CH 5. Alkenes & Alkynes

A. Structures and Stability

1. Unsaturation, reactive species and reaction energy

a. Structures, molecular formulas and orbital geometry

• Alkenes contain at least one carbon–carbon double bond; alkynes contain at least one carbon–carbon triple bond.
• An acyclic monoalkene has molecular formula CₙH₂ₙ; an acyclic monoalkyne has CₙH₂ₙ₋₂. These expressions do not apply to every ring-containing or multiply unsaturated structure.
• Unsaturation changes orbital geometry and enables reactions at the π bond.

• Alkene carbons are approximately sp² and trigonal planar; alkyne carbons are sp and linear. A multiple bond contains one σ bond plus one or two π bonds.

• Twisting the ends of C=C disrupts π overlap.

b. Selecting the Parent

• Choose the parent framework containing the multiple bond and number it to assign appropriate low locants to unsaturation.
• Use -ene and -yne endings with locants for the position of the double or triple bond.

c. Multiple Unsaturations

• Two double bonds are described by the suffix -diene; three by -triene.
• A molecule containing both C=C and C≡C bonds uses an en-yne name and appropriate locants.

d. Addition and Elimination

• In addition, atoms or groups add across a multiple bond and its π bond is converted to σ bonds.
• In elimination, the substrate loses atoms or groups and a π bond can form.

e. Substitution and Rearrangement

• Substitution replaces a leaving group with another group.
• Rearrangement changes the connectivity of atoms within a molecular framework without necessarily changing molecular formula.

f. Degree of unsaturation

• For a usual neutral closed-shell formula containing C, H, N and halogens, DBE = (2C + 2 + N − H − X)/2. Oxygen and divalent sulfur do not enter this count.
• A ring or double bond contributes one DBE; a triple bond contributes two. DBE constrains possibilities but does not determine a unique structure.

Feature	DBE contribution
Ring	1
C=C or C=O	1
C≡C or C≡N	2
Benzene ring	4

g. Pair-electron reactions

• A nucleophile donates an electron pair; an electrophile accepts one. A lone pair, negative charge or pi bond can provide the electron source.
• A curved arrow begins at electrons and ends where a bond or lone pair forms. It never begins at an isolated positive charge.

• An alkene π bond can act as the electron-rich partner in electrophilic addition.

h. Single-electron reactions

• Homolytic bond cleavage forms radicals. Radical mechanisms use fishhook arrows and often form chains with initiation, propagation and termination.
• In heterolysis, the entire bonding pair remains on one fragment. The resulting ions have charges that must balance the starting material.

i. Energy diagrams

• A transition state is an energy maximum along the reaction coordinate; an intermediate is a local minimum between maxima.
• The activation free energy controls rate. The free-energy difference between reactants and products controls equilibrium. A catalyst changes the pathway and rate but not the equilibrium constant.

• A transition state is not an isolable species.

j. Temperature and the Hammond idea

• Increasing temperature usually increases reaction rate. For closely related pathways, a higher barrier generally gives a slower reaction at the same temperature.
• A transition state tends to resemble the adjacent species closest to it in energy. Late, product-like radical abstraction transition states often show greater selectivity.

Quantity	Meaning
ΔG‡	Activation free energy
ΔG°	Overall free-energy change
ΔH ≈ ΣD(broken) − ΣD(formed)	Approximate bond-enthalpy estimate

k. Carbocations and radicals

• Simple carbocations are usually planar and electron deficient; alkyl substitution often stabilizes them by hyperconjugation and induction. Allylic and benzylic resonance can dominate the alkyl trend.
• A carbocation can undergo a 1,2-hydride or alkyl shift if the resulting pathway is favorable. Check connectivity as well as the final functional group.

