Alkyne Notes

Chemistry HYDROCARBONS ALKYNE GENERAL INTRODUCTION A triple bond gives an alkyne four fewer hydrogen atoms then the corresponding alkane. Therefore, the triple bond contributes two degrees of unsaturation (DU). Alkynes are not as common in nature as alkenes, but some plants do use alkynes to protect themselves against disease or predators. Acetylene is by far the most important commercial alkane. Acetylene is an important industrial feedstock, but its largest use is as the fuel the oxyacetylene welding torch. STRUCTURE AND BONDING IN ALKYNES (a) Alkynes are also hydrocarbons that contain carbon-carbon triple bond. (b) Alkynes are also called acetylenes because they are derivatives of acetylene. (c) The general formula is: CnH2n+2. (One triple bond) (d) In alkyne C ≡ C bond length is 1.20 Å (e) Its bond energy is 192 kcal. mol–1 (f) The hybridization of carbon atoms having triple bond (C ≡ C) in alkynes is sp. (g) Overlapping of these sp hybrid orbitals with each other and with the hydrogen orbitals gives the sigma bond framework which is linear (180°) structure. (h) Two π bonds result from overlap of the two remaining unhybridized p orbits on each carbon atom. these orbitals overlap at right angles (90°) to each other, forming one πbond with electron density above and below th4e C-C sigma bond, and the other with electron density in format and in back of the sigma bond. This result in a cylindrical π electron cloud around σ bonded structure. 1.20Å 1.06Å 180° H C C H H C C H Note: Any type of stereoisomerism does not arise in acetylenic bond due to linearity of C ≡ C bond. 1 Chemistry IUPAC NOMENCLATURE OF ALKYNES S.No. Compound Name 1. CH  CH Ethyne 2. CH2 – C  H Propyne 3. HC  C – CH2 – CH3 But-1-yne 4. CH3 – C  C – CH3 But-2-yne 5. CH3 Br | | CH3  CH  C C  CH2  CH  CH3 6-Bromo-2methylhept-3-yne ISOMERISM IN ALKYNES Type (i) Category Examples Chain isomerism Structural CH3 – CH2 – CH2 – C ≡ CH & CH3  CH C CH | CH3 (ii) Positional Isomerism CH3 – CH2 – CH2 – C ≡ CH & CH3 – C ≡ C– CH3 (iii) Functional group isomerism CH3 – CH2 – C ≡C – CH3 & CH3– C = C = CH – CH3 & METHOD OF PREPARATION (1) Dehydrohalogenation (a) From Gem Dihalides X H X | | | NaNH2  R  CH  C  H   R C  CH R  C  C  H alc.KOH.  HX HX | | H X   KOH  C2H5OH  C2H5 OK  H2O 2 Chemistry (b) From Vicinal Dihalides H H H | | |  C  C  C  C  | | | X X X NaNH2 alc .KOH Vinly  halide   C  C  Vic  dihalide  HX  HX ( Very Unreactive ) The elimination of one molecule of hydrogen halide yields vinyl halide which is very unreactive. Under mild conditions, the dehydrohalogenation stops at vinylic halide stage but more vigorous conditions – like the use of a stronger base like amide ion (NH2–) – are required for alkyne formation. (c) From Tetra haloalkanes (Dehalogenation) X X | | R CCH | | X X X X | |   Zn(dust )   R  C  C  H    R C  C  H 3000 C Zn ( dust ) In the above reaction it is necessary that the four halogen atoms must be attached at vicinal carbons. 