Alkenes & Alkynes – Chemistry Study Notes

General Methods of Preparation

To master alkenes and alkynes for competitive exams, you must be thoroughly familiar with elimination and reduction pathways that construct multiple bonds. Alkenes are commonly prepared via the dehydrohalogenation of alkyl halides using alcoholic potassium hydroxide ($\text{KOH}$). This process follows an E2 elimination mechanism.

Another primary industrial and laboratory route is the partial catalytic hydrogenation of alkynes. Treating an alkyne with Lindlar’s catalyst (palladium deactivated with barium sulfate or quinoline) yields a *cis*-alkene. In contrast, reduction using dissolving metals like sodium in liquid ammonia ($\text{Na/liq. NH}_3$) yields a *trans*-alkene through a radical-anion intermediate.

Alkynes are typically prepared through double dehydrohalogenation reactions. When a vicinal or geminal dihalide is treated with strong bases such as sodamide ($\text{NaNH}_2$) in liquid ammonia, two successive eliminations occur to form a carbon-carbon triple bond.

Additionally, the alkylation of terminal alkynes is a vital synthetic tool for extending carbon chains. When a terminal alkyne is treated with a strong base like $\text{NaNH}_2$, it forms a metal acetylide nucleophile. This nucleophile can then perform an $\text{S}_\text{N}2$ nucleophilic substitution on primary alkyl halides to yield higher alkynes.

• Dehydrohalogenation: $\text{R-CH}_2\text{-CH}_2\text{-X} + \text{alc. KOH} \rightarrow \text{R-CH=CH}_2 + \text{KX} + \text{H}_2\text{O}$
• Birch Reduction: Alkyne + $\text{Na(s)} + \text{liq. NH}_3 \rightarrow \text{*trans*-alkene}$
• Lindlar Reduction: Alkyne + $\text{H}_2/\text{Pd-BaSO}_4 \rightarrow \text{*cis*-alkene}$
• Chain Elongation: $\text{R-C}\equiv\text{CH} + \text{NaNH}_2 \rightarrow \text{R-C}\equiv\text{C}^- \text{Na}^+ \xrightarrow{\text{R’X}} \text{R-C}\equiv\text{C-R’}$

Electrophilic Addition Reactions & Markovnikov’s Rule

The hallmark reaction of unsaturated hydrocarbons is electrophilic addition. Because the $\pi$-electron cloud of alkenes and alkynes is electron-rich and exposed above and below the sigma framework, it acts as a nucleophile attacking electrophiles.

In the addition of unsymmetrical reagents (such as $\text{HBr}$) to unsymmetrical alkenes, the regioselectivity is governed by Markovnikov’s rule. This empirical rule states that the negative part of the addendum adds preferentially to the carbon atom of the double bond possessing the lesser number of hydrogen atoms.

Mechanistically, Markovnikov’s rule is explained by the stability of the carbocation intermediate formed in the rate-determining step. The electrophile ($\text{H}^+$) attacks first to generate the most stable carbocation—tertiary being more stable than secondary, which is more stable than primary—facilitating subsequent nucleophilic attack.

However, when hydrogen bromide ($\text{HBr}$) is added to alkenes in the presence of organic peroxides (such as benzoyl peroxide), the reaction proceeds via a free-radical chain mechanism rather than an ionic one. This leads to an inversion of regiochemistry known as the anti-Markovnikov’s rule or peroxide effect.

“In any polar addition across a multiple bond, the electrophile adds to yield the most stable intermediate carbocation.”

Ozonolysis of Alkenes and Alkynes

Ozonolysis is an indispensable oxidative cleavage technique used structurally to locate the exact position of multiple bonds in unknown organic compounds. The process involves treating an alkene or alkyne with ozone ($\text{O}_3$) in an inert solvent like dichloromethane, followed by reductive workup using zinc dust and water ($\text{Zn/H}_2\text{O}$) or oxidative workup.

