1) Bonding, Formal Charge, Resonance (Exam Core)
1.1 Formal Charge (fast checklist)
Formal charge = valence e⁻ − (nonbonding e⁻ + ½ bonding e⁻)
Common patterns
- N (amine): usually 3 bonds + lone pair → FC 0
- N (ammonium): 4 bonds → FC +1
- O (alcohol/ether): 2 bonds + 2 lone pairs → FC 0
- O (oxonium): 3 bonds + 1 lone pair → FC +1
- O⁻: 1 bond + 3 lone pairs → FC −1
1.2 Resonance (what matters on tests)
- Only electrons move (π bonds, lone pairs, formal charges).
- Best contributors: full octets, minimal charge separation, negative charge on electronegative atoms.
warning
Resonance is NOT equilibrium between structures; the real molecule is a hybrid.
Classic resonance system: allyl
SMILES
C=CC
2) Functional Groups (Recognition + Reactivity)
2.1 Functional Group Map (most-tested)
| Group | Diagram | Key Reactivity |
|---|---|---|
| Alkene | C=C | π bond nucleophilic; electrophilic additions |
| Alcohol | ROH | weakly nucleophilic O; can be activated → leaving group |
| Alkyl halide | R–X | electrophile at carbon; SN1/SN2/E1/E2 |
| Carbonyl (aldehyde/ketone) | C=O | electrophilic carbon; nucleophilic addition |
| Carboxylic acid derivatives | acyl-X | substitution at acyl carbon (addition–elimination) |
Carbonyl examples
SMILES
CC(=O)C
SMILES
CC=O
3) Acid–Base in Orgo (Rules You Use Constantly)
3.1 What makes an acid "more acidic"
- Conjugate base stabilized by: resonance > inductive > hybridization
- sp (more s-character) stabilizes negative charge better than sp²/sp³.
Hybridization acidity rule
C–H acidity: sp (alkyne) > sp² (alkene) > sp³ (alkane)
3.2 Quick "direction" rule
Acid–base reactions favor the side with the weaker acid (higher pKa).
tip
On exams, you rarely need exact pKa values—just relative stabilization logic.
4) Stereochemistry (R/S + Reactions That Change It)
4.1 Chirality quick test
- sp³ carbon with 4 different substituents → chiral center likely
- Look for symmetry planes (kills chirality)
Example: 2-butanol
RDKit.js
CCC(O)C
4.2 SN2 vs SN1 stereochemistry
- SN2: backside attack → inversion
- SN1: carbocation planar → racemization tendency (often not perfectly 50/50)
info
If a question asks "what happens to stereochemistry," find the mechanism first.
5) Conformations (Newman + Cyclohexane)
5.1 Newman projections (most stable patterns)
- anti (180°) usually lowest energy
- gauche (60°) slightly higher
- eclipsed highest
Butane conformations (conceptual)
SMILES
CCCC
5.2 Cyclohexane chair rules
- Bulky groups prefer equatorial
- Ring flip swaps axial ↔ equatorial but keeps up/down
tip
When comparing chairs, count axial substituents (especially bulky ones). Fewer axial = more stable.
6) Substitution & Elimination Decision Table (HIGH VALUE)
6.1 Substrate, nucleophile/base, solvent → outcome
| Condition | Likely Path |
|---|---|
| Primary + strong nucleophile | SN2 |
| Tertiary + weak nucleophile + protic solvent | SN1/E1 |
| Strong bulky base (any) | E2 (often Hofmann) |
| Strong base + heat | elimination favored |
6.2 SN1/SN2/E1/E2 mini-cheatsheet
| SN1 | SN2 | E1 | E2 | |
|---|---|---|---|---|
| Steps | 2 | 1 | 2 | 1 |
| Key intermediate | carbocation | none | carbocation | none |
| Needs strong nucleophile | no | yes | no | base yes |
| Stereochem | racemization | inversion | n/a | anti-periplanar |
| Favored substrate | 3° | 1° (best) | 3° | 2°/3° |
warning
E2 requires anti-periplanar geometry: in cyclohexanes that means "trans-diaxial" leaving group + β-H.
