Alkenes & Alkynes – Chemistry Study Notes

Structure, Bonding, and Physical Properties

Unsaturated hydrocarbons such as alkenes and alkynes exhibit characteristic bonding configurations that dictate their geometry and physical behavior. An alkene contains at least one carbon-carbon double bond, consisting of one strong sigma ($\sigma$) bond formed by $sp^2$ hybridization and one weaker pi ($\pi$) bond formed by the lateral overlap of unhybridized $p$-orbitals. This results in a planar geometry around the $sp^2$ carbons with a bond angle of approximately $120^\circ$.

Alkynes feature a carbon-carbon triple bond comprised of one sigma ($\sigma$) bond and two mutually perpendicular pi ($\pi$) bonds, arising from $sp$ hybridization. The spatial arrangement around these carbons is linear with a bond angle of $180^\circ$. The presence of diffuse electron clouds above and below (or around) the internuclear axis makes both classes of compounds electron-rich, predisposing them to attack by electrophiles.

In terms of physical properties, alkenes and alkynes exhibit weak intermolecular forces, specifically London dispersion forces. Consequently, their boiling points and melting points increase regularly with molecular weight due to an increasing surface area. They are essentially insoluble in polar solvents like water but dissolve readily in non-polar organic solvents such as benzene, carbon tetrachloride, and ether. Cis-alkenes generally possess a higher dipole moment and slightly different boiling points compared to their trans-counterparts due to vector addition of bond dipoles.

Methods of Preparation

Mastering the synthetic pathways for alkenes and alkynes is vital for competitive exams. Alkenes are routinely prepared via the dehydrohalogenation of alkyl halides using alcoholic potassium hydroxide ($KOH$). This process typically follows an $E2$ mechanism, obeying Zaitsev’s rule, which states that the more substituted and more stable alkene is formed as the major product. Another reliable route is the acid-catalyzed dehydration of alcohols using concentrated sulfuric acid ($H_2SO_4$) or phosphoric acid ($H_3PO_4$), which proceeds through a carbocation intermediate and is therefore susceptible to molecular rearrangements.

For selective laboratory preparation of cis-alkenes, catalytic hydrogenation of alkynes using poisoned catalysts like Lindlar’s catalyst (palladium on calcium carbonate deactivated with quinoline or lead acetate) is employed. Conversely, trans-alkenes are synthesized via the reduction of alkynes using Birch reduction conditions, which utilize sodium metal in liquid ammonia ($Na/liquid\ NH_3$).

Alkynes are primarily synthesized through the dehydrohalogenation of vicinal or geminal dihalides using strong bases such as sodamide ($NaNH_2$) in liquid ammonia. Another industrial and laboratory-scale preparation involves the alkylation of terminal alkynes. When a terminal alkyne is treated with a strong base like sodium amide or a Grignard reagent, it forms a metal acetylide. Subsequent reaction with a primary alkyl halide yields a substituted internal alkyne, effectively lengthening the carbon chain.

Electrophilic Addition Reactions and Markovnikov’s Rule

The hallmark chemical property of alkenes and alkynes is electrophilic addition. Because the $\pi$ electrons are loosely held, they act as nucleophiles and attack approaching electrophiles. The addition of unsymmetrical reagents (such as $HBr$, $HCl$, or $H_2O$) to unsymmetrical alkenes or alkynes is governed by Markovnikov’s rule.

Markovnikov’s Rule: During the ionic addition of an unsymmetrical reagent to an unsymmetrical multiple bond, the negative part of the addendum preferentially attaches itself to the carbon atom carrying the lesser number of hydrogen atoms.

From a mechanistic standpoint, Markovnikov’s rule is explained by the stability of the intermediate carbocation. The initial electrophile ($H^+$) adds to the double bond to generate the most stable carbocation possible (tertiary > secondary > primary), which is then captured by the nucleophile (e.g., $Br^-$). However, students must remember the anti-Markovnikov addition (peroxide effect or Kharasch effect), which occurs exclusively with $HBr$ in the presence of organic peroxides. This specific reaction proceeds via a free radical chain mechanism rather than a carbocation intermediate, reversing the regioselectivity.

