Organic Chemistry Hydrocarbons: Alkenes, Alkynes, and Aromatics Flashcards

24 sample cards from the Hydrocarbons: Alkenes, Alkynes, and Aromatics deck

Sampled from the end of the deck, so these are different cards from the ones shown on the syllabus page.

1. What reagents reduce an internal alkyne to a cis (Z) alkene?

   $\ce{H2}$ with Lindlar's catalyst (Pd poisoned with $\ce{CaCO3}$/Pb and quinoline). This partial syn hydrogenation gives the cis alkene.

2. What reagents reduce an internal alkyne to a trans (E) alkene?

   Dissolving-metal reduction: $\ce{Na}$ (or $\ce{Li}$) in liquid $\ce{NH3}$. The radical-anion mechanism delivers the two hydrogens anti, giving the trans alkene.

3. What product results from full hydrogenation of an alkyne with $\ce{H2}$ and $\ce{Pt}$ or $\ce{Pd}$?

   Complete reduction to the alkane; two equivalents of $\ce{H2}$ add across both $\pi$ bonds.

4. Describe the alkylation of an acetylide anion, including the reaction type and its limitation.

   An acetylide ($\ce{RC#C-}$) acts as a nucleophile in an $S_N2$ reaction with a primary alkyl halide to form a new C–C bond, giving an internal alkyne. Limitation: only works well with methyl/primary halides; secondary/tertiary give elimination.

5. How can a terminal alkyne be used to build a longer carbon chain? Outline the two-step sequence.

   (1) Deprotonate with $\ce{NaNH2}$ to form the acetylide; (2) react the acetylide with a primary alkyl halide ($S_N2$) to alkylate, extending the chain and forming an internal alkyne.

6. Define conjugated, isolated, and cumulated dienes with an example arrangement of double bonds.

   Conjugated: alternating double–single–double ($\ce{C=C-C=C}$). Isolated: double bonds separated by two or more $sp^{3}$ carbons ($\ce{C=C-C-C=C}$). Cumulated: adjacent double bonds sharing a carbon ($\ce{C=C=C}$, an allene).

7. Why is a conjugated diene more stable than an isolated diene?

   Delocalization of $\pi$ electrons across the four p-orbitals (resonance/conjugation) lowers the energy. This extra stabilization appears as a smaller heat of hydrogenation than expected.

8. When a conjugated diene undergoes electrophilic addition, what are the 1,2- and 1,4-addition products?

   After the electrophile forms an allylic (resonance-stabilized) cation, the nucleophile can add to the nearer carbon (1,2-addition) or to the far end of the allylic system (1,4-addition, giving a new internal double bond).

9. Which product (1,2 vs 1,4) dominates under kinetic versus thermodynamic control in diene addition?

   Kinetic control (low temperature) favors the 1,2-addition product (formed faster). Thermodynamic control (higher temperature, reversible) favors the more stable 1,4-addition product (more substituted double bond).

10. Describe the Diels–Alder reaction: what reacts, what forms, and the mechanism type.

    A conjugated diene reacts with a dienophile (alkene/alkyne) in a concerted [4+2] cycloaddition to form a cyclohexene ring. Three $\pi$ bonds convert to two new $\sigma$ bonds and one $\pi$ bond.

11. What conformation must the diene adopt for the Diels–Alder reaction, and what makes a good dienophile?

    The diene must be in the s-cis conformation. A good dienophile is electron-poor, bearing electron-withdrawing groups (e.g., $\ce{-C#N}$, $\ce{-CHO}$, $\ce{-CO2R}$) conjugated to its double bond.

12. State the stereochemical features of the Diels–Alder reaction (suprafacial nature and endo rule).

    It is stereospecific and suprafacial on both components: cis/trans relationships in reactants are preserved (syn addition). The endo product is kinetically favored due to secondary orbital overlap.

13. State Hückel's rule for aromaticity.

    A cyclic, planar, fully conjugated ring is aromatic if it contains $(4n+2)$ $\pi$ electrons, where $n$ is a non-negative integer ($2, 6, 10, \ldots$ $\pi$ electrons).

14. List the four requirements a compound must meet to be aromatic.

    (1) Cyclic; (2) planar; (3) fully conjugated (continuous ring of overlapping p-orbitals, each ring atom having a p orbital); (4) contains $(4n+2)$ $\pi$ electrons (Hückel's rule).

15. Define antiaromatic and nonaromatic compounds.

    Antiaromatic: cyclic, planar, fully conjugated but with $4n$ $\pi$ electrons — highly destabilized. Nonaromatic: not fully conjugated or not planar (e.g., an $sp^{3}$ center interrupts the ring), so aromaticity criteria don't apply.

16. Classify cyclopentadienyl anion, cyclopentadienyl cation, and cycloheptatrienyl (tropylium) cation by aromaticity.

    Cyclopentadienyl anion: aromatic (6 $\pi$ e$^-$). Cyclopentadienyl cation: antiaromatic (4 $\pi$ e$^-$). Tropylium cation: aromatic (6 $\pi$ e$^-$).

17. Why is benzene ($\ce{C6H6}$) unusually stable, and what is its resonance/aromatic description?

    Benzene has 6 delocalized $\pi$ electrons in a planar, cyclic, fully conjugated ring, satisfying Hückel's rule ($4n+2$, $n=1$). Delocalization gives it large resonance (aromatic) stabilization energy.

18. Name the common benzene-derived substituent groups: $\ce{C6H5-}$ and $\ce{C6H5CH2-}$.

    $\ce{C6H5-}$ is the phenyl group; $\ce{C6H5CH2-}$ is the benzyl group.

19. How are disubstituted benzene ring positions designated with ortho, meta, and para?

    Ortho (o-) = 1,2 (adjacent); meta (m-) = 1,3 (one carbon between); para (p-) = 1,4 (opposite).

20. Give the common names for these benzene derivatives: methylbenzene, hydroxybenzene, aminobenzene, and methoxybenzene.

    Methylbenzene = toluene; hydroxybenzene = phenol; aminobenzene = aniline; methoxybenzene = anisole.

21. Outline the general mechanism of electrophilic aromatic substitution (EAS).

    Step 1: the aromatic ring's $\pi$ electrons attack an electrophile $\ce{E+}$, forming a resonance-stabilized, non-aromatic arenium ion (sigma complex). Step 2: loss of $\ce{H+}$ from that carbon restores aromaticity, yielding the substituted arene.

22. List the five classic electrophilic aromatic substitution reactions and their key reagents.

    Halogenation ($\ce{X2}$, $\ce{FeX3}$); nitration ($\ce{HNO3}$, $\ce{H2SO4}$); sulfonation ($\ce{SO3}$, $\ce{H2SO4}$); Friedel–Crafts alkylation ($\ce{RX}$, $\ce{AlCl3}$); Friedel–Crafts acylation ($\ce{RCOCl}$, $\ce{AlCl3}$).

23. Distinguish activating/ortho-para directors from deactivating/meta directors in EAS.

    Activators (electron-donating: $\ce{-OH}$, $\ce{-NH2}$, $\ce{-OR}$, alkyl) speed up EAS and direct ortho/para. Deactivators (electron-withdrawing: $\ce{-NO2}$, $\ce{-C#N}$, $\ce{-CO2R}$, $\ce{-SO3H}$) slow EAS and direct meta. Halogens are exceptions: deactivating but ortho/para directing.

24. Why is Friedel–Crafts acylation often preferred over Friedel–Crafts alkylation?

    Acylation forms a resonance-stabilized acylium ion, so no carbocation rearrangements occur, and the product ketone deactivates the ring preventing polysubstitution. Alkylation suffers from carbocation rearrangements and over-alkylation.

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