Physical and Chemical Properties — Revision Notes

⚡ 30-Second Revision

General formula: $C_nH_{2n-2}$
Hybridization: $sp$ (linear geometry, $180^circ$ bond angle)
Acidity of terminal alkynes: $R-C equiv C-H$ (acidic H due to $sp$ carbon)
Key Reagents & Products:

- $H_2/$ Lindlar's catalyst: Alkyne $ightarrow$ cis-alkene - $Na/liq. NH_3$: Alkyne $ightarrow$ trans-alkene - $H_2/Pt/Pd/Ni$: Alkyne $ightarrow$ Alkane - $X_2/CCl_4$: Alkyne $ightarrow$ Tetrahaloalkane - $HX$ (excess): Alkyne $ightarrow$ Geminal dihalide (Markovnikov) - $H_2O/HgSO_4, H_2SO_4$: Alkyne $ightarrow$ Ketone/Aldehyde (via enol, Markovnikov) - Cold, dilute $KMnO_4$: Alkyne $ightarrow$ Vicinal diketone - Hot, conc.

$KMnO_4$: Alkyne $ightarrow$ Carboxylic acids/$CO_2$ - $NaNH_2$: Terminal alkyne $ightarrow$ Acetylide - Tollens' reagent ($[Ag(NH_3)_2]OH$): Terminal alkyne $ightarrow$ White Ag-acetylide ppt. - Ammoniacal $Cu_2Cl_2$: Terminal alkyne $ightarrow$ Red Cu-acetylide ppt.

2-Minute Revision

Alkynes are hydrocarbons with a $C equiv C$ triple bond, featuring $sp$ hybridized carbons and a linear structure. Physically, they are nonpolar, insoluble in water, and their boiling points increase with molecular weight.

A critical property is the weak acidity of terminal alkynes ($R-C equiv C-H$) due to the high s-character of the $sp$ carbon, allowing them to react with strong bases like $NaNH_2$ to form acetylides.

Chemically, the electron-rich triple bond undergoes various addition reactions. Selective hydrogenation with Lindlar's catalyst yields cis-alkenes, while $Na/liq. NH_3$ gives trans-alkenes. Complete hydrogenation forms alkanes.

Halogenation leads to tetrahaloalkanes. Hydrohalogenation and hydration follow Markovnikov's rule, producing geminal dihalides and carbonyl compounds (ketones/aldehydes via enol tautomerism), respectively.

Strong oxidation with hot $KMnO_4$ cleaves the triple bond to carboxylic acids or $CO_2$. Terminal alkynes are uniquely identified by their reactions with Tollens' reagent (white ppt) and ammoniacal cuprous chloride (red ppt).

5-Minute Revision

Alkynes, characterized by their $C equiv C$ triple bond, are unsaturated hydrocarbons with $sp$ hybridized carbons, resulting in a linear geometry. Their physical properties include nonpolarity, insolubility in water, and increasing boiling/melting points with molecular mass.

The most distinctive physical-chemical property is the acidity of terminal alkynes ($R-C equiv C-H$). The $sp$ carbon's high s-character makes the C-H bond weakly acidic, allowing reaction with strong bases like $NaNH_2$ to form acetylides ($R-C equiv C^-Na^+$).

This acidity is also the basis for qualitative tests: terminal alkynes form a white precipitate with Tollens' reagent ($AgNO_3/NH_3$) and a red precipitate with ammoniacal cuprous chloride ($Cu_2Cl_2/NH_3$).

Chemically, alkynes are highly reactive due to their two $pi$ bonds, primarily undergoing electrophilic addition reactions:

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Hydrogenation — Complete reduction to alkanes occurs with $H_2/Pt, Pd, Ni$. Partial hydrogenation is stereoselective: Lindlar's catalyst ($Pd/CaCO_3$ poisoned) yields cis-alkenes via syn-addition, while $Na/liq. NH_3$ yields trans-alkenes via anti-addition.

Example: $CH_3-C equiv C-CH_3 xrightarrow{H_2/\text{Lindlar}} \text{cis}-CH_3-CH=CH-CH_3$

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Halogenation — Addition of $X_2$ (e.g., $Br_2/CCl_4$) occurs in two steps, forming a dihaloalkene then a tetrahaloalkane.
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Hydrohalogenation — Addition of $HX$ (e.g., $HCl$) follows Markovnikov's rule, yielding geminal dihalides after two additions.

Example: $CH_3-C equiv C-H + 2HCl \rightarrow CH_3-CCl_2-CH_3$

1
Hydration — Addition of $H_2O$ in the presence of $HgSO_4/H_2SO_4$ follows Markovnikov's rule, forming an unstable enol that rapidly tautomerizes to a ketone (for most alkynes) or an aldehyde (only for ethyne).