• Protonation consumes the alkene π bond. For simple alkyl carbocations, tertiary > secondary > primary is a starting stability trend; resonance can override it. A planar carbocation can be attacked on either face and may give a stereoisomer mixture.

l. Carbanions and carbenes

• Carbanion stability depends strongly on resonance, electronegative substituents, hybridization and solvent. A simple alkyl trend cannot be applied across unlike structures.
• A carbene is a neutral divalent carbon species. Carbene and carbenoid additions can convert an alkene into a cyclopropane.

m. Chain sequence

• Light can cleave X–X to initiate radicals. A halogen radical abstracts H, then the carbon radical reacts with X₂ to form product and regenerate X•.
• Termination combines radicals without regenerating a chain carrier. Product ratios depend on both the number of available hydrogens and their relative reactivities.

n. Chlorination and bromination

• Bromination is usually more selective for the site forming the more stable radical; chlorination is less selective.
• Radical halogenation can yield mixtures and is not a universal way to obtain a single alkyl halide. Allylic bromination with NBS is treated with conjugated systems.

o. Comparing heats of hydrogenation

• Compare heats of hydrogenation only for genuinely comparable molecules and products.

B. Electrophilic Addition

1. Ionic and radical addition pathways

a. Ionic HX addition and carbocation regioselectivity

• The alkene π bond donates an electron pair to H from HX; halide then captures the carbocation to form C–X. For an unsymmetrical alkene, the favored pathway commonly forms the more stable carbocation and gives Markovnikov orientation.

• Hydride or alkyl shifts can change the carbon skeleton before capture. This stepwise carbocation pathway is generally not stereospecific.

b. HBr addition to ethene: a concrete ionic example

• In a simplified ionic addition of HBr to ethene, the π bond forms a C–H bond as the H–Br bond cleaves heterolytically.
• Bromide subsequently bonds to the carbocationic carbon, forming bromoethane.
• More complex substrates may rearrange or display other selectivity depending on reagents and conditions.

c. HBr with peroxides

• Under appropriate radical conditions, Br adds so that the more stable carbon radical forms; H abstraction from HBr completes anti-Markovnikov addition.
• The useful peroxide effect is characteristic of HBr. Do not extend it automatically to HCl or HI.

Conditions	Orientation	Intermediate
HX, ordinary ionic conditions	Usually Markovnikov	Carbocation
HBr, ROOR, radical conditions	Anti-Markovnikov	Carbon radical

d. Dihalide formation

• Br₂ or Cl₂ adds through a bridged halonium ion. Backside opening commonly gives anti addition across the former double bond.
• A halonium pathway avoids ordinary hydride and alkyl shifts. The substrate geometry controls the stereochemical relationship of products.

• For cyclic substrates, use the starting geometry and facial accessibility when assigning stereochemical relationships.

• In an achiral medium, an achiral alkene can give a racemic pair when the addition creates new stereocenters.

e. Water as nucleophile

• With Br₂/H₂O, water opens the bromonium ion, followed by deprotonation. OH normally appears at the more substituted carbon and Br at the other.
• The OH and halogen are anti. Formation of an enantiomeric pair depends on substrate symmetry and whether a chiral influence is present.

f. Chain growth from alkene monomers

• Alkene molecules can combine into long chains by converting π bonds into new σ bonds.
• A radical polymerization proceeds through initiation, chain propagation, and termination.
• The polymer repeat unit retains the carbon framework of the monomer; polyethylene has the repeat unit –CH2–CH2–.
• The average polymer length and properties depend on the initiation, propagation, transfer, and termination processes.

C. Hydration and Hydroboration

1. Regioselectivity and hydration mechanisms

a. Addition of water

• Dilute aqueous acid hydrates alkenes by protonation, water attack, and deprotonation.
• Regioselectivity generally corresponds to Markovnikov addition, with OH at the more substituted former alkene carbon.
• Since a carbocation may form, hydride or alkyl shifts can compete in susceptible substrates.
• The hydration/dehydration equilibrium depends on conditions and substrate.

b. Acid and oxymercuration

• Dilute aqueous acid gives Markovnikov hydration through protonation, water attack and deprotonation; rearrangements may occur.
• Oxymercuration–demercuration uses Hg(OAc)₂/H₂O then NaBH₄ to give Markovnikov alcohols without typical free-carbocation rearrangements. Overall stereochemistry is not reliably a simple anti rule.

c. Hydroboration–oxidation: syn anti-Markovnikov hydration

• BH₃, commonly supplied in THF, adds H and B across C=C in a concerted syn process. B generally attaches to the less substituted carbon.

