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Conjugation and resonance structures

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Intros
Lessons
  1. Conjugation and resonance.
  2. What is a conjugated system?
  3. Resonance forms.
  4. Molecules don't 'resonate': terms explained.
  5. The resonance hybrid.
  6. How functional groups affect resonance forms.
Topic Notes
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Introduction to Conjugation and Resonance Structures

Conjugation and resonance structures are fundamental concepts in organic chemistry that play a crucial role in understanding molecular behavior and reactivity. The introduction video provides a comprehensive overview of these topics, serving as an essential starting point for students and enthusiasts alike. Conjugation refers to the interaction of p-orbitals in alternating single and double bonds, while resonance structures represent different electron distributions within a molecule. These concepts are vital for predicting molecular stability, reactivity, and properties. By mastering conjugation and resonance structures, students gain a deeper understanding of organic compounds and their behavior in various chemical reactions. The video emphasizes the significance of these topics in organic chemistry, highlighting their applications in predicting reaction outcomes, understanding aromaticity, and explaining spectroscopic data. As core principles, conjugation and resonance structures form the foundation for more advanced organic chemistry concepts, making them indispensable for anyone pursuing a career in chemistry or related fields.

Understanding Conjugated Systems

Conjugated systems are a fundamental concept in organic chemistry, playing a crucial role in the behavior and properties of many molecules. These systems are characterized by alternating single and multiple bonds, typically involving carbon atoms, which create a unique electronic structure. At the heart of conjugated systems are delocalized electrons, which contribute significantly to the stability and reactivity of these molecules.

To understand conjugated systems, we must first grasp the concept of delocalized electrons. In a typical covalent bond, electrons are localized between two atoms. However, in conjugated systems, electrons are not confined to a single bond but instead spread out over several atoms. This delocalization occurs due to the overlap of p orbitals, which are electron-containing regions perpendicular to the plane of the molecule.

P orbitals play a crucial role in conjugation. These orbitals are shaped like figure-eights and extend above and below the plane of the molecule. When p orbitals on adjacent atoms overlap, it allows electrons to move freely between them, creating a delocalized electron cloud. This electron cloud is responsible for many of the unique properties of conjugated systems.

One of the most famous examples of a conjugated system is benzene. Benzene is a six-carbon ring with alternating single and double bonds. However, experimental evidence shows that all carbon-carbon bonds in benzene are of equal length, falling between the typical lengths of single and double bonds. This is due to the delocalization of electrons in the molecule.

In benzene, each carbon atom contributes one electron to the delocalized system. These six electrons form a continuous ring of electron density above and below the plane of the molecule. This arrangement is often represented by a circle within the hexagonal structure of benzene, symbolizing the delocalized nature of these electrons.

The delocalization of electrons in conjugated systems like benzene contributes significantly to their stability. This stability arises from the fact that delocalized electrons have lower energy than localized ones. In benzene, for instance, the delocalized electrons are spread over a larger area, reducing electron-electron repulsion and lowering the overall energy of the molecule.

It's important to distinguish between localized and delocalized charges. In molecules with localized charges, the positive or negative charge is concentrated on a specific atom or small group of atoms. In contrast, delocalized charges are spread out over a larger portion of the molecule. This delocalization of charge, like the delocalization of electrons, contributes to the stability of the molecule by distributing the charge over a larger area.

The concept of conjugation extends beyond simple hydrocarbons like benzene. Many biological molecules, including chlorophyll and beta-carotene, contain extensive conjugated systems. These systems are responsible for the absorption of light in these molecules, leading to their characteristic colors and their roles in processes like photosynthesis.

Understanding conjugated systems is crucial for predicting and explaining the behavior of organic molecules. The presence of conjugation can affect a molecule's reactivity, spectroscopic properties, and even its physical characteristics. For instance, conjugated systems often absorb light in the visible spectrum, leading to colored compounds. They can also influence a molecule's conductivity, making some conjugated polymers useful in electronic applications.