Example: $CH_3-C equiv C-H + H_2O xrightarrow{HgSO_4, H_2SO_4} CH_3-CO-CH_3$ (Acetone)

1
Oxidation — Cold, dilute $KMnO_4$ forms vicinal diketones. Hot, concentrated $KMnO_4$ or ozonolysis leads to oxidative cleavage, forming carboxylic acids (from internal alkynes) or carboxylic acids and $CO_2$ (from terminal alkynes).

Understanding these specific reagents, their mechanisms, and the products formed is crucial for NEET.

Prelims Revision Notes

Physical Properties:

State — Lower alkynes (C2-C4) are gases, C5-C17 are liquids, higher are solids.
Solubility — Nonpolar, insoluble in water, soluble in organic solvents (benzene, ether).
Boiling/Melting Points — Increase with molecular weight. Branching decreases boiling point.
Density — Less dense than water, increases with molecular weight.
Acidity of Terminal Alkynes ($R-C equiv C-H$) — Due to $sp$ hybridized carbon (50% s-character), which is more electronegative, polarizing the C-H bond. Order of acidity: Carboxylic acids > Alcohols > Water > Terminal Alkynes > Ammonia > Alkenes > Alkanes.

Chemical Properties (Reactions of the Triple Bond):

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Hydrogenation (Reduction)

* Complete: $R-C equiv C-R' + 2H_2 xrightarrow{Pt/Pd/Ni} R-CH_2-CH_2-R'$ (Alkane) * Partial (cis-alkene): $R-C equiv C-R' + H_2 xrightarrow{\text{Lindlar's Catalyst}} \text{cis}-R-CH=CH-R'$ (Syn-addition) * Partial (trans-alkene): $R-C equiv C-R' + 2Na xrightarrow{liq. NH_3} \text{trans}-R-CH=CH-R'$ (Anti-addition)

1
Halogenation (Addition of $X_2$) — $R-C equiv C-R' + 2X_2 xrightarrow{CCl_4} R-CX_2-CX_2-R'$ (Tetrahaloalkane). Decolorizes $Br_2$ water.
2
Hydrohalogenation (Addition of $HX$) — $R-C equiv C-H + 2HX \rightarrow R-CX_2-CH_3$ (Geminal dihalide, Markovnikov's rule).

* With peroxides, $HBr$ can show anti-Markovnikov addition, but less efficiently than alkenes.

1
Hydration (Addition of $H_2O$) — $R-C equiv C-R' + H_2O xrightarrow{HgSO_4, H_2SO_4} \text{Enol} \rightarrow \text{Ketone/Aldehyde}$ (Markovnikov's rule).

* Ethyne $ightarrow$ Acetaldehyde ($CH_3-CHO$) * Terminal alkynes (except ethyne) $ightarrow$ Methyl Ketones ($R-CO-CH_3$)

1
Oxidation Reactions

* **Cold, dilute, alkaline $KMnO_4$ (Baeyer's reagent)**: $R-C equiv C-R' \rightarrow R-CO-CO-R'$ (Vicinal diketone). * **Hot, concentrated $KMnO_4$ or Ozonolysis ($O_3$ then $H_2O_2$)**: Oxidative cleavage. * Internal alkyne: $R-C equiv C-R' \rightarrow R-COOH + R'-COOH$ (Carboxylic acids) * Terminal alkyne: $R-C equiv C-H \rightarrow R-COOH + CO_2 + H_2O$

Reactions of Acidic Hydrogen (Terminal Alkynes Only):

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Formation of Acetylides — $R-C equiv C-H + NaNH_2 \rightarrow R-C equiv C^-Na^+ + NH_3$
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Tollens' Test — $R-C equiv C-H + [Ag(NH_3)_2]OH \rightarrow R-C equiv C-Ag downarrow$ (White ppt of silver acetylide)
3
Ammoniacal Cuprous Chloride Test — $R-C equiv C-H + [Cu(NH_3)_2]Cl \rightarrow R-C equiv C-Cu downarrow$ (Red ppt of cuprous acetylide)

Polymerization:

Ethyne $xrightarrow{\text{Red hot Fe tube}}$ Benzene (Cyclic trimerization)

Vyyuha Quick Recall

To remember alkyne hydrogenation products: Lindlar's gives Cis, Na/liq. Ammonia gives Trans. (LC NAT - 'L' for Lindlar, 'C' for Cis; 'N' for Na/liq. Ammonia, 'T' for Trans).