ochem week 5 part 2
Course context and classroom approach
Instructor setting: casual, students in a fourth-semester chemistry class; acknowledges discrepancies between teaching methods and resources across instructors.
Safety-net policy: if you learn/react differently, you won’t be docked for it; honesty about different representations is okay.
Emphasis on clarity and transparency: the goal is understanding, not drama; the instructor prefers direct discussion and safe space to ask questions.
Meta discussion about resonance and counting electrons: there are debated points, but for this course, the following approach is used:
If resonance exists, electrons contributing to the pi-system are counted in the appropriate framework; hindsight debates may arise about specific lone-pair contributions, but the instructor will count them in the presented framework.
Two specific debated scenarios (not exhaustively listed here) about whether to count certain electrons or resonance forms; the instructor will err on the side of counting electrons in the pi-system for the purposes of this class, unless told otherwise.
Important nuance: atoms don’t “desire” things; resonance exists or doesn’t; the goal is to determine whether a resonance form is valid and contributes to the pi-system.
Exam policy and practice notes:
The instructor avoids no-reaction-only problems to avoid undermining confidence; focus on reactivity trends (fastest/slowest) rather than trick questions.
If an inconsistency is encountered (e.g., naming/nomenclature changes or exceptions), those are discussed, but exams focus on established content taught in class.
If a topic isn’t covered in class, it won’t appear on the test, even if it exists in literature.
Counting pi electrons and resonance basics
Key concept: aromaticity follows the Hückel rule: a planar cyclic π-system is aromatic if it contains π-electrons (n = 0,1,2,…).
For counting π electrons in rings with heteroatoms, the fate of lone pairs matters:
In pyrrole, the nitrogen lone pair is part of the π-system and contributes 2 electrons to aromaticity.
In pyridine, the nitrogen lone pair is not part of the π-system (it resides in an sp^2 orbital orthogonal to the ring π-system).
Practical takeaway: when evaluating sites for electrophilic aromatic substitution (EAS) in heterocycles, distinguish between lone-pair contributions that participate in aromaticity and those that do not.
Examples discussed in class framing:
Allyl cation is resonance-stabilized (multiple contributing forms with charge delocalized over three carbons).
Vinyl cation is not resonance-stabilized in the same way; placing a positive charge on a vinyl carbon is much less favorable due to limited stabilization.
Take-home rule used in class: count π electrons from the π-system (including lone pairs that participate in the ring’s aromaticity) when assessing aromaticity/reactivity; exclude lone pairs that are not part of the π-system unless the mechanism explicitly involves their participation.
Phenol oxidation and the para-diol (hydroquinone) story
Phenol oxidation pathway (conceptual):
Phenol can be oxidized with a strong oxidant to give a diketone on the aromatic ring, yielding a benzoquinone-type structure (two carbonyls at the 1,4-positions).
The intermediate/product is benzoquinone (often described as a quinone structure with two C=O groups opposite each other).
The oxidation stops at the diketone stage due to octet considerations; further oxidation to the fully oxidized diacid is not favorable here because of octet and stability concerns.
Reduction of the benzoquinone product:
Benzoquinone can be reduced back to hydroquinone (para-dihydroxybenzene, i.e., two OH groups at the 1,4-positions).
The reduction is reversible: a reductant can convert hydroquinone back to benzoquinone, and strong oxidants can re-oxidize hydroquinone to the diketone.
A specific redox couple can be manipulated (the instructor notes that reduction to the two alcohols is reversible via a reductant such as fluoride-mediated conditions in their example, and back to the diketone by oxidation).
Practical takeaway:
This redox couple provides a clear example of reversible oxidation-reduction chemistry on a benzene ring and demonstrates para-difunctionality (para-diol or para-diketone forms).
The sequence is a useful flashcard topic for oxidation/reduction (oxidation = “ox”; reduction = “redox”).
Additional note: the discussion foreshadows more oxygen-containing chemistries in Chapter 15 and the idea that reactions with oxygen-containing species will be more common and varied.
Heterocycles and EAS: pyrrole vs pyridine; base vs aromaticity
Core idea: introducing nitrogen into a benzene ring (to form pyrrole or pyridine) changes both aromaticity and reactivity, particularly for EAS and basicity.
