Organic chemistry is the chemistry of carbon compounds. CBSE Class 11 has a full chapter on the basics — structure, isomerism, nomenclature, reaction mechanisms and analysis. This is the foundation for everything in Class 12 organic.
Core Concepts
Tetravalency of carbon
Carbon has 4 valence electrons and forms 4 covalent bonds. This is the source of organic chemistry’s variety — chains, rings, branches, multiple bonds.
Why carbon and not silicon (which also has 4 valence electrons)? Carbon atoms are small enough to form strong C-C bonds (). Silicon-silicon bonds are much weaker (). Carbon also forms strong double and triple bonds; silicon does not. This is why life is carbon-based, not silicon-based.
Carbon can hybridise in three ways:
- sp3: 4 sigma bonds, tetrahedral, 109.5° (methane, ethane)
- sp2: 3 sigma + 1 pi, trigonal planar, 120° (ethene, benzene)
- sp: 2 sigma + 2 pi, linear, 180° (ethyne, CO2)
Count the number of sigma bonds + lone pairs around the carbon:
- 4 → sp3 (tetrahedral)
- 3 → sp2 (trigonal planar)
- 2 → sp (linear)
The number of pi bonds does not affect hybridisation — they use unhybridised p orbitals.
Functional groups
Part of a molecule responsible for its characteristic reactions. Alcohol (-OH), aldehyde (-CHO), ketone (C=O), carboxylic acid (-COOH), amine (-NH2), halide (-X), ether (-O-), ester (-COO-). Molecules with the same functional group behave similarly.
| Group | Formula | Example | IUPAC Suffix |
|---|---|---|---|
| Alcohol | -OH | Ethanol (C2H5OH) | -ol |
| Aldehyde | -CHO | Ethanal (CH3CHO) | -al |
| Ketone | C=O | Propanone (CH3COCH3) | -one |
| Carboxylic acid | -COOH | Ethanoic acid (CH3COOH) | -oic acid |
| Amine | -NH2 | Ethanamine (C2H5NH2) | -amine |
| Ether | -O- | Diethyl ether (C2H5OC2H5) | -oxy (prefix) |
| Ester | -COO- | Ethyl ethanoate | -oate |
| Halide | -X | Chloromethane (CH3Cl) | halo- (prefix) |
| Nitro | -NO2 | Nitrobenzene | nitro- (prefix) |
| Nitrile | -CN | Ethanenitrile (CH3CN) | -nitrile |
Homologous series
Series of compounds with the same functional group and differing by -CH2. Alkanes (CH4, C2H6, C3H8…), alkenes, alkynes, alcohols. Members show gradation in physical properties and similar chemical properties.
Properties that change gradually within a homologous series:
- Boiling point increases (more London forces with larger molecules)
- Melting point increases (generally)
- Density increases (approaches but stays below water for hydrocarbons)
- Solubility in water decreases (longer hydrocarbon chain is more hydrophobic)
Properties that stay the same:
- Chemical reactions (same functional group, same reactions)
- General formula (CnH2n+2 for alkanes, CnH2n for alkenes)
IUPAC nomenclature
Longest chain gives the parent. Functional groups determine the suffix. Substituents get prefixes with locants. Numbering gives lowest locants to principal groups.
The word roots for carbon chain length: meth (1), eth (2), prop (3), but (4), pent (5), hex (6), hept (7), oct (8), non (9), dec (10). These are worth memorising completely — they appear in every naming question.
Reaction mechanisms
Electrophile — electron-seeking. Nucleophile — nucleus-seeking (electron-rich). Homolytic cleavage gives radicals. Heterolytic cleavage gives ions. Arrows show electron movement.
Bond breaking:
Homolytic cleavage: (each atom gets one electron — radicals form). Happens in non-polar bonds or with UV light.
Heterolytic cleavage: (more electronegative atom takes both electrons — ions form). Happens in polar bonds in polar solvents.
Common electrophiles: H+, Cl+, NO2+, carbocations (R+), BF3, AlCl3
Common nucleophiles: OH-, CN-, NH3, H2O, R-O-, carbanions (R-)
Electrophiles have an empty orbital or a positive charge. Nucleophiles have a lone pair or a negative charge. This one rule explains which species attacks which in 90% of organic reactions.
Inductive effect
Electron-donating groups (+I effect): alkyl groups (-CH3, -C2H5). They push electrons toward the functional centre, stabilising positive charges.
Electron-withdrawing groups (-I effect): halogens, -NO2, -COOH, -CN. They pull electrons away, destabilising positive charges but stabilising negative charges.
The +I effect of alkyl groups explains why tertiary carbocations are more stable than secondary, which are more stable than primary:
Resonance
Delocalisation of electrons across multiple atoms through overlapping p orbitals. Resonance structures are not real — the actual molecule is a hybrid of all contributing structures.
Benzene is the classic example: each C-C bond is neither single nor double but has a bond order of 1.5. The electrons are delocalised over the entire ring, which makes benzene unusually stable (aromatic stability).
