Chemistry Concept Notes for JEE: Organic Chemistry
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Reaction Intermediates
| Intermediate | Electrons on carbon | Shape | Stability order |
|---|---|---|---|
| Carbocation | 6 | Planar, sp2 | Allyl, benzyl and triphenylmethyl are the most stable; then 3 degrees > 2 degrees > 1 degree > methyl |
| Carbanion | 8 (a lone pair) | Pyramidal, sp3 | Methyl > 1 degree > 2 degree > 3 degree; benzyl and carbanions next to C=O or \( \ce{NO2} \) are strongly stabilised; more s-character stabilises |
| Free radical | 7 | Nearly planar | 3 degrees > 2 degrees > 1 degree > methyl; allyl and benzyl are very stable; radicals are paramagnetic |
| Carbene | 6 | Bent or linear | Neutral and very reactive; inserts into C-H bonds |
Bond fission: homolytic fission gives free radicals (favoured by light, heat and peroxides in non-polar media). Heterolytic fission gives ions (favoured by polar solvents and polarised bonds). The shared pair goes to the more electronegative atom.
Reagents
1. Electrophiles are electron-pair acceptors: \( \ce{H^+} \), \( \ce{NO2^+} \), \( \ce{R^+} \), \( \ce{SO3} \), \( \ce{BF3} \), \( \ce{AlCl3} \), \( \ce{BCl3} \), carbenes.
2. Nucleophiles are electron-pair donors: \( \ce{OH^-} \), \( \ce{RO^-} \), \( \ce{CN^-} \), \( \ce{NH3} \), \( \ce{H2O} \), \( \ce{R^-} \) (as in Grignard reagents). Nucleophilicity falls across a period as electronegativity rises and rises down a group in protic solvents, I^- > Br^- > Cl^-.
3. Ambident species such as \( \ce{CN^-} \) and \( \ce{NO2^-} \) can attack through two different atoms.
Types of Reaction
| Reaction | Typical substrate and reagent | Key points |
|---|---|---|
| Free radical substitution | Alkanes with \( \ce{Cl2} \) or \( \ce{Br2} \) and light | Initiation, propagation, termination. Bromine is more selective: 3 degrees > 2 degrees > 1 degree |
| SN1 | Tertiary halides in polar protic solvents | Two steps; planar carbocation; racemisation; rate depends only on the substrate |
| SN2 | Primary and methyl halides with strong nucleophiles | One step; backside attack; inversion of configuration; rate depends on both reactants |
| Electrophilic aromatic substitution | Benzene with \( \ce{NO2^+} \), \( \ce{SO3} \), \( \ce{R^+} \) or \( \ce{RCO^+} \) | Activating groups (\( \ce{OH} \), \( \ce{NH2} \), \( \ce{OR} \), alkyl) direct ortho and para; \( \ce{NO2} \), \( \ce{CN} \), \( \ce{CHO} \), \( \ce{COOH} \) direct meta; halogens deactivate but direct ortho and para |
| Electrophilic addition | Alkenes with HX, \( \ce{X2} \) | Markovnikov addition through the more stable carbocation; with peroxide HBr adds by a radical path (anti-Markovnikov) |
| Nucleophilic addition | Aldehydes and ketones with HCN, \( \ce{NaHSO3} \), Grignard reagents | Reactivity: HCHO > RCHO > ketones, since alkyl groups donate and crowd the carbonyl |
| Elimination (E1, E2) | Alkyl halides with alcoholic KOH | Saytzeff's rule: the more substituted alkene is the major product |
Friedel-Crafts reactions need a Lewis acid such as \( \ce{AlCl3} \) to generate the electrophile, so \( \ce{NaCl} \) cannot be used. Aryl and vinyl halides resist nucleophilic substitution because the C-X bond has partial double-bond character and the carbon is sp2. A tertiary alcohol dehydrates most easily because it gives the most stable carbocation.
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Organic Chemistry Concepts and Summary for JEE Chemistry
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