3 Chemistry If they are attached at the two ends then the product alkene is obtained. (2) From Kolbe’s Synthesis O ||   H  C  C  OK ||   H  C  C  OK || O  O ||  HCCO ||  2K   HCCO || O Potassium Maleate At Anode O ||   H  C  C  OK ||   H  C  C  OK || O  O || H  C  C  O. ||  2e H  C  C  O. || O  H C |||  2CO2 H C At Cathode 2K+ + 2e– ⟶ 2K• 2K• + 2H2O ⟶ 2KOH + H2 (3) From Alkynes: (To form higher Alkyne) (i) With Na: When acetylene or 1-alkyne react with Na in presence of liq. NH 3 then an intermediate compound sodium acetylide or sodium alkyne is formed which gives higher alkyne with alkyl halide.  liq.NH3 X R 2H  C  C  H  2Na   2H  C  CNa   H C  C R H3  liq .NH3 R 'X 2R  C  C  H  2Na   2R  C  CNa   R  C  C  R ' NaX H2 (ii) With Grimard reagent: When acetylene or 1–alkyne react with GR then alkane and unsaturated GR is formed which further react with alkyl halide and form higher alkyne. 4 Chemistry RX H – C ≡ CH + RMgX ⎯⎯⎯ HC ≡ C MgX   H–C≡C–R  MgX2 Acetylene R – C ≡ C – H + R Mg X ⎯⎯⎯ R – C ≡ C MgX ⎯⎯⎯⎯ R – C ≡ C – R' (4) Preparation of Ethyne of Acetylene (a) From Metal carbide [Laboratory method]: Acetylene is prepared in the laboratory by the action of water on calcium carbide. CaC + 2H O → CH ≡ CH + Ca(OH)   Ca 2  C  C 2H  2OH  CH  CH  Ca (OH)2 (b) Manufacture: Acetylene is manufactured by heating methane or natural gas at 1500°C in the electric arc. 2CH ⎯⎯⎯⎯⎯⎯⎯⎯⎯ CH ≡ CH + 3H (c) Berthelot’s process: Acetylene is synthesized by striking an electric arc between ∘ carbon electrodes in presence of hydrogen. 2C + H 2 ⎯⎯⎯⎯⎯ CH ≡ CH (d) From haloform [CHI3, 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 method at high temperature. ° 4CH4 + 3O2 ⎯⎯⎯⎯⎯ 2CH ≡ CH + 6H2O PHYSICAL PROPERTIES (a) Alkynes are colourless, odourless and tasteless. (b) Lower alkynes are partially soluble in H2O. (It is due to its polarizability). (c) Higher alkynes are insoluble in water due to more % of covalent character. (d) Completely soluble in organic solvents. (e) Melting point and boiling point increases with molecular mass and decreases with number of branches. (f) Up to C4 alkynes are gaseous C5 – C11 are liquid, C12 & above are solids. (g) Pure acetylene is odourless and impure acetylene has odour like garlic. It is due to impurities of Arsene (AsH3) & Phosphine (PH3). (h) Acetylene & 1-alkyne are acidic in nature. It is due to greater electronegativity of sp hybridized ‘C’. (i) Acetylene has two acidic hydrogen atoms. It can neutralise two equivalents of base at the same time. So, it is also called as dibasic acid but the base should be very stronger as ‘NH2 or –CH3 etc. 5 Chemistry CHEMICAL PROPERTIES The chemical properties of alkyne are due to two factors: Presence of 𝛑 electrons: Due to presence of loosely bond π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 reaction. In addition to electrophilic additions, alkynes also undergo nucleophilic addition with nucleophiles 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 I-alkynes are considered as weak acids. Ex: The amounts of s-character in various types of C–H bods