More resonance structures = more stable molecule. The energy difference between the most stable resonance structure and the actual hybrid is called resonance energy. Benzene has a resonance energy of about 150 kJ/mol.
Worked Examples
Longest chain with OH is propan-1-ol. A methyl group is on C2. So 2-methylpropan-1-ol. IUPAC rules applied step by step.
Substitution (one atom replaced), addition (two atoms add across a multiple bond), elimination (atoms removed to form double bond), rearrangement (internal reorganisation). Four major reaction classes.
CH3COOH has two carbons. The methyl carbon (CH3-) is bonded to 4 atoms — sp3 hybridised. The carbonyl carbon (C=O with OH) has 3 sigma bonds — sp2 hybridised. Different carbons in the same molecule can have different hybridisations.
After losing H+, the acetate anion (CH3COO-) is stabilised by resonance — the negative charge is delocalised over both oxygen atoms equally. The ethoxide anion (CH3CH2O-) has no such resonance stabilisation — the charge is localised on one oxygen. Greater stabilisation of the conjugate base means stronger acid.
Common Mistakes
Writing that all organic compounds have hydrogen. Some do not — CCl4 has no H.
Confusing electrophile and nucleophile. Electrophile likes electrons; nucleophile has electrons to donate.
Saying IUPAC name is always the longest. Locants should be as low as possible even if it means a different numbering direction.
Confusing resonance with tautomerism. Resonance structures differ only in electron distribution (same atom positions). Tautomers differ in atom positions (a proton has actually moved).
Treating +I and -I effects as the same as resonance. Inductive effect operates through sigma bonds and weakens with distance. Resonance operates through pi bonds and can be strong even over several atoms.
Exam Weightage and Revision
Organic basics (General Organic Chemistry or GOC) carries 2-3 questions in NEET and 3-5 marks in CBSE Class 11 boards. JEE gives it even more weight — 3-4 questions per paper. This chapter is the grammar of organic chemistry; every subsequent chapter builds on it.
| Question Type | NEET Frequency | JEE Frequency |
|---|---|---|
| Hybridisation identification | Most years | Every year |
| Acid/base strength comparison | Every year | Every year |
| Electrophile/nucleophile identification | Most years | Most years |
| Inductive/resonance effect | Every year | Every year |
| Carbocation stability order | Most years | Most years |
The most reliable NEET question from this chapter: “Arrange the following in order of acidity/basicity.” To answer, compare conjugate base stability using resonance and inductive effects. Carboxylic acid > phenol > alcohol > water > alkane.
Practice Questions
Q1. Arrange the following in order of decreasing acidity: ethanol, phenol, acetic acid, water.
Acetic acid > phenol > water > ethanol. Acetic acid’s conjugate base has resonance over two equivalent oxygens. Phenol’s conjugate base has resonance with the benzene ring. Water’s conjugate base (OH-) has no resonance stabilisation. Ethanol’s conjugate base (ethoxide) is destabilised by the +I effect of the ethyl group.
Q2. Identify the hybridisation of each carbon in CH2=CH-CHO.
Carbon 1 (CH2=): sp2 (3 sigma bonds). Carbon 2 (=CH-): sp2 (3 sigma bonds). Carbon 3 (-CHO): sp2 (3 sigma bonds). All three carbons are sp2 hybridised because each has one double bond (pi bond) associated with it.
Q3. Which is a stronger nucleophile — OH- or H2O? Why?
OH- is a stronger nucleophile than H2O. OH- carries a negative charge, making it more electron-rich and more willing to donate electrons to an electrophilic centre. H2O is neutral and its lone pairs are held more tightly. In general, anions are better nucleophiles than their corresponding neutral molecules.
Q4. Why is the C-Cl bond in chlorobenzene shorter than in chloroethane?
In chlorobenzene, the lone pair on chlorine participates in resonance with the benzene ring, giving the C-Cl bond partial double bond character. This shortens and strengthens the bond. In chloroethane, no such resonance is possible (only sigma bonds to an sp3 carbon), so the C-Cl bond is a pure single bond and longer.
FAQs
Why is organic chemistry called “organic”?
Historically, chemists believed organic compounds could only be made by living organisms (the “vital force” theory). Friedrich Wohler disproved this in 1828 by synthesising urea (an organic compound) from ammonium cyanate (an inorganic salt) in the lab. The name stuck even after the theory was disproved.
How many organic compounds exist?
Over 100 million are known, compared to about 2 million inorganic compounds. Carbon’s ability to form four strong bonds and chain with itself endlessly explains this extraordinary diversity.
What is the difference between structural formula and molecular formula?
Molecular formula shows only the number of each type of atom (C2H6O). Structural formula shows how the atoms are connected (CH3-CH2-OH vs CH3-O-CH3). Two compounds can have the same molecular formula but different structural formulae — these are isomers.
Memorise eight functional groups and their IUPAC suffixes. Covers most NEET naming questions.
Organic basics is the grammar of organic chemistry. Learn the rules once, and the rest of the subject becomes vocabulary and practice.