The alkynes are the third homologous series of organic compounds of hydrogen and carbon, where there is at least one triple-bond between the atoms in the molecules.
The alkenes are said to be unsaturated because of the existence of a multiple bond in the molecule. The general structure of the alkene series of hydrocarbons is CnH2n-2. The first member of the ethene series is ethyne (previously called acetylene). The names of all alkynes end in "-yne". Rules for the systematic naming of alkynes are similar to those for alkenes. In the case of higher members of the alkene series, the triple bond may be between the terminal carbon atoms of the chain, or may be between internal carbon atoms in the chain.
Ethyne (Acetylene) HCCH Propyne HCCCH3 1-Butyne HCCCH2CH3 1-Pentyne HCC(CH2)2CH3 1-Hexyne HCC(CH2)3CH3 1-Heptyne HCC(CH2)4CH3 1-Octyne HCC(CH2)5CH3 1-Nonyne HCC(CH2)6CH3 1-Decyne HCC(CH2)7CH3 2-Butyne CH3CCCH3 2-Pentyne CH3CCCH2CH3The bond formed between the hydrogen atom and the unsaturated carbon atom, and first bond between the unsaturated carbon atoms in the ethynes are s bonds (sigma bonds) and these bonds are formed by the end-on overlap of sp hybrid orbitals of the carbon atoms and the bonds are arranged as far apart in space as possible (i.e. at 180 degree) to form a linear molecule. The second and third bonds that makes up the triple bond of the unsaturated carbon atoms in alkenes are p-bonds (pi-bonds), formed by the side-on overlap of the two p-orbitals on each of the carbon atoms. The p-bonds (pi-bonds) are much more reactive than the s bonds (sigma bonds), and react readily in addition reactions.
Acetylene is a linear molecule, all four atoms lying along a straight line. This linear structure can only be explained by the existence of sp hybridisation of the orbitals of the carbon atoms of ethyne.
The carbon-carbon triple bond is thus made up of one strong bond and two weaker (bonds; it has a total strength 123 kcal. It is stronger than the carbon-carbon double bond of ethylene 100 kcal or the single carbon-carbon bond of ethane 83 kcal, and therefore is shorter than either.
The C-C distance is 1.2 A, as compared with 1.34 A in ethylene and 1.54 A in ethane and is a more electronegative grouping than that formed by carbon atoms joined by either a double or a single bond.
The hydrogen attached to the carbon-carbon triple bond in ethyne or in any alkyne where the carbon-carbon triple bond is situated at the end of a carbon chain is able to separate from the rest of the molecule as a hydrogen ion; the electronegative carbon is able to retain both electrons from the broken covalent bond.
A significant result of this bonding is that ethyne can unite with metals and so be distinguished from alkenes by chemical means.
The linear structure does not permit geometric isomerism of ethyne.
2 C2H2 + 5 02 ==> 4 CO2 + 2 H2O
KMnO4 HCCH ==> O = COH Ethyne O = COH Oxalic Acid
Ni 150degC HCCH + H2 ===> Ethyne Ni 150degC H2C=CH2 + H2 ===> C2H6 Ethene Ethane
HCCH + Cl2 ==> ClHC=CHCl + Cl2 ==> Cl2HCCHCl2
HCCH + HCl ==> H2C=CHCl + HCl ==> CH3CHCl2
HgSO4 60 degC HCCH + H2O ==> CH3CHO
573 degK HCCH + NH3 ==> CH3CN + H2 Ethyne Ethanonitrile
When ethyne is passed through a glass tube at 4000C a little benzene is formed. This is not a suitable way to make benzene in quantity but it is an example of direct conversion from an open chain to an aromatic compound, (i.e. one with a closed-ring benzenoid structure)
400 degC 3 C2H2 ==> C6H6 Ethyne Benzene
Two molecules of ethyne can be combined to produce vinyl ethyne, HC(CCH=CH2, by passing the ethyne into a saturated solution of cuprous chloride in ammonium chloride continuously in such a way that low conversions of starting material occur.
Cu2Cl2 NH4Cl 2 HC CH ==> HCCCH = CH2 Ethyne Vinyl Ethyne
This linear polymerisation can be extended by altering the conditions of reaction. For example,
HCCCH=CH2 + HCCH ==> CH2=CHC(CCH=CH2 Vinyl Ethyne Ethyne DiVinyl Ethyne
When ethyne is passed through a solution of sodium in liquid ammonia then sodium acetylide is formed and hydrogen is liberated.
liq.NH3 HCCH + 2Na ==> 2HCCNa + H2 Sodium Acetylide
The other hydrogen atom in ethyne can be similarly replaced. When ethyne is passed into a solution of cuprous chloride in ammonia, cuprous acetylide is produced.
