Organic Chemistry I – Alkynes

Alkynes

Hydrocarbons that have two carbon atoms in a triple bond are called alkynes

Naming Alkynes

These are hydrocarbons with a general formula C_nH_2n-2 where n is the number of Carbon atoms in a molecule. The C≡C triple bond as the functional group.
A functional group is the reacting site of a molecule /compound.
The carbon atoms are linked by at least one triple bond to each other and single bonds to hydrogen atoms.

NOTES ON ALKYNES

• Since carbon is tetravalent, each atom of carbon in the alkyne MUST always be bonded using four covalent bond /four shared pairs of electrons including at the triple bond.
• Since Hydrogen is monovalent, each atom of hydrogen in the alkyne MUST always be bonded using one covalent bond/one shared pair of electrons.
• One member of the alkyne, like alkenes and alkanes, differ from the next/previous by a CH2 group (molar mass of 14 atomic mass units).
• They form a homologous series.
• The -C≡C- triple bond in alkyne is the functional group. The functional group is the reacting site of the alkynes.
• The -C≡C- triple bond in alkyne can easily be broken to accommodate more /four more monovalent atoms. The -C≡C- triple bond in alkynes make it thus unsaturated like alkenes.

Isomers of Alkynes

Isomers of alkynes have the same molecular general formula but different molecular structural formula.
Isomers of alkynes are also named by using the IUPAC (International Union of Pure and Applied Chemistry) system of nomenclature/naming.
The IUPAC system of nomenclature of naming alkenes uses the following basic rules/guidelines:

• Identify the longest continuous/straight carbon chain which contains the -C≡C- triple bond to get/determine the parent alkene.
• Number the longest chain form the end of the chain which contains the -C≡C- triple bond so the -C≡C- triple bond lowest number possible.
• Indicate the positions by splitting “alk-positions-yne” e.g. but-2-yne, pent-1,3-diyne.
• The position indicated must be for the carbon atom at the lower position in the -C≡C- triple bond.
• Determine the position, number and type of branches. Name them as methyl, ethyl, propyl e.tc. according to the number of alkyl carbon chains attached to the alkyne. Name them fluoro-,chloro-,bromo-,iodo- if they are halogens
• Use prefix di-,tri-,tetra-,penta-,hexa- to show the number of triple -C≡C- bonds and branches attached to the alkyne.

Preparation of Alkynes

Ethyne is prepared from the reaction of water on calcium carbide.
The reaction is highly exothermic and thus a layer of sand should be put above the calcium carbide to absorb excess heat to prevent the reaction flask from breaking.
Copper(II)sulphate(VI) is used to catalyze the reaction.

CaC₂(s) + 2H₂O(l) → Ca(OH)₂ (aq) + C₂H₂ (g) p> 
Lab Preparation of Alkyne

Physical Properties of Alkynes

• Like alkanes and alkenes, alkynes are colourles gases, solids and liquids that are not poisonous.
• They are slightly soluble in water. The solubility in water decrease as the carbon chain and as the molar mass increase but very soluble in organic solvents like tetrachloromethane and methylbenzene.
• Ethyne has a pleasant taste when pure.
• The melting and boiling point increase as the carbon chain increase. This is because of the increase in van-der-waals /intermolecular forces as the carbon chain increase.
• The density of straight chain alkynes increase with increasing carbon chain as the intermolecular forces increases reducing the volume occupied by a given mass of the alkyne.

Chemical Properties of Alkynes

Combustion/Burning

Alkynes burn with a yellow/ luminous very sooty/ smoky flame in excess air to form carbon(IV) oxide and water.

Alkyne + Air → Carbon (IV) Oxide + Water
2C₂H₂(g)+ 5O₂(g) → 4CO₂(g) + 2H₂O(l/g)

Alkynes burn with a yellow/ luminous verysooty/ smoky flame in limited air to form carbon(II) oxide/carbon and water.

Alkyne + Air → Carbon (II) Oxide/Carbon + Water
2C₂H₂(g)+ O₂(g) → 2CO₂(g) + C + 2H₂O(l/g)

Addition Reactions of Alkynes

An addition reaction is one which an unsaturated compound reacts to form a saturated compound. Addition reactions of alkynes are also named from the reagent used to cause the addition/convert the triple C≡C to single C-C bond

1. Hydrogenation
   Hydrogenation is an addition reaction in which hydrogen in presence of Palladium/Nickel catalyst at 150^oC temperatures react with alkynes to form alkenes then alkanes.
   During hydrogenation, two hydrogen atom in the hydrogen molecule attach itself to one carbon and the other two hydrogen to the second carbon breaking the triple bond to double and then to single.
   Hydrogenation of Alkynes
2. Halogenation
   Halogenation is an addition reaction in which a halogen (Fluorine, chlorine, bromine, iodine) reacts with an alkyne to form a halogenoalkene then halogenoalkane.
   The reaction of alkynes with halogens is faster than with alkenes. The triple bond in the alkyne break and form a double then single bond.The color of the halogen fades as the number of moles of the halogens remaining unreacted decreases. Two bromine atoms bond at the 1st carbon in the triple bond while the other two goes to the 2nd carbon.
   Halogenation of Alkynes
3. Hydrohalogenation
   Hydrogen halides reacts with alkyne to form a halogenoalkene then halogenoalkane.
   The triple bond in the alkyne break and form a double then single bond. The main compound is one which the hydrogen atom bond at the carbon with more hydrogen (Markonikoff’s rule).
   Ethyne reacts with hydrogen bromide to form dibromoethane.
   Both But-1-yne and But-2-yne react with hydrogen iodide to form 2-iodobutane
   Hydrohalogenation of Alkynes
4. Oxidation by acidified KMnO₄/K₂Cr₂O₇
   Bromine water, Chlorine water and Oxidizing agents acidified KMnO₄/K₂Cr₂O₇ change to unique colour in presence of -C≡C-
   Acidified KMnO₄ decolorizes.
   Orange colour of acidified K₂Cr₂O₇ turns green.
   Bromine water is decolorized.
   Chlorine water is decolorized.

Uses of Ethyne

• Ethyne burns in oxygen to give a very luminous light. Hawkers use this as lamps.
• Ethyne is used for oxy-acetylene flame used for industrial welding.
• Ethyne is used for manufacture of synthetic plastics, synthetic rubbers, and synthetic fibers.
• Ethyne is also used making many industrially useful organic compounds like acetaldehyde, acetic acid, etc.