Question

Classify ethynes as acids

Answer 1

Oxidation of acids giverise acids like oxalic acid and acetic acid
 Chemical properties of alkynesAlkynes contain a triple bond (). A triple bond has one and two bonds.Some characteristic reactions of alkynes are,CombustionAlkynes burns in air or oxygen with smoky flame.Electrophilic addition reactionsCarbon-carbon triple bond, C=C, is a combination of one and two bonds. Alkynes give electrophilic addition reactions as they show reactivity due to the presence of bonds. This property is similar to alkenes but alkynes are less reactive than alkenes towards electrophilic addition reactions due to the compact CC electron cloud. Some typical electrophilic addition reactions given by alkynes are:Addition of hydrogenAn alkyne reacts with hydrogen in the presence of catalyst (Pt or Ni) at 250°C, first forming alkenes and finally alkane.For example, ethyne gives ethane in two steps.ethyne etheneethaneEthane is obtained in good yields if hydrogenation is done with a calculated amount of hydrogen in the presence of palladium or barium sulphate.Propyne gives,
Addition of halogensAlkynes react with halogens (Cl2 or Br2) in the dark, forming dihaloalkenes first and finally tetrahaloalkanes. The reaction gets accelerated in the presence of light or halogen carriers.RCCHRCX=CHXRCX2CX2alkyne dihaloalkene tetrahaloalkaneFor example, ethyne (acetylene) with chlorine gives,ethyne dichloroethene tetrachloroethaneDilute bromine water with ethyne gives dibromo, while liquid bromine gives tetrabromo derivative.tetrabromoethane ethyne 1,2-dibromoethene(acetylene)propyne gives,The order of reactivity is Cl2 > Br2 > I2.Addition of halogen acidsAlkynes reacts with halogen acids according to the Markownikoff's rule i.e. the carbon atom carrying the least number of hydrogen atoms will have the negative part of the addendum attached to it.For example, ethyne (acetylene) with HBr gives,With diluted HCl at 65°C and in the presence of Hg2+ (mercuric ion) ethyne gives vinyl chloride.vinyl chloridePropyne givespropyne 2-bromopropene 2,2-dibromopropaneThe rate of addition of halogen acids follows the order, HI > HBr > HCl
MechanismAddition of hypochlorous acidAlkynes react with hypochlorous acid according to the Markownikoff's rule.For example, ethyne with HOCl gives,dichloroethanalIn the presence of peroxides the addition of HBr takes place according to the anti-MarkowniKoff's rule.Addition of sulphuric acidAlkynes add up two molecules of sulphuric acid. For example, ethyne givesNucleophilic addition reactionsAlkynes also give the following nucleophilic addition reactions.Addition of waterIn the presence of sulphuric acid (42%) and 1 % mercuric sulphate at 60°C, alkynes add on one water molecule to give aldehydes or ketones. For example,alkyne ketoneEthyne gives ethanal and propyne gives acetone.ethyne (acetylene) ethanal (acetaldehyde)
Addition of HCNAlkynes add one molecule of HCN in the presence of Ba(CN)2. For example,Ethyne givesethyne vinyl cyanide
Addition of ozoneOzone adds up across the triple bond to give ozonides. After hydrolysis, ozonides give diketones and carboxylic acids.Ethyne gives glyoxal and formic acid,glyoxal formic acid
Substitution reactionsDue to their acidic nature, alkynes form metallic salts called alkynides e.g., sodium, silver and copper(ous) salts. Examples are,Ethyne (acetylene) has two acidic hydrogen atoms, hence it finally gives dimetal salts.Acidic hydrogen in 1-alkynesHydrogen atoms in ethyne and 1-alkynes, linked to the carbon atom having a triple bond on it, are acidic in nature. For example, ethyne (acetylene) is a weak acid: weaker than water but stronger than ammonia.