Exam Master Review Sheet - Alkynes

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EXAM MASTER FORMULA SHEET

Hydrocarbons: Alkynes

Triple Bond Chemistry, Acidity & Hydration Pathways

1. Structure & Acidic Nature

Unsaturated hydrocarbons containing at least one $\ce{C\equiv C}$ triple bond. The carbons are sp hybridized (50% s-character), making them highly electronegative.

Acidity Master Order

Terminal alkynes are sufficiently acidic to form salts with strong bases like $NaNH_2$.

\ce{H2O > ROH > HC\equiv CH > NH3 > CH2=CH2 > CH3-CH3}

Terminal vs Non-Terminal

Only terminal alkynes ($\ce{R-C\equiv C-H}$) possess acidic hydrogen and react with Tollens' or $\ce{Cu2Cl2}$.

Crucial for distinguishing alk-1-ynes from alk-2-ynes!

2. Methods of Preparation

1. From Calcium Carbide

Industrial method for preparing Ethyne (Acetylene).

\[ \ce{CaC2 + 2H2O -> Ca(OH)2 + C2H2} \]

2. Synthesis of Higher Alkynes

Using terminal alkynes and alkyl halides ($1^\circ$ only). $S_N2$ mechanism.

\[ \ce{HC\equiv CH ->[NaNH2] HC\equiv C^-Na+ ->[R-X] HC\equiv C-R} \]

*If $2^\circ/3^\circ$ RX is used, elimination dominates to give alkene.*

3. Dehydrohalogenation of Dihalides

Requires strong bases to remove two moles of HX from vicinal or geminal dihalides.

\[ \ce{R-CH(Br)-CH2(Br) ->[Alc. KOH][-\ce{HBr}] R-C(Br)=CH2 ->[NaNH2][-\ce{HBr}] R-C\equiv CH} \]

Note: Alc. KOH is not strong enough to remove the second HBr molecule from the less reactive vinyl halide, hence the stronger base $\ce{NaNH2}$ (sodamide) is used.

3. Addition Reactions

Alkynes undergo electrophilic addition ($Ad_E$), but are less reactive than alkenes because the intermediate vinyl cation ($\ce{R-C^+=CH2}$) is less stable than an alkyl carbocation.

1. Partial Hydrogenation

Lindlar's Catalyst ($\ce{Pd/BaSO4}$ + Quinoline):
Stereochemistry: SYN Addition.
Product: CIS-Alkene.

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Birch Reduction ($\ce{Na}$ / liq. $\ce{NH3}$):
Stereochemistry: ANTI Addition.
Product: TRANS-Alkene.

2. Addition of $\ce{X2}$ and $\ce{HX}$

Addition of $\ce{Br2/CCl4}$: Reddish-brown color discharges. Proceeds via anti-addition forming tetrahalides ($\ce{R-CBr2-CBr2-R'}$).

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Addition of $\ce{HX}$: Follows Markovnikov's rule. Forms gem-dihalides ($\ce{R-CX2-CH3}$).
($\ce{HBr}$ + peroxide gives anti-Markovnikov vicinal product).

3. Hydration of Alkynes (Highly Tested)

Kucherov Reaction (Acidic Hydration)

Reagent: $\ce{1\% HgSO4 / dil. H2SO4}$ at 333K.

Follows Markovnikov. Initial Enol tautomerizes to highly stable Keto form.

$\ce{R-C\equiv CH ->[\ce{Hg^2+, H+}] R-CO-CH3}$ (Ketones)

*Ethyne ($\ce{HC\equiv CH}$) gives Acetaldehyde.*

Hydroboration-Oxidation (HBO)

Reagent: Bulky borane (e.g., Sia$_2$BH) then $\ce{H2O2/OH^-}$.

Follows Anti-Markovnikov. Prevents double addition.

$\ce{R-C\equiv CH ->[HBO] R-CH2-CHO}$ (Aldehydes)

4. Oxidation & Identification Tests

Ozonolysis & Oxidation

Unlike alkenes, reductive and oxidative ozonolysis of alkynes both yield Carboxylic Acids.

\[ \ce{R-C\equiv C-R' ->[O3][H2O] R-COOH + R'-COOH} \]

Terminal alkynes give $\ce{CO2}$ gas + Carboxylic Acid. Hot $\ce{KMnO4}$ gives identical products.

Tests for Terminal Alkynes

Tollens'

Ammoniacal $\ce{AgNO3}$ gives a White Precipitate of Silver Acetylide ($\ce{R-C\equiv C-Ag}$).

Cu₂Cl₂

Ammoniacal $\ce{Cu2Cl2}$ gives a Red Precipitate of Copper Acetylide ($\ce{R-C\equiv C-Cu}$).

5. Cyclic & Linear Polymerization

Cyclic Trimerization

Passing alkynes through a Red Hot Iron/Copper Tube at 873K yields aromatic compounds.

$\ce{3 HC\equiv CH ->[Fe, \Delta] \text{Benzene } (C6H6)}$

$\ce{3 CH3-C\equiv CH ->[Fe, \Delta] \text{Mesitylene}}$

(1,3,5-Trimethylbenzene)

Linear Polymerization

Under specific conditions ($\ce{Cu2Cl2/NH4Cl}$), ethyne undergoes linear dimerization/trimerization.

\[ \ce{2 HC\equiv CH ->[Cu2Cl2][NH4Cl] CH2=CH-C\equiv CH} \]

Vinyl Acetylene

Reacts further with $\ce{HCl}$ to form Chloroprene (monomer of Synthetic Rubber / Neoprene).

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