Alkynes

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		CH3CCCH2CH3
The 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.


Alkynes Chemical Properties

Combustion of Alkynes
Ethyne burn in air with a luminous, smoky flame, (forming carbon dioxide and water).


	2 C2H2   +   5 02       ==>     4 CO2   +   2 H2O               

The ethynes are highly dangerously explosives when mixed with air or oxygen.

Oxidation of Alkynes
Ethyne is oxidised by a dilute aqueous solution of potassium permanganate to form oxalic acid. Thus, if ethyne is bubbled through a solution of potassium permanganate the solution is decolourised. This is Baeyer's test for unsaturated organic compounds.


				KMnO4           

		HCCH         	==>          O = COH 

		Ethyne                          O = COH 

						Oxalic Acid     
Addition Reactions of Alkynes
Because of the unsaturated nature of ethyne addition reactions can occur across the triple bond.
Addition of Hydrogen
When acetylene and hydrogen are passed over a nickel catalyst at 150 degC, (or over platinum black catalyst at room temperature) ethene is first formed and then this is further reduced to ethane.


		Ni              

		       		150degC            

	HCCH    +    H2  	===>     

	Ethyne  

				Ni      

				150degC   

	H2C=CH2  +   H2       ===>     	C2H6    

	Ethene                          	Ethane  

Addition of Halogens
Ethyne reacts explosively with chlorine at room temperature, forming hydrogen chloride and carbon. To control the reaction, acetylene and chlorine (also bromine) are added in retorts filled with kieselguhr (hydrated silica) and iron filings.


   HCCH   +   Cl2   ==>   ClHC=CHCl   +   Cl2   ==>   Cl2HCCHCl2      

				

Addition of Hydrogen Halides.
Ethyne reacts with the halogen acids. Hydrogen iodide adding on the most readily, at room temperature. A similar reaction occur with hydrogen bromide at 100 degC. Reaction with hydrogen chloride occurs very slowly.


   HCCH   +   HCl   ==>   H2C=CHCl   +   HCl   ==>   CH3CHCl2


Addition of Water (Hydration)
Hydration of ethyne occurs when the gas is passed into dilute sulphuric acid at 60 degC. Mercuric sulphate is used as a catalyst for the reaction, and the product formed is ethanal (i.e. acetaldehyde).


			HgSO4   

			60 degC 

   HCCH   +   H2O   ==>   CH3CHO  


Nitrile Formation
When a slight excess of ethyne and ammonia are passed over an alumina catalyst at 573 degK, ethanonitrile (i.e. acetonitrile) is produced.


				573 degK        

		HCCH   +   NH3   ==>     CH3CN + H2      

		Ethyne                  Ethanonitrile   


Polymerisation of Alkynes due to Triple Bond
The products obtained by polymerising ethyne depend on the conditions used.

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  

Substitution Reaction of Alkynes
The reactions of ethynes indicate acidic properties for the hydrogens which are attached to the carbon atoms involved in the triple bond. Ethynes readily form compounds with metal.

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 

Warning : Methyl acetylides are explosive when dry so great care should be taken in their preparation. The metal acetylides can be destroyed when they are still wet by warming with dilute acid which will regenerate the parent ethyne.



	HCC-Na(+)   +   HNO3   ==>    HCCH   +   NaNO3 


Alkynes Physical Properties

Alkynes are compounds which have low polarity, and have physical properties that are essentially the same as those of the alkanes and alkenes.

  1. They are insoluble in water.
  2. They are quite soluble in the usual organic solvents of low polarity (e.g. ligroin, ether, benzene, carbon tetrachloride, etc.).
  3. They are less dense than water.
  4. Their boiling points show the usual increase with increasing carbon number.
  5. They are very nearly the same as the boiling points of alkanes or alkenes with the same carbon skeletons.

Table of the physical properties of Alkynes



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


Alkynes Preparation

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    	         Alkyne  
The groups that are eliminated and the reagents used are essentially the same as in the preparations of alkenes.

Dehydrohalogenation of Alkyl Dihalides
This reaction is particularly useful since the dihalides are readily obtained from the corresponding alkenes by addition of halogen. This amounts to conversion by several steps of a double bond into a triple bond.

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.

Reaction of Sodium Acetylides with Primary Alkyl Halides
This reaction permits conversion of smaller alkynes into larger ones. In practice, the reaction is limited to the use of primary halides because of the great tendency for secondary and tertiary halides to undergo a side reaction, elimination.


		NaNH2   

	HCCH 	==>	HCC(-)Na(+)  +  RX           


Dehalogenation of Tetrahalides
This reaction is severely limited by the fact that these halides are themselves generally prepared from the alkynes. As is the case with the double bond and a dihalide, the triple bond may be protected by conversion into a tetrahalide with subsequent regeneration of the triple bond by treatment with zinc.


Alkynes Reactivity

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.


Start of Hypertext .... Elements .... Compounds .... Index
Hypertext Copyright (c) 2000 Donal O'Leary. All Rights Reserved.