Alkenes and Alkynes

IUPAC nomenclature for alkenes (C=C double bond) and alkynes (C≡C triple bond) follows the same rules as alkanes with the following variations:

• Suffix -ene for alkenes, -yne for alkynes
• The main chain must contain the double/triple bond (even if it is not the longest)
• Numbering starts from the end closest to the multiple bond, which takes priority over substituents
• The position of the multiple bond is indicated by the number of the first carbon involved
• For compounds with both double and triple bonds: suffix -en-yne; the double bond takes priority in numbering

Examples: CH₂=CH-CH₃ → propene (not 1-propene, the double bond is in position 1 by default). CH₃-CH=CH-CH₃ → 2-butene (exists as cis and trans isomers). CH₃-C≡C-CH₃ → 2-butyne.

E/Z isomerism applies to C=C double bonds with different substituents on each carbon. The CIP (Cahn-Ingold-Prelog) system already described is used: Z (zusammen, together) when the two higher priority substituents are on the same side of the double bond; E (entgegen, opposite) when they are on opposite sides. The E/Z system is more general than cis/trans (e.g., when the substituents are not the same on both carbons).

Real-world applications: many biologically active compounds contain double bonds with defined E/Z isomerism. Retinal (11-cis-retinal) is the chromophore of rhodopsin in vision; the photoinduced cis-to-trans isomerization triggers the nerve signal.

The characteristic reaction of alkenes is electrophilic addition to the C=C double bond, which is electron-rich (the $pi$ bond acts as a nucleophile). The general mechanism has two stages:

1. Electrophilic attack: the electrophile (E⁺) is attracted by the $pi$ cloud of the double bond and forms a carbocation intermediate, which is more stable the more substituted it is (3° > 2° > 1°). The $pi$ bond breaks and a C—E $sigma$ bond forms.
2. Nucleophilic attack: the nucleophile (Nu⁻) attacks the carbocation, completing the addition. The product is a substituted alkane.

Markovnikov's rule: in the addition of an asymmetric reagent H—X (HCl, HBr, HI, H₂O) to an asymmetric alkene, the hydrogen bonds to the carbon of the double bond that already has more hydrogen atoms (the less substituted carbon), and X bonds to the more substituted carbon (the one that forms the more stable carbocation).

Example: propene + HBr → 2-bromopropane (Markovnikov product). The secondary carbocation CH₃—C⁺H—CH₃ is more stable than the primary one CH₃—CH₂—C⁺H₂.

Anti-Markovnikov addition: obtained in the presence of peroxides (ROOR) with HBr (peroxide effect): the mechanism becomes radical and the addition occurs with opposite regioselectivity (Br to the less substituted carbon). The peroxide effect does not work with HI and HCl.

Catalytic hydrogenation: addition of H₂ to the double bond in the presence of a metal catalyst (Pd/C, Pt, Ni). It is a syn addition (the two H atoms bond from the same side of the molecular plane). Used to saturate unsaturated fats (margarine production).

Real-world applications: Markovnikov's rule guides the synthesis of alcohols, alkyl halides, and other industrial intermediates. Catalytic hydrogenation is used in the food industry (partial hydrogenation of vegetable oils) and in fuel production (hydrotreating).

Hydration (addition of H₂O) of alkenes produces alcohols. It requires an acid catalyst (typically dilute H₂SO₄) and follows Markovnikov's rule: $$mathrm{R-CH=CH_2 + H_2O \xrightarrow{H_2SO_4} R-CH(OH)-CH_3}$$ The mechanism involves: (1) protonation of the double bond (formation of the most stable carbocation), (2) attack of water on the carbocation, (3) deprotonation to give the alcohol. Caution: carbocations can undergo rearrangements (1,2 migration of H or an alkyl group) leading to unexpected products.

Hydroboration-oxidation: a complementary method that gives anti-Markovnikov with syn addition.

1. Hydroboration: $mathrm{BH_3}$ (borane, often as THF-BH₃ complex) adds to the double bond. Boron (electrophile) bonds to the more substituted carbon, hydrogen to the less substituted one (anti-Markovnikov). It is a syn addition (B and H from the same side).
2. Oxidation: $mathrm{H_2O_2/OH^-}$ oxidizes the C—B bond, replacing boron with —OH, retaining the stereochemistry (syn). The net product is the syn addition of H—OH with anti-Markovnikov orientation.

Alkynes: electrophilic addition to alkynes is analogous but can occur once or twice. Hydration of terminal alkynes (in the presence of HgSO₄/H₂SO₄) produces ketones (keto-enol tautomerism). Hydration of acetylene gives acetaldehyde (Kutscherov reaction).

Real-world applications: hydroboration-oxidation (discovered by H.C. Brown, Nobel 1979) is a stereospecific and regioselective method for synthesizing alcohols with anti-Markovnikov orientation. Hydration of acetylene produces acetaldehyde (industrial intermediate for acetic acid, solvents).

Dienes are compounds with two C=C double bonds. If the double bonds are separated by a single bond (C=C—C=C), they are called conjugated, and exhibit special properties due to electron delocalization.

Resonance stabilization: the conjugated system of 1,3-butadiene (CH₂=CH—CH=CH₂) has its $pi$ electrons delocalized over all four carbons. The length of the central C₂—C₃ bond is intermediate between single and double (≈ 147 pm vs 154 pm for a pure single bond), indicating partial double bond character.

Electrophilic addition to conjugated dienes: the attack of one equivalent of HBr on 1,3-butadiene produces a mixture of two isomers:

• 1,2-Addition: Br bonds to C₂ (adjacent to the carbocation) → direct addition product
• 1,4-Addition: Br bonds to C₄, with the double bond forming between C₂ and C₃ (conjugate addition product). The intermediate is an allylic carbocation stabilized by resonance, with positive charge delocalized between C₂ and C₄.

The proportion between 1,2 and 1,4 addition depends on temperature:

• Low temperature (−80°C): the 1,2 product prevails (kinetic control, the primary carbocation is more reactive)
• Room temperature or higher: the 1,4 product prevails (thermodynamic control, the 1,4 product is more stable due to the more substituted internal double bond)

Diels-Alder: one of the most important reactions in organic synthesis, it is a [4+2] cycloaddition between a conjugated diene and a dienophile (substituted alkene or alkyne). Forms a substituted cyclohexene with high stereospecificity (syn addition). Concerted reaction (no intermediates), allows construction of 6-membered rings in a single step.

Real-world applications: the Diels-Alder reaction is fundamental in the synthesis of steroids, alkaloids, and complex drugs (e.g., total synthesis of cholesterol and vitamin D). Conjugated dienes are important monomers for synthetic polymers (synthetic rubber: polybutadiene and styrene-butadiene copolymers SBR).