In conclusion, conjugated systems, characterized by delocalized electrons moving freely through overlapping p orbitals, are a cornerstone of organic chemistry. These systems, exemplified by molecules like benzene, demonstrate unique stability and reactivity due to their electronic structure. The concept of delocalization, whether of electrons or charges, is key to understanding the behavior of many organic compounds and forms the basis for numerous applications in chemistry and related fields.

Resonance Structures and Resonance Hybrid

Resonance structures are a fundamental concept in organic chemistry that helps explain the behavior and properties of certain molecules. These structures represent different ways to distribute electrons within a molecule, particularly in cases where multiple valid Lewis structures can be drawn. The concept of resonance is crucial for understanding molecular stability, reactivity, and bonding patterns.

A resonance hybrid is the actual structure of a molecule that exists as an average or combination of multiple resonance structures. It's important to note that resonance structures are not separate, isolable entities, but rather theoretical representations that contribute to the overall electronic distribution in the molecule.

To illustrate the concept of resonance structures and resonance hybrid, let's consider the example of an imine. An imine is a functional group containing a carbon-nitrogen double bond (C=N). In its simplest form, we can draw two resonance structures for an imine:

1. The first structure shows a double bond between carbon and nitrogen, with a lone pair on the nitrogen atom.

2. The second structure depicts a single bond between carbon and nitrogen, with a positive charge on nitrogen and a negative charge on carbon.

These two structures represent different electron distributions within the imine molecule. The actual structure of the imine is a resonance hybrid, which is an intermediate between these two extreme forms. This hybrid structure exhibits properties that are a blend of both resonance structures.

Understanding resonance is crucial for predicting molecular reactivity. Resonance structures provide insights into the distribution of electron density within a molecule, which directly influences its chemical behavior. For instance, in the imine example, the resonance hybrid suggests that the nitrogen atom has partial positive character, while the carbon atom has partial negative character. This information helps chemists anticipate how the imine might react with various reagents or participate in different chemical reactions.

Moreover, resonance structures help explain the enhanced stability of certain molecules. When multiple resonance structures can be drawn for a molecule, it often indicates greater stability due to the delocalization of electrons across a larger portion of the molecule. This delocalization effect can lead to lower overall energy and increased resistance to chemical reactions.

It's important to recognize that a single Lewis structure is often insufficient to represent conjugated systems accurately. Conjugated systems are molecules or parts of molecules with alternating single and multiple bonds. In these cases, the electrons are delocalized across multiple atoms, and the true electronic structure cannot be captured by a single Lewis representation.

For example, in molecules like benzene or other aromatic compounds, a single Lewis structure fails to depict the true nature of the bonding. The actual structure is a resonance hybrid of multiple contributing structures, which explains the unique stability and reactivity of these compounds.

In more complex organic chemistry molecules, understanding resonance becomes even more critical. Many biological molecules, such as amino acids and nucleic acids, contain conjugated systems where resonance plays a significant role in determining their properties and functions. The ability to recognize and interpret resonance structures is essential for predicting how these molecules will behave in various biochemical processes.

Furthermore, resonance concepts extend beyond organic chemistry and play crucial roles in inorganic chemistry as well. Many inorganic compounds and complexes exhibit resonance, which influences their stability, color, and reactivity. Understanding resonance in these contexts is vital for fields such as materials science and catalysis.

In conclusion, resonance structures and the concept of a resonance hybrid are powerful tools for chemists to understand and predict molecular behavior. By recognizing that a single Lewis structure is often inadequate, especially for conjugated systems, chemists can gain deeper insights into molecular properties and reactivity. The imine example demonstrates how multiple resonance structures contribute to a more accurate representation of electron distribution. As students and researchers delve deeper into chemistry, mastering the principles of resonance becomes increasingly important for tackling complex molecular systems and developing new materials and chemical processes.

