Alkanes, Alkenes, and Alkynes

Hydrocarbon Structure

Alkanes are saturated hydrocarbons with the general formula CnH2n+2. Every carbon is sp3 hybridized with tetrahedral geometry (109.5 degree bond angles). Alkanes adopt staggered conformations to minimize torsional strain; the energy difference between staggered and eclipsed ethane is ~12 kJ/mol (~3 kcal/mol). Cycloalkanes introduce angle strain (Baeyer strain) when bond angles deviate from 109.5 degrees: cyclopropane (60 degrees) and cyclobutane (~88 degrees) are highly strained; cyclopentane and cyclohexane are nearly strain-free. Cyclohexane adopts the chair conformation, which eliminates all angle and torsional strain; substituents occupy axial (perpendicular to ring plane, 1,3-diaxial interactions) or equatorial (around ring perimeter) positions, with equatorial being more stable for bulky groups. Alkenes contain at least one C=C double bond (general formula CnH2n). Each sp2 carbon is trigonal planar (120 degrees) with one unhybridized p orbital forming the pi bond. The pi bond is weaker than the sigma bond (~264 kJ/mol vs. ~350 kJ/mol for C=C) and is the site of reactivity. Alkenes exhibit geometric isomerism (cis/trans or E/Z) because the pi bond prevents rotation. The E/Z system assigns priority based on Cahn-Ingold-Prelog rules: higher atomic number = higher priority. Alkynes contain a C triple bond C (general formula CnH2n-2). Each sp carbon is linear (180 degrees) with two perpendicular pi bonds. Terminal alkynes (R-C triple bond C-H) are weakly acidic (pKa ~ 25) and can be deprotonated by strong bases (NaNH2, n-BuLi) to form acetylide nucleophiles.

Radical Reactions

Alkanes are relatively unreactive because they lack polar bonds and pi electrons. Their primary MCAT reaction is radical halogenation: alkane + X2 -> alkyl halide + HX, initiated by heat (delta) or light (h-nu). The mechanism has three phases: (1) Initiation: homolytic cleavage of X2 by light/heat generates two halogen radicals (X dot). (2) Propagation: a halogen radical abstracts a hydrogen from the alkane, forming HX and an alkyl radical, which then attacks another X2 molecule to form the alkyl halide product and a new halogen radical, sustaining the chain. (3) Termination: any two radicals combine to form a stable molecule. Selectivity follows radical stability: tertiary > secondary > primary > methyl, because the more-substituted radical is stabilized by hyperconjugation. For bromination, selectivity is high (Br dot is less reactive, more selective): tertiary:secondary:primary is approx 1600:80:1. For chlorination, selectivity is lower (Cl dot is more reactive) ~5:4:1. Allylic and benzylic radicals are especially stable due to resonance delocalization, making allylic/benzylic positions the preferred sites for radical halogenation; a critical MCAT distinction. NBS (N-bromosuccinimide) is used for selective allylic/benzylic bromination because it maintains a low, steady concentration of Br2.

Alkene Addition Reactions

Alkenes undergo electrophilic addition across the pi bond. The general mechanism: the pi electrons act as a nucleophile, attacking the electrophile (E+) to form a carbocation intermediate; a nucleophile (Nu-) then adds to the carbocation. Markovnikov's rule states that in addition of HX to an unsymmetrical alkene, the hydrogen adds to the less-substituted carbon (the carbon with more hydrogens), placing the carbocation on the more-substituted carbon; this is driven by carbocation stability (3 deg > 2 deg > 1 deg > methyl). Key addition reactions: (a) Hydrohalogenation (HX): H+ adds first to form the most stable carbocation, then X- adds. Rearrangements (hydride or alkyl shifts) occur if a more stable carbocation is accessible. (b) Acid-catalyzed hydration (H2O/H+): Markovnikov addition of H2O to form an alcohol. (c) Oxymercuration-demercuration: Hg(OAc)2/H2O then NaBH4; achieves Markovnikov hydration WITHOUT rearrangement because the mercurinium ion intermediate prevents carbocation formation. (d) Hydroboration-oxidation: BH3-THF then H2O2/NaOH; delivers ANTI-Markovnikov addition of H and OH across a double bond (the boron adds to the less-substituted carbon because both steric and electronic factors favor it). Syn addition; no rearrangements. (e) Halogenation (X2): Br2 or Cl2 adds anti across the double bond via a cyclic halonium ion intermediate. In water, halohydrin formation occurs (Br2/H2O) with OH adding to the more-substituted carbon (Markovnikov-like, driven by the partial positive charge on the more-substituted halonium carbon). (f) Carbene addition: CH2I2/Zn(Cu) (Simmons-Smith) adds a CH2 group syn to form cyclopropane.