Pyrrole (five-membered ring with nitrogen):
Aromatic even with nitrogen; the N–H or N-substituted forms still maintain aromaticity with the nitrogen lone pair contributing to the π-system.
EAS on pyrrole occurs at the C-2 position (the position adjacent to nitrogen) because it provides the most stabilized arenium ion upon attack (best resonance stabilization for the σ-complex).
The N–H in pyrrole has a very acidic character (pKa ≈ 0.5 in the instructor’s note). The conjugate base is resonance-stabilized; the nitrogen's involvement in aromaticity means its lone pair is not available to act as a base in the same way as in non-aromatic amines.
Pyridine (six-membered ring with one N):
Aromatic ring, but the nitrogen lone pair is not part of the π-system (it resides in an sp^2 orbital and is available as a base).
The lone pair can act as a base and is relatively basic, but the ring’s aromaticity competes with protonation; the conjugate acid of pyridine (pyridinium, [pyridine-H]^+) has a pKa around 5.25 (as given in class notes).
Comparative acidity/basicity (as discussed in class):
Pyrrole: very weak base due to involvement of N lone pair in aromatic sextet; pKa of the relevant protonated form is low, and the base strength is weak.
Pyridine: lone pair is in an sp^2 orbital, not used for aromaticity, making it a somewhat stronger base than pyrrole, but still weaker than typical aliphatic amines because of the aromatic stability which resists protonation.
For a standard amine (alkylamine, sp^3 N): extremely basic due to high electron density and availability of the lone pair; pKb and related acidity/basicity trends correlate with s-character (more s-character → weaker base).
Hybridization and acidity/base trends (recalled from ORGO1 and foundational principles):
More s-character in the nitrogen’s hybrid orbital makes for a stronger conjugate acid (more s-character stabilizes the cation); conversely, it makes the base weaker.
In general, sp^2 (pyridine-like) nitrogens are less basic than sp^3 (amine) nitrogens due to lower availability of the lone pair for donation.
EAS behavior of pyridine: highly deactivated for EAS because the lone pair can coordinate with Lewis acids, disturbing electrophilic attack; the nitrogen can form complexes with acids, effectively deactivating the ring toward EAS under typical conditions.
Special note on pyridine in electrophilic aromatic substitution:
If forced to react under harsh conditions (high temperature), EAS can occur at the 3-position (meta relative to the nitrogen) in some cases, but this is not typical and requires strong forcing conditions.
In most practical conditions for EAS, pyridine is not an ideal substrate due to deactivation by the lone pair.
Polyheteroaromatics (e.g., indole, carbazole) versus pyridine/pyrrole cores:
Indole (a fused pyrrole–benzene system) behaves differently: the indole nitrogen can be reactive as a nucleophile, and indole is a good nucleophile in many contexts.
Indole and related cores are relevant in biochemistry (tryptophan contains the indole side chain) and in biologically important contexts.
Takeaway: nitrogen incorporation into heteroaromatic rings drastically changes both aromaticity and reactivity, affecting EAS, base strength, and potential for complexation with Lewis acids.
Amine activation, amide protection, and diazonium chemistry (Diazotization and Sandmeyer reactions)
Aniline as a test case for activation and subsequent manipulation:
Aniline is strongly activating for EAS (it donates electron density into the ring), but its lone pair also coordinates with Lewis acids, potentially causing side reactions and deactivation in some contexts.
Because of these issues, a common tactic is to temporarily “hide” the amine’s lone pair by converting aniline into an amide (via acylation).
Amide formation: converting the amine to an amide reduces donation via resonance and reduces basicity, enabling smoother EAS and later deprotection steps.
Diazotization: converting an amine (like aniline) to a diazonium salt
Reagents and general process: Aniline (Ar–NH2) is treated under diazotizing conditions (acidic medium with nitrous acid, typically generated in situ from NaNO2 and acid) to form the diazonium salt Ar–N2+ X− (a diazonium salt; X− is often Cl−, BF4−, etc.).
