Hydrocarbons (Alkynes)

Alkynes

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Alkynes are unsaturated hydrocarbons characterized by the presence of at least one carbon-carbon triple bond ($C \equiv C$). They are more reactive than alkenes due to the presence of the triple bond.

Nomenclature And Isomerism

IUPAC Nomenclature:

1. Parent Chain: Identify the longest continuous carbon chain containing the triple bond.
2. Numbering: Number the chain starting from the end nearest to the triple bond, giving it the lowest possible number.
3. Suffix: Replace the '-e' ending of the parent alkane with '-yne'. Indicate the position of the triple bond by the lower number of the two triply bonded carbon atoms.
4. Substituents: Name and number substituents as usual and list them alphabetically.

Examples:

• $HC \equiv CH$: Ethyne (common name: acetylene)
• $CH_3C \equiv CH$: Propyne
• $CH_3CH_2C \equiv CCH_3$: Hex-3-yne

Isomerism in Alkynes:

• Chain Isomerism: Possible for alkynes with four or more carbon atoms (e.g., $C_4H_6$ has but-1-yne and but-2-yne).
• Position Isomerism: Possible for alkynes with four or more carbon atoms (e.g., but-1-yne and but-2-yne).
• Functional Isomerism: Alkynes are functional isomers of alkadienes (hydrocarbons with two double bonds, general formula $C_nH_{2n-2}$), e.g., but-1-yne ($C_4H_6$) and buta-1,3-diene ($C_4H_6$).

Structure Of Triple Bond

Hybridization: Carbon atoms involved in a triple bond are $sp$ hybridized.

Bond Formation:

• Sigma ($\sigma$) Bond: Formed by the head-on overlap of one $sp$ hybrid orbital from each carbon atom.
• Pi ($\pi$) Bonds: Formed by the lateral overlap of the two remaining unhybridized $p$ orbitals from each carbon atom. These two $\pi$ bonds are perpendicular to each other and to the $\sigma$ bond.

Geometry: The $sp$ hybridization leads to a linear geometry around each carbon atom involved in the triple bond.

Bond Angles: The bond angles ($C-C \equiv C$) are 180°.

Acidity of Terminal Alkynes:

• The $C-H$ bonds in terminal alkynes (where the triple bond is at the end of the chain, $R-C \equiv C-H$) are weakly acidic.
• The $sp$ hybridized carbon atom is more electronegative than $sp^2$ or $sp^3$ hybridized carbons. It can stabilize the negative charge on the conjugate base (acetylide ion, $R-C \equiv C^-$), making the parent alkyne acidic.
• Terminal alkynes react with strong bases like sodium amide ($NaNH_2$) or organometallic reagents (like Grignard reagents) to form acetylides.

$HC \equiv CH + NaNH_2 \rightarrow HC \equiv C^-Na^+ + NH_3$

$RC \equiv CH + NaNH_2 \rightarrow RC \equiv C^-Na^+ + NH_3$

• These acetylides can react with alkyl halides to form longer alkynes (alkyne synthesis).

$HC \equiv C^-Na^+ + CH_3CH_2Br \rightarrow HC \equiv CCH_2CH_3 + NaBr$

Preparation

Alkynes are typically prepared by methods that introduce a triple bond:

1. From Dihalides (Dehydrohalogenation): Elimination of two molecules of HX (e.g., $HBr$ or $HCl$) from vicinal dihalides (halogens on adjacent carbons) or geminal dihalides (both halogens on the same carbon). This requires strong bases like alcoholic $KOH$ or sodium amide ($NaNH_2$).
• Vicinal Dihalide:

$CH_2BrCH_2Br + 2KOH(alc) \xrightarrow[2 \ steps]{\Delta} CH \equiv CH + 2KBr + 2H_2O$

• Geminal Dihalide:

$CH_3CHBr_2 + 2KOH(alc) \xrightarrow[2 \ steps]{\Delta} CH_3C \equiv CH + 2KBr + 2H_2O$

2. From Calcium Carbide: Reaction of calcium carbide ($CaC_2$) with water yields ethyne (acetylene), the simplest alkyne. This is the main industrial method for producing ethyne.

$CaC_2(s) + 2H_2O(l) \rightarrow Ca(OH)_2(aq) + C_2H_2(g)$

Properties

Physical Properties:

• Ethyne ($C_2H_2$) is a colorless gas.
• Higher alkynes are liquids or solids.
• They have characteristic odors (pure ethyne is odorless, but impure samples have a garlic-like smell).
• Solubility: Sparingly soluble in water but soluble in organic solvents.
• Boiling and Melting Points: Generally increase with molecular weight, but less regularly than alkanes and alkenes. They are higher than corresponding alkanes due to greater linearity and stronger van der Waals forces.

Chemical Properties: Alkynes are more reactive than alkenes due to the presence of two $\pi$ bonds.

1. Addition Reactions: They undergo addition reactions across the triple bond, typically adding two molecules of the reagent across the two $\pi$ bonds.

• Addition of Hydrogen (Hydrogenation): Leads to alkenes (using Lindlar's catalyst for cis-alkene) or alkanes (using $Ni, Pt, Pd$).

$CH \equiv CH + H_2 \xrightarrow{Lindlar's \ catalyst} CH_2=CH_2$

$CH \equiv CH + 2H_2 \xrightarrow{Ni} CH_3CH_3$

• Addition of Halogens: Addition of $X_2$ ($Cl_2$, $Br_2$).

$CH \equiv CH + Br_2 \rightarrow CHBr=CHBr$ (1,2-dibromoethene)

$CHBr=CHBr + Br_2 \rightarrow CHBr_2CHBr_2$ (1,1,2,2-tetrabromoethane)

• Addition of Hydrogen Halides: Addition of $HX$. Follows Markovnikov's rule.

$CH_3CH_2C \equiv CH + HBr \rightarrow CH_3CH_2CBr=CH_2$ (Major product)

• Addition of Water (Hydration): Addition of water in the presence of mercuric sulfate ($HgSO_4$) and dilute sulfuric acid catalyst at 333 K. Follows Markovnikov's rule. Produces a product that tautomerizes to a ketone.

$CH \equiv CH + H_2O \xrightarrow{HgSO_4, H_2SO_4, 333K} [CH_2=CHOH] \rightarrow CH_3CHO$ (Ethyne forms ethanal)

$CH_3C \equiv CH + H_2O \xrightarrow{dil. H_2SO_4, HgSO_4} CH_3C(OH)=CH_2 \rightarrow CH_3COCH_3$ (Propyne forms propanone)

• Oxidation: Strong oxidizing agents like hot acidic $KMnO_4$ or $K_2Cr_2O_7$ cleave the triple bond, yielding carboxylic acids or $CO_2$.

$CH \equiv CH + [O] \rightarrow 2CO_2 + 2H_2O$

2. Acidity of Terminal Alkynes: Terminal alkynes ($R-C \equiv C-H$) are weakly acidic and react with strong bases like sodium amide ($NaNH_2$) to form sodium acetylides ($R-C \equiv C^-Na^+$).

3. Polymerisation:

• Linear Polymerization: Under certain conditions, alkynes can polymerize to form long chains.
• Cyclic Polymerization: Ethyne polymerizes to form benzene.

$3C_2H_2(g) \xrightarrow{red \ hot \ Fe \ or \ Ni \ tube} C_6H_6(g)$

• Alkynes can also form cyclic polymers with more rings, like cyclooctatetraene ($C_8H_8$).