Alkynes

Alkynes undergo the same electrophilic addition reactions as alkenes but with two equivalents of reagent (one per pi bond). Key reactions: (a) Hydrohalogenation: 2 HX adds to give a geminal dihalide; with 1 equivalent, Markovnikov addition gives a vinyl halide. (b) Hydration: HgSO4/H2SO4/H2O catalyzes Markovnikov addition of water to an alkyne; the initial enol tautomerizes to a ketone (for terminal alkynes, a methyl ketone; for internal alkynes, the more-substituted enol becomes the more-substituted ketone). Hydroboration-oxidation of terminal alkynes gives an anti-Markovnikov enol that tautomerizes to an aldehyde. (c) Halogenation: 2 X2 adds to give a tetrahalide. (d) Acidity of terminal alkynes: RC triple bond CH has pKa ~ 25, deprotonated by NaNH2 or n-BuLi to form RC triple bond C- (acetylide). Acetylides are strong nucleophiles that undergo SN2 reactions with primary alkyl halides to form new C-C bonds (longer-chain alkynes); a reaction unique to terminal alkynes. Internal alkynes lack this acidic proton.

Hydrogenation

Hydrogenation adds H2 across pi bonds. For alkenes: H2 + metal catalyst (Pt, Pd, Ni, or Pd/C) yields the corresponding alkane via syn addition (both hydrogens add to the same face). The reaction is exothermic; the heat of hydrogenation measures alkene stability: less-substituted alkenes release less heat and are more stable (hyperconjugation). Stability order: tetrasubstituted > trisubstituted > disubstituted (trans > cis) > monosubstituted. For alkynes, catalytic hydrogenation is stepwise: alkyne -> alkene -> alkane. Two equivalents of H2 with Pt/Pd/Ni fully reduce to alkane. With a poisoned (deactivated) catalyst (Lindlar's catalyst: Pd/CaCO3 with Pb(OAc)2 or quinoline), hydrogenation stops at the alkene, giving syn addition to yield the cis (Z) alkene. Dissolving metal reduction (Na or Li in liquid NH3) also reduces alkynes to alkenes but with anti addition, yielding the trans (E) alkene; a critical MCAT distinction from Lindlar. The mechanism involves a radical anion intermediate that adopts the most stable (anti) conformation.

Oxidation and Reduction

Oxidation increases the oxygen content or oxidation state of carbon; reduction increases hydrogen content. Key alkene oxidations: (a) Syn dihydroxylation: OsO4 (catalytic with NMO co-oxidant) or cold, dilute KMnO4 adds two OH groups to the same face, yielding a cis-1,2-diol. (b) Anti dihydroxylation: epoxidation (mCPBA or peroxyacid) followed by acid-catalyzed ring opening with H2O yields a trans-1,2-diol. (c) Oxidative cleavage: hot, concentrated KMnO4 or ozonolysis (O3 then Zn/H2O or Me2S) cleaves the C=C bond, converting each sp2 carbon to a carbonyl. Ozonolysis with reductive workup (Zn or Me2S) gives aldehydes/ketones; with oxidative workup (H2O2) gives carboxylic acids/ketones. These reactions are powerful structure-determination tools: the carbonyl products reveal the position and substitution pattern of the original double bond. Alkyne oxidation: KMnO4 cleaves the triple bond to give carboxylic acids (terminal alkynes give CO2 + carboxylic acid). Alkane oxidation: combustion (complete oxidation to CO2 + H2O) is thermodynamically favorable but kinetically slow without ignition; the MCAT rarely tests alkane oxidation beyond combustion stoichiometry.

Markovnikov Orientation

Markovnikov's rule is the single most-tested reactivity principle for alkenes and alkynes on the MCAT. Statement: In the addition of HX to an unsymmetrical alkene, the hydrogen adds to the carbon with the greater number of hydrogens (less-substituted carbon), while X adds to the more-substituted carbon. The mechanistic basis is carbocation stability: the pathway that forms the more stable carbocation intermediate is lower in energy and dominates. Carbocation stability order (3 deg > 2 deg > 1 deg > methyl) is explained by hyperconjugation: neighboring C-H and C-C sigma bonds donate electron density into the empty p orbital. The more alkyl groups attached, the more hyperconjugative stabilization. Rearrangements (1,2-hydride shifts and 1,2-alkyl shifts) occur when a more stable carbocation is one shift away; this is an MCAT trap. Anti-Markovnikov additions occur when the mechanism bypasses carbocation formation: hydroboration-oxidation (BH3 adds to the less-hindered carbon for both steric and electronic reasons, with the more electropositive H from B-H adding to the more-substituted carbon) and radical addition of HBr (only HBr in the presence of peroxides, ROOR) where the bromine radical adds first to the less-substituted carbon, forming the more stable radical intermediate; the peroxide effect. The MCAT frequently tests the contrast between Markovnikov HBr addition (no peroxides, ionic mechanism) and anti-Markovnikov HBr addition (with peroxides, radical mechanism).