Factors Affecting Resonance Stability

Resonance stability is a crucial concept in organic chemistry that plays a significant role in determining the overall stability and reactivity of molecules. Several factors influence the stability of resonance structures, including localized charges, incomplete octets, and electronegativity. Understanding these factors is essential for predicting the behavior and properties of molecules with multiple resonance forms.

Localized charges are one of the primary factors affecting resonance stability. In general, structures with fewer localized charges are more stable than those with more localized charges. This is because localized charges create areas of high electron density or deficiency, which can lead to increased reactivity. For example, in the resonance structures of the carbonate ion (CO3^2-), the form with two negative charges on oxygen atoms is more stable than the form with a negative charge on the carbon atom.

Incomplete octets also play a crucial role in determining the stability of resonance structures. Atoms, particularly those in the second row of the periodic table (C, N, O), tend to be more stable when they have a complete octet of electrons. Resonance forms that maintain complete octets for these atoms are generally more stable than those with incomplete octets. For instance, in the resonance structures of the nitrate ion (NO3^-), the forms with a complete octet on the nitrogen atom contribute more to the overall resonance hybrid than the form with an incomplete octet on nitrogen.

Electronegativity is another essential factor influencing resonance stability. More electronegative atoms are better able to accommodate negative charges, while less electronegative atoms are better suited for positive charges. This principle affects the relative stability of different resonance forms. For example, in the resonance structures of phenol, the form with the negative charge on the oxygen atom is more stable than the form with the negative charge on the carbon atom, due to oxygen's higher electronegativity.

The overall stability of the resonance hybrid is determined by the combined effects of these factors across all contributing resonance structures. More stable resonance forms contribute more significantly to the hybrid, while less stable forms have a smaller influence. This concept is crucial for understanding the properties and reactivity of molecules with multiple resonance structures.

To illustrate how these factors work together, let's consider the example of the acetate ion (CH3COO^-). The two main resonance structures differ in the location of the negative charge: one has the charge on one oxygen atom, while the other has it on the other oxygen atom. Both of these structures are relatively stable because:

1. The negative charge is localized on an electronegative oxygen atom, which can better accommodate the extra electron density.

2. All atoms have complete octets in both structures.

3. The charge is distributed over two equivalent oxygen atoms, further stabilizing the overall molecule.

In contrast, a less stable resonance form would place the negative charge on the carbon atom. This form contributes less to the overall hybrid because carbon is less electronegative than oxygen and therefore less suited to bear a negative charge.

Another example that demonstrates these principles is the benzene molecule. The six resonance structures of benzene are all equally stable because:

1. There are no localized charges in any of the structures.

2. All carbon atoms maintain complete octets in every resonance form.

3. The pi electrons are evenly distributed throughout the ring in all structures.

This equal stability of all resonance forms contributes to benzene's unique aromatic properties and high overall stability.

Understanding these factors is crucial for predicting the behavior of molecules in various chemical reactions. For instance, in electrophilic aromatic substitution reactions, the stability of different resonance forms of reaction intermediates can help predict the preferred sites of substitution on aromatic rings.

In conclusion, the stability of resonance structures is influenced by a complex interplay of factors including localized charges, incomplete octets, and electronegativity. By considering these factors, chemists can predict the relative contributions of different resonance forms to the overall resonance hybrid, providing valuable insights into molecular properties and reactivity. This understanding is fundamental to many areas of organic chemistry, from predicting reaction outcomes to designing new molecules with specific properties.

Electron-Donating and Electron-Withdrawing Groups

In organic chemistry, the behavior of conjugated systems is greatly influenced by the presence of electron-donating groups (EDGs) and electron-withdrawing groups (EWGs). These groups play a crucial role in determining the distribution of electron density within molecules, which in turn affects their reactivity and chemical properties. Understanding the effects of EDGs and EWGs is essential for predicting and explaining various chemical reactions and phenomena.

