Hydrocarbon. Alkynes

Hydrocarbon ALKYNES Alkynes are unsaturated hydrocarbons and General Methods of Preparation : characterised by the presence of a triple bond 1. From Gem dihalides ( by dehydrohalogenation) between the two carbon atoms ( C C). The triple Dehydrohalogenation agents are : NaNH 2 bond is also called acetylinic bond. It consists of a (Sodamide) or Alc. KOH or ROH + RONa. strong  and two weak  bonds. Alkynes are isomers of alkadienes and cycloalkenes . Property General formula C—C Bond energy 198 K cal/mole Hybridisation on C sp Bond angle 1800 C—C Bond length 1.20 A0 C—H Bond length 1.08 A0 shape linear      Characteristics of alkynes CnH 2n–2 R—C CH + NaX + NH3 (Stable by resonance) (Vinyl halide) (a) Due to stability of vinyl halide by resonance there is partial double bond in which elimination does not take place by alc. KOH so stronger base NaNH2 is used. (b) Basic strength :  is stronger base then  Ex.24 The minimum number of carbon atoms present in alkyne which contains one side chain , are Sol. 5C, (c) Trans elimination takes place in forming of alkynes. 2. From Vicinal dihalides (by dehydrohalogenation) Ex.25 The minimum number of carbon atoms present in alkyne which contains two side chain , are        Sol. 6C, Ex26 The minimum number of carbon atoms present in alkyne which contains three side chain , are R—C CH (a) Elimination of Vic. dihalides gives also alkadiene (1, 2 and 1, 3 alkadienes) but the major product is alkyne.   CH 2 Ex. Sol. 8C, CH CH2+CH2 CH— C CH—CH3+ 181 Hydrocarbon (b) Non terminal gem dihalide gives 2-Alkyne in presence of alc. KOH while gives 1-alkyne in Ex.27 Is PH of solution changed in Kolbe's electrolysis. presence of NaNH2. Sol. The concentration of NaOH solution increased so pH of solution is increased with time. 5. Ex. Preparation of higher alkynes by Grignard reagent : By this method lower alkyne is converted in to higher alkyne   CH 3.   —Br    + CH4  Dehalogenation of tetrahalo alkane : By heating 1, 1, 2, 2 - tetra halo alkane with Zn dust. + R —C CH  R—C CH + 2ZnX2 R—C CH + CH3Mg—Br  4. + CH4   From Kolbe's electrolysis : By the electrolysis of aqueous solution of sodium or potassium fumarate or maleate, acetylene is formed at anode.  R' —C + CO2 6 Mechanism :   +2K+ at anode (Alkyl and CO2 gas is formed)   Preparation of Ethyne or Acetylene: (a) From Metal carbide [ Laboratory method] Acetylene is prepared in the laboratory by the action of water on calcium carbide. CaC2 + 2H2O CH CH + Ca(OH)2 Ca+2 +   + 2H+ + 2OH–  CH (Oxygen free radical)    at cathode (KOH and H2 gas is formed) 2K+ + 2e— C—R + 2K 2K + 2H2O 2KOH + H2 CH + Ca(OH)2 (b) Manufacture : Acetylene is manufactured by heating methane or natural gas at 1500 0C in an electric arc 2CH4   CH CH + 3H2 (c) Berthelot's process : Acetylene is synthesized by striking an electric arc between carbon electrodes in presence of hydrogen.  2C + H2   CH CH 182 Hydrocarbon Since s electrons are closer to the nucleus than the p electrons, the electrons present in a bond having more s-character will be more closer to nucleus. Due to high s-character of the C—H bond in alkyne (s 50%) the electrons constituting this bond are more strongly held by the carbon nucleus, with the result the H present on CH can be easily removed as proton (d) From haloform [CHI 3 , CHCl3] : Pure acetylene is obtained when iodoform or chloroform is heated with Silver powder CHI3 + 6Ag + I3CH CH CH + 6AgI (e) Partial oxidation of methane : A recent method for manufacturing of acetylene is the controlled partial oxidation of methane at high temperature.  