Organic Chemistry – Some Basic Principles and Techniques: NEET notes

1. Carbon's tetravalence and the shapes of organic molecules

NCERT § "Tetravalence of Carbon: Shapes of Organic Compounds"

• Carbon is tetravalent and forms covalent bonds using hybrid orbitals: sp³ (tetrahedral, as in CH₄), sp² (trigonal planar, as in C₂H₄) and sp (linear, as in C₂H₂).
• s character is 25% in sp³, 33% in sp² and 50% in sp; more s character holds electrons closer to the nucleus.
• An sp-hybridised carbon is therefore more electronegative than sp², which is more electronegative than sp³.
• More s character also means shorter and stronger bonds formed by that carbon.
• A π bond forms by sideways overlap of unhybridised p orbitals; the two atoms joined by it and the atoms attached to them must lie in one plane, and rotation about the double bond is restricted.
• π electrons lie above and below the bond axis and are loosely held, so multiple bonds are reactive centres attacked by electron-seeking reagents.

2. Structural representations

NCERT § "Structural Representations of Organic Compounds"

• A complete (Lewis or dash) structural formula shows every bond as a dash and may show lone pairs.
• A condensed formula omits some or all dashes and groups identical units, for example CH₃(CH₂)₆CH₃.
• In bond-line formulas carbon and hydrogen atoms are not written; each line end or bend is a carbon, and hydrogens are assumed to fill each carbon's valence.
• Heteroatoms such as O, N and Cl, and the hydrogens on them, are always written explicitly in bond-line formulas.
• Three-dimensional structures are drawn with solid wedges for bonds coming towards the viewer, dashed wedges for bonds going away, and plain lines for bonds in the plane of the paper.

3. Classification of organic compounds

NCERT § "Classification of Organic Compounds"

• Acyclic or open-chain (aliphatic) compounds have straight or branched chains, such as ethane and acetic acid.
• Alicyclic compounds have rings made only of carbon but behave like aliphatic compounds, such as cyclohexane.
• Aromatic compounds include benzenoid compounds (benzene and its derivatives) and non-benzenoid compounds such as tropone.
• Heterocyclic compounds have one or more non-carbon atoms (N, O or S) in the ring; furan, thiophene and pyridine are examples, and they can be aromatic.
• A functional group is the atom or group that gives a family of compounds its characteristic chemical properties, such as –OH, –CHO or –COOH.
• A homologous series is a family with the same functional group in which successive members differ by a –CH₂ unit; members show similar chemistry and a gradual change in physical properties.

4. IUPAC nomenclature

NCERT § "Nomenclature of Organic Compounds"

• An IUPAC name is built from a root word (length of the parent chain), a primary suffix (ane, ene, yne), a secondary suffix for the principal functional group and prefixes for substituents.
• Choose the longest carbon chain as the parent; if two chains are equally long, choose the one with more substituents.
• Number the chain so that the substituents (or the principal group, or multiple bonds) get the lowest set of locants.
• Substituent prefixes are written in alphabetical order; multiplying prefixes such as di-, tri- and tetra- are ignored when alphabetising.
• When several functional groups are present, the principal group takes the suffix; the order of preference is –COOH > –SO₃H > –COOR > –COCl > –CONH₂ > –CN > –CHO > >C=O > –OH > –NH₂ > >C=C< > –C≡C–.
• Halogens (–F, –Cl, –Br, –I), –NO₂ and –OR are always written as prefixes (fluoro, chloro, bromo, iodo, nitro, alkoxy), never as suffixes.
• Example: CH₂=CH–CH₂–CHBr–CH₃ is numbered from the end nearer the double bond, giving 4-bromopent-1-ene rather than 2-bromopent-4-ene.
• Ring compounds take the prefix cyclo-, as in cyclohexane.
• In disubstituted benzenes, 1,2-, 1,3- and 1,4- positions are also called ortho (o-), meta (m-) and para (p-).

