Fundamentals
The core facts every aspirant should own — each a titled nugget with a real-world story, the concept in plain words, and a memory trick. Works even when the internet doesn't.
300 fundamentals
“Your phone battery drains faster on a hot day because reactions run quicker.”
Increasing temperature raises the rate constant k (Arrhenius: k ∝ e^(-Ea/RT)). A common rule of thumb is roughly double the rate for each 10 °C rise, though the exact factor depends on Ea.
Memory trick: ≈ double rate each +10 °C.
“A carbon chain is like a string of paperclips, each clip (atom) firmly holding the next without slipping.”
Carbon atoms form strong C–C single bonds (bond energy ~350 kJ mol⁻¹) and have four valence orbitals, allowing long chains and rings. This stability makes catenation a hallmark of organic chemistry. Exam trap: remember that catenation drops down the group; elements heavier than carbon have weaker M–M bonds and form shorter chains.
Memory trick: C = Strong, Stable, Single bond
“A smartphone’s camera module decides its photo ability, just as a functional group decides a molecule’s reactions.”
A functional group is a specific atom or group (-OH, -COOH, -NH2…) that governs the chemical behavior of the whole molecule. It defines the class and reactivity. Do not mix-up similar groups; -CHO is aldehyde, -COOH is acid.
Memory trick: Group = reaction guide
“Identical twins wearing different outfits – same DNA (formula) but different looks (structure).”
Isomers share the same molecular formula but differ in connectivity (structural) or spatial arrangement (stereoisomers). This leads to distinct physical and chemical properties. In stereochemistry, missing a chiral centre loses marks.
Memory trick: Same formula, different fashion
“Passing a basketball (electron pair) to a teammate is a nucleophile; the teammate who wants the ball is the electrophile.”
An electrophile is electron-poor and seeks a pair of electrons; a nucleophile is electron-rich and donates a pair to any electrophilic centre, not only carbon. Many bases are nucleophiles, but nucleophilicity depends on charge, solvent and hardness. Trap: don’t equate every base with a nucleophile or limit nucleophiles to carbon attacks.
Memory trick: E-electron-poor, N-electron-rich
“A blended smoothie combines fruits into one flavor, just as resonance blends several structures into one real molecule.”
Resonance delocalises π-electrons over two or more atoms, represented by contributing structures; the real molecule is a hybrid that is more stable than any single form. Keep total electrons constant across all contributors.
Memory trick: Resonance = electron sharing
“A crowded party stabilizes a guest; more alkyl groups around a carbocation spread the positive charge.”
Carbocation stability rises with alkyl substitution because hyperconjugation and inductive effects disperse the positive charge: 3° > 2° > 1° > methyl. Allylic and benzylic carbocations are even more stable than simple tertiary ones.
Memory trick: More alkyl = calmer cation
“Gasoline that runs a scooter is mainly alkanes, while ethene’s C=C double bond makes it a key plastic precursor.”
Alkanes contain only C–C single bonds (saturated) and follow CnH2n+2. Alkenes have one C=C double bond (unsaturated) with formula CnH2n. Alkynes have one C≡C triple bond with formula CnH2n-2. The suffix –ane, –ene, –yne tells the bond type; a common trap is forgetting the hydrogen count when a multiple bond is present.
Memory trick: -ane single, -ene double, -yne triple
“A fully inflated balloon (saturated hydrocarbon) has no extra space, while a deflated one (unsaturated) has a gap representing a double bond.”
Saturated hydrocarbons have only single C–C bonds, giving the maximum hydrogen count (CₙH₂ₙ₊₂). Unsaturated hydrocarbons contain one or more C=C or C≡C bonds, reducing hydrogen to CₙH₂ₙ or CₙH₂ₙ₋₂. Multiple bonds increase chemical reactivity.
Memory trick: No double = full; double/triple = missing H.
“A candle’s flame spreads faster on the thicker wick, just as H adds to the carbon already bearing more H atoms in HX addition.”
In electrophilic addition of HX to an unsymmetrical alkene, H attaches to the carbon that already has more hydrogens (the less-substituted carbon). This leaves a more substituted, more stable carbocation on the other carbon, so X ends up on that carbon. The common exam trap is to reverse the direction; remember the rule only flips in peroxide-initiated radical addition of HBr.
Memory trick: H to H-rich, X to C-rich (more substituted).
“Benzene is like a perfectly balanced round table where six friends (π electrons) share seats equally, making it stable and unwilling to let anyone sit extra (add).”
Benzene is a planar ring with six delocalised π electrons, satisfying Huckel’s 4n+2 rule (n=1). This aromatic stabilization forces electrophilic substitution rather than addition, preserving the aromatic sextet. Loss of planarity or electron count destroys aromaticity.
Memory trick: Aromatic → substitutes, not adds.
“A gas stove ignites methane (an alkane) and the flame shows carbon turning into invisible CO₂ while hydrogen becomes water droplets on the glass.”
Complete combustion of a hydrocarbon in excess O₂ gives CO₂ and H₂O with heat: CₓHᵧ + (x + y/4) O₂ → x CO₂ + (y/2) H₂O + ΔH. Incomplete combustion yields CO or soot. The O₂ amount follows from balancing the equation.
Memory trick: Fuel + O₂ → CO₂ + H₂O + energy.
“When you rinse a glass with rubbing alcohol, its –OH groups let it mix with water and grease alike.”