Electron-donating groups, as the name suggests, have the ability to release electrons into a conjugated system. Common examples of EDGs include -OH, -NH2, -NR2, and -OR groups. When attached to a conjugated system, these groups increase the electron density, particularly at specific positions. This increased electron density can stabilize certain intermediates and influence the molecule's reactivity. On the other hand, electron-withdrawing groups have the opposite effect. EWGs, such as -NO2, -CN, -COOH, and halogens, pull electrons away from the conjugated system, creating areas of lower electron density.

The distribution of electron density in conjugated systems is particularly important when considering compounds like enols and ketones. Enols, which contain a hydroxyl group (-OH) attached to a carbon-carbon double bond, exhibit interesting behavior due to the electron-donating nature of the hydroxyl group. The -OH group pushes electrons into the π system, increasing the electron density at specific carbons. This electron-rich character makes enols more nucleophilic and reactive towards electrophiles.

Ketones, on the other hand, contain a carbonyl group (C=O) which acts as an electron-withdrawing group. The electronegative oxygen atom in the carbonyl group pulls electrons away from the surrounding carbon atoms, creating a partial positive charge on the carbon directly attached to the oxygen. This carbon is known as the alpha (α) carbon, and its electron-deficient nature makes it susceptible to nucleophilic attack. The carbon adjacent to the alpha carbon is called the beta (β) carbon, and it also plays a significant role in many reactions involving ketones.

The presence of EDGs and EWGs in conjugated systems can have profound effects on the reactivity of molecules. For instance, in electrophilic aromatic substitution reactions, EDGs activate the aromatic ring by increasing its electron density, making it more susceptible to attack by electrophiles. Conversely, EWGs deactivate the ring by decreasing its electron density, making it less reactive towards electrophiles. This principle is widely used in organic synthesis to control the regioselectivity of reactions.

In the case of enols and ketones, the distribution of electron density influenced by EDGs and EWGs affects their acid-base properties and reactivity. Enols, with their electron-rich character, tend to be more basic than ketones. This increased basicity makes enols more reactive towards proton donors. Ketones, with their electron-withdrawing carbonyl group, have increased acidity at the alpha position. This acidity is the basis for many important reactions in organic chemistry, such as aldol condensations and enolate formations.

The concept of alpha and beta carbons in relation to carbonyl groups is crucial for understanding the reactivity of ketones and related compounds. The alpha carbon, being adjacent to the carbonyl group, is particularly reactive due to its ability to form enolates. Enolates are resonance-stabilized anions formed by the removal of an alpha hydrogen, and they serve as important intermediates in many organic reactions. The beta carbon, while not as directly affected by the carbonyl group, can participate in reactions such as Michael additions and conjugate additions.

Understanding the effects of EDGs and EWGs on conjugated systems is not only important for predicting reactivity but also for designing and synthesizing molecules with specific properties. By strategically placing these groups, chemists can tune the electronic properties of molecules, influencing their physical characteristics, spectroscopic properties, and biological activities. This knowledge is invaluable in fields such as drug design, materials science, and the development of new synthetic methodologies.

In conclusion, the interplay between electron-donating and electron-withdrawing groups in conjugated systems is a fundamental concept in organic chemistry. Their effects on electron distribution, as exemplified in enols and ketones, and the significance of alpha an

Resonance and Reactivity in Organic Molecules

Resonance is a fundamental concept in organic chemistry that profoundly affects the reactivity of molecules. This phenomenon occurs when a molecule can be represented by multiple Lewis structures, each contributing to the overall electronic distribution. Understanding resonance is crucial for predicting and explaining the behavior of organic compounds in various chemical reactions.

The impact of resonance on reactivity is closely tied to the concepts of nucleophiles and electrophiles. Nucleophiles and electrophiles are electron-rich species that donate electrons in chemical reactions, while electrophiles are electron-poor and accept electrons. Resonance can enhance or diminish the nucleophilic or electrophilic character of specific regions within a molecule, thereby influencing its reactivity.