5. Isomerism

NCERT § "Isomerism"

• Isomers have the same molecular formula but different properties because their structures differ.
• Structural isomers differ in the order in which atoms are bonded.
• Chain isomers differ in the carbon skeleton: pentane, 2-methylbutane and 2,2-dimethylpropane are all C₅H₁₂.
• Position isomers differ in the position of a substituent or functional group on the same skeleton, such as propan-1-ol and propan-2-ol.
• Functional group isomers have different functional groups, such as ethanol and methoxymethane (both C₂H₆O).
• Metamerism comes from sharing the alkyl groups differently between the two sides of the same functional group: methoxypropane and ethoxyethane are both C₄H₁₀O ethers.
• Stereoisomers have the same bonding sequence but differ in the spatial arrangement of atoms; they are classed as geometrical and optical isomers.
• When counting isomers of a formula, check each carbon skeleton and then each distinct position; C₃H₈O, for example, has two alcohols (propan-1-ol, propan-2-ol) and one ether (methoxyethane).

6. Bond cleavage and reactive intermediates

NCERT § "Fundamental Concepts in Organic Reaction Mechanism"

• In an organic reaction an attacking reagent reacts with the substrate; the bond to the carbon under attack breaks and a new one forms.
• Heterolytic cleavage leaves both bonding electrons on one fragment, giving ions; homolytic cleavage gives one electron to each fragment, forming free radicals.
• A carbocation has a carbon with only six valence electrons and a positive charge; it is sp² hybridised and trigonal planar, with an empty p orbital perpendicular to the plane.
• Carbocation stability: (CH₃)₃C⁺ (3°) > (CH₃)₂CH⁺ (2°) > CH₃CH₂⁺ (1°) > CH₃⁺, because alkyl groups release electrons by inductive and hyperconjugation effects.
• A carbanion carries a negative charge on carbon with eight valence electrons; it is sp³ hybridised and pyramidal.
• Free radicals have an unpaired electron; alkyl radicals follow the stability order tertiary > secondary > primary > methyl.
• A nucleophile (electron-pair donor, 'nucleus-seeking') attacks electron-poor centres: OH⁻, CN⁻, carbanions, and neutral molecules with lone pairs such as H₂O and R₃N.
• An electrophile (electron-seeking) attacks electron-rich centres: carbocations, and neutral species with an electron-deficient atom such as the carbonyl carbon or the carbon bonded to halogen in an alkyl halide.

7. Electron displacement effects

NCERT § "Electron Displacement Effects in Covalent Bonds"

• Inductive effect: a permanent shift of σ-bond electrons towards a more electronegative atom, polarising the chain; it weakens quickly and is usually negligible beyond three bonds.
• −I (electron-withdrawing) groups include –NO₂, –CN, –COOH, –COOR, halogens and aryloxy (–OAr); among halogens the −I effect is F > Cl > Br > I.
• +I (electron-releasing) groups are mainly alkyl groups such as –CH₃ and –C₂H₅.
• Resonance (mesomeric) effect: a permanent polarity caused by delocalisation of π electrons or lone pairs through a conjugated system.
• +R groups push electron density into the conjugated system: halogens, –OH, –OR, –OCOR, –NH₂, –NHR, –NR₂, –NHCOR.
• −R groups pull electron density out of it: –COOH, –CHO, >C=O, –CN, –NO₂.
• Electromeric effect: a temporary, complete transfer of a π-electron pair to one atom of a multiple bond, occurring only while an attacking reagent is present and disappearing when it is removed.
• +E: π electrons move to the atom to which the reagent attaches; −E: they move to the atom away from the attacking reagent.
• Hyperconjugation: delocalisation of σ electrons of a C–H bond on the carbon next to (alpha to) a carbocation or a π system; more alpha hydrogens mean more stabilisation.
• Hyperconjugation is also called no-bond resonance, and it adds to the explanation of the 3° > 2° > 1° > CH₃⁺ carbocation order and of the stability of more substituted alkenes.

8. Resonance in organic molecules

NCERT § "Resonance Structure"

• When one Lewis structure cannot describe a molecule, several contributing structures are drawn; the true molecule is their resonance hybrid.
• All six C–C bonds in benzene measure 139 pm, a value lying between a C–C single bond (154 pm) and a C=C double bond (134 pm), which shows the π electrons are delocalised.
• Contributing structures must have the same positions of nuclei and the same number of unpaired electrons.
• The more stable a contributing structure, the more it contributes: structures with more covalent bonds, octets on all atoms, less charge separation and negative charge on the more electronegative atom are favoured.
• Resonance energy is the difference between the energy of the real molecule and that of the most stable contributing structure; resonance always lowers the energy.
• In the carboxylate ion (e.g. acetate), the negative charge is spread equally over both oxygens and the two C–O bonds are identical.
• In the allyl cation the two contributing structures are equivalent, so they contribute equally and the charge is shared by the two end carbons.

