| Molecule | Bond | CC / A� |
|---|---|---|
| Ethane | CH3−CH3 | 1.54 |
| Ethene | CH2=CH2 | 1.33 |
| Ethyne | CH≡CH | 1.20 |
Alkynes are hydrocarbons with at least one CC triple bond. This of course requires a sp−hybridisation for the valence orbitals of the carbon to give two pairs of p−orbitals perpendicular to each other. The two pairs of adjacent p−orbitals then interact to give the δ−bond. Note that the δ−bond is in quantum mechanics a single bond though in practice we often think of it as two π−bonds. So a CC triple bond should be viewed as a σ−bond plus a δ−bonds.
REACTIONS
ELECTROPHILIC ADDITION
The δ−bond of alkyne shows the same chemistry as the π−bond of alkenes. It reacts by electrophilic addition. So if HCl is added to alkyne we will first obtained the alkene chloride then the alkyl chloride.
| R−C≡C−H + HCl � R−C(Cl)=CH2 | (1) |
| R−C(Cl)=CH2 + HCl � R−CCl2−CH3 | (2) |
The reaction obeys Markovnikov's rule since a carbonium ion intermediate is involved. It must also be noted that the carbonium ion in this case is a vinyl cation, R−C(+)=CH2 which is less stable than an alkyl carbonium ion. So alkynes are generally less reactive than alkene towards electrophilic addition.
Vinyl chloride
Vinyl chloride, commercially known as VCM (vinyl chloride monomer), is in high demand for the preparation of poly(vinyl chloride) or PVC. One reaction used to prepare this compound is:
H−C≡C−H + HCl � H−C(Cl)=CH2
The reaction is carried out in the vapor phase over a mercuric chloride / activated charcoal catalyst. The reaction is highly exothermic so it is carried out in a multi−tubular reactor at a pressure of between 5 to 10 bars, with the temperature maintained between 100 and 180�C. The product is washed with aqueous NaOH to remove unreacted HCl, and then cooled to −40�C. Vinyl chloride is obtained by fractional distillation.
For some electrophilic addition reactions there are certain interesting deviation after the first addition. When 4−hexyne is reacted with sulphuric acid we expect to obtain a vinyl alcohol;
CH3CH2−C≡C−CH2CH3 + H2O / H2SO4 � CH3CH2−C(OH)=CH−CH2CH3
But vinyl alcohol is not stable and the hydroxyl group immediately gave its proton to the adjacent carbon to form the more stable ketone. Now everyone is happy.
CH3CH2−C(OH)=CH−CH2CH3 � CH3CH2−C(O)−CH2CH2CH3
This is a good reminder: "do not take chemistry for granted". Walk through the reaction carefully and observe all its laws and you would be all right. For the electrophilic addition of alkynes remember that the addition goes by way of a carbonium ion and the product of the first stage must be sufficiently stable for it to proceed smoothly to the second stage.
SYN ADDITION − HYDROBORATION
Diborane reacts with alkynes at 0�C to give a trivinylborane.
| RC≡CR' + BH3 | � | (RCH=CR'-)3B |
| (RCH=CR'−)3B + H2O2 / NaOH | � | 3 RCH=CR'−OH |
We are now aware that the vinyl alcohol would immediately rearrange to the ketone.
| RCH=CR'−OH | � | RCH2−C(O)R' |
HYDROGENATION
Hydrogenation of π− and δ−bonds with the help of nickel, palladium or platinum occurs readily to give the corresponding alkane. The hydrogen atoms add to the bond from the same side of the molecular plane as the reaction is a syn addition.
However alkynes, unlike alkenes, can also be hydrogenated by systems like Na/liquid ammonia and LiAlH4. This is not a syn addition, and the addition is one atom of hydrogen at a time. With these systems it is possible to stop at the at the alkene stage, which is very difficult with the metal catalysts. The alkene would of course have the thermodynamically more stable configuration, that is the trans-stereoisomer.
AS A BASE − in ELIMINATION REACTION
The terminal −C≡CH alkyne unit is weakly acidic and the proton can dissociate to react with a base. This is because the greater the amount of s-character in the hybrid orbital the more ownership will the carbon has on the electron, so the proton can leave if it wants to. (Technospeak: The more s−character the less basic is the carbanion and so the more acidic is the corresponding conjugate acid.) So terminal alkynes can form ionic compounds with Group I elements like RC≡CLi, RC≡CNa and RC≡CAg. However these salts are explosive so they are seldom isolated. They are kept at low temperature (below 0�C) or kept moist.
Another application for this reaction is the purification of terminal alkynes from other organic by-products. The salts are prepared and the other organic products separated before the terminal alkynes is regenerated.
Since the carbanion is negatively charged it behaves as a base and so is able to execute an Elimination Reaction on an alkyl halide to produce an alkene.
            |
� |
 |
| CH3(CH2)2C≡C‾Li+ + CH3CH(Br)CH3 | HMPT, 25�C � |
CH3(CH2)3C≡CH + CH2=CHCH3 (85%) + CH3(CH2)3C≡CCH(CH3)2 (6%) |
Note: HMPT = hexamethylphosphoric triamide, [(CH3)2N]3PO.
AS A NUCLEOPHILE − in SUBSTITUTION REACTION
Instead of pulling out a proton it can also push out the halide atom.
| CH≡CH + NaNH2 | liq NH3, −33�C � |
CH≡C‾Na+ + NH3 |
| CH≡C‾Na+ + CH3(CH2)2CH2Br | � | CH3(CH2)2CH2C≡CH + NaBr |
The terminal alkyne is added to the sodium amide/ammonia solution and stirred for a few minutes. Ammonia boils at −33�C so a dry ice condenser (−78�C) must be used. The alkyl bromide is then added and the mixture stirred for a few hours before water is added to give the 1−hexyne. Yield is ca 90%.
ELIMINATION versus SUBSTITUTION
In the reaction of the carbanion with isopropyl bromide the main product is the alkene, while for the n−butyl bromide the main reaction is the SN Reaction.
NUCLEOPHILIC ADDITION
Acetylene can react with alkoxides in an alcoholic solution to become an alkene. This is a rather unique chemistry in the sense that the δ−bond, a bond rich in electron, will do business with a nucleophile, a negatively charged species. Before we proceed it must be clear that alkoxide is the nucleophile, not a carbanion. So this is an attack of a δ−bond by an nucleophile followed by the attachment of a proton. So the reaction is classified as a Nucleophilic Addition of acetylene.
| RO‾ + CH≡CH | ROH � |
RO−CH=CH‾ |
| RO−CH=CH‾ + ROH | � | RO−CH=CH2 + RO‾ |
Tutorial 5.1
What do you understand by "terminal alkyne is a nucleophile in the nucleophilic substitution of alkyl halide" and "acetylene can undergo nucleophilic addition"? Answer
ACETYLENE BLACK
Acetylene black is a carbon produced through self−heating pyrolysis of acetylene gas in the absence of oxygen. As such there is negligible amount of hydrogen as undecomposed residual matter or as a functional group; and functional groups containing oxygen are not found. So acetylene black is very pure carbon composed of larger crystallite. This gave it high electrical and thermal conductivity, low moisture absorption, and high liquid absorption. It is useful as the basic material for dry cell, as additive to the antistatic and electrically conductive rubber or plastic material which are used in various industrial fields, such as electric wires and cables, tires, belt, hoses, paints, adhesives and many electronics parts.