For instance, consider the case of benzene. Its resonance structures distribute electron density evenly across the ring, making it less reactive than expected for a compound with three double bonds. This resonance stabilization explains why benzene undergoes substitution reactions rather than addition reactions typical of alkenes.

Carbonyl compounds provide another excellent example of how resonance affects reactivity. In aldehydes and ketones, the resonance between the carbon-oxygen double bond and the adjacent carbon-carbon single bond creates a partial positive charge on the carbonyl carbon. This resonance-induced polarization makes the carbonyl carbon more electrophilic, increasing its susceptibility to nucleophilic attack.

Conversely, resonance can enhance nucleophilicity in certain molecules. The carboxylate ion exemplifies this effect. The negative charge is delocalized over both oxygen atoms through resonance, making the carboxylate a more stable and, therefore, less reactive nucleophile compared to a simple alkoxide ion.

Phenols demonstrate how resonance can influence both nucleophilicity and electrophilicity within the same molecule. The lone pair on the oxygen can participate in resonance with the aromatic ring, increasing electron density at the ortho and para positions. This resonance effect makes these positions more nucleophilic and reactive towards electrophilic aromatic substitution.

Amides present another intriguing case of resonance-affected reactivity. The resonance between the nitrogen lone pair and the carbonyl group results in a partial double bond character for the C-N bond. This resonance stabilization makes amides less reactive towards nucleophiles compared to other carbonyl compounds, explaining their stability in biological systems.

The concept of tautomerization represents an extreme case of resonance where the interchange between two structural isomers involves the movement of a proton and the shifting of bonding electrons. Keto-enol tautomerism is a classic example, where a carbonyl compound exists in equilibrium with its enol form. While not strictly a resonance phenomenon, tautomerization shares similarities in how it affects electron distribution and, consequently, molecular reactivity.

In the keto-enol system, the enol form often exhibits enhanced nucleophilicity at the α-carbon due to the presence of the π electrons. This increased reactivity plays a crucial role in various organic reactions, including aldol condensations and halogenation of ketones. Understanding tautomerization is essential for predicting reaction outcomes and designing synthetic strategies in organic chemistry.

The interplay between resonance and reactivity extends to more complex systems as well. In aromatic heterocycles like pyridine, resonance affects the distribution of electron density around the ring, making certain positions more susceptible to nucleophilic or electrophilic attack. This resonance-driven reactivity pattern is fundamental to the behavior of many biologically active compounds and pharmaceuticals.

In conclusion, resonance profoundly influences the reactivity of organic molecules by altering the electronic distribution and, consequently, the nucleophilic and electrophilic properties of different regions within a compound. This understanding is crucial for predicting reaction outcomes, designing synthetic pathways, and explaining the behavior of complex organic systems. By considering resonance effects, chemists can better manipulate and control chemical reactions, leading to more efficient and targeted synthesis of desired compounds.

Conclusion

Conjugation and resonance structures are fundamental concepts in organic chemistry that significantly impact molecular stability and reactivity. Understanding these principles is crucial for predicting chemical behavior and designing effective synthetic strategies. Conjugation involves the delocalization of electrons across multiple bonds, while resonance structures represent different electron arrangements within a molecule. These concepts explain why certain compounds exhibit unexpected properties or react in specific ways. To solidify your grasp of these essential topics, we encourage you to rewatch the introduction video for a comprehensive overview. For further exploration, consider delving into advanced topics such as aromaticity, pericyclic reactions, and molecular orbital theory. These areas build upon the foundation of conjugation and resonance, offering deeper insights into organic chemistry. Remember, mastering these concepts will greatly enhance your ability to analyze and predict organic reactions, making you a more proficient chemist.

Conjugation and Resonance

Conjugation and resonance. What is a conjugated system?