• Subsequent H₂O₂/OH⁻ oxidation replaces C–B with C–OH with retention at that carbon. The overall hydration is syn and anti-Markovnikov, without a freely rearranging carbocation.

Method	OH position	Rearrangement
H₃O⁺	More substituted	Possible
Hg(OAc)₂/H₂O; NaBH₄	More substituted	Normally absent
BH₃; H₂O₂/OH⁻	Less substituted	Absent

• Track the carbon skeleton and both possible faces when a new stereocenter can form.

D. Oxidation and Reduction

1. Hydrogenation, oxidation and alkyne transformations

a. Reduction of carbon–carbon double bonds

• H2 in the presence of Pt, Pd, or Ni converts many C=C bonds into C–C single bonds.
• Hydrogen is transferred from the metal surface and the net addition to an alkene is syn.
• Catalytic hydrogenation decreases unsaturation without changing the number of carbon atoms.
• When a cyclic alkene is reduced, approach to the two faces may be influenced by steric accessibility.

b. Epoxides, diols, and oxidative cleavage

• Peroxyacids such as mCPBA convert alkenes to three-membered epoxide rings in a concerted oxygen-transfer step.
• Syn dihydroxylation can produce a vicinal diol with two OH groups added to the same alkene face.
• Ozonolysis cleaves the C=C bond to carbonyl compounds under reductive workup.
• The oxidative workup of ozonolysis can convert aldehyde products into carboxylic acids.

c. Terminal alkynes as carbon nucleophiles

• Terminal alkynes are more acidic than comparable alkenes or alkanes because the conjugate base has negative charge in an sp orbital.
• A sufficiently strong base such as NaNH2 converts RC≡CH into an acetylide anion, RC≡C−.
• Acetylide anions form C–C bonds by SN2 attack on methyl and many primary alkyl halides.
• Secondary and tertiary halides are poor substrates for acetylide SN2 alkylation because competing elimination or steric obstruction dominates.
• Alkylation extends the carbon framework; check how many new carbon atoms are added.

Reagent	Net result	Regiochemistry
HBr	Alkyl bromide	Often Markovnikov
H3O+	Alcohol	Markovnikov
BH3; H2O2/OH−	Alcohol	Anti-Markovnikov
Br2	Vicinal dibromide	Anti addition
Br2/H2O	Halohydrin	OH at more substituted carbon
H2/Pd	Alkane	Syn reduction

Reagent	Net result	Selectivity
H2/Lindlar	cis alkene	Syn partial reduction
Na/NH3(l)	trans alkene	Anti partial reduction
HgSO4/H2SO4/H2O	Ketone	Terminal (except acetylene) → methyl ketone; acetylene → ethanal
9-BBN; H2O2/OH−	Aldehyde	Terminal → aldehyde
NaNH2; primary R–X	Higher alkyne	C–C bond formation

d. Carbene and carbenoid cyclopropanation

• A suitable carbene or carbenoid can add across an alkene to form a cyclopropane. Simmons–Smith conditions use CH₂I₂ and a zinc reagent and retain the alkene’s relative stereochemistry.

e. Epoxide and diol formation

• A peroxyacid such as mCPBA transfers oxygen in a concerted epoxidation; the alkene’s relative geometry is retained in the epoxide.
• OsO₄ followed by suitable workup gives a syn vicinal diol. Epoxidation followed by aqueous acid opening gives an anti diol overall.

f. Ozonolysis

• O₃ followed by reductive workup, such as dimethyl sulfide, cleaves a C=C into carbonyl fragments. A carbon bearing H becomes an aldehyde; a carbon with two carbon substituents becomes a ketone.
• An oxidative workup can oxidize aldehyde fragments further to acids. Always state the workup before assigning final products.

