Organic chemistry Study Guide

📖 Core Concepts Line‑angle (skeleton) diagram – every line end or vertex = a carbon; H atoms are omitted but are assumed to satisfy carbon’s tetravalence. Carbon’s versatility – tetravalent; can form single, double, triple bonds and delocalised π‑systems (aromaticity). Functional groups – specific atom groups (‑OH, ‑COOH, ‑NH₂, etc.) that dominate reactivity and physical properties. Reaction families – addition, elimination, substitution, pericyclic, rearrangement, redox. Arrow‑pushing – curved arrows show electron flow from nucleophiles (electron‑rich) to electrophiles (electron‑poor). Retrosynthetic analysis – planning synthesis by repeatedly “disconnecting” the target into simpler, commercially available precursors. Key spectroscopic tools – NMR (connectivity + stereochemistry), MS (molecular weight + fragmentation), IR/UV‑Vis (functional‑group signatures), X‑ray (3‑D structure). --- 📌 Must Remember Carbon valence = 4 → max of four single bonds or combinations (e.g., one double = two bonds). Aromaticity rule – planar, cyclic, fully conjugated system with 4n + 2 π electrons (Hückel’s rule). IUPAC naming – locate parent chain/ring → add suffix for principal functional group → add prefixes/locants for substituents. Nucleophile vs electrophile – nucleophile = electron‑pair donor (often basic, low pKa); electrophile = electron‑pair acceptor (often positively polarized, high pKa). Zaitsev’s rule (elimination) – the more substituted alkene is usually favored. Chromatography choice – GC for volatile, thermally stable compounds; HPLC for non‑volatile, polar, or thermally labile molecules. Solubility cue – neutral organics → hydrophobic (prefer organic solvents); presence of ionizable groups → increased water solubility. --- 🔄 Key Processes Retrosynthetic Analysis Identify the target molecule. Disconnect strategic bonds → generate simpler synthetic equivalents (retro‑disconnections). Repeat until all fragments correspond to commercially available or easily prepared precursors. Assemble a synthetic tree to compare alternative routes. Typical Substitution (SN1/SN2) Mechanism Step 1: Leaving group (X) departs → carbocation (SN1) or nucleophile attacks simultaneously (SN2). Step 2: Nucleophile (Nu) forms new bond → product Nu‑R. Arrow‑pushing in an Aldol Reaction Deprotonate carbonyl α‑hydrogen → enolate (negative charge on α‑C). Enolate attacks electrophilic carbonyl carbon of another molecule → C‑C bond formation → β‑hydroxy carbonyl (aldol product). NMR Structural Determination (quick workflow) Acquire ¹H spectrum → count signals → integrate for proton count. Use splitting patterns (n + 1 rule) to infer neighboring protons. Combine with ¹³C data to assign carbon skeleton. --- 🔍 Key Comparisons Line‑angle vs. Condensed formula Line‑angle: shows connectivity only; fast for large molecules. Condensed: explicit atoms; useful for counting H’s. Alkane vs. Alkene vs. Alkyne Alkane: only C–C single bonds (saturated). Alkene: at least one C=C double bond (unsaturated, planar). Alkyne: at least one C≡C triple bond (linear geometry). Alcohol vs. Carboxylic Acid Alcohol: –OH attached to sp³ carbon; pKa ≈ 16; hydrogen‑bond donor/acceptor. Carboxylic acid: –COOH; pKa ≈ 4–5; stronger H‑bond donor, can deprotonate in water. GC vs. HPLC GC: gas phase, requires volatility, high‑temperature stability. HPLC: liquid phase, handles polar, high‑MW, thermally labile analytes. Nucleophile vs. Electrophile Nucleophile: electron‑rich, often negative or lone‑pair donor (e.g., OH⁻, NH₃). Electrophile: electron‑poor, often positively polarized (e.g., C=O carbon, alkyl halide carbon). --- ⚠️ Common Misunderstandings “All hydrogens are drawn” – In line‑angle diagrams they are implicit; forgetting this leads to wrong formulas. Aromaticity = any ring – Only rings that