is as– ≡C–H =C–H –C–H 50% 33% 25% Since s electrons are closer to the nucleus than the p electrons, the electrons in a bond having more s-character will be 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. The acidic nature of the three types of – C–H bonds as ≡ C–H > = C–H > – C–H sp sp2 sp3 Relative acidic order H2O > ROH > HC ≡ CH > HNH2 > CH2 = CH2 > CH3 – CH3 (1) Addition Reaction Electrophilic Addition: Addition reactions where the addition is initiated electrophilic (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 π electrons are tightly held by carbon nuclei and so they are less easily available for reaction for reaction with electrophiles. Reactivity order of hydrocarbons for electrophilic addition Alkenes > Alkynes > Alkanes. Another reason is: The intermediates when an electrophile attack on alkene and alkynes a are: H |  H (i) R  C  C  R  R  C  C R 6 Chemistry (ii)   H R  C  CH  R   R  CH  CH  R | H Stability of intermediates  R  CH  C R (  ) ve on more En   R  CH 2  C H  R More stable Atoms are less stable So, we can say that alkenes are more reactive towards electrophilic addition reaction. (a) ADDITION OF HYDROGEN Hydrogenation H | Ni Ni R  C  C  H  H2   R  C  CH  H2    CH2  CH3 HighTemp. HighTemp. | H The above reaction is called as Sabatier Sanderson’s reaction. Hydrogenation in the presence of lindlar’s Catalyst Addition up to alkene takes place in cis manner. (i) H  VC  C  H  H2     PdBaSO4  H H C=C H (ii) CH − C ≡ C − CH + H H ⎯⎯⎯⎯⎯⎯⎯⎯⎯ Hydrogenation by Na + NH3 (liq.) Addition up to alkene takes place in trans manner (i) Na CH  CH   NH ( liq ) (ii) R − C ≡ C − R′ ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ / 7 Chemistry Mechanism  Na  N a  e R – C C – R’ + e– ⟶ 𝛩 𝑅 − 𝐶 = 𝑅′ ⊖ −𝑁𝐻2 ↓ 𝐿𝑖𝑞. 𝑁𝐻3 • R − C = C − R′ | H ⊕ ↓ Na → N a + e− ⊖ R − C = C − R′ | H ⊖ −NH2 ↓ Liq. NH3 H | R − C = C − R′ | H (b) ADDTION OF BH3 / THF OR B2H6 (HYDROBORATION): THF – Tetrahydrofuran is used as solvent. BH3 3R-C Ξ C – R    R  C  C  3 B THF | | H H Since BH3 is not available as monomer so a solvent THF is used for the stability of BH3. 𝛅 𝛅 𝐑−𝐂𝚵 𝐂 −𝐑+ 𝛅 𝐁 𝐇𝛅 𝐇𝛅 𝐇𝛅 ⟶ 𝐑 − 𝐂𝐇 = 𝐂 − 𝐁𝐇𝟐 | 𝐑 R  CH  C  BH2  2R  C  C  R  (R  CH  C)3 B | | R R H2O2/(-OH) Basic Oxidation (R − C = C −)3 B | H | R H2/H2O R − C = C − OH + B(OH)3 → R  CH2  C  R | | || H R O (enol form) R R +B C=C | | (O H H H) (cis-Alkene) (Ketones) 8 Chemistry Ex.  H2O / OH BH CH3  C  C  CH3   (CH3  C  C)3 B   CH3  CH C −OH → CH − CH C CH3 THF | | | || H CH3 − (𝐤𝐞𝐭𝐨𝐟𝐨𝐫𝐦 (𝐞𝐧𝐨𝐥𝐟𝐨𝐫𝐦 𝐥𝐞𝐬𝐬𝐬𝐭𝐚𝐛𝐥𝐞) 𝐦𝐨𝐫𝐞𝐬𝐭𝐚𝐛𝐥𝐞) (c) ADDITION OF HALOGENS (HALOGENATION) In presence of Lewis acid as a catalyst alkyne form tetrahaloderivative with halogen. X X | | X2 X | FeX3 R  C  C  H  X  X   R − C = C − H ⎯⎯⎯ R − C − C − H X  F ,Cl ,Br − FeX3 + X2 → FeX 4 + X | X + | X | X X+ R–C =C-H +X − R-C=C-H | X X-X+FeX3 X-X+ | R-C-C-H | X +X𝑋−𝑋 R-C Ξ C-H ⎯⎯⎯ X X | | R-C-C-H | | X X Reaction with dilute Br2 or bromine water CH ≡ CH + Br2 ⎯⎯⎯ Br – CH = CH – Br CH = CH + Br2 ⎯⎯⎯⎯⎯ Br − C − C − Br | | | | Reaction with iodine CH ≡ CH + I2 ⎯⎯⎯⎯⎯⎯ CH ≡ CH + I2 ⎯⎯⎯ CHI = CHI (addition reaction) I – C ≡ C – I + NH4I (substitution reaction) Di diiodoacetylene 9 Chemistry (d) ADDITION OF HALOGEN ACIDS (H-X) {HYDROHALOGENATION} Addition according to Marconi-KOff’s Rule. Reactivity order of H – X: HI > HBr > HCl > HF | R – C ≡ C – H ⎯⎯⎯ | R−C−C−H | | (Gem dihalides major product) Mechanism +  H R  C  CH   R−C =C−H [H X ] | H (Intermediate is carbocation) ⊕ X | X− R – C = CH2 ⎯⎯⎯ R − C = CH2 Further X | X | R  C  C H2  | H H+ R − C = CH2 ⎯⎯ ( morestabledueto Mof X ) X |   R  C C H 2 | H (only 1of X ) So X | X R  C  CH   3  X | R  C  CH 3 | X (Major product ) (e) ADDITION OF HOX: Alkynes react with hypohalous acids according to markwonikoff’s rule ant form gem diol, which are unstable, lose a molecule of water and form halo aldehyde or halo ketones. Reactivity order HOCl > HOBr > HOl R − C ≡ CH + HOCl → R − C − CHCl || 10 Chemistry Mechanism +  O   R − C = C H → R – C = CH [ HOCl ] R − C ≡ CH + | :Cl: + : Θ OH R – C = CH ⎯⎯⎯ + Cl Cl Cyclic Cation OH | R – C = CH | Cl Further OH | Cl ⊕ R − C = C H ⎯⎯⎯⎯ | Cl OH | R  C CH | | OH Cl ( morestbledueto  Mof OHgroup )  Cl |  R  C  CH | | Cl OH ( less  MNof Cl  Atom ) So Cl OH |  |  H2O OH R  C  C H   R  C  CHCl   R  C CHCl2 2 || | | | O (Dichloroketones ) OH Cl OH ( Unstable) (2) Nucleophilic Addition Reaction In these reactions some heavy metal cation like Hg+2, Pb+2, Ba+2 is used. This cation attracts the π– e– of alkynes and decrease the e– density and hence a nucleophilic can attack an alkyne. (a) 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 → 11 Chemistry A structure in which –OH group is attached to double bond carbon is called as enol (enol + – ol). This reaction is used for preparation of aldehyde and ketone. dil. H SO (1%) 2 4  R  C  CH R – C ≡ CH  3 dil HgSO (40%) || 4 O Mechanism ⊕ R – C ≡ CH ⎯⎯⎯ R − C = CH → R – C = CH | +2 : Hg : : | R – C = CH ⎯⎯⎯⎯⎯⎯⎯ ⎯⎯⎯ R − C = CH +2 | Hg ⟶ R − C − CH || (Keto more stable) OH Ex. O || | H / H2O CH3 – C≡ C – CH3  CH3  C  C  CH3  CH3  C CH2  CH3 Hg 2 | H 2 but an one (b) ADDITION OF HCN Addition takes place in the presence of CuCl Cucl H  C  C  H  H  CN   CH2  CH  C  N 770 (c) ADDITION OF ACETIC ACID: Acetate acetylene combines with acetic acid in presence of mercurocuprate. It first forms vinyl acetate and then ethylidene acetate. CH ||| H | 𝐻𝑔 +2 C H + OOCCH3 ⎯⎯⎯⎯ CH2 || H | CH 2 || C H + OOCCH3 C HOOCCH3 vinylacetate CH 3 COOH ⎯⎯⎯⎯⎯⎯⎯ CH 3 || C H(OOCCH3 )2 Ethylidene acetate 12 Chemistry 𝐂𝐇𝟐 = 𝐂𝐇𝐎𝐂 − 𝐂𝐇𝟑 || 𝐎 𝐏𝐨𝐥𝐲𝐦𝐞𝐫𝐢𝐬𝐚𝐭𝐢𝐨𝐧 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ Vinyl Acetate (Monomer) 𝐇𝐲𝐛𝐫𝐨𝐥𝐲𝐬𝐢𝐬 (-CH2-CH-)n ⎯⎯⎯⎯⎯⎯⎯ | O – C – CH3 || O Poly viny 1acetate (Polymer: adhesive compound) (−𝐂𝐇𝟐 ) − 𝐂𝐇 −)𝐧 + 𝐧𝐂𝐇𝟑 𝐂𝐎𝐎𝐇 | 𝐎𝐇 𝐏𝐨𝐥𝐲𝐯𝐢𝐧𝐲𝐥𝐚𝐥𝐜𝐨𝐡𝐨𝐥 When ethylidene acetate is heated it gives acetaldehyde and acetic anhydride O CH3-CH CH3 – C OC – CH3 O OC-CH3 O ∆ ⎯ CH3 – C CH3 – CHO + O O (d) ADDITION OF ALCOHOLS: In presence of BF3 and HgO alkynes react with alcohols and form acetal and ketal OCH3 CH≡CH+CH3O H