesomeric effect \, (+M) is an electron-donating group that ‘pushes’ electrons onto the carbon atom it is bonded to, usually via a lone pair that can make a resonance structure. This increases electron density on carbon and beyond. See the image below:

        To find +M groups, look for single bonds to atoms with lone pairs. Examples are:
        • The alkoxide anion (-O-) and alcohol group (-OH)
        • Amino group (-NH2) and alkyl analogues (-NR2)
        • Ether (-OR)

      • A group with a negative mesomeric effect \, (-M) is an electron-withdrawing group that ‘pulls’ electrons out from the carbon atom and the rest of the structure it is attached to.
        To do this a group needs pi orbital overlap to delocalize electrons; double bonds to electronegative atoms that ‘want’ electrons make this more likely. See the image below:

      To find -M groups, look for double bonds to oxygen and nitrogen!
      Examples are:
      • Nitro groups (-NO2)
      • Cyano groups (-CN)
      • Carbonyl groups such as aldehydes (-CHO) and ketones (-COR)
      • Esters (-COOR)

    • The inductive effect \, (I) is a group’s ability to polarize a sigma bond through electronegativity. This is more straightforward than the mesomeric effect:
      • A group with a positive inductive effect (+I) will increase electron density by polarizing the sigma bond. This is normally seen as a weak effect due to:
        • Alkyl groups (-R)
        • Aromatic ring substituents (-C6H5)

      • A group with a negative inductive effect (-I) decreases electron density on the carbon atom by polarizing the sigma bond. This is the effect seen when carbon bonds to electronegative atoms like:
        • Halogens (-F, -Cl. -Br, -I)
        • Oxygen (-OR)
        • Nitrogen (-NR2)
        See the image below for examples of both:

  • The mesomeric and inductive effects don’t directly impact one another, but both must be considered when looking at group attachments.
    The mesomeric effect dominates over the inductive effect in most cases. For example, ethers or alcohols, which are -I but +M, are both widely recognized as electron-donating groups (EDGs) because of their mesomeric effect.