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Hydrocarbons - Alkynes - Nomenclature, Structure of Triple Bond, Preparation and Properties

Grade 11CBSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

πŸ”‘Concepts

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Alkynes are unsaturated hydrocarbons containing at least one carbon-carbon triple bond (C≑CC \equiv C). Their general formula is CnH2nβˆ’2C_nH_{2n-2}.

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In the structure of the triple bond, each carbon atom is spsp hybridized. The bond consists of one Οƒ\sigma bond (formed by spβˆ’spsp-sp overlap) and two Ο€\pi bonds (formed by lateral overlap of 2p2p orbitals).

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The geometry around the triple bond is linear with a bond angle of 180∘180^\circ. The C≑CC \equiv C bond length is approximately 120120 pm, which is shorter than both Cβˆ’CC-C and C=CC=C bonds.

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Nomenclature: The IUPAC name is derived by replacing the suffix '-ane' of the corresponding alkane with '-yne'. The position of the triple bond is indicated by the lower number of the two triple-bonded carbons.

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Acidic Character: Terminal alkynes (like HC≑CHHC \equiv CH or Rβˆ’C≑CHR-C \equiv CH) are acidic because the spsp hybridized carbon has 50%50\% ss-character, making it highly electronegative and capable of releasing the H+H^+ ion.

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Preparation: Alkynes can be prepared by the reaction of calcium carbide with water (CaC2+2H2OCaC_2 + 2H_2O) or by dehydrohalogenation of vicinal dihalides using alcoholic KOHKOH followed by NaNH2NaNH_2.

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Addition Reactions: Alkynes undergo electrophilic addition of H2H_2, X2X_2 (halogens), HXHX, and H2OH_2O. Addition of water (hydration) in the presence of Hg2+/H+Hg^{2+}/H^+ produces carbonyl compounds (aldehydes or ketones).

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Cyclic Polymerization: Ethyne undergoes cyclic polymerization when passed through a red-hot iron tube at 873873 K to form Benzene (C6H6C_6H_6).

πŸ“Formulae

CnH2nβˆ’2C_nH_{2n-2}

CaC2+2H2O→Ca(OH)2+C2H2CaC_2 + 2H_2O \rightarrow Ca(OH)_2 + C_2H_2

CH2Brβˆ’CH2Br+2KOH(alc.)β†’Ξ”CH≑CH+2KBr+2H2OCH_2Br-CH_2Br + 2KOH (alc.) \xrightarrow{\Delta} CH \equiv CH + 2KBr + 2H_2O

HC≑CH+H2Oβ†’Hg2+/H+,333K[CH2=CHβˆ’OH]β†’tautomerismCH3CHOHC \equiv CH + H_2O \xrightarrow{Hg^{2+}/H^+, 333K} [CH_2=CH-OH] \xrightarrow{tautomerism} CH_3CHO

3HC≑CHβ†’RedΒ hotΒ ironΒ tube,Β 873KC6H63HC \equiv CH \xrightarrow{\text{Red hot iron tube, } 873K} C_6H_6

πŸ’‘Examples

Problem 1:

Write the IUPAC name of CH3βˆ’C(CH3)2βˆ’C≑Cβˆ’CH3CH_3-C(CH_3)_2-C \equiv C-CH_3.

Solution:

4,4βˆ’dimethylpent-2-yne4,4-\text{dimethylpent-2-yne}

Explanation:

The longest carbon chain containing the triple bond has 5 carbons (pent-). Numbering starts from the right to give the triple bond the lower locant (2). Two methyl groups are present at position 4.

Problem 2:

How can you distinguish between Ethene (C2H4C_2H_4) and Ethyne (C2H2C_2H_2) using a chemical test?

Solution:

Ammoniacal Silver Nitrate (Tollenβ€²sΒ reagentTollen's\ reagent) test.

Explanation:

Ethyne reacts with ammoniacal silver nitrate to give a white precipitate of silver acetylide (Agβˆ’C≑Cβˆ’AgAg-C \equiv C-Ag), whereas ethene does not react because it lacks acidic hydrogens.

Problem 3:

Predict the product of the reaction: CH3βˆ’C≑CH+H2Oβ†’HgSO4/H2SO4CH_3-C \equiv CH + H_2O \xrightarrow{HgSO_4/H_2SO_4}.

Solution:

CH3βˆ’COβˆ’CH3CH_3-CO-CH_3 (Propanone/Acetone)

Explanation:

According to Markovnikov's rule, the βˆ’OH-OH group attaches to the carbon with fewer hydrogen atoms. The resulting enol (CH3βˆ’C(OH)=CH2CH_3-C(OH)=CH_2) undergoes tautomerization to form a stable ketone (Propanone).

Alkynes - Nomenclature, Structure of Triple Bond, Preparation and Properties Revision - Class 11…