4CH4 + 3O2   2CH CH + 6H2O Physical Properties : . (i) First three members [C2 to C4] are gases, from C5 to C11 are liquid and after that they are solid. (ii) Alkynes are slightly soluble in H2O but soluble in CCl4, benzene, acetone and alcohol. (iii) B.P., M.P. and densities of alkynes are comparatively more than alkenes and alkanes due to more polarisation.  1.  Chemical Properties : The chemical properties of alkynes are due to two factors (a) Presence of electrons : Due to presence of loosely bonded electrons, alkynes like alkenes, undergo easily electrophilic addition reaction. Carbon-carbon triple bond is less reactive than the carbon-carbon double bond towards electrophilic addition reactions. Addition reaction : (a) Addition of hydrogen : Alkynes reacts with hydrogen in presence of a catalyst.In presence of Pt., Pd or Ni alkynes give alkanes with H2 R—C CH  R—CH CH2      R—CH2—CH3 In presence of Lindlar's catalyst [ quinoline or Nickle boride] alkynes give cis – alkene  R—C C—R'  In addition to electrophilic additons, alkynes also undergo nucleophilic addition with nucleophiles (b) Presence of acidic hydrogen atom : The hydrogen atom attached to the triple bonded carbon can be easily removed by a strong base and hence acetylene and 1-alkynes are considered as weak acids. Explanation : The amounts of s-character in various types of C—H bonds is asC—H C—H 50% 33% —C—H + cis - alkene (Stereo specific reaction) In presence of Na/NH3 alkynes give trans-alkene. (Birch Recuction) R—C C—R'   trans-alkene (Stereo specific reaction) 25% 183 Hydrocarbon (b) Electrophilic addition : Addition reactions where the addition is initiated by electrophile (positive group).The characteristic reaction of alkynes is electrophilic addition but the reactivity of alkynes towards electrophilic addition is less than alkenes because in C C, the  electrons are tightly held by carbon nuclei and so they are less easily available for reaction with electrophiles. Mechanism : Cl2 + FeCl3 Cl+ + Reactivity order of hydrocarbons for electrophilic addition . Another reasons is : The intermediates when an electrophile attack on alkene and alkynes are : (i) R—C C—R    (ii) R—CH CH—R  Stability of intermediates : R—CH  —R < R—CH2— (+) ve on more EN  —R more stable atoms is less stable So we can say that alkenes are more reactive towards electrophilic addition reaction. (1) Addition of Halogens : Reactivity order of Halogens Cl2 > Br2 > I2 Alkynes react with Cl2 or Br2 in dark in presence of metal halide and form di and tetra halo derrivatives. Reaction with dilute Br2 or bromine water: CH CH + Br2  Br—CH CH—Br R—C  CH  CH CH + Br2  184 Reaction with Iodine : CH CH + I2   CHI CHI (addition reaction) CH CH + I2  I—C Hydrocarbon (3) Addition of HOX : Alkynes react with hypohalous acids according to markownikoff's rule and form gem diol, which are unstable, lose a molecule of water and form halo aldehyde or halo ketones. C—I + NH4I Di iodoacetylene (substitution reaction) (2) Addition of halogen acids (H – X) : Addition according to Markowni-Koff's Rule. R—C Reactivity order of H – X : R—C Mechanism :  CH    C—H   (Cyclic cation) (Gem dihalides major product)   Mechanism : R—C CH Further       (Intermediate is carbocation) R—  CH2   Further +    (more stable due to +M of OH group) (less + M of Cl - Atom) so +  (more stable due to +M of X)  (only –I of X) so       (Major Product) (Unstable) (Dichloro ketones) 185 (4) Addition of BH3 / THF or B2H6 (Hydroboration) : THF - Tetrahydrofurane is used as solvent. 