Organic chemistry tricks that make functional groups stick

Alcohol=OH. Aldehyde=CHO (H on carbonyl). Ketone=CO (no H on carbonyl). Carboxylic acid=COOH. Amine=NH2. These 5 appear on almost every orgo exam.

Groups that form H-bonds with water are water soluble. Alcohols, amines, carboxylic acids — all soluble in small molecules. As carbon chain grows, hydrophobic character WANES solubility.

The carbonyl carbon (C=O) is electron-deficient — nucleophiles attack here. The oxygen is electron-rich — it can donate electrons. This determines ALL carbonyl reaction mechanisms.

Broad absorption 3200–3600 cm⁻¹ = OH or NH. Sharp strong peak ~1710 cm⁻¹ = C=O (ketone). ~1735 = ester. ~1715 = carboxylic acid. ~1680 = amide.

Electron Withdrawing Groups (EWG): NO2, CN, halogens — pull density away from ring, deactivate. Electron Donating Groups (EDG): OH, NH2, alkyl — push density into ring, activate.

When a molecule has multiple functional groups, the highest priority group determines the suffix (parent name). Priority order (highest to lowest): carboxylic acid (-oic acid) > ester (-oate) > amide (-amide) > aldehyde (-al) > ketone (-one) > alcohol (-ol) > amine (-amine). Lower priority groups become prefixes (e.g., amino-, hydroxy-, oxo-). Only one suffix allowed — pick the highest priority group.

A good leaving group departs as a stable anion. Stability = weak base = conjugate of strong acid. Order: I⁻ (best) > Br⁻ > Cl⁻ > F⁻ (worst). Tosylate (OTs) and mesylate (OMs) are excellent leaving groups — convert OH to a good LG. OH⁻ and NH₂⁻ are poor leaving groups (strong bases). Protonation converts OH to H₂O (good LG) — acid catalysis. In SN2: I⁻ > Br⁻ > Cl⁻ >> F⁻.

Nucleophilicity = rate of attack on electrophile. In polar protic solvents: I⁻ > Br⁻ > Cl⁻ > F⁻ (opposite of basicity — larger, less solvated atoms are better nucleophiles). In polar aprotic solvents: follows basicity order (F⁻ best nucleophile). Negatively charged > neutral. Less sterically hindered = better nucleophile. Good nucleophiles are not always good bases: RS⁻ (excellent nucleophile, weak base), PhO⁻ (moderate nucleophile, weak base).

DoU formula: (2C + 2 + N – H – X) / 2. Oxygen does not appear (no net effect). Examples: C₆H₆ (benzene) = (12+2-6)/2 = 4 DoU (3 double bonds + 1 ring). C₄H₈ = (8+2-8)/2 = 1 DoU (one ring or one double bond). C₄H₄ = (8+2-4)/2 = 3 DoU. If DoU ≥ 4 with 6 carbons → likely aromatic ring. DoU = 0 → fully saturated. Nitrogen adds 1 to numerator; halogens subtract like H.

Resonance structures: same compound, same atom positions, only electrons differ — connected by double-headed arrow (↔). Tautomers: different compounds, atoms in different positions (usually H moves) — connected by equilibrium arrow (⇌). Example: keto ⇌ enol tautomers (H moves from C to O). Resonance structures cannot be separated; tautomers can be separated (in principle, often fast equilibrium). Both described as 'contributing structures' or 'forms' but mean fundamentally different things.

Inductive effect: electron withdrawal or donation through sigma bonds. Decreases with distance (drops off as 1/r²). Example: Cl withdraws electrons inductively — makes alpha carbon electrophilic. Resonance effect: delocalization through pi bonds/conjugation. Does not drop off with distance within conjugated system. Example: -NO₂ withdraws by resonance ortho/para to itself. Compounds can have both effects working together or in opposition (e.g., halogens: withdraw inductively, donate by resonance).

Sigma (σ) bonds: head-on orbital overlap, single bonds, freely rotating, always present in any bond. Pi (π) bonds: side-by-side p orbital overlap, present in double and triple bonds, restrict rotation. Double bond = 1σ + 1π. Triple bond = 1σ + 2π. Pi bonds are weaker than sigma bonds (less overlap), more reactive, and responsible for E/Z isomerism (restricted rotation). Electrophiles attack pi bonds preferentially.

H-bonding: requires N-H, O-H, or F-H + lone pair — strongest IMF after covalent/ionic. Alcohols, carboxylic acids, amines (N-H). Dipole-dipole: polar molecules without H-bond donors (ketones, aldehydes, ethers, esters). London dispersion (van der Waals): all molecules — increases with size/surface area. Boiling point: H-bond >> dipole > dispersion. Water solubility: functional groups that H-bond with water are soluble; long carbon chains reduce solubility.