The reaction proceeds through an unstable cyclic intermediate known as a molozonide, which rapidly rearranges via a 1,3-dipolar cycloaddition reversal into a stable ozonide. Upon reductive cleavage of the ozonide, the carbon-carbon double bond is completely cleaved, and each former $\text{sp}^2$ carbon atom becomes bonded to an oxygen atom, forming carbonyl compounds (aldehydes or ketones).

If the cleavage involves an alkyne, the resulting cleavage products are typically $\alpha$-dicarbonyl compounds ($\text{R-CO-CO-R’}$), which can be further oxidized to carboxylic acids if oxidative workup is employed. JEE and NEET frequently test this concept via inverse problem-solving, asking students to deduce the original alkene structure from the given aldehyde or ketone fragments.

• Monosubstituted Alkene ($\text{R-CH=CH}_2$): Yields an aldehyde ($\text{R-CHO}$) and formaldehyde ($\text{HCHO}$).
• Disubstituted Alkene ($\text{R}_2\text{C=CH-R}$): Yields a ketone ($\text{R}_2\text{C=O}$) and an aldehyde ($\text{R-CHO}$).
• Alkyne Cleavage: $\text{R-C}\equiv\text{C-R’} \xrightarrow{\text{O}_3} \text{Ozonide} \xrightarrow{\text{Zn/H}_2\text{O}} \text{R-CO-CO-R’}$

Acidity of Terminal Alkynes

Unlike alkanes and alkenes, terminal alkynes exhibit a distinct weakly acidic character. This unique acidity arises from the hybridization of the carbon atoms involved in the triple bond. A sp-hybridized carbon in a terminal alkyne ($\text{R-C}\equiv\text{CH}$) has 50% s-character.

Because s-orbitals are spherical and held closer to the positively charged nucleus than p-orbitals, electrons residing in sp orbitals experience a higher effective nuclear charge and are held more tightly. Consequently, the sp-hybridized carbon possesses high electronegativity, stabilizing the negative charge of the conjugate base (the acetylide ion, $\text{R-C}\equiv\text{C}^-$) when a proton is abstracted.

Terminal alkynes can readily react with strong bases like sodamide ($\text{NaNH}_2$), Grignard reagents ($\text{RMgX}$), or ammoniacal solutions of silver nitrate and cuprous chloride. Notably, non-terminal (internal) alkynes lack an acidic hydrogen atom and consequently do not react with these metal reagents, providing a reliable laboratory method to distinguish terminal from internal alkynes.

Important Facts / Formulas

Key Points to Remember

• Markovnikov’s addition proceeds via the most stable carbocation intermediate.
• Anti-Markovnikov addition (Peroxide effect) is restricted exclusively to $\text{HBr}$ due to energetic favorability.
• Lindlar’s catalyst yields *cis*-alkenes, whereas dissolving metal reduction ($\text{Na/liq. NH}_3$) yields *trans*-alkenes.
• Ozonolysis breaks both $\sigma$ and $\pi$ components of the multiple bond, converting them into carbonyl functionalities.
• Terminal alkynes react with ammoniacal $\text{AgNO}_3$ to form white precipitates of silver acetylide.
• Internal alkynes cannot react with $\text{NaNH}_2$ or form metallic acetylides due to the absence of acidic protons.
• Baeyer’s reagent ($\text{cold alkaline KMnO}_4$) hydroxylates alkenes to form *syn*-1,2-diols with a color change from purple to brown.

Quick Revision Summary

• Alkenes and alkynes undergo characteristic electrophilic addition due to their electron-dense $\pi$-systems.
• Markovnikov’s rule dictates regioselectivity based on carbocation stability; the peroxide effect reverses this for $\text{HBr}$.
• Catalytic hydrogenation stereoselectively produces *cis* or *trans* alkenes depending on the reducing agent used.
• Terminal alkynes are acidic because of the high 50% s-character of the sp carbon atom.
• Metal acetylide formation allows terminal alkynes to undergo nucleophilic substitution to elongate carbon chains.
• Ozonolysis is a powerful diagnostic and synthetic technique for cleaving multiple bonds into carbonyl fragments.
• Distinguishing tests like ammoniacal $\text{AgNO}_3$ help separate terminal alkynes from internal alkynes and alkenes.