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https://docs.google.com/document/d/1ts2KjX27bln5tXUumLsIuSnDQU-P0bfN/edit?usp=sharing&ouid=109474854956598892099&rtpof=true&sd=true preparation, properties and reactions. Alkanes - Conformations : Sawhorse and Newman projections (of ethane); Mechanism of halogenation of alkanes. Alkenes - Geometrical isomerism; Mechanism of electrophilic addition: addition of hydrogen, halogens, water, hydrogen halides (Markownikoff’ s and peroxide effect); O zonolysis, oxidation, a nd polymerization. Alkynes - Acidic character; Addition of hydrogen, ha logens, water a nd hydrogen halides; Polymerization. Aromatic hydrocarbons - Nomenclature, benzene - structure and aromaticity; Mechanism of electrophilic substitution: halogenation, nitration, Friedel – Craft’s alkylation and acylation, directive influence of functional group in mono-substituted benzene. HYDROCARBONS AND THEIR CLASSIFICATIONS Compounds of only Carbon and Hydrogen are called hydrocarbons. They are parent organic compounds and all other organic compounds have been derived by replacing one or more H atoms from hydrocarbon. Hydrocarbons CHAPTER INCLUDES Alkane Alkene Acyclic (open chain hydrocarbon) Alicyclic Cyclic Aromatic CH3 Alkyne Aromatic hydrocarbons Saturated hydrocarbon/ Alkanes/Paraffins Unsaturated hydrocarbons (having= and ≡) (cycloalkanes) i.e., , etc. i.e., , Toluene ALKANES Methods of Preparation : Reactions where number of carbon atoms are increased Wurtz Reaction 2R − X + 2Na ⎯⎯dry⎯eth⎯er →R − R + NaX Here other metals in the finely divided state may also be used such as Cu, Ag etc. Methane cannot be prepared by this method. Only symmetrical alkane can be prepared by this method in good yield. TEACHING CARE Online Live Classes https://www.teachingcare.com/ +91-9811000616 Corey-House Synthesis: R - X + Li ⎯⎯→RLi + LiX RLi + CuI ⎯⎯→R2LiCu (a) (b) R2LiCu + 2R − X ⎯⎯→ 2R − R + LiX + CuX R2LiCu + 2R′ − X ⎯⎯→ 2R − R′ + LiX + CuX etc. It can be used for preparing both symmetrical and unsymmetrical alkanes. Kolbe’s Electrolytic Decarboxylation 2RCOONa (aq) ⎯⎯ele⎯ct⎯rol⎯yti⎯c →R - R + 2NaOH + 2CO2 + H2 Mechanism: RCOONa RCOO– + Na+ 2H O 2H+ + 2OH– Anodic Reaction 2RCOO– 2RCOO• + 2e– 2RCOO• R - R + 2CO Cathodic Reaction 2H+ + 2e– H Methane cannot be prepared by this method Unsymmetrical hydrocarbon (alkane) cannot be prepared Reactions where number of carbon atoms are retained Sabatier - Sanderen’s Reduction H H C = C + H2 Raney Ni 200 - 300°C C – C | | H H – C ≡ C – + 2H Raney Ni 200 - 300°C | | — C — C — | | H H Reduction of Alkyl Halides R − X + Zn ⎯⎯H⎯Cl →R − H+ HX Zn – Cu and C2H5OH or Na and alcohol can also be used 4R–X + LiAlH4 ⎯⎯→ 4RH + LiX+AlX This is a nucleophilic substitution reaction with the nucleophilic H– coming from LiAlH . R – X + (n – C4 H9)3SnH R – H + (n –C4H9)3SnX. Reduction of Alcohols, Aldehydes, Ketones and Carboxylic Acids with HI/Red P. ROH + 2HI R ⎯⎯Re d⎯⎯P →R − H + I2 + H2O 150°C (b) H R1 C = O + 4HI 2H 2H Red P 150°C R–CH3 + H2O + 2I2 C = O R2 + 4HI ⎯⎯Re d⎯P → 150°C R1 – CH2 R2 + H2O + 2I2 2H R — C 2H O + 6HI ⎯⎯Red⎯P →R − CH3 + 2H2O + 3I2 150°C OH H Clemmensen’s Reduction CH 3 − C − H + 4H ⎯ ⎯Zn−⎯Hg⎯/ H⎯Cl → CH 3 − CH 3 + H 2 O || Δ O R1 − C− R2 + 4H ⎯⎯Zn−⎯Hg⎯/ H⎯Cl →H2O + R1 − CH2 − R2 || O Clemmensen’s reduction should not be used when the carbonyl compound has a basic end in it. Wolf-Kishner’s Reduction: R1 – C– H + NH2 || NH2 → R – C = N − NH2 | H2O KOH+Glycol 453 k − 473 k R – CH3 + N2 O CH3 – C– C2H5 + NH2 – NH2 || O H CH3 C H Hydrazone C=N–NH2 +H O ⎯⎯KO⎯H⎯+ ⎯Gl⎯yc⎯o⎯l → CH3CH2C2H5 + N2 2 453 K –473 K 2 5 Hydrazone This reaction should not be used when the carbonyl compound has an acidic end in it. (a) Using Grignard’s Reagent R - X + Mg ⎯⎯eth⎯er → RMgX RMgX + H – OH R – H + MgX(OH) + H – NH2 R – H + MgX(NH2) + H – OR1 R – H + MgX(OR1) +H–O–C–R′ || O R–H +| MgX (O–C–R′) || O (b) Using Alkyl lithium compound R1 NH + R–Li (R1NH) Li + R – H. Reaction where number of carbon atom are decreased Sodalime Decarboxylation RCOONa + NaOH CaO 630 k R – H + Na2CO3 CH3 CaO CH—CH —COONa + NaOH CH3 CH—CH + Na CO 2 CH3 630 k CH3 3 2 3 Some other methods of preparation Preparation of Methane from carbides (a) Al4C3 + 12H2O 4Al(OH)3 + 3CH4 Be2C + 4H2O 2Be(OH)2 + CH4 Methane from carbon monoxide CO + 3H2 ⎯⎯Ni +⎯C → 250°C CH4 + H2O This is also called Sabatier Sanderen’s reduction Physical Properties Boiling Point (B.P.) B.P. increases with the increase of molecular mass. Among the isomers, straight chain alkane have higher b.p. than branched chain alkane. Melting Point (M.P.) The melting points do not show regular variation with increase in molecular size. The even number members have higher m.p. as compared to next alkanes with odd number of carbon atoms (ALTERATION EFFECT). Solubility They are soluble in non polar solvents but insoluble in polar solvents such as water. Chemical Properties Alkanes are generally inert towards acids, bases, oxidising and reducing agents but they give following reactions: Halogenation. Alkanes undergoes substitution reaction with halogen. Cl2 and Br2 only in presence of ultra violet light or high temperature (573 – 773K). But in presence of direct sunlight reaction is as CH + 2Cl ⎯⎯⎯⎯⎯⎯→ C + 4HCl direct sunlight CH4 + Cl2 ⎯⎯h⎯ν → CH3Cl + HCl CH3Cl + Cl2 ⎯⎯h⎯ν → CH2Cl2 + HCl CH2Cl2 + Cl2 ⎯⎯h⎯ν → CHCl3 + HCl CHCl3 + Cl2 ⎯⎯h⎯ν → CCl4 + HCl Decreasing order of reactivity of halogens towards alkanes. F2 > Cl2 > Br2 > I2 CH – CH – CH + Cl 298 K CH CH – CH + CH CH CH –Cl 3 2 3 CH3 2 Light 3 Cl 3 3 2 2 CH – C – H + Cl 298 K (CH ) C – Cl + (CH ) CHCH Cl 3 CH3 2 Light 3 3 3 2 2 Similarly CH3 CH – C – H + Br 298 K (CH ) C – Br + (CH ) CH–CH Br 3 CH3 2 Light 3 3 3 2 2 99% 1% General mechanism : Halogenation of alkanes takes place in 3 steps : Chain initiation steps : X + X sunlight Propagation : Termination R − H + X∙ ⎯⎯→R∙ + HX R∙ + X − X ⎯⎯→RX + X∙ X∙ + X∙ ⎯⎯→ X − X R∙ + R∙ ⎯⎯→R − R R∙ + X∙ ⎯⎯→R − X Among these three steps, propagation step is rate determing step. The relative rates of abstraction of various types of hydrogen follow the order : 3ºC : 2ºC : 1ºC Chlorination → 5 : 3.8 : 1 Bromination → 1600 : 82 : 1 Liquid phase For higher alkanes – Fuming HNO3, at 413 K Nitration : e.g., : C6H13 – H + HONO2 C6H13NO2 + H2O Vapour phase For lower alkanes – conc. HNO3, at 670-750 K e.g., : CH3 − NO2 ⎯⎯→CH3NO2 + H2O Oxidation : Alkanes undergo oxidation under special conditions to yield a variety of products. (a) (b) 2CH + O ⎯⎯⎯C⎯u ⎯⎯→ 2CH OH 573 / 1100 atm CH + O ⎯⎯⎯⎯→HCHO + H O Mo2O3 2R – CH + 3O Manganese acetate O 2R – C – OH + 2H O 3 2 373-430 K 2 or Ag2O Isomerization : In presence of Anhy. AlCl3 + HCl or AlBr3 + HBr, straight chain alkanes, get converted in branched alkane. CH – CH CH CH AlCl3 + HCl CH3 CH – CH – CH 3 3 3 3 200ºC, 35 atm 3 3 CH (CH ) – CH AlCl3 + HCl CH3 – CH – CH2 – CH2 – CH3 + CH3 CH3 – CH2 – CH– CH2 – CH3 CH3 2-methyl pentane 3-methyl pentane Aromatization : Alkanes having minimum 6 or more carbons when heated at 773 K under high pressure (10-20 atm) in presence of Cr2O3, V2O3, Mo2O3 supported on Alumina gets converted into aromatic hydrocarbon. CH ( CH ) CH Cr2O3 + 4H ↑ 3 2 4 3 773K, 10-20 atm 2 Benzene n-heptane Cr2O3 773K, 10-20 atm Reaction with steam : CH4 + H2O ⎯⎯Ni → CO + 3H2 Conformations of Alkanes CH3 Toluene + 4H2↑ Conformations isomers/conformers are compounds which arises due to rotation around C–C. In fact C–C rotation is hindered by an energy barrier of 1 to 20 kJ × mol–1. There are infinite number of conformers possible. Out of infinite number of conformers extremes can be discussed as Conformers of ethane : H H H H H H H H H H H H H H H H H H Eclipsed Staggered Eclipsed Staggered Sawhorse projection Newmann projection It may be noted that one extreme conformation of ethane can be converted into other extreme conformer by rotation of 60º about C–C bond. Conformers lying between two extreme are called skew conformations. 0 60º 120º 180º dihedral angle Staggered> Eclipsed Decreasingorder of stability Conformers of propane : CH3 H H CH3 H H H H H H H H Eclipsed Staggered Conformers of Butane : CH3 CH3 CH3 CH CH3 H H H CH3 H 3 60º 60º H 60º H H H CH3 (I) H CH3 H (II) H H H (III) H H H (IV) Staggered Partially eclipsed Gauche (V) Gauche CH3 H 60º Fully eclipsed CH3 H CH3 H H H (VI) Partially eclipsed Stability order : I > III ≈ V > II ≈ VI > IV ALKENES Alkene have the structural unit 〉C = C〈 . The carbon atoms carrying the unsaturation are sp2 hybridized with the p orbital laterally overlaping to form π-bonds. They have the general formula CnH2n. They are isomeric with cycloalkanes e.g. – C4H8 CH3 – CH2 – CH = CH2 Butene Nomenclature : CH2 CH2 CH2 CH2 Cyclobutane The IUPAC rules for naming alkenes are similar in many respects to those for naming alkanes. Hence few solved examples are taken. 1 2 3 4 5 (i) CH3 – CH = CH– CH2 – CH3 2 - pentene CH3 C2H5 CH3– C = CH– CH2 – CH – CH2–CH2–CH3 1 2 3 4 5 6 7 8 5-Ethyl–2–methyl–2–octene Alkene – H = Alkenyl e.g., CH2 = CH– ethenyl (Vinyl group); CH2 = CH – CH2 – propenyl (Allyl group) e.g., CH2 = CH – Cl - Vinyl chloride; CH2 = CH–CH2–OH Allyl alcohol Preparations of Alkenes From Alcohols By heating alcohol with H2SO4 or H3PO4 at 170°C. CH – CH OH 95% H2SO4 CH = CH 3 2 Mechanism : 170°C 2 2 1st step : H2SO4 ⎯⎯→ H+ + HSO – 2nd step : CH – CH – OH + H+ + 3 2 3rd step : CH – CH ⊕ H CH3 – CH2 – OH2 ⊕ CH3 – CH2 + H2O 4th step : + CH2 – CH2 H – CH2 = CH2 +H2SO4 HSO4 Here the 3rd step i.e. formation of carbocation is rate determining step. The ease of dehydration of alcohols depends on the stability of carbocations formed. Hence the order of reactivity of alcohols is ter–> sec–> pri–> CH3 – OH because the incipient carbonium ion stability is ter–> sec–> pri–> CH +. Hydride shift or Example (Hydride shift) CH3 – CH – CH – CH3 CH3 OH Mechanism : H2SO4 170°c + CH3 – C = CH2 – CH3 + CH3 (Major) CH2 = C – CH2 – CH3 CH3 (Minor) CH3 – CH – CH – CH3 + H CH3 O.. H H CH3– C – CH – CH3 CH3 – CH – CH – CH2 CH3 OH2 + + CH3 – C – CH – CH3 CH3 – C – CH2 – CH3 CH3 CH3 sec-carbocation ter-carbocation (Less stable) (More stable) H (a) CH – C = CH – CH (A) H – CH2 – C – CH – CH3 CH3 CH3 CH2 = C – CH2 – CH3 CH3 (B) Dehydration of alcohols follow Saytzeff’s rule. Hence (B) product is maximum. Saytzeff’s Rule: It states that “During dehydration of alcohols and dehydrohalogenation of alkyl halides the product formed is preferentially the one in which maximum number of alkyl groups are attached to the doubly bonded carbon atoms. Dehydration by passing over alumina (Lewis Acid) CH – CH OH Al2O3 CH = CH + H O 3 2 CH3 350°C 2 2 2 Al O CH3 2 3 250°C CH3 – CH = CH2 (CH ) C – OH Al2O3 CH – C = CH 3 3 250°C 3 2 The experimental conditions change with the structure of alcohols pri - alcohols ⎯⎯con⎯c. H⎯2S⎯O⎯4 → alkene 180 C sec - alcohols ⎯⎯85%⎯H⎯3PO⎯3 → 170°C alkene OH ⎯8⎯5%⎯H3⎯P⎯O3→ 170°C ter - alcohols CH2 – OH ⎯2⎯0%⎯H2⎯S⎯O4→ 85°C H+ / heat alkene Dehydrohalogenation of alkyl halides β α CH3 – CH2 – CH2 – CH2 – Br alc KOH Δ CH3 – CH2 – CH = CH2 β α β CH – CH – CH – CH alc KOH CH – CH = CH – CH + CH – CH – CH = CH 2. 3 2 | 3 Δ 3 3 3 2 2 Br (Major) (Minor) CH3 CH3 CH2 | | || CH – CH – C – CH alc KOH CH3 – C – CH – CH3 + (CH3)2 C = C(CH3)2 3. 3 | 3 Δ Br (Minor) | CH3 (Major) The base used may be strongly basic anions like OH–, RO–, C2H5O– (CH3)3CO– etc.) The group leaving (i.e., halogen) is a good leaving group if it is the conjugate base of a strong acid. (i.e., weakly basic halide ion). One may also use sulphonates. Dehydrohalogenation of 2° and 3° alkyl halides follow Saytzeff’s Rule. Exception to Saytzeff’s Rule (Hofmann Rule) When dehydrohalogenation is carried out with potassium, tertiary butoxide there is formation of less substituted alkene. CH3 | (CH ) COH CH3 CH – CH – C = CH (CH ) CO– + CH CH C – Br 3 3 CH – CH = C 3 2 | 2 3 3 3 2 | CH3 75°C 3 (Minor) CH3 CH3 (Major) Dehalogenation Reactions | | alc | | – C – C – | | Br Br + Zn KOH – C = C – + ZnBr2 VICINAL 2 R – CH Br alc + Zn KOH Br R – CH = CH – R + 2ZnBr2 GEMINAL By controlled hydrogenation of alkynes (i) CH3 – C ≡ C – CH3 + H2 Pd/BaSO4 LINDLAR’S CATALYST CH3 C = C CH3 Cis - 2 - Butene H H Here we may use small amounts of sulphur or quinoline also. CH – C ≡ C – CH Na/LiqNH3 CH3 H C = C 3 3 Δ H CH3 trans-2-Butene By heating Quaternary Ammonium hydroxide (C2H5)NOH Δ CH2 = CH2 + (C2H5)3N + H2O Kolbe’s Electrolysis CH2COONa | CH2COONa + 2H2O electrolysis CH2 = CH2 + 2CO2 + 2NaOH + H2 Sodium succinate Mechanism is