This diazonium salt is highly versatile and can be used as a key intermediate for multiple transformations.
Sandmeyer reactions (copper-mediated substitutions from diazonium salts):
CuX reactions replace the diazonium group with halide or nitrile:
Ar–N2+ + CuBr → Ar–Br (bromination)
Ar–N2+ + CuCl → Ar–Cl (chlorination)
Ar–N2+ + CuCN → Ar–CN (cyano substitution)
The three direct Sandmeyer products commonly used are aryl bromides, aryl chlorides, and benzonitriles.
There is also an iodine variant: Ar–N2+ + NaI (or CuI/iodide reagents) → Ar–I.
Hydrolysis/phenol formation from diazonium salts:
Aqueous hydrolysis of the diazonium salt can yield phenol (Ar–OH) under mild conditions, offering a direct route to phenols from anilines.
This approach avoids harsher conditions that were previously required to form phenols from anilines, providing a gentler route in practice.
De-diazotization (loss of N2) to form Ar–H:
The diazonium group can be replaced by hydrogen, effectively removing the diazonium substituent and yielding the parent arene (Ar–H).
This de-diazotization pathway highlights how diazonium salts can act as a versatile leaving group to introduce or remove functional groups.
Diazotization as a platform for functional-group interconversion:
The diazonium intermediate enables a variety of transformations beyond Sandmeyer, including alkyne/aryl substitutions and other coupling strategies (see azo coupling below).
Five major transformations via diazonium chemistry (overview):
Sandmeyer halogenation and nitrile introduction (Br, Cl, CN)
Iodination (I) via NaI or CuI
Phenol formation via hydrolysis of the diazonium salt
De-diazotization to yield Ar–H
Azo coupling (see next section) to form azo dyes
Important safety note: diazonium salts are often unstable and can be hazardous (explosive under certain conditions); handle with care in the lab.
Azo coupling and color chemistry (coupling of diazonium salts)
Azo coupling involves reacting a diazonium salt with another aromatic system that bears an activating group, forming an azo linkage (–N=N–) between two aromatic rings.
The coupling partner is typically an activated arene (e.g., one with an electron-donating group) and the diazonium salt acts as the diazo component.
The reaction typically occurs at the para position relative to the activating group on the coupling partner due to stabilization of the arenium intermediate and resonance considerations.
The resulting azo compounds are highly conjugated and display vivid colors, which is why this chemistry underpins azo dyes used in textiles and coloring processes.
Conjugation across the diazo linkage and the two aromatic rings gives extended π-system, leading to intense color.
Practical summary of azo chemistry steps:
Start with aniline or another aniline derivative; diazotize to form Ar–N2+.
Perform azo coupling with a second activated arene to form Ar–N=N–Ar′.
The color and properties depend on substituents on both rings and the extent of conjugation.
Additional notes from the lecture:
The diazonium salt’s utility lies in its ability to participate in multiple transformations with nucleophiles including halides, cyanide, phenol formation, and aryl-aryl coupling via azo formation.
This chemistry illustrates how functional-group interconversion is a central theme in organic synthesis: turning an amine into a diazonium, then into many other functionalities.
Final practical takeaway: azo dyes are a quintessential example of how extended conjugation leads to color, making diazonium chemistry a powerful tool in materials and pigment chemistry.
Indole, pyrrole, pyridine, and bio-relevance
Indole and related polyheterocycles: indole has a fused bicyclic framework that includes a pyrrole-type ring, which is highly reactive as a nucleophile in many contexts.
Indole is a common motif in biochemistry and drug design; it is a good nucleophile and participates readily in various substitution and coupling reactions.
Indole-containing amino acids (like tryptophan) are biologically important; the indole ring is common in biochemistry and pharmaceuticals.
Pyridine vs indole in biological contexts:
Pyridine-like nitrogen (in pyridine) is basic and can coordinate with metals or Lewis acids; however, in the heterocycle core, it may deactivate certain reactions unless conditions are adjusted.
Indole (pyrrole-like nitrogen) behaves differently due to its aromatic stabilization and its N–H (or N-substituted) situation; reactivity is directed differently compared to pyridine.