3R— C C—R  Since BH3 is not available as monomer so a solvent THF is used for the stability of BH3. Hydrocarbon (c) Nucleophilic addition reaction : In these reactions some heavy metal cation like Hg+2, Pb+2, Ba+2 are used. These cation attracts the – e– of alkynes and decrease the e– density and hence a nucleophile can attack an alkynes. (1) Addition of dil. H2SO4 (Hydration) : The addition of water takes place in the presence of Hg+2 and H2SO4 [1% HgSO4+40% H2SO4]. In this reaction carbonyl compounds are obtained . CH  CH + H—OH CH2 CH—OH      CH3—CHO enol unstable CH3—C CH + H—OH      enol (unstable)  A structure in which –OH group is attached to double bond carbon is called as enol (ene + – ol). This reaction is used for preparation of aldehyde and ketone. R—C  CH  Mechanism :   R—C CH  Ex. CH3—C C—CH3      (enol form less stable) (keto form more stable) 186 Ex. CH3—C   C—CH3    CH CH + CH3OH  2-butanone (2) Addition of HCN : The addition of HCN in presence of barium cyanide to form vinyl cyanide. CH CH + HCN   CH2 CHCN The vinyl cyanide is used for making polymers such as orlon and Buna-N rubber. CH2 Hydrocarbon (4) Addition of alcohols : In presence of BF3 and HgO alkynes react with alcohols and form acetal and ketal CH2 CHOCH3  Methylal (acetal) R—C CH + CH3—OH   CHCN  Orlon (3) Addition of acetic acid : Acetate acetylene combines with acetic acid in presence of murcuricsulphate. It first forms vinyl acetate and then ethylidene acetate. +    vinyl acetate Ethylidene acetate Ketal Acetylene forms acetal while other alkynes form ketal. (d) Addition of AsCl3 : In presence of AlCl3 or HgCl2 acetylene combines with AsCl3 to yield Lewisite gas. It is four times poisonous than mustard gas. CH CH + Cl—AsCl2  2-Chlorovinyl dichloro arsine (Lewisite gas) The action of Lewsite may be checked by its antidote BAL (British Anti Lewisite). BAL combines with the Lewisite to form a cyclic non toxic compound. 2, 3–Dimercapto–1–propanol [BAL] When ethylidene acetate is heated it give acetaldehyde and acetic anhydride (e) Carbonylation : Reaction of alkynes with CO in presence of Ni(CO)4 Alkyne + CO + H2O  Olefinic acid Alkyne + CO + R–OH  Olefinic ester + CO + H2O  Acrylic acid + CO +  Alkyl acrylate 187 Hydrocarbon 2 OXIDATION REACTIONS : (a) Combustion : CnH2n-2 +  Ex.28 An alkyne on oxidation with acidic KMnO4, only acetic acid is obtained what is given alkynes ? O2 nCO2 + (n-1) H2O + Heat 2HC Sol. In Oxidation of alkynes two moles of mono carboxylic acids are obtained.  CH3—C C —CH3 CH + 5O2  4CO2 + 2H2O + 312 K.cal The combustion of acetylene is used for welding and cutting of metals in which oxy-acetylene flame having high temp (30000C) is produced. (b) Oxidation with alkaline KMnO4: Oxidation with alkaline KMnO4 gives carboxylic acids. 2–Butyne (d) Oxidation with selenium dioxide : Selenium dioxide oxidises alkynes to the dicarbonyl compounds.   + 4[O]  Glyoxal Acetylene CH3—C Oxalic acid CH + 4[O] CH3—COOH + HCOOH (c) Oxidation with acidic KMnO4 or K2Cr2O7 : In presence of acidic KMnO4 or acidic K2Cr2O7. Alkynes are oxidised to monocarboxylic acids. R—C    2–Butyne Butane - 2, 3 -dione (e) Oxidation with ozone (O3) : In the ozonolysis both sp-C-atoms are converted into group. C—R' + 2[O]     RCOOH + R'COOH + 2[O]      2HCOOH glyoxal CH3—C Ozonide (Addition of ozone) (Acids) In this reaction H2O2 is oxidant which oxidise into acids. CH + 2[O]     CH3COOH + HCOOH But if we use some amount of Zn as reductant with H2O then it reduce H2O2 so oxidation does not take place H2O2 + Zn ZnO + H2O 188 Hydrocarbon R—C C—H + AgNO3 + NH4OH  R— C CAg + NH4NO3 + H2O Tollen's reagent White ppt. These reactions are used for detecting the presence of acetylenic hydrogen.These are test for distinguish alkenes and alkynes or 1-alkynes and 2-alkynes. Ex. 4. 