similar to preparation of alkanes. Pyrolysis CH CH CH CH ⎯⎯Δ → CH –CH=CH + CH 3 2 2 3 770 K 3 2 4 CH CH CH CH CH CH ⎯⎯Δ → CH CH=CH + CH CH CH 3 2 2 2 2 3 770 K 3 2 3 2 3 By heating saturated hydrocarbon with SiO2 C H – CH CH – C H ⎯⎯SiO⎯2 → C H –CH=CH–C H + H 6 5 2 2 6 5 Δ 6 5 6 5 2 Physical Properties Solubility They are insoluble in water but soluble in organic solvents. Boiling point The boiling point of cis–alkenes is usually higher than corresponding trans–alkenes (More polarity). Melting point The melting point of trans-alkenes is usually greater than cis-alkene. (trans form is more symmetrical). Chemical Properties In alkenes C = C bond is made of stable σ-bond and reactive π-bond. As π-bond can easily be broken, alkenes undergo addition reactions. C = C + XY C – | C | X Y Being electron rich species they react with electrophiles in three ways. Ionic Mechanism C = C E – Nu C – C + – + Nu | E Free Radical Mechanism C = C E – E C – C + E | E Transition State C = C+ E – Nu C – C E Nu Reactions : Hydrogenation : C = C + H2 Pd or Pd or Ni Δ C – C H H The relative rates of hydrogenation is CH2 = CH2 > RCH = CH2 > R2 C = CH2 > RCH = CHR > R2 C = CR2. This is due to the fact that as number of alkyl groups increase the steric hindrance increases and there by rate decreases. Halogenation : X CH = CH + X ⎯⎯⎯C⎯C⎯l4 ⎯⎯→ | 2 2 2 Or inert solvent CH2 – CH2 | X CH = CH Cl Cl ⎯⎯⎯C⎯C⎯l4 ⎯⎯→ | 2 2 2 Or inert solvent CH2 – CH2 | Cl The addition always leads to the formation of trans addition product. Mechanism : δ– X δ+ X C = C + X2 C …— C Transition State (T.S.) δ– X C …– C C – C + X X | C — C | X X e.g., CH3 – CH = CH – CH3 + Cl2 – 9°C CH3– CH – CH – CH3 | | Cl Cl Addition of Halogen Acid C = C + HX C — C H X The reactivity order of halogen acid is HI > HBr > HCl Mechanism : R–CH=CH2 + HX b R–CH–CH2 T.S. (a) – R–CH–CH3 + X – (b) R– CH2–CH2 + X Here the transition state cleaves to from the most stable carbocation hence (a) cleavage takes place and hence, + – are formed R – CH2 – CH3+ X i.e., R – CH – CH3+ X R – CH – CH3 | X Markownikoff’s Rule may therefore be applied. It states that, “During the addition of unsymmetrical reagents to unsymmetrical alkenes, the negative part of the addendum goes to carbon of double bond with least number of atoms”. Example : δ— X H b CH3–CH=CH2 + HX (a) CH3– CH – CH2 CH3–CH–CH3 + X A CH3–CH2–CH2 + X B Since A is more stable than B. Hence A is formed and we get + CH3 – CH – CH3 X CH3 – CH – CH3 | X CH3 – CH = CH2 + HBr CH3– CH– CH3 | Br Kharasch - Mayo Effect If the above reaction is carried out in the presence of some peroxide then addition takes place contrary to Markownikoff’s Rule CH3 – CH = CH2+ HBr ⎯P⎯ero⎯x⎯ide→ CH3– CH2 – CH2 – Br Explanation This can be explained on the basis of free radical formation • Step 1 : R – O – O – R 2R – O • • Step 2 : R – O + HBr ROH + Br CH – CH – CH • (A) | 2 Step 3 : CH3 – CH = CH2 + Br Br 1° Carbon free radical • CH3 – CH – CH2 – Br (B) (B) is more stable than (A). 