Alkynes undergo electrophilic addition similarly to alkenes, though they are generally less reactive toward electrophiles because their $sp$ carbons hold the $\pi$ electron cloud more tightly. Addition of one molecule of halogen or hydrogen halide gives a haloalkene or dihaloalkene; further addition can yield tetrahalides or geminal dihalides.

Ozonolysis and Acidity of Terminal Alkynes

Ozonolysis is a powerful oxidative cleavage technique used to determine the exact position of double and triple bonds in unknown organic structures. When an alkene is treated with ozone ($O_3$) followed by reductive workup with zinc dust and water ($Zn/H_2O$), the carbon-carbon double bond is completely cleaved, yielding carbonyl compounds (aldehydes or ketones). Oxidative workup using $H_2O_2$ instead of zinc further oxidizes any resulting aldehydes into carboxylic acids.

Similarly, ozonolysis of alkynes cleaves the triple bond entirely to yield diketones or carboxylic acids, depending on the reaction conditions and workup. This reaction serves as an indispensable diagnostic tool in structural elucidation problems frequently encountered in JEE and NEET exams.

A distinctive chemical property differentiating terminal alkynes from internal alkynes and alkenes is their acidity. Due to the high $s$-character (50%) of the $sp$ hybridized carbon atoms in terminal alkynes, the electrons of the $C-H$ bond are held closer to the carbon nucleus, making the hydrogen atom appreciably acidic. Consequently, terminal alkynes react readily with strong bases like sodium amide ($NaNH_2$), Grignard reagents ($RMgX$), or heavy metal ions such as ammoniacal silver nitrate ($AgNO_3$) and ammoniacal cuprous chloride ($Cu_2Cl_2$) to form insoluble metal acetylides (silver and copper acetylides), which precipitate out of solution. Internal alkynes lack acidic hydrogens and do not give positive tests with these reagents.

Key Points to Remember

• Hybridization: Alkenes are $sp^2$ ($120^\circ$, planar); alkynes are $sp$ ($180^\circ$, linear).
• Lindlar’s Catalyst: $Pd/CaCO_3$ poisoned with quinoline reduces alkynes specifically to cis-alkenes.
• Birch Reduction: $Na/liquid\ NH_3$ reduces alkynes stereospecifically to trans-alkenes.
• Markovnikov Addition: Follows carbocation stability; positive ion goes to the carbon with more hydrogens.
• Peroxide Effect: Only applies to $HBr$ in the presence of peroxides, yielding anti-Markovnikov products via free radicals.
• Acidity: Terminal alkynes possess acidic protons due to 50% $s$-character, reacting with $AgNO_3$ and $Cu_2Cl_2$.
• Ozonolysis: Cleaves unsaturated bonds to form carbonyl compounds; invaluable for locating multiple bonds.
• Zaitsev Rule: Elimination reactions preferentially yield the most substituted and stable alkene.

Quick Revision Summary

• Alkenes and alkynes feature reactive $\pi$ bonds that undergo characteristic electrophilic addition reactions.
• Zaitsev’s rule dictates elimination product distributions toward the most substituted alkene.
• Markovnikov’s rule predicts regioselectivity based on relative carbocation stability.
• Anti-Markovnikov addition occurs exclusively for $HBr$ under free-radical peroxide conditions.
• Catalytic hydrogenation strategies allow precise stereochemical control (Lindlar for cis, Birch for trans).
• Terminal alkynes exhibit unique acidity, enabling identification via metal acetylide precipitation.
• Ozonolysis provides a definitive method for locating unsaturation points by cleaving chains into carbonyl fragments.
• Chain elongation of terminal alkynes is achieved through strong base deprotonation followed by alkylation.