General Organic Chemistry flash cards

1. 1.What is the inductive effect?

   The permanent displacement of $\sigma$-bonding electrons along a chain of atoms due to an electronegativity difference. It is transmitted through $\sigma$ bonds, is permanent, and dies out rapidly with distance (negligible beyond 3 carbons).

   Hint: $\sigma$-electron shift; through-bond; distance-dependent

2. 2.Distinguish $-I$ and $+I$ groups with examples.

   $-I$ (electron-withdrawing): pull electrons toward themselves, e.g. $-NO_2, -CN, -COOH, -F, -Cl, -NR_3^+$. $+I$ (electron-donating): push electrons away, e.g. alkyl groups $-CH_3, -C_2H_5$, and $-COO^-, -O^-$.

   Hint: Withdraw vs release along $\sigma$

3. 3.Why does the inductive effect weaken with distance?

   Each successive $\sigma$ bond transmits only a fraction of the charge displacement (electrostatic induction diminishes). By the third carbon from the substituent the effect is negligibly small.

   Hint: Fractional relay through each bond

4. 4.Arrange $+I$ order of alkyl groups.

   $-C(CH_3)_3 > -CH(CH_3)_2 > -CH_2CH_3 > -CH_3$. More alkyl branching (more C–H/C–C bonds) means a stronger electron-releasing $+I$ effect.

   Hint: tert > iso > ethyl > methyl

5. 5.What is resonance (mesomerism)?

   The description of a molecule whose real electronic structure is a hybrid of two or more Lewis structures (canonical forms) differing only in the arrangement of $\pi$/lone-pair electrons, not of nuclei. The real molecule is more stable than any single form (resonance/delocalisation energy).

   Hint: Delocalised electrons; nuclei fixed

6. 6.List the conditions for valid resonance structures.

   (1) Same positions of all nuclei; (2) same number of unpaired electrons; (3) planar (or nearly planar) system allowing p-orbital overlap; (4) only electrons (mostly $\pi$ and lone pairs) move; (5) obey octet where possible.

   Hint: Move e$^-$, not atoms

7. 7.What features make a resonance structure more stable (major contributor)?

   More covalent bonds; complete octets on all atoms; minimal charge separation; negative charge on the more electronegative atom and positive charge on the more electropositive atom; like charges not on adjacent atoms.

   Hint: More bonds, octets, least charge separation

8. 8.Define the mesomeric effect ($M$ or $R$).

   The permanent polarisation produced by delocalisation of $\pi$ or lone-pair electrons in a conjugated system. $+M$ groups donate electrons into the system (e.g. $-OH, -NH_2, -OR, -Cl$); $-M$ groups withdraw electrons (e.g. $-NO_2, -CN, -CHO, -COOH$).

   Hint: $\pi$/lone-pair delocalisation; conjugation-dependent

9. 9.How does the mesomeric effect differ from the inductive effect?

   Inductive acts through $\sigma$ bonds, involves partial $\sigma$-electron shift, and dies with distance. Mesomeric acts through $\pi$/conjugated systems, involves complete transfer of $\pi$/lone-pair electrons, needs conjugation, and can operate over long conjugated chains without dying out.

   Hint: $\sigma$ vs $\pi$; distance-limited vs conjugation-transmitted

10. 10.Give an example where a group shows $+M$ but $-I$ simultaneously.

    Halogens ($-Cl, -Br$) and $-OH, -OR, -NH_2$: they withdraw electrons inductively ($-I$) through the $\sigma$ bond but donate lone-pair electrons into a conjugated $\pi$ system ($+M$). In phenol/aniline/chlorobenzene, $+M$ controls o/p-directing behaviour.

    Hint: Lone pair donates, but electronegative pulls

11. 11.What is hyperconjugation (Baker–Nathan / no-bond resonance)?

    Delocalisation of $\sigma$-electrons of a C–H bond (adjacent to a $\pi$ bond, positive carbon, or radical) into the empty/partly filled p orbital or $\pi$ system. It stabilises alkenes, carbocations and radicals; represented by no-bond resonance structures.

    Hint: $\sigma_{C-H}$ overlap with adjacent p/$\pi$

12. 12.How does the number of $\alpha$-hydrogens relate to hyperconjugative stability?

    More $\alpha$-H atoms (C–H bonds on carbons adjacent to the cationic/unsaturated centre) means more hyperconjugative structures and greater stabilisation. This underlies $3° > 2° > 1° > CH_3^+$ carbocation stability and Markovnikov/alkene stability trends.

    Hint: Count $\alpha$ C–H bonds

13. 13.Why is $2$-butene more stable than $1$-butene?

    $2$-butene (internal, more substituted) has more $\alpha$-hydrogens (6 hyperconjugative C–H bonds) than $1$-butene (2). Greater hyperconjugation plus more alkyl $+I$ donation to the $\pi$ system lowers its energy.

    Hint: More $\alpha$-H, more substituted alkene

14. 14.What is the electromeric effect ($E$)?

    A temporary, complete transfer of a shared $\pi$-electron pair to one of the bonded atoms, occurring only in the presence of an attacking reagent. It is instantaneous and reversible; when the reagent is removed the molecule reverts. Types: $+E$ (toward attacking reagent) and $-E$.

    Hint: Temporary $\pi$ shift on reagent attack

15. 15.How does the electromeric effect differ from the mesomeric effect?

    Electromeric is temporary and appears only when a reagent attacks (reversible); mesomeric is permanent and present in the ground state of a conjugated molecule. Both involve $\pi$-electron shift.