HCCH + Cu2Cl2 + NH4OH ==> CuCCCu Copper Acetylide
Silver acetylide is formed when ethyne is passed into an ammoniacal solution of silver nitrate.
AgNO3 NH4OH HCCH ==> AgCCAg + 2 HNO3 Silver Acetylide
These substitution reactions which ethynes undergo to form compounds with metals are not occur with the alkenes. These reactions can be used as tests to distinguish between acetylene and ethylene. When acetylene is passed through an ammonical solution of silver nitrate or cuprous chloride, at room temperature, precipitates of silver acetylide (white) or cuprous acetylide (red) are formed.
In addition to distinguishing ethyne from ethene by chemical means, these reactions provide a useful method for the preparation of higher alkynes:
HCC-Na+ + CH3I ==> HCCCH3 + NaI Propyne
HCC-Na(+) + HNO3 ==> HCCH + NaNO3
Alkynes are compounds which have low polarity, and have physical properties that are essentially the same as those of the alkanes and alkenes.
Name Formula MP degC BP degC Density(20C) ========= =========== ======= ======= ============ Acetylene HCCH -82 -75 Propyne HCCCH3 -101.5 -23 1-Butyne HCCCH2CH3 -122 91 1-Pentyne HCC(CH2)2CH3 -98 40 0.695 1-Hexyne HCC(CH2)3CH3 -124 72 0.719 1-Heptyne HCC(CH2)4CH3 -80 100 0.733 1-Octyne HCC(CH2)5CH3 -70 126 0.747 1-Nonyne HCC(CH2)6CH3 -65 151 0.763 1-Decyne HCC(CH2)7CH3 -35 182 0.770 2-Butyne CH3CCCH3 -24 27 0.694 2-Pentyne CH3CCCH2CH3 -101 55 0.714
The carbon-carbon triple bond of the alkynes is formed in the same way as a double bond of the alkenes, by the elimination of atoms or groups from two adjacent carbons.
W X W X HC - CH ==> HC = CH ==> HCCH X X Alkane Alkene AlkyneThe groups that are eliminated and the reagents used are essentially the same as in the preparations of alkenes.
Dehydrohalogenation can be carried out in two stages. The halides thus obtained, with halogen attached directly to double bonded carbon, are called vinyl halides, and are very unreactive. Under mild conditions, therefore, dehydrohalogenation stops at the vinyl halide stage; more vigorous conditions, use of stronger base is required for alkyne formation. If only the first step of this reaction is carried out, it is a valuable method for preparing unsaturated halides.
NaNH2 HCCH ==> HCC(-)Na(+) + RX
The unsaturated nature of alkynes means that most of their reactions will be similar to those of alkenes (i.e. electrophilic addition), because of the availability of the loosely held pi-electrons. The carbon to carbon triple bond is less reactive than the carbon to carbon double bond towards electrophilic reagents. As well as the addition reactions, alkynes undergo reactions that are due to the acidity of a hydrogen atom attached to the triple bonded carbon.
The carbon-carbon triple bond in ethyne is thus made up of one strong sigma-bond and two weaker pi-bonds. It has a total strength 123 kcal/mole. This is stronger than the carbon-carbon double bond of ethylene which has a total strength of 100 kcal/mole or the single carbon-carbon bond of ethane which has a total strength of 83 kcal/mole.
The carbon-carbon bond lengths, which depend on the strengths of the bonds are
Ethyne CHCH 1.20 Angstrom Units Ethylene CH2CH2 1.34 Angstrom Units Ethane CH3CH3 1.54 Angstrom Units.The ethynyl radical, CHC*, is a more electronegative group than that formed by carbon atoms joined by either a double or a single bond. Thus, the hydrogen attached to the carbon-carbon triple bond in ethyne, or in any alkyne where the carbon-carbon triple bond is situated at the end of a carbon chain, is able to separate from the rest of the molecule as a hydrogen ion, so that the alkyne shows acidic properties. The electronegative carbon is able to retain both electrons from the broken covalent bond. A significant result of this bonding is that ethyne can form compounds with metals and so be distinguished from alkenes by chemical means.