9. Types of organic reactions

NCERT § "Types of Organic Reactions and Mechanisms"

• Substitution: an atom or group is replaced by another, as in the conversion of an alkyl halide to an alcohol.
• Addition: a reagent adds across a multiple bond, as in the addition of hydrogen halides to alkenes.
• Elimination: atoms or groups are removed from adjacent carbons to create a multiple bond, as in dehydrohalogenation.
• Rearrangement: atoms or groups within a molecule shift to give a structural isomer.
• Reaction mechanisms describe the sequence of steps, the intermediates and the electron movement; curved arrows show movement of an electron pair, and fish-hook (half-headed) arrows show movement of single electrons.

10. Methods of purification

NCERT § "Methods of Purification of Organic Compounds"

• Sublimation separates a solid that passes directly into vapour on heating from non-sublimable impurities.
• Crystallisation relies on the compound being much more soluble in a hot solvent than in the cold one; impurities stay in solution. Repeated crystallisation removes impurities of similar solubility.
• Simple distillation separates liquids with large boiling point differences, or a volatile liquid from non-volatile impurities.
• Fractional distillation uses a fractionating column to separate liquids whose boiling points are close; it is used to separate crude oil fractions.
• Distillation under reduced pressure lets liquids that decompose at or below their normal boiling point boil at a lower temperature; glycerol is recovered from spent-lye this way in soap manufacture.
• Steam distillation purifies substances that are volatile in steam and immiscible with water, such as aniline; the mixture boils when the sum of the vapour pressures of water and the compound equals atmospheric pressure, below 373 K.
• Differential extraction shakes an aqueous solution with an organic solvent in which the compound is more soluble and which does not mix with water; the layers are separated in a separating funnel.
• Chromatography separates components by their different affinities for a stationary phase and a moving (mobile) phase.
• Adsorption chromatography uses a solid adsorbent such as silica gel or alumina, in column chromatography or thin layer chromatography (TLC); partition chromatography, as in paper chromatography, uses water held in the paper as the stationary phase.
• Retardation factor Rf = distance moved by the substance from the baseline / distance moved by the solvent front; colourless spots are located under UV light, with iodine vapour or with a reagent such as ninhydrin for amino acids.

11. Qualitative analysis: detecting elements

NCERT § "Qualitative Analysis of Organic Compounds"

• To detect carbon and hydrogen, heat the compound with copper(II) oxide: its carbon ends up as CO₂ (lime water turns milky) and its hydrogen as water (white anhydrous copper sulphate turns blue).
• In Lassaigne's test the compound is fused with sodium metal, turning N, S and halogens into NaCN, Na₂S and NaX, which are then extracted with water as the sodium fusion extract.
• Nitrogen: iron(II) sulphate is added to the extract, which is boiled and then acidified with sulphuric acid; a Prussian blue colour, from iron(III) hexacyanoferrate(II), Fe₄[Fe(CN)₆]₃·xH₂O, confirms nitrogen.
• Sulphur: acetic acid and lead acetate give a black precipitate of PbS; sodium nitroprusside gives a violet colour.
• When both N and S are present, NaSCN forms and gives a blood-red colour with Fe³⁺ instead of Prussian blue.
• Halogens: acidify the extract with nitric acid, boil it to destroy any cyanide or sulphide, then add silver nitrate; chlorine gives a white precipitate that dissolves in ammonia, bromine a pale yellow one sparingly soluble in ammonia, and iodine a yellow one insoluble in ammonia.
• Phosphorus: heating the compound with sodium peroxide, an oxidising agent, turns its phosphorus into phosphate; after boiling with nitric acid, this gives a yellow precipitate with ammonium molybdate.

