Organic Chemistry Hydrocarbons: Alkenes, Alkynes, and Aromatics Syllabus

Rank the acidity of a terminal alkyne, an alkene, and an alkane C–H, and explain the trend.

Alkyne ($\mathrm{p}K_a \approx 25$) > alkene ($\approx 44$) > alkane ($\approx 50$). Higher s-character of the carbanion orbital ($sp > sp^{2} > sp^{3}$) increases stability of the conjugate base and thus acidity.

What base is commonly used to deprotonate a terminal alkyne to form an acetylide, and why must it be strong?

$\ce{NaNH2}$ (sodium amide) is common; $\ce{NH3}$ has $\mathrm{p}K_a \approx 38$, well above the alkyne's ~25, so deprotonation is favorable. $\ce{n\text{-}BuLi}$ can also be used.

Give the products of adding one versus two equivalents of HX to a terminal alkyne.

One equivalent adds Markovnikov to give a vinyl (geminal-oriented) halide, e.g., 2-haloalkene. Two equivalents give a geminal dihalide (both halogens on the more substituted carbon).

What are the reagents and product for acid-catalyzed hydration of a terminal alkyne, including the key intermediate?

$\ce{H2O}$, $\ce{H2SO4}$, $\ce{HgSO4}$ give Markovnikov hydration to an enol, which tautomerizes to a methyl ketone. Terminal alkynes give ketones, not aldehydes.

How do you convert a terminal alkyne into an aldehyde by hydration?

Use hydroboration–oxidation: (1) disiamylborane or $\ce{BH3}$, then (2) $\ce{H2O2}$, $\ce{NaOH}$. This anti-Markovnikov hydration gives an enol that tautomerizes to an aldehyde.

What is keto-enol tautomerization and why is it relevant to alkyne hydration?

An enol ($\ce{C=C-OH}$) rapidly isomerizes to its more stable keto form ($\ce{C-C=O}$) by proton transfer. Alkyne hydration first yields an enol, which tautomerizes to the aldehyde or ketone.

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.

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.

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.

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.

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.

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).

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.

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).

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).

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.

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.

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.

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).

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).

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.

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$^-$).

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.

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

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

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).

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

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

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.

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 ($\ce{RCOCl}$, $\ce{AlCl3}$).

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.

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.