2° Carbon free radical Addition of Hypohalous Acid C = C+ HOX CH2 = CH2 + HOCl CH2 – CH = CH2 + HOBr Addition of Water C — C OH X CH2 — CH2 OH Cl CH2 – CH – CH2 OH Br This reaction also takes place via carbocation mechanism (Rearrangement possible). CH3 (CH3)2 C = CH2 + H2O H+ 10% H2SO4 CH3 – C – OH CH3 Addition of cold and conc. H2SO4 Carbocation Mechanism Followed, (Rearrangement Possible) R – CH = CH2 + H2SO4 R – CH – CH3 H2O Heat R – CH – CH3 HSO4 OH Oxy-Mercuration - Demercuration Synthesis of alcohols from alkenes is in accordance with Markownikoff’s Rule (No carbocation formed) R – CH = CH Hg(OAc)2/THF,H2O R – CH – CH 2 ii) NaBH /OH– | 3 OH Addition of Oxygen: Ag CH2 CH2 CH2 = CH2 + ½ O2 570 K O Ozonolysis: C = C + O3 O CCl4 C Zn/H2O –H2O2 2 C = O O— O Example : CH3CH = CH2 + O3 CCl4 O CH — CH CH Zn/H2O –H2O2 CH3 – CHO + HCHO O — O CH3 + O || O3/CCl4 Zn/H2O (–H2O2) CH3 Ozonalysis helps to locate the positions of double bonds in alkene. Hydroboration Oxidation R – CH = CH2 + B2H6 0°C ether (R – CH2 – CH2)3 – B H2O2 OH– R – CH2 – CH2 – OH + H3BO3 (Product is Antimarkownikov product.) Oxidation Reactions Reaction with Baeyer’s Reagent (Cold dilute Alkaline KMnO4, Hydroxylation) C = C + alk KMnO C – C | | OH OH The addition is a syn addition to form vicinal dihydroxy compounds. With hot KMnO4 or acidic KMnO4 CH2 = CH2 ⎯⎯[O⎯] → Hot KMnO 4 2HCOOH ⎯⎯→ 2CO2 + 2H2O CH3 − CH = CH2 ⎯⎯KM⎯nO⎯4 →CH3COOH + CO2 + H2O H (CH ) C = C(CH ) ⎯⎯Ho⎯t → 2 CH3 3 2 3 2 KMnO4 CH3 C = O Substitution Reaction CH3–CH = CH2 + Cl2 ⎯⎯500⎯−60⎯0⎯°C → Cl–CH2–CH=CH2 + HCl– This type of reaction takes place at a carbon atom attached to double bond carbon. This is called allylic substitution. Mechanism Step - 1 : Cl – Cl ⎯5⎯00 ⎯−60⎯0⎯°C→ 2Cl• Step - 2 : CH =CH–CH +Cl• HCl + CH = CH– CH• 2 3 ⎯⎯→ 2 2 Step - 3 : CH2 = CH - CH• +Cl2 ⎯⎯→ CH2 = CH – CH2 Cl + Cl• Wohl Zeigler Reaction CH3 – CH = CH2 + Br2 ⎯L⎯ow ⎯con⎯c. o⎯f B⎯r2→ CH2 = CH–CH2–Br The low concentration of Br2 is obtained from NBS O || CH3 – CH = CH2 +CH2 — C CH2 — C || O N – Br CH2 = CH – CH2Br + O || CH2 — C | CH2 — C || O N – H Addition of Carbenes (i) CH2 – N ≡ N hν .. CH2 + N2 (ii) CH3 – CH = CH2 + Isomerization : C&H2 ⎯⎯h⎯ν → CH3 – CH – CH2 CH2 CH – CH – CH – CH = CH 700 - 970 atm CH – CH – CH = CH – CH 2 2 2 Polymerization : 2 or Al2 (SO4)3; 470 K 3 2 3 CH = CH Pressure O2 ( CH2 – CH2)n CF2 = CF2 Trans O2 ( F2C – CF2)n ALKYNES Compounds containing the structural unit – C≡C– are called Alkynes. Like the double bond it is unsaturated and highly reactive towards the reagent that double bonds react with and also towards others. The simplest member of the alkyne family is acetylene, C2H2.Each of the carbon atoms carrying the triple bond are sp hybridized. 1s 2s 3p C sp C sp Alkynes the compounds having general formula CnH2n – 2 where n ≥ 2 it can be categorize by two ways. Terminal alkynes : Alkynes having triple bond at one end of the carbon attached to H e.g. CH3–C ≡ C–H, CH3 – CH2–C≡CH. Terminal hydrogen is acidic in nature. Non-terminal alkynes : Alkynes in which both triple bonded carbons are attached to alkyl group. Preparation of Alkynes From Dehydrohalogenation of vicinal or geminal dihalides CH2 | — CH2 | alc. KOH Δ CH ≡ CH + 2H2O + KBr Br Br R– CH – CH alc. KOH R – C ≡ CH + 2H O + KBr | | 2 Δ 2 Br Br Dehalogenation Reaction (a) X X | | – C – C – | | X X Zn dust Δ – C ≡ C + 2ZnX2 X 2R – C X X Zn dust Δ R – C ≡ C – R + 3ZnX2 Kolbe’s Electrolytic Decarboxylation R COOK C || C R COOK + 2H O Electrolysis R – C ≡ C – R + 2KOH + CO2 + H2 cis or trans Formation of Higher alkyne CH ≡ CH NaNH2 CH ≡ C Na CH ≡ C Na + CH3Cl CH ≡ C – CH3 + NaCl R–C ≡ CH + NaNH2 RC ≡ CNa + NH3 R–C ≡ C Na + R′ X R–C ≡ C–R′ + NaX Chemical Properties Alkynes undergo electrophilic addition generally but in the presence of salt of heavy metals which forms complexes with multiple bonds it undergo nucleophilic addition reaction. Addition Reaction Electrophilic addition reaction : Addition of halogen CH + Cl2 CH CHCl = CHCl Cl2 acetylene dichloride CHCl2 CHCl2 acetylene tetrachloride (westron) CH3 – C ≡ CH Br2/CCl4 CH3 – C = C Br Br Br2/CCl4 H Br Br CH3 – C – C – H Br Br Trans-1, 2-dibromopropene 1, 1, 2, 2-tetra bromo propane Addition of halogen acids HC ≡ CH HCl CH = CHCl HCl CH CHCl CH3COOH 2 Vinyl chloride CH3COOH 2 2 Ethylidene chloride CH ≡ CH Cl2/H2O or CH = CHOH OH HOCl CHCl – CH – OH –H2O HCCl2 – CHO HOCl Cl Dichloro acetaldehyde Nucleophilic addition reaction : Because of greater electronegativity of sp hybridized C as compared to sp2 hybridized carbons, Alkynes are more susceptible to nucleophilic addition reactions than alkenes. It is due to formation of some sort of complex of heavy metal ion with π electrons like Hg2+ and this results decrease in electron density around triply bonded carbon atoms and this can be attacked by nucleophiles. Addition of H2O or hydration of alkyne or Kucherov reaction + 2+ H , Hg H OH O tautomerise + H2O CH 333 K H – C = C – H Vinyl alcohol CH3 – C – H Acetaldehyde R – C ≡ C – H + H2O Addition of HCN + 2+ H , Hg OH R – C = CH2 tautomerise O R – C – CH3 Ketone CH + HCN Ba(CN)2 CH CH2 CHCN Vinyl cyanide Similarly alkynes adds acids in presence of lewis acid catalyst or Hg2+ give vinyl ester. O HC≡C – H + H3CCOOH Hg2+/353 H CH3 – C – O – CH = CH2 Vinyl ethanoate O O R R – C ≡ CH + CH – C – OH BF3 CH – C – O – C = CH a-alkyl vinyl ethanoate Reaction of Acidic H Atom Alkynes having acidic H atom reacts with metals like Na, K, evolves H2 gas. CH + Na CH (1 mol) C – Na C – H + 1/2 H2↑ CNa C – H + Na Monosodium acetylide C – Na + 1/2 H2↑ C – Na Disodium acetylide HC ≡ C − H + NaNH2 → HC ≡ C − Na + NH3 ↑ ⎯⎯NaN⎯H⎯2 →NaC ≡ C − Na + NH3 ↑ Reaction with Tollens reagent : When alkyne reacts with tollens reagent (Ammonical AgNO3 solution) at forms white precipitate of silver acetylide. C – H C – H + 2AgNO3 + 2NH4OH (Tollens reagent) C – Ag C – Ag ¯ + 2NH4Cl + 2H2O (White precipitate) These acetylide are not decomposed by H2O like acetylide of Na but by mineral acids like dil HNO3. C – Ag C – Ag + 2HNO3 CH 2AgNO3 + CH R − C ≡ CH + Ag+ → R − C ≡ C − Ag + H⊕ Reaction with Ammonical Cuprous Chloride : HC + Cu2Cl2 + 2NH4OH HC CCu CCu (red ppt) ↓+ 2NH4Cl + 2H2O copper acetylide These reactions are used to distinguish terminal alkynes from other alkynes. Polymerization Reaction When acetylene is passed in red hot cutube or retube. It converted into benzene. CH 773K 3 CH C6H6 CH3 Similarly CH3 – C ≡ CH red hot Cu or Fe tube H3C CH3 (Mesitylene) CH 2 CuCl/NH4Cl CH = CH – C ≡ CH HCl Cl CH = CH – C = CH CH Cu2Cl2/NH4Cl 2 Vinyl acetylene 2 2 Chloroprene Chloroprene on polymerization gives polymer called neoprene; used as artificial rubber. Under high pressure and in presence Ni(CN)2 acetylene tetramerises. 