Step 1: Introduction to Conjugation and Resonance

In this section, we will explore the concept of resonance in organic molecules and understand what a conjugated system is in organic chemistry. The primary objective is to comprehend how conjugated systems affect the reactivity of a molecule compared to non-conjugated systems. Additionally, we will delve into the nature of a resonance hybrid and clarify the terminology associated with resonance structures.

Step 2: Understanding Resonance and Conjugation

Resonance in organic molecules involves the delocalization of electrons across multiple atoms, which can stabilize the molecule. A conjugated system is a specific type of resonance where alternating single and double bonds allow for the delocalization of π (pi) electrons across the entire system. This delocalization can significantly influence the molecule's reactivity and stability.

Step 3: The Role of Electron Density

Organic molecules are held together by covalent bonds, where electrons are shared between atoms. However, in some cases, such as with carbon-halogen bonds or pi systems involving double bonds, the electrons are not shared equally. This unequal sharing creates regions of high and low electron density, which can affect the molecule's reactivity. High electron density areas are nucleophilic (electron-rich) and can attack electrophilic (electron-poor) sites, while low electron density areas can be attacked by nucleophiles.

Step 4: Delocalization of Electrons

Molecules with pi bonds can stabilize themselves by delocalizing electrons. Delocalization means spreading the electrons across a larger area rather than keeping them localized between specific atoms. For example, in a benzene ring, the double bonds are not fixed between specific carbon atoms but are spread across the entire ring. This delocalization creates a more stable system by evening out the electron density.

Step 5: Characteristics of a Conjugated System

A conjugated system is characterized by the presence of alternating single and double bonds, which allows for the delocalization of pi electrons. This delocalization can be visualized as a continuous overlap of p orbitals, creating a system where the electrons are shared across multiple atoms. This even distribution of electron density makes the molecule more stable and less reactive.

Step 6: Experimental Evidence of Conjugation

Experimental evidence supports the concept of conjugation. For instance, in benzene, all the carbon-carbon bond lengths are equal, indicating that the electrons are delocalized across the entire ring. This equal bond length is a key indicator of a conjugated system, where the electrons are not confined to specific bonds but are spread evenly across the molecule.

Step 7: Formation of Conjugated Systems

Conjugated systems form when p orbitals overlap and share pi electrons. This typically requires double bonds or sp2 hybridization. For example, in a molecule with alternating single and double bonds, the p orbitals can overlap, allowing the pi electrons to delocalize across the entire system. This delocalization creates a conjugated system, where the electron density is evenly distributed.

Step 8: Stability and Reactivity of Conjugated Systems

Conjugated systems are more stable than non-conjugated systems due to the even distribution of electron density. This stability reduces the molecule's reactivity, as there are no highly electron-rich or electron-poor areas that can readily participate in chemical reactions. The delocalization of electrons lowers the chemical potential energy of the system, making it more stable.

Step 9: Visualizing Conjugated Systems

To visualize a conjugated system, consider a benzene ring with alternating single and double bonds. The p orbitals of the carbon atoms overlap, allowing the pi electrons to delocalize across the entire ring. This creates a stable, conjugated system where the electron density is evenly distributed, and all the carbon-carbon bond lengths are equal.

Step 10: Conclusion

In summary, a conjugated system is a type of resonance structure where alternating single and double bonds allow for the delocalization of pi electrons across the entire system. This delocalization stabilizes the molecule by evening out the electron density, making it less reactive and more stable. Understanding conjugated systems is crucial in organic chemistry, as it helps explain the reactivity and stability of various organic molecules.

FAQs

  1. What is the meaning of conjugate in chemistry?

    In chemistry, conjugation refers to the interaction of p-orbitals in a system of alternating single and multiple bonds. This results in the delocalization of electrons across the conjugated system, leading to increased stability and unique chemical properties.

  2. What is a conjugated molecule in organic chemistry?

    A conjugated molecule in organic chemistry is a compound with a system of alternating single and multiple bonds, typically involving carbon atoms. These molecules have delocalized electrons that can move freely across the conjugated system, affecting the molecule's reactivity and spectroscopic properties.