Alcohols are organic molecules that contain a hydroxyl (–OH) group attached to a saturated carbon atom. The polar –OH makes them good solvents for both polar and some non-polar substances. On oxidation, primary alcohols give aldehydes (and further to acids with strong oxidants), secondary give ketones, while tertiary alcohols resist oxidation.
Memory trick: OH = "Oh! It mixes" → solvent power
“Cracking an egg shows the carbonyl at the edge (aldehyde), while the middle of a sugar hides a carbonyl (ketone).”
Aldehydes contain a terminal –CHO group, so the carbonyl carbon is bonded to at least one hydrogen. Ketones have the carbonyl carbon bonded to two other carbons (C–CO–C). Because of this, aldehydes are oxidised to acids by mild reagents (Tollen’s, Fehling’s) while ketones do not.
Memory trick: Aldehyde = End, Ketone = Middle → END = E.
“The sour bite of lemon juice comes from its carboxyl group –COOH releasing H+.”
Carboxylic acids possess the –COOH group. The acidic hydrogen is released because the conjugate base –COO⁻ is resonance-stabilized, making them weak acids (pKa ≈ 4–5). They are stronger than alcohols but much weaker than mineral acids; they give a sour taste and turn blue litmus red.
Memory trick: COOH = C for Citrus sour.
“When perfume is made, an acid and an alcohol shake hands, giving a sweet-smelling ester and water.”
Esterification (Fischer esterification) is an acid-catalysed equilibrium between a carboxylic acid and an alcohol producing an ester (RCOOR′) and water. Protonation of the carbonyl makes it electrophilic, allowing nucleophilic attack by the alcohol. The reaction is reversible; removing water or using excess alcohol drives it to the right.
Memory trick: Ester = 'E' for 'Evaporate water' to push forward.
“When you sniff the sharp smell of rotting fish, you’re sensing volatile amines that behave like weak bases in water.”
Amines are organic derivatives of ammonia where one or more H atoms are replaced by carbon groups. The nitrogen keeps a lone pair, so it can accept a proton, giving basic character. Exam trap: amide nitrogens and aromatic amines are far less basic because resonance delocalises the lone pair.
Memory trick: A-M-I-N-E: A (has) MINE (lone pair) → basic.
“Glucose fuels your brain the way a car uses gasoline to run.”
Carbohydrates are polyhydroxy aldehydes (monosaccharides) or ketones that polymerise to disaccharides and polysaccharides such as starch and cellulose. They provide rapid energy because they are readily oxidised to CO₂ and H₂O. In exams, remember that cellulose is indigestible due to β-1,4 linkages, unlike starch.
Memory trick: C for CHO, Energy = C+H+O.
“Your muscles are built like LEGO bricks, each brick an amino acid linked by peptide bonds.”
Proteins are polymers of 20 amino acids joined by peptide bonds, forming a specific 3-D structure that determines function. The peptide bond forms by condensation, releasing one H₂O per link. A common trap is to confuse denaturation with hydrolysis; only hydrolysis breaks the backbone.
Memory trick: Peptide = Pea + tide (water) removed.
“When you boil an egg, the clear albumin turns opaque – the protein’s shape unravels.”
Denaturation is the disruption of a protein’s native conformation by heat, pH or chemicals. The unfolded chain may refold (reversible) or stay unfolded (irreversible) depending on the protein and conditions. In exams avoid stating denaturation is always irreversible; check if renaturation is mentioned.
Memory trick: Denature = change shape; can be ‘re-set’ or not.
“Vitamin D is like a solar panel, converting sunlight into a hormone that helps calcium absorption.”
Vitamins are essential micronutrients required in minute amounts. Fat-soluble vitamins (A, D, E, K) are stored in adipose tissue and can accumulate to toxicity, whereas water-soluble vitamins (B-complex, C) are excreted in urine and excess is lost. Remember the solubility rule in deficiency vs overdose questions.
Memory trick: FAT vitamins stay, WATER wash.
“Building a LEGO chain just adds bricks, like addition polymers; baking a cake releases steam, like condensation polymers shedding H₂O.”
Addition polymers grow by successive addition of monomers across unsaturated bonds, giving a saturated backbone with no by-product. Condensation polymers form by stepwise reactions between functional groups, each step releasing a small molecule such as H₂O or HCl. In exams, look for a leaving small molecule – its presence signals condensation; absence means addition.
Memory trick: Add = No loss; Condense = Lose H₂O/ HCl.
“A diamond-cut watch glass stays clear while a pencil lead leaves a dark streak on paper.”
Diamond’s carbon atoms form a 3D tetrahedral lattice giving a wide band-gap (~5.5 eV), so it behaves as an insulator. In graphite the atoms lie in layers with delocalised π-electrons, making it a good conductor. Other allotropes (e.g., fullerenes, amorphous carbon) can be semiconductors or insulators, so never assume all non-graphite carbon is insulating.
Memory trick: Diamond = D (dielectric), Graphite = G (good conductor).
“Lithium and magnesium both make lightweight alloys used in aircraft frames.”
Elements diagonal in the periodic table (Li-Mg, Be-Al) show similar properties because they have comparable charge density and effective nuclear charge. This similarity arises from small atomic size and high Z_eff. Exam trap: only period-2 and period-3 diagonal pairs exhibit this; don’t extend it to heavier rows.
Memory trick: Li-Mg, Be-Al: small-size, high-Z → similar