How it works

Hydrocarbon reactivity is governed by bond order. Alkanes (all sigma bonds, sp3) are inert to polar reagents; only radical halogenation works, and then with selectivity controlled by radical stability. Alkenes (one pi bond, sp2) and alkynes (two pi bonds, sp) are electron-rich and react by electrophilic addition; the pi electrons attack E+, forming a carbocation (or bridged ion), and Nu- captures it. Markovnikov's rule follows directly from carbocation stability (3 deg > 2 deg > 1 deg). Reactions that avoid carbocations (hydroboration, oxymercuration, radical HBr, halonium ions) give non-Markovnikov or stereospecific outcomes. Alkynes do everything alkenes do, but with two pi bonds: stoichiometry control (1 vs. 2 equivalents) and tautomerization (enol -> carbonyl) are the key differences. Hydrogenation progressively saturates pi bonds; catalyst choice (Lindlar vs. dissolving metal vs. Pt/Pd/Ni) controls stereochemistry and degree of saturation. Oxidation spans a spectrum from syn dihydroxylation (adds OH, preserves the C-C bond) to oxidative cleavage (breaks the C-C bond to carbonyls). Every reaction type reappears in later topics: alcohols/epoxides (OC-005) are made from alkenes, carbonyls (OC-006) come from alkyne hydration and ozonolysis, and biological alkene chemistry follows the same electrophilic addition logic.

How it works

Hydrocarbon reactivity is governed by bond order. Alkanes (all sigma bonds, sp3) are inert to polar reagents; only radical halogenation works, and then with selectivity controlled by radical stability. Alkenes (one pi bond, sp2) and alkynes (two pi bonds, sp) are electron-rich and react by electrophilic addition; the pi electrons attack E+, forming a carbocation (or bridged ion), and Nu- captures it. Markovnikov's rule follows directly from carbocation stability (3 deg > 2 deg > 1 deg). Reactions that avoid carbocations (hydroboration, oxymercuration, radical HBr, halonium ions) give non-Markovnikov or stereospecific outcomes. Alkynes do everything alkenes do, but with two pi bonds: stoichiometry control (1 vs. 2 equivalents) and tautomerization (enol -> carbonyl) are the key differences. Hydrogenation progressively saturates pi bonds; catalyst choice (Lindlar vs. dissolving metal vs. Pt/Pd/Ni) controls stereochemistry and degree of saturation. Oxidation spans a spectrum from syn dihydroxylation (adds OH, preserves the C-C bond) to oxidative cleavage (breaks the C-C bond to carbonyls). Every reaction type reappears in later topics: alcohols/epoxides (OC-005) are made from alkenes, carbonyls (OC-006) come from alkyne hydration and ozonolysis, and biological alkene chemistry follows the same electrophilic addition logic.

Comparisons

• C/P (Alkenes to Alcohols): Hydration, oxymercuration, and hydroboration reactions directly produce alcohols; the bridge to OC-005 (Alcohols, Ethers, and Epoxides).
• C/P (Carbonyls): Alkyne hydration (HgSO4) gives ketones; ozonolysis gives aldehydes/ketones; these connect to OC-006 (Carbonyl Chemistry) where nucleophilic addition at carbonyls is the next major reactivity unit.
• C/P (Thermodynamics): Heats of hydrogenation measure alkene stability; hyperconjugation and steric effects explain stability trends; connects to GC-007 (Thermochemistry).
• C/P (Stereochemistry): Syn vs. anti addition outcomes (hydrogenation, dihydroxylation, bromination) directly test stereochemical reasoning from OC-002 (Stereochemistry).
• C/P (Mechanisms): Carbocation stability, rearrangements, and Markovnikov/anti-Markovnikov logic are the central mechanistic reasoning tested on the MCAT; built from OC-003 (Reaction Mechanisms).
• B/B (Biochemistry): Hydrogenation and oxidation of unsaturated fatty acids (partial hydrogenation produces trans fats); beta-oxidation of fatty acids mirrors alkane oxidation logic; alkene isomerization occurs in beta-oxidation of unsaturated fatty acids.