  3. How do you know if a compound is conjugated?

    To identify a conjugated compound, look for alternating single and multiple bonds in the molecular structure. The compound should have at least three p-orbitals in a row that can overlap, allowing for electron delocalization. Common examples include alkenes with alternating double bonds, aromatic compounds, and molecules with carbonyl groups adjacent to double bonds.

  4. What are conjugated examples in organic chemistry?

    Examples of conjugated systems in organic chemistry include: 1. 1,3-butadiene (CH2=CH-CH=CH2) 2. Benzene and other aromatic compounds 3. Beta-carotene (the pigment in carrots) 4. Retinal (involved in vision) 5. Conjugated polymers like polyacetylene

  5. How does conjugation affect a molecule's properties?

    Conjugation significantly impacts a molecule's properties: 1. Increased stability due to electron delocalization 2. Lower overall energy compared to non-conjugated analogs 3. Absorption of light at longer wavelengths, often resulting in color 4. Enhanced conductivity in some conjugated polymers 5. Altered reactivity, particularly in electrophilic addition reactions

Prerequisite Topics

Understanding conjugation and resonance structures is a crucial aspect of organic chemistry that builds upon several fundamental concepts. While there are no specific prerequisite topics provided for this subject, it's important to recognize that a strong foundation in basic chemistry principles is essential for grasping these more advanced ideas.

To fully comprehend conjugation and resonance structures, students should have a solid understanding of atomic structure, chemical bonding, and electron configuration. These foundational concepts provide the necessary framework for exploring how electrons are shared and distributed within molecules, which is at the heart of conjugation and resonance.

Additionally, familiarity with Lewis structures and VSEPR theory is invaluable when studying conjugation and resonance. These topics help students visualize molecular geometry and electron distribution, which are critical for identifying conjugated systems and drawing resonance structures accurately.

A good grasp of organic chemistry basics, including the nature of carbon bonds and the concept of hybridization, is also crucial. These principles lay the groundwork for understanding how conjugation occurs in organic molecules and why certain structures can resonate.

Moreover, knowledge of electronegativity and polarity is essential for predicting electron movement in conjugated systems and resonance structures. These concepts help explain why electrons delocalize in certain ways and how this affects a molecule's stability and reactivity.

Understanding orbital theory, particularly the concepts of sigma and pi bonds, is also highly beneficial. This knowledge allows students to visualize how electrons are shared in conjugated systems and how they can move to create different resonance structures.

Lastly, a solid foundation in acid-base chemistry can be helpful when studying conjugation and resonance, as these concepts often play a role in explaining the stability and reactivity of conjugated molecules and resonance hybrids.

By ensuring a strong grasp of these underlying principles, students will be better equipped to tackle the complexities of conjugation and resonance structures. These topics are not isolated concepts but rather build upon and integrate various aspects of chemistry. As such, revisiting and reinforcing these foundational ideas can significantly enhance one's understanding and application of conjugation and resonance in more advanced chemical studies.

In conclusion, while there may not be a specific list of prerequisites provided, the interconnected nature of chemistry means that a comprehensive understanding of basic principles is crucial for mastering more advanced topics like conjugation and resonance structures. Students are encouraged to review and strengthen their knowledge in these areas to ensure a solid foundation for their studies in organic chemistry.

In this lesson, we will learn:

  • To understand how a conjugated system affects the reactivity of molecules.
  • To understand the nature of a resonance hybrid and contributing resonance structures.
  • To predict important resonance forms of organic molecules by the presence of electron-donating and withdrawing groups.

Notes:

  • Organic molecules are held together by shared electrons between atoms. However, many bonds in organic molecules don’t evenly share the electrons that make them. Because it is electrons that make (and by moving, break) bonds, molecules become reactive when electrons are concentrated or sparse in one specific region of a molecule (more detail in Nucleophiles and electrophiles).
    Knowing where electrons are in a molecule, then, is very important in understanding any reactivity.