3 CH3—CH2—C SUBSTITUTION REACTION : (Formation of metallic derivatives) Only 1-alkynes give substitution reaction and show  acidic characters  Acetylene is dibasic acid where as propyne is monobasic means acetylene can give two H+ where as propyne can give one H+. (a) Formation of sodium acetylides : Acetylene and 1-alkynes react with sodamide to form acetylides 5. H—C C—H  NaC C —H  Mono sodium acetylide NaC CNa Disodium acetylide Dry alkynides are generally unstable and explosive. These are easily converted in to original alkynes when heated with dilute acids. NaC CNa + 2HNO3  HC CH + 2NaNO3 This reaction can be used for the purification, seperation and identification of 1-alkynes. (b) Formation of copper and silver acetylides Copper and silver acetylides are obtained by passing 1-alkynes in the ammonical solution of cuprous chloride and silver nitrate (Tollen's reagent) respectivley. R—C ISOMERISATION: When alkyne-1 is heated with alc. KOH alkyne-2 is obtained. C—H + Cu2Cl2 + NH4OH  R—C CCu + NH4Cl + H2O ammonical cuprous- copper acetylide chloride [Red ppt.] CH     CH3—C C—CH3 1–Butyne 2–Butyne When alkyne -2 is heated with NaNH2 alkyne -1 is obtained CH3—C C—CH3    CH3—CH2—C CH 1–Butyne 2–Butyne POLYMERISATION : (a) Linear polymerisation : Dimerisation : When two molecules of acetylene passed through a solution of Cu2Cl2 and NH4Cl a vinyl acetylene is obtained. 2HC C—H  CH2 CH—C C—H mono vinyl acetylene When vinyl acetylene react with HCl then chloroprene is obtained. CH2 CH—C C—H   Neoprene (Synthetic rubber) 2- chloro-1,3-butadiene [chloroprene] Trimerisation : 3 molecules of acetylene.  3CH CH  CH2 CH—C C—CH Divinyl acetylene CH2 189 Hydrocarbon (b) Cyclic polymerisation : When alkyne is passed through red hot metallic tube, cyclic polymerisation takes place with the formation of aromatic compound Other reactions of acetylene :  3CH CH  Benzene 3 CH3—C  CH  Mesitylene 3 CH3—C  C—CH3  Hexa methyl benzene 3 HC CH   (vi) Reaction with HCHO : This reaction is called ethynylation. CH CH + HCHO   benzene (90% yield) 4CH CH  HC CCH2—OH  propargyl alcohol (vii) Reaction with NaOCl : (Substitution reaction) H—C   C—H + NaOCl  H—C C—Cl + NaOH H—C   C—Cl + NaOCl  1,3,5,7–cyclooctatetraene Cl—C C—Cl + NaOH Dichloro acetylene 190 USES OF ACETYLENE : Hydrocarbon Seperation of ethane, ethene and ethyne (i) Oxyacetylene flame used in welding and cutting (ii) Acetylene is used as an illuminant (iii) Acetylene is used for artificial ripenning of fruits (iv) Used for manufacture of acetaldehyde, acetic acid, ethyl alcohol, westron, westrosol, PVC, PVA, Chloroprene, butadiene, Lewisite etc. (v) It is used as a general anaesthetic. Test for alkynes : (i) Decolourization of Br2 in CCl4 solution. (ii) Decolourisation of 1% alkaline KMnO4 solution. (iii) 1- alkynes give white ppt. with ammonical AgNO3 and red ppt with ammonical cuprous chloride solution. Note : (i) and (ii) tests are used for determination of unsaturation (i.e, presence of double or triple bond in any compound) (iii) Test is used for distinguish between alkenes and 1-alkynes or 1-alkyne and 2-alkyne. 191