Practical connection: understanding how the presence and type of nitrogen in heterocycles affects reactivity helps predict behavior in biological systems and medicinal chemistry contexts (e.g., amino acids with heterocyclic rings, nucleophilic sites in enzymes, etc.).
Aniline-directed substitutions and practical synthesis strategies
Directing effects: aniline is strongly activating and ortho/para-directing for EAS due to electron donation from the amino group.
Complexation with Lewis acids: the amine’s lone pair can coordinate with Lewis acids used as catalysts, which can complicate reactions; the strategy to control this is to temporarily mask the amine (e.g., convert to an amide).
Amide protection strategy: converting aniline to an amide reduces donation and decreases the likelihood of unwanted coordination, allowing cleaner EAS chemistry and later deprotection to reveal the amine.
Practical use of amide protection in EAS:
The amide allows EAS to proceed with fewer side reactions and under milder conditions.
Later, deprotection can restore the amino substituent, enabling subsequent transformations.
Nitration vs diazonium chemistry context:
Before diazotization, aniline is prepared for diazonium chemistry; diazonium salts then enable a wide range of transformations (Sandmeyer, azo coupling, hydrolysis to phenol, etc.).
Summary of “how this helps you replace functional groups”: diazonium chemistry provides a flexible, two-step route to interconvert anilines to a variety of other functionalities (bromo, chloro, cyano, iodide; phenol; hydrogen; azo dyes), enabling rapid diversification of aromatic compounds for synthesis and materials chemistry.
Practical and exam-oriented tips drawn from the lecture
For EAS with heterocycles, pay close attention to:
Whether the ring nitrogen’s lone pair participates in aromaticity (pyrrole) or is available for conjugation/base (pyridine).
The directing effects of substituents: EDGs direct ortho/para; EWGs direct meta; however, in heterocycles, actual outcomes may be nuanced due to ring electronics and a given reagent’s conditions.
When discussing ring reactivity:
In pyrrole, substitution tends to occur at C-2 (the position adjacent to nitrogen).
In pyridine, EAS is strongly deactivated; if forced, substitution at the 3-position is possible under harsh conditions, but it’s not the norm.
Naming and nomenclature notes from the class:
Diazotization uses diazonium salts Ar–N2+ X−; this intermediate is key for downstream transformations.
Sandmeyer reactions provide a straightforward path to Ar–X (X = Br, Cl, CN, or I under appropriate conditions).
Azo coupling yields Ar–N=N–Ar′ and gives vivid colors due to extended conjugation; this is the basis for azo dyes.
Safety and lab practice reminders:
Diazonium salts can be hazardous and potentially explosive; handle with care.
Diazotization reactions are typically done under controlled, cooled conditions with proper reagents and quenchers.
Real-world relevance: the content connects to biochemistry (indole and tryptophan), materials chemistry (azo dyes), and synthetic strategy (functional-group interconversion via diazonium chemistry), illustrating how fundamental ideas translate to real-world applications.
Quick formula and concept recap (LaTeX-framed)
Aromaticity rule: a planar cyclic π-system is aromatic if it contains π-electrons (n = 0,1,2,…).
For pyrrole, the nitrogen lone pair contributes to the π-system, adding 2 electrons to the aromatic sextet.
For pyridine, the nitrogen lone pair does not contribute to the π-system; it sits in an sp^2 orbital and can be protonated or act as a base.
Phenol oxidation product (conceptual): phenol → benzoquinone (two C=O groups, para-disubstituted) → hydroquinone upon reduction, i.e., para-diol (p-dihydroxybenzene).
Diazotization general form (conceptual):
Ar–NH2 → Ar–N2+ X− (diazonium salt) under nitrous acid conditions.
Sandmeyer reactions (examples):
Ar–N2+ + CuBr → Ar–Br
Ar–N2+ + CuCl → Ar–Cl
Ar–N2+ + CuCN → Ar–CN
Azo coupling (general form): Ar–N2+ + Ar′–X (activated arene) → Ar–N=N–Ar′ (azo dye core).
Title
Organic Chemistry Lecture Notes: Resonance, EAS, Heterocycles, and Diazonium Chemistry