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Alkynes - Organic Chemistry Tutor

The acyclic hydrocarbons with one triple bound in the molecule are called alkynes. The still customary denomination of acetylene comes from the series first hydrocarbon, acetylene.

Structure

Acetylene has the composition C₂H₂: it contains two carbon atoms connected through a triple bound: CHCH. The C-H bounds of acetylene are formed by the interpenetration of an orbital sp from the carbon atom with an orbital s from the hydrogen atom; the triple bound is made of one sigma bound (formed by the interpenetration of two orbital sp’s from each carbon atom) and two pi bounds (result of the interpenetration of two pairs of orbital p’s).

Same as for ethylene, for detaching the pi bounds a smaller quantity of energy is necessary unlike for detaching the sigma bounds. The isomers for compounds with triple bound are unknown, because all the atoms are positioned collinear. Acetylene derivates (obtained through the substitution of one or both hydrogen atoms with radical alkyls) can be considered the homologues of acetylene. Therefore, by substituting one hydrogen atom with one methyl radical you obtain propine C₃H₄: CHC-CH3.

Also, if one hydrogen atom of acetylene is substituted with a –C₂H₅ radical, you obtain ethyl acetylene, meaning 1-butine. The isomer of 1-butine is 2-butine, in other words the hydrocarbon containing the triple bound is between C2-C3. This compound can be taken as a deriving from acetylene by the replacement of the two hydrogen atoms with a CH₃ radical, resulting a dimethylacetylene: CH₃-CH₂-CH₂-CC-CH₃ is 2-hexine.

Sometimes more alkynes are denominated as acetylene substitution products; for example, dimethylacetylene as 2-butine. The derivate radicals of alkynes are called alkynyls, for example CHC-CH2 is called 2-propinyl.

General formula

Unsaturated hydrocarbons with a triple bound in the molecule, containing 2 atoms less than the corresponding saturated hydrocarbons, have the general formula C_nH_2n-2.

Physical properties

At usual temperature, the C2-C4 alkynes are gases, C4-C14 are liquids, and the superior ones are solids. Their boiling points are similar, yet a bit higher than the alknes and alkenes points. Their solubility is higher than their corresponding saturated hydrocarbons.

Chemical properties

The chemical behavior of the alkynes is characterized by their capacity of giving addition reactions, polymerization reactions and metallic derivates forming reactions. A great part of reactions are known as belonging to acetylene.

Obtaining methods

1. Methane pyrolize

2CH4 CHCH + 3H2 (acetylene)

2. Obtaining alkynes out of alkenes (X=Cl₂, Br₂, I₂)

R-CH=CH-R' + X₂ R-CHX-CHX-R' R-CC-R' + 2HX

3. Obtaining alkynes out of halogenated derivates

R-X + NaCC-R' NaX + R-CC-R' (alkyne)

Chemical reactions

1. H2 addition (hydrogenating)

R-CC-R' + H₂ R-CH=CH-R' (alkene)
R-CC-R' + 2H₂ R-CH₂-CH₂-R' (alkane)

2. X2 addition (in action less solvent)

C_nH_2n-2 + X₂ nC + 2n-2HX (without a solvent, a explosion may take place)
R-CC-R' + X₂ R-CX=CX-R' (dihalogenated derivate) - using a action less solvent)

Examples:
CHCH + Cl₂ 2C + 2HCl - explosion
CHCH + Cl₂ CHCl=CHCl (1,2 dichloretene) + Cl2 CHCl2-CHCl2 (1,1,2,2 -tetraloretane)

3. HX addition

R-CCH + HX R-CX=CH₂ + HX R-CX₂-CH₃ (geminal dihalogenated derivate)

Examples:
CHCH + HCl CHCl=CH₂ + HCl (vinyl chloride) + HX R-CX₂-CH₃
CHC-CH₃ + HCl CH₂=CCl-CH₃ + HCl CH₃-CCl₂-CH₃

4. H₂O addition (hydration, catalyser: HgSO₄ and H₂SO₄)

R-CCH + H₂O R-COH=CH₂ (instable enol) (tautomeryzation) R-C=O-CH3

5. Prussic acid addition (HCN - catalyser Cu₂Cl₂, NH₄Cl)

R-CHCH + HCN R-CCN=CH₂

6. CH₃-COOH addition (catalyser: Zn(CH₃COO)₂ / 200 grade)

R-CCH + CH₃-COOH R-COOCH₃=CH₂ (acetate)

7. Poly addition reactions (polymerization) - dimerization, trimerization, tetramerization

a) Dimerization reaction
CHCH + CHCH CHC-CH=CH2 (acetylene vinyl)

b) Trimerization reaction (it takes place in ceramic tubes at temperatures within 600-800 Celsius degrees)

3CHCH

8. Oxidation reaction

a) Complete: C_nH_2n-2 + 3n-1/2 O2 nCO2 + n-1H₂O + Q

b) Incomplete
CHCH + 4[O] COOH-COOH (oxalic acid)
CH3-CCH + 3[O] CH₃-C=O-COOH (piruvic acid)

Use

» acetylene was and still is one of the most used raw material of the of the organic chemistry industry
» from acetylene you can obtain rubber, vinyl poly chloride and many more.

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