Generated by AskSia.ai — graphs, formulas, traps
| Type | Geometry | Angle | π bonds | Example |
|---|---|---|---|---|
| sp³ | tetrahedral | 109.5° | 0 | CH₄, C–C single |
| sp² | trigonal planar | 120° | 1 | C=C, benzene, C=O |
| sp | linear | 180° | 2 | C≡C, C≡N, allene C |
Steric # = (σ bonds) + (lone pairs) → sp³ if 4, sp² if 3, sp if 2s-character determines acidity of C–H: more s = more electronegative C = more acidic. sp (50% s) > sp² (33%) > sp³ (25%). Terminal alkyne C–H pKa ≈ 25; alkane C–H ≈ 50.
Skeletal: each vertex = C, lines = bonds, H implicit to fill octetTo find hybridization, count σ bonds + lone pairs, not just atoms attached. NH₃ has 3 bonds + 1 LP = 4 = sp³. H₂O has 2 bonds + 2 LP = 4 = sp³ (bent shape). Forgetting lone pairs assigns wrong geometry.
To compare conjugate-base stability (= acid strength), check in this order: A·R·I·O.
| Factor | What it does | Example |
|---|---|---|
| Atom (size + EN) | down a column: bigger = more stable; across: more EN = more stable | HI > HBr > HCl > HF |
| Resonance | spreads charge over more atoms | RCOO⁻ vs RO⁻ |
| Inductive | EN groups withdraw e⁻ density | CCl₃COOH > CH₃COOH |
| Orbital (s-character) | more s = more stable anion | HC≡CH (sp) > H₂C=CH₂ (sp²) |
pKa = −log Ka lower pKa = stronger acid ΔpKa > 4: rxn favors weaker acidEnergy diagrams: y-axis = potential energy. Hill = transition state; valleys = intermediates. Highest hill = rate-determining step. ΔG = product − reactant; activation energy = peak − reactant.
Drawing a 'resonance structure' that moves H or C is a tautomer, not a resonance contributor. Real resonance keeps σ-framework frozen. Only π / LP / charge migrate. Drawing illegal arrows costs the entire problem.
A chiral center = sp³ carbon with 4 different substituents. Chiral molecules are non-superimposable on their mirror image.
2ⁿ stereoisomers max for n chiral centers (less if symmetry)1. CIP priority by atomic number (higher Z = higher priority).
2. If tie at first atom, work outward — first point of difference wins.
3. Lowest priority pointing AWAY: read 1→2→3.
4. Clockwise = R, counterclockwise = S. If lowest is toward you, reverse.
| Term | Definition |
|---|---|
| Enantiomers | non-superimposable mirror images (R/S inverted at all centers) |
| Diastereomers | stereoisomers, NOT mirror images |
| Meso | has chiral centers BUT internal mirror plane → achiral |
| Racemic | 50:50 mix of enantiomers → no optical rotation |
If the lowest-priority group points toward you (out of page), the visible 1→2→3 rotation is opposite the true R/S. Reverse what you see. Most R/S errors come from skipping this check.
H–X across C=C: H goes to the carbon with more H's (less substituted). X goes to the more substituted C (forms the more stable carbocation).
CH₃CH=CH₂ + HBr → CH₃CHBrCH₃ (2° carbocation, not 1°)| Reagent | Adds | Regio / stereo |
|---|---|---|
| HX (HCl, HBr, HI) | H + X | Markovnikov |
| HBr + ROOR (peroxide) | H + Br | anti-Mark (radical) |
| H₂O, H₂SO₄ | H + OH | Markovnikov |
| BH₃ then H₂O₂/OH⁻ | H + OH | anti-Mark, syn |
| Br₂, Cl₂ | X + X | anti addition |
| Br₂ / H₂O | OH + Br | Mark, anti (halohydrin) |
| OsO₄ / KMnO₄ cold | 2 OH | syn diol |
| O₃ / Zn | cleaves C=C | 2 carbonyls |
| H₂ / Pd or Pt | 2 H | syn, full reduction |
Alkynes add twice if reagent is in excess (alkyne → alkene → alkane). With H₂O / H₂SO₄ / Hg²⁺: Markovnikov hydration gives a ketone (via enol tautomerization).