  • Molecules with pi bonds can become stable by delocalizing electrons throughout the structure.
    • This is what benzene does; the alternating single/double (pi) bonds form a conjugated system that delocalizes pi electrons across the p orbitals on carbon.
    A conjugated system \, is made when p orbitals combine and delocalize electrons among them. In a conjugated system the electrons are not localized to specific atoms or their orbitals, but instead delocalized, shared amongst multiple atoms. Instead of a painting with only polarizing black (electron-rich) or white (electron-poor) regions, a conjugated system combines to make a single shade of grey.
    It is not accurate to draw conjugated systems with just one molecular structure, so chemists represent it by drawing multiple resonance forms. Each resonance form ‘contributes’ to the true structure which is a mix or resonance hybrid \, of them.

    In benzene, both forms contribute equally because they are chemically equivalent.
    Depending on the compound though, some resonance forms contribute more to the true structure than others.
    In the example of an imine (think C=O carbonyl but with C=N instead) this isn’t true:

    This imine has three resonance forms but the red form is extremely unstable and the blue form is somewhat less stable than the form in black. We would say this black resonance form contributes most to the structure and the blue form contributes a small amount; the resonance hybrid is closer to the black structure than the other two, but most importantly its real structure is somewhere in between and can’t be described by one single structure.

  • When drawing resonance forms DO NOT use equilibrium arrows. There is no chemical reaction happening!
    In this sense, the word ‘resonance’ is misleading; the electrons are not ‘resonating’ or constantly shuffling between the different resonance forms (constantly going between black and white), they are in a structure settled somewhere between these extremes (a solid shade of grey).

  • Like the imine, most compounds with resonance forms are not contributed to equally (like the two benzene resonance forms). There are some issues to consider when finding important resonance forms:
    • Resonance forms with localized charges are less stable.
      Molecules are reactive when they have areas of high charge density, be it positive or negative. Resonance forms stabilize a molecule because charge is dispersed (think back to black and white becoming a shade of grey). Charged atoms are the opposite of this delocalizing effect!
    • Incomplete octets make resonance forms less stable. This is especially true of electronegative atoms like nitrogen and oxygen. These atoms ‘demand’ electrons more than other atoms, so a form which leaves them electron-poor will be extremely unstable.
    • Stable cations/anions make resonance forms more important. If all resonance forms have localized charge of some sort, consider how stable the ions are.
      For a positive charge:
      • An ion with a filled octet is more stable.
      • A more substituted carbon is more stable (primary < secondary < tertiary) due to the positive inductive effect of alkyl groups.
      • Adjacent electron donating groups (EDGs) stabilize positive ions; more on this below.
      For a negative charge:
      • More electronegative atoms hold negative charge more easily.
      • Adjacent electron withdrawing groups (EWGs) stabilize negative ions; more on this below.

  • Electron-withdrawing groups and electron-donating groups make some resonance forms very important in organic molecules. This is because by donating or accepting pi electrons they change the overall look of the conjugated system – the ‘shade of grey’ the conjugated molecule reaches is made a lot darker or lighter by taking out (EWGs) or putting in (EDGs) electrons!
    Identifying EDGs and EWGs in molecules and drawing resonance forms of these molecules is therefore extremely helpful when finding good nucleophiles and electrophiles.
    • Electron-donating groups adjacent to double bonds make an important resonance form where the further carbon in the double bond is negatively charged. This is because the EDG is donating pi electrons; the molecule is nucleophilic at this carbon atom and can attack electrophilic sites from here. See the diagram below:
    • Electron-withdrawing groups make the ?-carbon (two carbons away from a functional group) electron-poor because of an important resonance form where the EWG is accepting pi electrons. This leaves a positive charge on the ?-carbon, and it can be attacked by nucleophiles here. This is the resonance structure described in red below: