Understanding Conjugation in Organic Chemistry
Explore the fundamental concepts of conjugation and resonance structures in organic chemistry. Learn how these principles influence molecular stability, reactivity, and properties to excel in your studies.

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Now Playing:Conjugation and resonance structures – Example 0a
Intros
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  1. Conjugation and resonance.
  2. What is a conjugated system?
  3. Resonance forms.
Reaction mechanisms
Notes

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:
Concept

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.

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

Prerequisites

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.