• Count carbon atoms in both carbonyl fragments to confirm conservation of the original alkene carbon skeleton.

g. Permanganate

• Cold dilute permanganate can give syn dihydroxylation; hot vigorous oxidative conditions can cleave the double bond.
• Under vigorous cleavage, an alkene carbon with H gives a carboxylic acid; a fully substituted alkene carbon gives a ketone; a terminal CH₂ carbon can be oxidized to CO₂.

h. Partial reduction

• H₂/Lindlar catalyst stops at the cis alkene for appropriate internal alkynes. Na in liquid NH₃ gives the trans alkene by dissolving-metal reduction.
• Excess H₂ with an active metal catalyst normally reduces the alkyne all the way to an alkane. Terminal alkene products do not have cis/trans geometry.

• Lindlar is a poisoned palladium catalyst; Na or Li in liquid ammonia can provide dissolving-metal partial reduction of suitable internal alkynes.

i. HX and X₂

• One equivalent of HX gives a vinyl halide; a second can give a geminal dihalide under ordinary Markovnikov conditions.
• One equivalent of X₂ gives a dihaloalkene, often with anti addition favored; excess X₂ can give a tetrahalide. Stoichiometry is essential.

Reagent	Typical product
H₂, Lindlar	cis alkene
Na, NH₃(l)	trans alkene
Excess H₂, Pd/Pt/Ni	Alkane
2 equivalents HX	Geminal dihalide

• For a terminal alkyne under ordinary Markovnikov conditions, X generally bonds to the internal carbon. Conditions and rates need not match those of an alkene.

j. Alkyne hydration and carbonyl products

• HgSO₄/H₂SO₄/H₂O hydrates an alkyne through an enol that tautomerizes to a carbonyl compound. A terminal alkyne other than acetylene usually gives a methyl ketone.
• A bulky borane followed by H₂O₂/OH⁻ gives the other hydration orientation; a terminal alkyne gives an aldehyde after tautomerization.

• The initially formed enol undergoes proton transfer and double-bond relocation to the keto tautomer.

• For an internal unsymmetrical alkyne, the hydration regioselectivity needs substrate-specific analysis.

k. Acetylide chemistry

• NaNH₂ can deprotonate a terminal alkyne. The acetylide adds to aldehydes/ketones or substitutes at a suitable methyl/primary halide to form a C–C bond.
• Secondary/tertiary halides usually favor elimination with acetylide. Internal alkynes lack the terminal acidic H.

E. Conjugated Dienes

1. Conjugation, allylic reactions and cycloaddition

a. Adjacent π systems

• A conjugated diene has alternating double and single bonds such as CH2=CH–CH=CH2.
• Four neighboring p orbitals permit electron delocalization over multiple atoms.
• Conjugated systems have different bond lengths and electronic behavior than isolated double bonds.
• Addition to a conjugated diene can give products from attack at the 1,2 and 1,4 positions; relative proportions depend on conditions.

b. 1,2- and 1,4-addition

• Conjugated dienes have alternating pi and sigma bonds and can form resonance-stabilized allylic intermediates. Electrophilic addition can give both 1,2- and 1,4-products.
• Kinetic and thermodynamic product preferences depend on barriers, product stability, temperature and reversibility; a product position alone does not define which control applies.
• NBS under radical conditions promotes allylic bromination while maintaining a low Br₂ concentration; resonance can lead to more than one allylic product.

c. Diels–Alder reaction

• A conjugated diene in an s-cis conformation reacts with a dienophile in a concerted [4+2] cycloaddition to make a six-membered ring.
• Relative cis/trans geometry on the dienophile is retained. Endo approach is often favored kinetically for suitable electron-withdrawing substituents, but is not an unconditional rule.

• Track the substituent positions and relative stereochemistry through the cycloaddition.