OCH3 OCH3 | R-C = CH2 R-C ≡ CH + CH3 – CH ⎯⎯⎯⎯⎯ OCH3 | R-C = CH2 | OCH3 Ketal Acetylene forms acetal while other alkynes form ketal. (3) 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 ⟶ CH − Cl || CHAsCl 2-Chlorovinyl dichlorvos arsine (Lewisite gas) 13 Chemistry The action of Lewisite may be checked by its antidote BAL (British Anti Lewisite). BAL combines with the Lewisite to form a cyclic nontoxic compound. | | 2,3-Dimercapto-1-propanol [BAL] | CH − CH− C H (4) Carbonylation Reaction of alkynes with CO in presence of Ni (CO)4 Alkyne + CO + H2O Alkyne + CO + R – OH + CO + H2O ( ) ( ) ⎯⎯⎯⎯⎯ Olefinic acid ⎯⎯⎯⎯⎯ ( Olefinic ester ) ⎯⎯⎯⎯⎯⎯ ( | H + CO + H2O | O ) ⎯⎯⎯⎯⎯⎯ (5) Oxidation Reactions (a) COMBUSTION CnH2n–2 + 2HC ≡ CH + 5O2 𝑂 ⟶ nCO2 + (n–1) H2O + Heat ⟶ 4CO2 + 2H2O + 312 K.cal The combustion of acetylene is used for welding and cutting of metals in which oxyacetylene flame having high temp (3000°C) is produced. (b) OXIDATION WITH ALKALINE KMNO4: Oxidation with alkaline KMnO4 gives carboxylic acids. 𝒂𝒍𝒌. 𝑲𝑴𝒏𝑶𝟒 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ COOH | COOH Oxalic acid 14 Chemistry . CH3 – C ≡ 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 ≡ C – R' + 2[O] → R C C R' H 2 O+[O] ⎯⎯⎯⎯⎯⎯⎯ RCOOH + R'COOH O O CH +2[O] → CH ≡ ≡ CH CH +2[O] [ ] 2HCOOH ⎯⎯⎯⎯⎯⎯ CH3 – C ≡ CH + 2[0] → CH3-C=O | CHO H 2 O+[O] ⎯⎯⎯⎯⎯⎯⎯ CH3COOH + HCOOH (d) OXIDATION WITH SELENIUM DIOXIDE: Selenium dioxide alkynes to the tricarbonyl compounds. CHO CH CHO 2[O] Glyoxal CH SeO 2 CH3 C CH3 CO CH3 C 2[O] SeO2 2-Butyne CH3 CO Butane-2, 3-dione (e) OXIDATION WITH OZONE (O3): In the ozonolysis both sp-C-atoms are converted into −C − C − group. || O || O O R-C ≡ C-R=O3 → R–C–C–R → | | O O → Ozonide (Addition of ozone) 𝐑– 𝐂– 𝐎𝐇 + 𝐑 − 𝐂 − 𝐎𝐇 || 𝐎 || 𝐎 (Acids) 15 Chemistry In this reaction H2O2 is oxidant which oxidase R − C − C − R into acids. || || But if we use some amount of Zn as reductant with H 2O then it reduces H2O2 so oxidation does not take place H2O2 + Zn → ZnO + H2O (3) Substitution Reaction: (Formation of metallic derivatives) Only 1-alkynes gives substitution reaction and show acidic characters ≡ C − H 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 so diamide to form acetylides H – C ≡ C – H ⎯⎯⎯⎯⎯ NaC ≡ C – H ⎯⎯⎯⎯⎯ NaC ≡ CNa Mono sodium Disodium acetylide acetylide Dry alkalinizes 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, separation and identification of 1alkynes. (b) FORMATION OF COPPER AND SILVER ACETYLIDES: Copper and silver acetylides are obtained by passing 1-alkynes in the ammoniacal solution of cuprous chlorides and silver nitrate (Tollen’s reagent) respectively. R – C ≡ C – H + Cu2Cl2 + NH4OH → R – C ≡ CCu + NH4Cl + H2O ammoniacal cuprouschloride copper acetylide [Red ppt.] 