7) Alkene Reactions (Pattern Recognition)
7.1 Markovnikov vs Anti-Markovnikov
- Markovnikov: H adds to the carbon that already has more H's.
- Anti-Markovnikov: special conditions (radical/peroxides for HBr; hydroboration-oxidation for hydration)
7.2 Core additions (what to expect)
| Reaction | Key outcome | Stereochem notes |
|---|---|---|
| H₂ (catalyst) | alkene → alkane | syn addition |
| X₂ | vicinal dihalide | anti addition (via halonium) |
| HX | alkyl halide | Markovnikov (usually) |
| Hydration (acid) | alcohol | Markovnikov; rearrangements possible |
Example alkene
SMILES
CC=CC
8) Carbonyl Chemistry (Orgo 1 Essentials)
8.1 Carbonyl reactivity summary
- Carbonyl carbon is electrophilic
- Oxygen is basic/nucleophilic (can be protonated)
- Nucleophiles add to carbonyl carbon
8.2 Aldehyde vs ketone
- Aldehydes: more reactive (less steric, less electron donation)
- Ketones: less reactive (more substituted)
Aldehyde
SMILES
CC=O
Ketone
SMILES
CC(=O)C
9) Spectroscopy (Open-Note Exam Power Section)
9.1 IR quick peaks
| Approx. (cm⁻¹) | Group |
|---|---|
| ~3300 broad | O–H (alcohol) |
| ~3300 sharp | N–H |
| ~1700 strong | C=O |
| ~2100–2260 | C≡C / C≡N |
tip
If you see a strong ~1700 peak, look for carbonyl-containing groups first.
9.2 ¹H NMR quick rules
- # signals = # unique proton environments
- splitting = n + 1 (neighbors)
- integration = relative # of protons
Ethyl group signature
CH₃ next to CH₂ → triplet + quartet pattern often
SMILES
CCO
10) Mechanism "Templates" (Arrow-Pushing Friendly)
This section is intentionally written so you can later render curved arrows using mapped SMILES, atom indices, and your mechanism overlay system.
10.1 SN2 Template
Logic
- Nucleophile attacks electrophilic carbon (backside)
- Leaving group departs (same step)
- Stereochemistry: inversion
Diagram
Reaction schematic
CCBr | strong nucleophile → CCO
10.2 SN1 Template
Logic
- Leaving group leaves → carbocation forms (rate-determining)
- Nucleophile attacks planar carbocation
- Proton transfer if needed
Reaction schematic
CC(C)(C)Br | weak nucleophile + protic solvent → CC(C)(C)O
10.3 E2 Template
Logic
- Strong base abstracts β-H while leaving group leaves
- Requires anti-periplanar arrangement
Reaction schematic
CC(C)CBr | strong base + heat → CC=CC
warning
"If a question asks "major product," check Zaitsev vs Hofmann AND stereochemistry constraints (anti-periplanar)."
11) "Exam Flowcharts" (Fast Decision Rules)
11.1 If you see an alkyl halide…
- Identify substrate: 1°, 2°, 3°
- Identify reagent: strong nuc? strong base? bulky base?
- Identify solvent: protic vs aprotic
- Decide: SN1/SN2/E1/E2
- Apply stereochem rule if needed
11.2 If you see an alkene…
- Identify reagent class:
- hydrogenation
- halogenation
- hydrohalogenation
- hydration
- Decide Markovnikov vs anti-Markovnikov
- Decide syn vs anti addition if relevant
"Most Common Mistakes" Checklist
- Forgetting carbocation rearrangements in SN1/E1 or acid hydration
- Treating E2 like it can happen without anti-periplanar geometry
- Mixing up strong base vs strong nucleophile
- Assigning R/S without putting the lowest priority group back
- Counting NMR neighbors incorrectly through heteroatoms or across π systems
Appendix A — Structure Snippets You Can Reuse
Common substrates (copy/paste)
- Ethanol:
CCO - 2-Butene:
CC=CC - Acetone:
CC(=O)C - Acetaldehyde:
CC=O - tert-Butyl bromide:
CC(C)(C)Br
CCO
CC=CC
CC(=O)C
CC=O
CC(C)(C)Br