4 CH ≡ CH Ni(CN)2 Cycloocta tetraene Reaction with S8, N2, NH3 and HCN Acetylene reacts with S8, N2, NH3 and HCN to form different heterocyclic compounds. CH CH CH + + CH 300°C S CH CH + N2 Electric spark 2HCN 1 S (Thiophene) 8 8 CH CH Δ + CH + CH N (iv) 2 CH + HCN red hot tube CH N (Pyridine) NH3 H (Pyrole) AROMATIC HYDROCARBONS Hydrocarbons which follow Huckel rule are termed as Aromatic hydrocarbons. Huckel's rule : A planar molecule having complete delocalisation of (4n + 2) π electrons is termed as aromatic hydrocarbon (where n is any integer) e.g., 4n + 2 = 6 n = 1 ⊕ 4n + 2 = 2 n = 0 4n + 2 = 6 n = 1 CH3 4n + 2 = 6 n = 1 4n + 2 = 6 n = 1 Homologues of Benzene They are all aromatic hydrocarbons. Aromaticity is present due to benzene ring. CH3 C2H5 etc. Electrophilic Aromatic substitution reactions in Benzene : (EAS) Benzene and its homologues readily undergo EAS. As a consequence of complete delocalization of π electrons in benzene, it has π electron cloud over benzene ring which makes electrophile attack over it. General Mechanism : + E+ H + E E + H+ Halogenation + Cl (resonance stabilised intermediate) Cl ⎯⎯FeC⎯l3 → + HCl Benzene 2 dark Reaction with I2 is reversible. I + I2 + HI Hence it is carried out in the presence of conc. nitric acid to oxidise the Hydrogen Iodide formed. Nitration + HNO3 ⎯⎯H2S⎯O4⎯co⎯n⎯c. → NO2 H2O ⎯⎯H2S⎯O4⎯co⎯n⎯c. → +HNO3 363−373K NO2 NO2 Sulphonation Nitro benzene SO3H SO3H + H2SO4 + SO3 ⎯⎯He⎯at → + H2O or + ClSO3H ⎯⎯He⎯at → ΗCl Friedel Crafts Reaction Chloro Sulphuric Acid Benzene sulphonic acid Alkylation : Reactive intermediate is carbocation which can undergo rearrangement. CH3 + CH3 – Cl ⎯⎯Anh⎯ydr⎯ous⎯Al⎯C⎯l3 → + HCl + CH3CH2CH2Cl ⎯⎯Anh⎯ydr⎯ous⎯Al⎯C⎯l3 → + HCl ⎛ O: ⎜ Acylation : Reactive intermediate is acylium ion ⎝R–C+ COCH3 which cannot undergo rearrangement. + CH3 – COCl ⎯⎯Anh⎯ydr⎯ous⎯Al⎯C⎯l3 → + HCl Acetyl chloride Acetophenone Ortho and para substitution : Electron releasing groups like - R (alkyl) — O.. H, — O.. R, — NHR, — NHCOR are activating groups i.e., they increase electron density at ortho and para position, therefore, are ortho and para directing towards electrophilic substitution reactions. Meta substitution : Electron withdrawing groups such as – NO2, – CHO, – COOH, – COCH3, – CN, – SO3H, – COOR are called deactivating groups. They decrease electron density at ortho and para- position, therefore, electrophillic substitution takes place at meta-position. Halogenation of side chain : CH3 Toluene or Methyl Benzene + Cl ⎯⎯su⎯n → 2 light CH2Cl Benzyl chloride or chlorophenyl methane + Cl2 ⎯⎯hv → CHCl2 Benzal chloride or dichlorophenyl methane ⎯⎯C⎯l2 → CCl3 + HCl Benzotrichloride Oxidation : or trichlorophenyl methane O O Benzene + 5O2 + H2O 2-Butene-1,4-dioic acid ❑ ❑ ❑