Chemistry, the high-yield way

Organic Reaction Mechanisms High-yield

• SN2 reactions occur in one concerted step: the nucleophile attacks from the back, causing inversion of configuration ('Walden inversion') — favoured by primary substrates, strong nucleophiles, and polar aprotic solvents.
• SN1 reactions occur in two steps via a carbocation intermediate, giving racemisation — favoured by tertiary substrates (stable carbocation) and polar protic solvents.
• Markovnikov's rule for HX addition to alkenes: H adds to the carbon that already has more hydrogens, the halide to the more substituted carbon (via the more stable carbocation).
• Anti-Markovnikov addition occurs only with HBr in the presence of peroxides (free-radical mechanism) — a classic 'exception' MCQ.
• Electrophilic aromatic substitution on benzene: nitration, halogenation, sulfonation, and Friedel–Crafts alkylation/acylation all proceed by an electrophile attacking the ring and losing H+ to restore aromaticity.
• Ring-activating groups (–OH, –NH2, –CH3) are ortho/para directors; ring-deactivating groups (–NO2, –COOH, –SO3H) are meta directors — memorise this pairing, it's tested repeatedly.
• Nucleophilic addition to carbonyls: aldehydes are generally more reactive than ketones toward nucleophiles due to less steric hindrance and less electron donation to the carbonyl carbon.
• Free-radical halogenation of alkanes proceeds via initiation (homolysis), propagation (chain reaction), and termination (radical combination) steps.
• Elimination (dehydrohalogenation) follows Zaitsev's rule: the more substituted (more stable) alkene is the major product.

Chemical Equilibrium & Kinetics High-yield

• Equilibrium constant expressions (Kc, Kp) are written as products over reactants, each raised to its stoichiometric coefficient; Kp = Kc(RT)^Δn relates the two.
• Le Chatelier's Principle: a system at equilibrium shifts to counteract any change in concentration, pressure/volume, or temperature.
• Increasing pressure shifts equilibrium toward the side with fewer gas moles; increasing temperature shifts an endothermic reaction forward (and an exothermic reaction backward).
• A catalyst speeds up both forward and reverse rates equally — it changes how fast equilibrium is reached, never where the equilibrium position lies.
• Rate law and order of reaction must be determined experimentally — they are not simply read off the balanced equation's coefficients (that's a very common misconception the exam tests).
• Molecularity (number of molecules colliding in an elementary step) is a theoretical, whole-number concept, distinct from the experimentally determined order of reaction.
• The Arrhenius equation shows rate constant k increases exponentially with temperature — a 10°C rise roughly doubles many reaction rates.
• Collision theory: a reaction only occurs when colliding particles have both sufficient energy (≥ activation energy) and the correct orientation.
• For a first-order reaction, half-life is constant and independent of initial concentration — useful for quick calculation MCQs.

s-, p- and d-Block Trends High-yield

• Across a period (left to right): atomic radius decreases, ionisation energy generally increases, electronegativity increases.
• Down a group: atomic radius increases, ionisation energy decreases, electronegativity generally decreases.
• s-block (Groups 1 & 2): alkali metals show low ionisation energy, +1 oxidation state, and reactivity that increases down the group; alkaline earth metals show +2 oxidation state and are less reactive than alkali metals.
• p-block elements show variable oxidation states and increasing metallic character down a group; heavier p-block elements (Tl, Pb, Bi) show the 'inert pair effect' — a preference for an oxidation state two less than the group valency.
• Halogens (Group 17) are strong oxidising agents; oxidising power decreases down the group (F2 > Cl2 > Br2 > I2).
• Noble gases (Group 18) have complete octets and very low reactivity, though heavier ones (Xe, Kr) can form limited compounds.
• d-block (transition) elements typically show variable oxidation states, form coloured compounds (due to d–d electron transitions), act as catalysts, and form complex ions with ligands.
• Diagonal relationships (Li–Mg, Be–Al, B–Si) — elements diagonally adjacent show unusually similar chemical properties despite being in different groups; a frequently tested exception to normal periodic trends.

Electrochemistry High-yield

• Oxidation is loss of electrons (occurs at the anode); reduction is gain of electrons (occurs at the cathode) — remember via 'OIL RIG' and 'AN OX, RED CAT'.
• A galvanic (voltaic) cell converts a spontaneous redox reaction into electrical energy; an electrolytic cell uses electrical energy to drive a non-spontaneous reaction.
• Standard electrode potential (E°) measures a species' tendency to be reduced; the more positive E°, the stronger the oxidising agent (and vice versa for negative E°/reducing agents).
• Cell EMF: E°cell = E°cathode − E°anode; a positive E°cell means the reaction is spontaneous as written.
• Faraday's laws of electrolysis: mass deposited/liberated at an electrode is directly proportional to the quantity of charge passed (and, for a fixed charge, to the equivalent weight of the substance).
• Corrosion (e.g. rusting of iron) is fundamentally an electrochemical oxidation process occurring in the presence of moisture and oxygen.
• Common batteries: the dry (Leclanché) cell and the lead–acid battery are classic exam examples of galvanic cells with practical applications.
• Electroplating uses electrolysis to deposit a thin metal layer onto an object (object = cathode).

Biochemistry Basics High-yield

• Carbohydrates: monosaccharides (glucose, fructose) join via glycosidic bonds to form disaccharides (sucrose, maltose, lactose) and polysaccharides (starch, glycogen — energy storage; cellulose — structural).
• Proteins are built from amino acids linked by peptide bonds; structure is organised into primary (sequence), secondary (α-helix/β-sheet), tertiary (3D folding), and quaternary (multiple subunits) levels.
• Lipids: fatty acids can be saturated (no C=C, solid at room temp) or unsaturated (has C=C, generally liquid); triglycerides store energy, phospholipids form the bilayer backbone of cell membranes.
• Nucleic acids: DNA is double-stranded and contains deoxyribose + thymine; RNA is generally single-stranded and contains ribose + uracil (replacing thymine).
• Base pairing: adenine pairs with thymine (or uracil in RNA) via 2 hydrogen bonds; guanine pairs with cytosine via 3 hydrogen bonds — purines (A, G) always pair with pyrimidines (C, T/U).
• Enzymes are biological catalysts, almost always proteins, that lower the activation energy of biochemical reactions without being consumed.
• Fat-soluble vitamins (A, D, E, K) are stored in body fat; water-soluble vitamins (B-complex, C) are not stored and need regular dietary intake — an easy, low-effort scoring area.