JEE Chemistry Hydrocarbons Notes Set 06

Alkynes: Structure and Acidity

Alkynes have a C#C triple bond and the general formula \( \ce{CnH2n-2} \). Each triple-bond carbon is sp hybridised (50% s-character), the C-C#C unit is linear (180 degrees), and the triple bond is one sigma bond plus two perpendicular pi bonds, giving a cylindrical electron cloud. The C#C bond is the shortest (120 pm) and strongest of the three carbon-carbon bonds.

A carbon with more s-character holds its bonding electrons closer, so the C-H bond is shorter and more acidic and the conjugate acetylide ion is more stable. Terminal alkynes (the \( \ce{#C-H} \) type) are acidic enough to react with sodium, sodamide, ammoniacal silver nitrate and ammoniacal cuprous chloride. They are much weaker acids than water, so NaOH does not deprotonate them, but they are stronger acids than ammonia.

The metal acetylides are explosive when dry. Dilute acid regenerates the alkyne. Alkylation of an acetylide with a primary alkyl halide builds up the chain: ethyne with \( \ce{NaNH2} \) then \( \ce{CH3I} \), then \( \ce{NaNH2} \) and \( \ce{C2H5I} \), gives pent-2-yne. Internal alkynes such as but-2-yne give none of these tests.

Preparation of Alkynes

Reactions of Alkynes

Alkynes add electrophiles more slowly than alkenes because the intermediate vinyl cation is less stable than an alkyl cation. In a molecule with both a C=C and a C#C, an electrophile such as \( \ce{Br2} \) adds to the double bond first, while nucleophiles such as \( \ce{CN^-} \) or \( \ce{RO^-} \) add more readily to the triple bond.

Uses of Acetylene

1. Oxy-acetylene flame for welding and cutting metals (about 3000 degrees C).
2. Raw material for ethanal, acetic acid, vinyl chloride, vinyl acetate, acrylonitrile, neoprene and benzene.
3. Illuminant, and for artificial ripening of fruit.

Telling Alkanes, Alkenes and Alkynes Apart

To separate a mixture of ethane, ethene and ethyne, pass it through ammoniacal silver nitrate to remove ethyne, then through conc. \( \ce{H2SO4} \) or bromine water to remove ethene, leaving ethane. Degree of unsaturation = (2C + 2 - H)/2 for a hydrocarbon: one for an alkene or ring, two for an alkyne or diene.

Cycloalkanes

Cycloalkanes have the formula \( \ce{CnH2n} \) (isomers of alkenes) and are named with the prefix cyclo. Small rings are strained, as Baeyer's strain theory describes: the angle in the planar ring differs from 109.5 degrees. Cyclopropane has the most angle strain and cyclobutane also has a lot; cyclopentane is nearly strain-free; cyclohexane in the chair form is free of angle strain.

Preparation: Freund reaction (1,3-dibromopropane with zinc gives cyclopropane, 1,4-dibromobutane with sodium gives cyclobutane); addition of a carbene to an alkene; hydrogenation of benzene to cyclohexane; Diels-Alder reaction for rings of six.