are planar, fully conjugated, and obey 4n + 2 π e⁻ are aromatic. All carbonyls are equally electrophilic – Electron‑withdrawing groups increase electrophilicity; amides are much less reactive than acyl chlorides. Distillation always purifies – Non‑volatile impurities or azeotropes can remain; sometimes chromatography is needed. Polymers are synthetic only – Biopolymers (proteins, DNA) are naturally occurring. --- 🧠 Mental Models / Intuition Carbon as a “four‑way junction” – visualize each carbon as a hub with up to four “roads” (bonds); adding/removing a road changes the molecule’s class. Functional group as a “reactivity tag” – think of each group as a label that tells you which “tools” (reagents) will work best. Retrosynthesis as a puzzle – start from the picture on the box (target) and work backward, snapping pieces off until you have standard LEGO bricks (commercial reagents). Arrow‑pushing = electron traffic flow – arrows point from the source of electrons to the destination, like a one‑way street. --- 🚩 Exceptions & Edge Cases Heteroaromaticity – not all heterocycles are aromatic; furan is aromatic (5 π e⁻), while pyrrole’s nitrogen contributes a lone pair. X‑ray crystallography – requires a good single crystal; amorphous solids or oils cannot be analyzed. Polarity vs. Boiling point – highly polar small molecules may have lower boiling points than larger, non‑polar ones due to size outweighing dipole interactions. Amines vs. Amides – amides are far less basic/nucleophilic because the lone pair is delocalised into the carbonyl. Solvent extraction limits – compounds must have a sizable partition coefficient; very polar compounds may stay in the aqueous layer despite “extractable” functional groups. --- 📍 When to Use Which Choose NMR when you need connectivity + stereochemistry (most organic structures). Choose IR for rapid identification of functional groups (e.g., carbonyl 1700 cm⁻¹). Choose MS for molecular weight and fragmentation pattern (especially when a crystal isn’t available). Use GC for volatile, thermally stable samples; HPLC for polar, high‑MW, thermally sensitive compounds. Select substitution over addition when the substrate already contains a good leaving group and the nucleophile is strong; select addition for alkenes/alkynes lacking leaving groups. Apply Zaitsev’s rule when multiple alkene products are possible in an elimination; use Hofmann’s rule (least substituted alkene) only with bulky bases. --- 👀 Patterns to Recognize IR carbonyl stretch 1700 cm⁻¹ → indicates C=O (ketone, aldehyde, ester, acid). ¹H NMR singlet around 9–10 ppm → aldehydic proton. Triplet (CH₃) + quartet (CH₂) pattern → ethyl group attached to a carbonyl or aromatic ring. Mass spec M⁺ peak + 15 Da (CH₃) series → alkyl chain fragmentation. GC elution order – lower boiling point = earlier elution; polarity can invert order on polar stationary phases. Elimination reactions → look for β‑hydrogen and a good leaving group on adjacent carbons. --- 🗂️ Exam Traps Distractor: “All aromatic compounds are planar.” – Some hetero‑aromatics (e.g., pyridine) are planar, but others (e.g., pyrrole) can be slightly puckered; the key is conjugation + 4n + 2 π e⁻. Distractor: “Hydrophobic = insoluble in any solvent.” – Hydrophobic molecules dissolve well in non‑polar organic solvents; the trap ignores solvent polarity. Distractor: “Any carbonyl reacts like an acyl chloride.” – Amides, esters, and ketones are much less reactive; only acyl chlorides are highly electrophilic. Distractor: “Distillation always yields a pure compound.” – Overlook azeotropes and thermally labile substances; often a follow‑up chromatography is required. Distractor: “A strong base always gives the most substituted alkene.” – Bulky strong bases (e.g., KO‑t‑Bu) favor the least substituted (Hofmann) alkene. ---