ALKYNES  Alkynes are hydrocarbons that contain a carbon – carbon triple bond (with no functional groups, only substituents present).  They are characterized by molecular formula C n H 2n- 2.  They are said to be unsaturated (having triple bond).  They are also called aliphatic hydrocarbons ( chain compounds)  Relatively not common in nature compared to alkenes.

SOME IMPORTANT TERMS FOR NAMING ALKYNES  For naming the alkynes sytematically first we have to study about the following four features:  (a) Root word  (b)Primary suffix  (c)Secondary suffix  (d)Prefix

ROOT WORD: (PARENT CHAIN/MAIN CHAIN) It means the carbon chain. Some commonly used root words in alkynes are : Chain length Root Word  C1 Meth-  C2 Eth-  C3 Prop-  C4 But-  C5 Pent-  C6 Hex-  C7 Hept  C8 Oct-  C9 Non-  C10 Dec-

Primary Suffix: The primary suffixes are added to the root word to show unsaturation(triple bond) in a carbon chain. Name of Carbon Primary Suffix IUPAC Chain name Unsaturated -yne Alkyne (CH ≡ CH)

PREFIX  Prefixes are added to the root word to indicate the presence of one or more substituents on the main carbon chain. These prefixes include:  fluoro(-F)  Chloro(-Cl)  bromo(-Br)  Ido(-I)  Nitro(-NO 2 )  Alkoxy(-O-R)  Alkyl groups

Arrangement of Prefixes, Root word and Suffixes  The prefixes, root words, primary and secondary suffixes are arranged as follows while writing the name of an alkyne.  IUPAC name = Prefixes + Root word + Primary Suffix + Secondary suffix

RULES FOR NAMING ALKENES The base or parent chain of alkyne is determined by the longest chain of carbon atoms containing the triple bond in the formula. The longest chain may be bend or twist. It is seldom horizontal. CH 3 CH 3 -CH 2 -CH 2 -CH 2 -C | ║ CH 3 -CH 2 -CH 2 CH 2 C | ║ | CH 3 -CH 2 -CH-CH 2 -C CH 3 7 CARBON CHAIN 8 CARBON CHAIN

If the chain contain only triple bond and no other substituents, the chain is numbered starting from the Carbon closest to the triple bond and the triple bond is given the lower number of its two triple-bonded carbon atoms. CH ≡ C – CH 2 – CH 3 1- Butyne OR But – 1 – yne A compound with two triple bonds is a diyne. A triyne has three triple bonds and a tetrayne has four bonds. Numbers are used to specify the locations of the triple bonds. CH≡C–C≡CH CH≡C–C≡CH 1,3-butadiyne 1,3-butadiyne buta-1,3-diyne buta-1,3-diyne

If one substituent or functional group is attached to the parent chain, then numbering should start from the side to give minimum number to the sum of substituent and the triple bond. e.g. 1 CH ║ 8 CH CH CH 2 2 C | 5 CH CH CH-CH 3 The name of this compound is 3- methy-l- octyne or 3- methyl-oct-1-yne. We will start the numbering from right side because from right side the sum of the numbers of carbon containing substituent and the triple bond is 1+3=4 but from left side it is 6+8=14.

If a double bond and triple bond are on the same parent chain, numbering will start from the side to give least sum of the number of carbons containing double and triple bonds but the structure will be named as an alkyne. 1 CH 2 ═ 2 CH – 3 C ≡ 4 C – 5 CH 3 1-pentene-3-yne pent-1-ene-3-yne

If the double bond and the triple bond are at equal distance from the ends of the chain, number the chain so that the double bond receives a lower number than the triple bond (because “ene” comes before “yne” in the alphabet). 1 CH 2 ═ 2 CH – 3 CH 2 – 4 CH 2 – 5 C ≡ 6 CH 1-hexene-5-yne Hex-1-ene-5-yne

EXAMPLES

Number and name the substituents and write the name. CH 3 – C ≡ C – CH – CH 3 | CH 3 2-methyl-3-pentyne 2-methyl-pent-3-yne

Identify the structure. Give its IUPAC name. 4-ethyl-2-hexyne 4-ethylhex-2-yne

Draw the structure of 6-bromo-2-methylhept-3-yne

Draw the structure of pent-2-yne

SYNTHESIS OF ALKYNES 1. Carbide process: Acetylene is prepared on industrial scale by the reaction of calcium carbide (CaC 2 ) with water. CaC 2 + H 2 O  CH ≡ CH + Ca(OH) 2 2. Dehydrohalogenation of vic – dihalide: Alkynes can be prepared by the reaction of vic- Dihalides with a strong base. CH 2 Br – CH 2 Br NaOH CHBr═CH 2 NaOH CH≡CH+ HBr – HBr

PHYSICAL PROPERTIES OF ALKYNES  Physical State: Alkynes with 1-4 Carbon atoms are gases. Alkynes with 5-17 Carbon atoms are liquids. Alkynes with more than 18 Carbon atoms are solids.  Melting & Boiling Point: Alkynes usually have melting and boiling points greater than those of corresponding alkenes.  Solubility: Alkynes being non – polar, are soluble in non-polar solvents such as CCl 4, ether and benzene. They are generally insoluble in water.

 Density: The density of alkynes is generally somewhat higher than those of the corresponding alkenes.  Reactivity: Since alkynes are also unsaturated hydrocarbons like alkenes, so their reactivity is more or less the same as that of alkenes.

REACTIONS OF ALKYNES

Uses Acetylene, the simplest alkyne, is used in torches to generate temperatures of around 3,000 degrees Celsius to weld metal by generating oxy-acetylene welding. It is also used as a starting material in the synthesis processes used to make polymers, and as a solvent. Alkynes are generally used as the starting materials for the manufacture of a large number of organic compounds of industrial importance such as, chloroprene, vinyl chloride etc. Alkynes are also highly important parts of some antitumor agents.