12. Quantitative analysis: estimating elements

NCERT § "Quantitative Analysis"

• Carbon and hydrogen are estimated by burning a known mass in oxygen and weighing the CO₂ (absorbed in KOH) and water (absorbed in anhydrous CaCl₂) produced.
• Dumas method: the compound is heated with copper(II) oxide in CO₂ atmosphere; the nitrogen gas released is collected over KOH solution and its volume measured.
• Kjeldahl's method: the compound is heated with concentrated sulphuric acid so that its nitrogen becomes ammonium sulphate; the ammonia released with NaOH is absorbed in a known volume of standard acid and the excess acid is titrated.
• Kjeldahl's method gives wrong results for nitro compounds, azo compounds and ring nitrogen (such as pyridine): digestion with sulphuric acid fails to turn that nitrogen into ammonium sulphate.
• Carius method for halogens: the compound is heated with fuming nitric acid and silver nitrate in a sealed tube, and the silver halide formed is weighed.
• Sulphur is estimated by heating with fuming nitric acid to form sulphuric acid, which is precipitated and weighed as barium sulphate.
• Phosphorus is converted to phosphoric acid and weighed either as ammonium phosphomolybdate or as Mg₂P₂O₇.
• Oxygen is usually found by difference: subtract the combined percentages of every other element from 100.

Must-know facts

1. s character: sp 50%, sp² 33%, sp³ 25%; electronegativity of carbon sp > sp² > sp³.
2. Principal group priority: –COOH > –SO₃H > –COOR > –COCl > –CONH₂ > –CN > –CHO > >C=O > –OH > –NH₂ > C=C > C≡C.
3. Halo, nitro and alkoxy groups are always prefixes.
4. Multiplying prefixes (di, tri) are ignored when alphabetising substituents.
5. Carbocations: sp², planar; carbanions: sp³, pyramidal.
6. Carbocation stability: 3° > 2° > 1° > CH₃⁺.
7. −I effect of halogens: F > Cl > Br > I; alkyl groups are +I.
8. Inductive effect fades rapidly and is negligible beyond about three bonds.
9. Electromeric effect is temporary and needs an attacking reagent; inductive and resonance effects are permanent.
10. Hyperconjugation = σ-π conjugation = no-bond resonance; more α-hydrogens, more stabilisation.
11. Benzene C–C bond length is 139 pm, between single and double bond lengths.
12. Steam distillation: aniline; reduced-pressure distillation: glycerol from spent-lye; fractional distillation: crude oil.
13. Rf = distance travelled by substance / distance travelled by solvent front.
14. Lassaigne: Prussian blue for N, violet with nitroprusside and black PbS for S, blood red for N + S.
15. AgCl white (soluble in NH₃), AgBr pale yellow (sparingly soluble), AgI yellow (insoluble).
16. Kjeldahl fails for nitro, azo and ring nitrogen (pyridine).
17. Carius method estimates halogens (as AgX) and sulphur (as BaSO₄).
18. Oxygen percentage is found by difference.

Common traps

Formulas

Key terms

Functional group

An atom or group that decides the characteristic reactions of a compound family.

Homologous series

Compounds with the same functional group differing successively by CH₂.

Bond-line formula

A skeletal drawing where line ends and bends are carbons and C–H hydrogens are implied.

Metamerism

Isomerism from different alkyl groups on the two sides of a functional group.

Heterolytic cleavage

Bond breaking in which one fragment keeps both electrons, producing ions.

Homolytic cleavage

Bond breaking in which each fragment keeps one electron, producing free radicals.

Carbocation

A positively charged carbon species with six valence electrons.

Nucleophile

An electron-pair donor that attacks electron-poor centres.

Electrophile

An electron-seeking species that attacks electron-rich centres.

Inductive effect

Permanent polarisation of σ bonds by an electronegative or electropositive group.

Electromeric effect

Temporary complete shift of π electrons in response to an attacking reagent.

Hyperconjugation

Delocalisation of alpha C–H σ electrons into an adjacent empty p orbital or π system.

Resonance energy

Energy by which the real molecule is more stable than its most stable contributing structure.

Sodium fusion extract

Aqueous extract of a compound fused with sodium, used to test for N, S, halogens and P.

Chromatography

Separation by differential distribution between a stationary and a mobile phase.