    Hint: Temporary/on-demand vs permanent

16. 16.Define homolytic and heterolytic bond fission.

    Homolytic: bond breaks symmetrically, each atom keeps one electron, forming free radicals ($A{-}B \rightarrow A^{\bullet} + B^{\bullet}$); favoured in nonpolar media/UV/heat. Heterolytic: bond breaks unsymmetrically, one atom takes both electrons, forming ions ($A{-}B \rightarrow A^+ + B^-$); favoured in polar media.

    Hint: One e$^-$ each vs both to one

17. 17.What is a free radical and how is it represented?

    A neutral species with an unpaired electron, formed by homolytic fission, e.g. $CH_3^{\bullet}$. It is electron-deficient (7 electrons on C), highly reactive, and shown with a single dot.

    Hint: Odd electron, single dot

18. 18.Define carbocation (carbenium ion).

    A positively charged carbon species with only 6 valence electrons, formed by heterolytic fission where carbon loses a bonding pair. The positive carbon is $sp^2$ hybridised, planar, with an empty p orbital.

    Hint: $C^+$, 6 e$^-$, $sp^2$ planar

19. 19.Define carbanion.

    A negatively charged carbon species with 8 valence electrons including a lone pair, formed by heterolytic fission where carbon keeps the bonding pair. It is $sp^3$, pyramidal, and electron-rich (nucleophilic).

    Hint: $C^-$, lone pair, pyramidal

20. 20.What is a carbene?

    A neutral divalent carbon intermediate with 6 valence electrons: two bonds plus a lone pair, e.g. $:CH_2$ (methylene). Singlet carbene has paired electrons ($sp^2$); triplet carbene has two unpaired electrons (more diradical-like).

    Hint: Neutral divalent C, $:CH_2$

21. 21.What is a nitrene?

    The nitrogen analogue of a carbene: a neutral, electron-deficient monovalent nitrogen species with 6 valence electrons, $R{-}\ddot{N}:$, an intermediate in Hofmann and Curtius rearrangements.

    Hint: N analogue of carbene

22. 22.Define electrophile and give examples.

    An electron-loving, electron-deficient reagent that accepts an electron pair (a Lewis acid). Examples: $H^+, NO_2^+, Cl^+, SO_3, BF_3, AlCl_3, {}^+CH_3, R{-}C{\equiv}O^+$, carbocations.

    Hint: Electron acceptor; Lewis acid

23. 23.Define nucleophile and give examples.

    A nucleus-loving, electron-rich reagent that donates an electron pair (a Lewis base). Examples: $OH^-, CN^-, NH_3, H_2O, RO^-, X^-, R{-}NH_2$, carbanions, alkenes.

    Hint: Electron donor; Lewis base

24. 24.Distinguish ambident nucleophiles with an example.

    Nucleophiles with two different donor atoms that can attack through either site. Example: cyanide $CN^-$ (attacks via C giving nitriles, or via N giving isocyanides); nitrite $NO_2^-$ (via N giving nitroalkanes, via O giving alkyl nitrites).

    Hint: Two attacking atoms, e.g. $CN^-$

25. 25.Classify the three broad reaction types in organic chemistry.

    (1) Substitution — an atom/group is replaced by another; (2) Addition — atoms/groups add across a multiple bond (unsaturation decreases); (3) Elimination — atoms/groups are removed to create a multiple bond (unsaturation increases). Plus rearrangements.

    Hint: Substitute, add, eliminate

26. 26.Compare $S_N1$ and $S_N2$ intermediates/mechanisms.

    $S_N1$: two steps via a planar carbocation intermediate, first-order, favoured by $3°$ substrates and polar protic solvents, gives racemisation. $S_N2$: one step, backside attack, transition state (no intermediate), second-order, favoured by $1°$ substrates and polar aprotic solvents, gives inversion.

    Hint: Carbocation/racemise vs backside/invert

27. 27.What is aromaticity and Hückel's rule?

    Aromatic compounds are cyclic, planar, fully conjugated rings with delocalised $\pi$ electrons that obey Hückel's rule: $(4n+2)\,\pi$ electrons ($n = 0,1,2,\dots$). They have extra thermodynamic stability.

    Hint: Cyclic, planar, conjugated, $(4n+2)\pi$

28. 28.State the four requirements for a compound to be aromatic.

    (1) Cyclic; (2) planar; (3) complete conjugation (each ring atom has a p orbital / continuous $\pi$ overlap); (4) $(4n+2)$ delocalised $\pi$ electrons (Hückel).

    Hint: Ring + planar + conjugated + $(4n+2)$

29. 29.Define antiaromatic and non-aromatic.

    Antiaromatic: cyclic, planar, fully conjugated but with $4n$ $\pi$ electrons — less stable than the open-chain analogue (e.g. cyclobutadiene). Non-aromatic: not cyclic, not planar, or lacking continuous conjugation (e.g. cyclohexane, 1,3-cyclohexadiene).

    Hint: $4n$ planar-conjugated vs broken conjugation

30. 30.Why is the cyclopentadienyl anion aromatic but the cation not?

    The anion has $6\,\pi$ electrons ($4n+2$, $n=1$) delocalised over five $sp^2$ carbons — aromatic. The cation has only $4\,\pi$ electrons ($4n$) — antiaromatic. Hence cyclopentadiene is unusually acidic.

    Hint: Anion $6\pi$ vs cation $4\pi$

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