HBr + peroxide reverses regiochemistry to anti-Markovnikov via radicals. This only works for HBr — HCl and HI don't follow it. Many students apply 'peroxide rule' to all H-X. Wrong.
| Feature | Tells you |
|---|---|
| chemical shift δ (ppm) | chemical environment (electron density) |
| # of signals | number of distinct H environments |
| integration | relative # of H's per signal |
| multiplicity (n+1 rule) | number of neighboring H's: doublet → 1 nbr, triplet → 2, quartet → 3 |
δ ranges: alkyl ~1, OH/NH ~2-5 broad, aromatic 6-8, aldehyde 9-10, COOH 10-13¹³C NMR: shows # of unique C environments. Ranges: alkyl 0-50, alkene 100-140, aromatic 120-140, carbonyl 170-220.
DEPT: distinguishes CH (up), CH₂ (down), CH₃ (up); quaternary C invisible.
For –CH₂–CH₃: the CH₂ sees 3 H's on the CH₃ → quartet (3+1). The CH₃ sees 2 H's on the CH₂ → triplet (2+1). You don't count your own H's. Off-by-one multiplicities lose questions.
| Feature | SN2 | SN1 |
|---|---|---|
| Mechanism | 1 step, concerted | 2 steps via carbocation |
| Rate law | k[R-X][Nu] | k[R-X] only |
| Substrate | 1° > 2° > 3° | 3° > 2° > 1° |
| Nucleophile | strong, anionic | weak, neutral |
| Solvent | polar aprotic (DMSO, acetone) | polar protic (H₂O, ROH) |
| Stereochem | inversion (Walden) | racemization (mix R/S) |
| Leaving group | better LG = faster (both) | better LG = faster (both) |
SN2 rate = k [substrate][Nu] SN1 rate = k [substrate]Leaving-group ability: stable as anion. Best: I⁻ > Br⁻ > Cl⁻ > TsO⁻ >> F⁻ ≈ HO⁻. Use OTs (tosylate) when you need a great LG on an alcohol.
SN2 needs backside attack — tertiary carbons are too crowded. Even with a great Nu, 3° won't go SN2. Reverse trap: 1° won't go SN1 (cation too unstable). Match substrate to mechanism first, every time.
| Feature | E2 | E1 |
|---|---|---|
| Steps | 1 (concerted) | 2 via cation |
| Rate | k[R-X][base] | k[R-X] |
| Substrate | 3° > 2° > 1° | 3° > 2° >> 1° |
| Base | strong (NaOEt, t-BuOK, DBU) | weak / solvent |
| Geometry req. | anti-periplanar H and LG (180°) | none |
| Regioselectivity | Zaitsev (most subst alkene); Hofmann with bulky base | Zaitsev |
Zaitsev: removes H from the C with FEWEST H's → more substituted alkeneSN vs E competition: heat + strong base favors E. 1° + strong nonbulky Nu → SN2 wins. 3° + strong base → E2. 3° + weak base → SN1/E1 mix.
For E2 on a cyclohexane, the H and LG must both be axial. Sometimes the only axial-axial geometry forces removal of a less-favored H — Zaitsev violated by stereochemistry. Always draw the chair.
| Substrate | Conditions | Mechanism |
|---|---|---|
| Methyl, 1° | Strong Nu, polar aprotic | SN2 |
| Methyl, 1° | Strong base, heat | E2 (rare for 1°, except bulky base) |
| 2° | Strong Nu, polar aprotic | SN2 |
| 2° | Strong base, polar aprotic, heat | E2 |
| 2° | Weak Nu, polar protic | SN1 + E1 mix |
| 3° | Strong Nu (rare) | E2 (SN2 blocked) |
| 3° | Weak Nu, polar protic, heat | SN1 + E1 mix |
| 3° | Strong bulky base (t-BuOK) | E2, Hofmann |
| Reagent / clue | Use § from | |
|---|---|---|
| HX adds to alkene → Markovnikov | § ⑤ | |
| HBr + peroxide → anti-Markovnikov | § ⑤ | |
| BH₃ then H₂O₂ → anti-Mark, syn OH | § ⑤ | |
| OsO₄ or cold KMnO₄ → syn diol | § ⑤ | |
| O₃ then Zn → cleave C=C to carbonyls | § ⑤ | |
| broad O-H ~3200-3600 cm⁻¹ + sharp C=O ~1710 → COOH | § ⑦ | |
| quartet ~4 ppm + triplet ~1 ppm → ethyl group adjacent to O | § ⑦ | |
| 'rank acidity' | § ⑥ | A·R·I·O analysis on conjugate base |
Whenever the mechanism passes through a carbocation (SN1, E1, HX addition), watch for 1,2-H or 1,2-alkyl shifts to a more stable C⁺. The 'expected' product may be wrong because the cation rearranged before the Nu attacked.
Mechanisms earn partial credit per arrow. Always draw curved arrows from electron source to electron sink. Skipping arrow notation = losing 50% even with the right product.