Common confusions

• Confusing hydroboration-oxidation (anti-Markovnikov, syn addition) with acid-catalyzed hydration (Markovnikov, rearrangement possible). BH3 adds to the LESS-substituted carbon; H+ adds to the LESS-substituted carbon to form the MORE-substituted carbocation.
• Forgetting that oxymercuration-demercuration gives Markovnikov addition WITHOUT rearrangement; students often incorrectly predict rearrangement products for oxymercuration.
• Mixing up Lindlar's catalyst (cis/syn alkyne to alkene) with dissolving metal reduction (trans/anti alkyne to alkene). Lindlar: Pd/CaCO3, Pb(OAc)2, Z-alkene. Na/NH3: E-alkene.
• Forgetting the peroxide effect is ONLY for HBr (not HCl or HI). HBr + ROOR -> anti-Markovnikov radical addition. HCl and HI with peroxides still give Markovnikov addition because the chain propagation steps are thermodynamically unfavorable.
• Assuming all alkyne additions proceed with 2 equivalents. With careful stoichiometry, 1 equivalent adds to give the vinyl intermediate; alkyne to alkene with Markovnikov orientation.
• Neglecting tautomerization after alkyne hydration. The enol is not the final product; it tautomerizes to the carbonyl (ketone for internal alkynes and terminal alkynes via HgSO4; aldehyde for terminal alkynes via hydroboration-oxidation).
• Overlooking carbocation rearrangements in alkene addition. If a more stable carbocation is one hydride or alkyl shift away, the rearrangement will occur; this is why oxymercuration was developed.
• Misidentifying the more-substituted carbon for Markovnikov prediction. Count the number of carbon atoms directly attached to each sp2 carbon; NOT the total number of substituent atoms.
• Confusing syn vs. anti dihydroxylation. OsO4 / cold KMnO4 = syn (cis-diol). Epoxidation then ring opening = anti (trans-diol).
• Forgetting that terminal alkynes are acidic (pKa ~ 25) and can be deprotonated by NaNH2 or n-BuLi to form acetylide nucleophiles; this is a unique reaction not available to alkenes or alkanes.

Quick review

• Alkanes: CnH2n+2, sp3, 109.5 deg. Unreactive; radical halogenation only. Selectivity: 3 deg > 2 deg > 1 deg (Br2 > Cl2 in selectivity).
• Radical halogenation mechanism: Initiation (X2 -> 2X dot), Propagation (X dot + RH -> HX + R dot; R dot + X2 -> RX + X dot), Termination (2 radicals combine).
• Alkenes: CnH2n, sp2, 120 deg. Undergo electrophilic addition. Markovnikov: H adds to less-substituted C -> carbocation on more-substituted C.
• Carbocation stability: 3 deg > 2 deg > 1 deg > methyl. Rearrangements (hydride/alkyl shifts) occur when a more stable carbocation is one shift away.
• Hydrohalogenation (HX): Markovnikov, rearrangements possible. HBr + ROOR (peroxides) -> anti-Markovnikov (radical, HBr ONLY).
• Acid-catalyzed hydration: H2O/H+ -> Markovnikov alcohol (rearrangements possible). Oxymercuration-demercuration: Markovnikov WITHOUT rearrangement.
• Hydroboration-oxidation: 1) BH3-THF, 2) H2O2/NaOH -> anti-Markovnikov alcohol, syn addition, no rearrangement.
• Halogenation: Br2 or Cl2 -> anti addition via halonium ion. Br2/H2O -> halohydrin (OH adds to more-substituted C).
• Hydrogenation: H2/Pt, Pd, Ni -> syn addition, alkane. Alkene stability: tetrasubstituted > tri > di (trans > cis) > mono. Heat of hydrogenation proportional to instability.
• Alkyne to alkene: Lindlar (Pd/CaCO3, Pb) -> Z (cis), syn addition. Na/NH3(l) -> E (trans), anti addition.
• Alkyne hydration: HgSO4/H2SO4 -> Markovnikov enol -> ketone (methyl ketone for terminal). Hydroboration-oxidation: anti-Markovnikov enol -> aldehyde (terminal).
• Terminal alkynes (R-C triple bond C-H): pKa ~ 25, deprotonated by NaNH2/n-BuLi -> acetylide (R-C triple bond C-), good nucleophile for SN2 with 1 deg alkyl halides.
• Syn dihydroxylation: OsO4/NMO or cold KMnO4 -> cis-1,2-diol. Anti: epoxidation (mCPBA) then H2O/H+ -> trans-1,2-diol.
• Oxidative cleavage: O3 then Zn/H2O (-> aldehydes/ketones) or H2O2 (-> carboxylic acids). Hot KMnO4 -> carboxylic acids/ketones. Used to determine alkene position.
• Cyclohexane chair: equatorial substituents more stable (avoid 1,3-diaxial interactions). Cis/trans isomerism in cycloalkanes.