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. (4) Reaction with NaOCl Acidic hydrogen is substituted by ‘Cl’ (i) (ii) NaOCl H  C  C  H   Cl  C  C  Cl NaOCl R  C  C  H    R  C  C  Cl 16 Chemistry Mechanism R-C≡C-H R-C≡C-Cl R–C≡ C (5) Other reactions of acetylene NH3 Electric spark NH3 High temp. 2HCN CH CH CH CH : N H Pyrole CH2N2 –N2 : HCN Red hot Fe N Pyridine CH CH Cold either solution CH CH CH N  NH CH2N2 Cold either solution CH CH CH2 Biocyclo (1,1,0) butane –N2 CH2 Cyclo propene CH2N2 CH2 CH2N2 H2 CH2 CH CH2 N  NH Pyrazoline CH CH CH N  S  Thiphene (6) Reaction with HCHO: This reaction is called ethynylation. OH OH | | Cu HCHO CH  CH  HCHO   HC  CCH2  OH    CH2  C  C  CH2 Uses of Acetylene (i) Oxyacetylene flame used in welding and cutting (ii) Acetylene is used as an illuminant (iii) Acetylene is used for artificial ripening of fruits 17 Chemistry (iv) Used for manufacture of acetaldehyde, acetic acid, ethyl alcohol, Wistron, webtool, PVC, PVA, Chloroprene, butadiene, Lewisite etc. (v) It is used as a general anaesthetic. (7) Polymerisation Polymerisation Reactions: Alkyne mainly shows addition polymerisation reactions. Dimerization and Cyclization (a) DIMERISATION: Two mole acetylene reacts with Cu2Cl2 & NH4Cl and forms vinyl acetylene. Note: If acetylene would be in excess then product would be divinyl acetylene and the reaction is called trimerization. OH | OH | Cu 2 Cl 2 +NH 4 Cl H | H | C H2 − C ≡ C − C H2 + H − C ≡ C − H ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ H − C = C − C ≡ C − H vinyl acetylene or buteonine H | H | H | H | H | H | H | H − C = C − C ≡ C − H+ C ≡ C − H → H − C = C − C ≡ C − C = C − H divinyl acetylene (b) TRIMERISATION: If three mole of acetylene is passed into red hot iron or Cu or quartz tube, then a cyclic trimer is formed which is called benzene. CH (Octa Carbonyl cobalt with bromine) CH Benzene 18 Chemistry \\\ CH CH | C CH → CH3 – C CH CH C – CH3 mesitylene (having 3-1°, 3-2º & 3-3° carbons) Important: Mesitylene can also be obtained from acetone by condensation polymerisation. (c) TETRAMERISATION: According to the name four moles of acetylene are heated with nickel tetra cyanide, then acetylene forms a cyclic tetramer cyclol octa tetraene. CH CH CH ||| CH = CH CH ||| CH CH (Nonaromatic) 𝐍𝐢(𝐂𝐍)𝟒 CH || CH ⎯⎯⎯⎯ CH || CH CH = CH Cyclooctatetraene (8) Coupling (a) Alkyne form respective cuprous alkyne with ammoniacal cuprous chloride solution. When cuprous alkyne is reacted with potassium ferry cyanide [K 3Fe (CN)6] they converted into conjugated dyne. R – C ≡ CH + Cu (NH3)2Cl ⟶ R – C ≡ C. Cu cuprous alkynide 2R – C ≡ C.Cu ∆ R–C≡C–C≡C–R diyne (b) Coupling is also done easily by 1-alkyne in the presence of Cu2Cl2 (cuprous compound) and amine (i.e., pyridine + air) cuprous alkynide is formed (this coupling is known as oxidative coupling or glass coupling) 1 Cu2Cl2 / NH3 2R  C  CH  O2   R  C  C  C  C  R  H2O 2 1 Cu2Cl2 / NH3 2CH3  C  CH O2    CH3  C  C  C  C  CH3  H2O 2 2,4Hexadyne 19 Chemistry (9) Isomerisation (a) When 1-alkyne is treated with alcoholic KOH 2-alkyne is formed. Alco.KOH R  CH2  C  CH    R  C  C  CH3  1alkyne 2Alklyne (b) When 2-alkyne is treated with iodamide then it is converted into 1-alkyne. NaNH2 H2O CH3  C  C  CH3   CH3  CH2  C  CNa   CH3  CH2  C  CH NH3 NaOH 20