Methods of Preparation of Alkenes PDF

ABHISHEK, VINAY&ARUNSCHEMISTK ARUN 986744504 SIR: VINAYALKENES 9833395472 ABHISHEK SIR PREPARATION OF METHODS OFreactio...

ABHISHEK, VINAY&ARUNSCHEMISTK ARUN 986744504 SIR: VINAYALKENES 9833395472 ABHISHEK SIR PREPARATION OF METHODS OFreaction:(1,2- orrp-elimination) A) Elimination ,2-elimination atoms or groun of -XY byremovng convertedinto C= C, Compounds X This reactioncontaining -Cbond is also the example adjacent carbon atoms. C is ofB-eliminationreactionsince-OH grouptakes Out one hydrogen atoms fro atom from the B-carbon atom. 1) From alkyl halides (By dehydrohalogenation): correspondingalkene. Inthis elimination,reacthenceion,itis alkali it gives When an alkyl halide is tboiled with alcoholic hydrogen atom from B-carbonatom are halogen atom from a-carbon atom and reaction, called dehydrohalogenation reaction. B-carbon atom, it is a B-elimination As hydrogen: atomiseliminated from B-climination -HX H X e.g. i) CH CH,Br + KOHale) ’CH, = CH, + KBr Ethyl bromide Ethylene ii) CH; - CH - CH, -Cl +KOH¡lc) CH-CH = CH, + KCI + H0 n-Propyl chloride Propylene The order of reactivity of alkyl halides towards dehydrohalogenation reaction is 3'> 20> 1 and iodides > bromides > chlorides. Saytzeff rule. Depending upon the structure, alkyl halides may give one or more isomeric alkenes. For example, dehydrohalogenation of 1-chlorobutane gives only one alkene, i.e., but-1-ene since only one type of ß-hydrogen is available on the left side of the molecule. CH;CH CH "CH, + KOHale) CH,CH;CH =CH, +KCI +H,0 1-Chlorobutane But-1-ene If, However, the structure of the alkyl halide is such that it has a B-hydrogen on either side of the carbon atom carrying the halogen, it can undergo elimination in two different ways giving two alkenes. The relative amounts of these two alkenes is governed by Saytzeff rule. According to this rule, whenever two alkenes are theoretically possible during a dehydrohalogenation more highly substituted alkene (i.e., having lesser number of hydrogen reaction, it is always the which predominates. atoms on the double bond) For example, CH CH= CH CH3 But-2-ene (80%) (more substituted CH;-CH CH-CH3 KOH(alc)A alkene; more stable) highly CH-CH CH= CH But-l-eneless(20%) Alkenes; stable)(Less highly substituted Where every student matters and every moment counts" 5  AREISEHEK, VINAY8 ARUN'S CHEMISTRY PVT. TUTORIALS ABHISHEK SIR: 9833395472 VINAY SIR: 9867442504 ARUN SIR: alcohols: (by dehydration) 98705ST7035 From are the hydroxyl derivatives of alkanes. They are represented by R-OH where, Ris CaH2n+|OH 2)Alcohol Dehydration means removal of water molecule. In this reaction, 0H IgroupasofB-climination hydrogenatomfrom B-carbon is eliminated as water molecule. It is known a-carbon atom and reaction, since hydrogen is eliminated from B-carbon - atom. B-climination -11,0 ’C=C H OH a. From alcohols (dehydration) The ease of dehydration of alcohols is in the order 3° > 20> o H 75°%H,SO, ’ H;C - H:C -Ç - 413 K CH -CH=CH, + H,0 Butan-1-ol But-1-ene (main product) HC----H 60% H,SO, 373K ’ H}C - CH = CH- CH; + HC - CH, - CH = CH+ H0 HÖH H (major) (minor) H ÇHH 20%H,SO, H-¢-¢-f-H 363K ’ HC-C= CH + H0 H OHH 2-Methylpropan-2-ol 2-Methylprop-1-ene bDehydration by alumina: When vapours of an alcohol are passed over heated alumina at 623K, it gives corresponding.alkene. Itis vapour phase dehydration. e.g. i) CH;CHOH 623K A;O, CH, = CH, + H0 Ethyl alcohol Ethylene i) CH; CH-CH)- OH Al,0, CH, CH = CH, + H0 623K n-Propyl alcohol Propylene 3) From vicinal dihalides (By dehalogenation): The dihalides in which two halogen atoms are attached to the adjacent carbon atoms are called vicinal dihalides. When vicinal dihalide is heated with zinc metal in presence of alcohol, it gives alkene. The reaction is called dehalogenation. i) Br-CH;-CH,-Br + Zn alcohol A CH, =CH+ ZnBr Ethylene dibromide Ethylene alcohol ’CH3 - CH= CH + ZnCh Propylene dichloride Propylene Where every student matters and every moment counts" 6  Puz z02 CHEMISTRYPV1 ABHISHEK, VVINAY VINArsiR: &ARUN'S 986742504 ARUN ORIALS 983339s472 ABHISHEK SIR 4) Addition reactionss From alkynes: partial reduction -C=C further reaction Alkenes Alkynes alkenes, which on give alkanes, Alkynes on catalytic hydrogenation givefirst CH-CH; H,/Pt CH= CH+Ha H,/Pt ’CH, = CH; Ethane Ethyne Ethene The reaction cannot be stopped at alkenestage, but continuesfurther to form an alkane. However, b reaction at. using certain specific catalysts and it reagents, is possibletostop the alkene stage. palladium deposited on partially poisoned with lead Lindlar's catalyst is usedIfor acetate.A partial reduction. better It is catalystmetallic is Pd-BaSO, calcium carbona. partially poisoned with sulphur These partiallv obtained are having cis geometry. palladised charcoal compounds is Lindlar's or quinoline. thus However, alkynes on reduction Alkenescatalyst. ammonia (Birch reduction) form trans alkenes. deactivatei with sodium in liquid R RC = CR + H2 Pd/C Lead acetate Alkyne H H cis-Alkene R H RC = CR + H2 Na/liquid NH, Alkyne H R C.g., trans-alkene i) CH =CH + H2 Pd/CaCO, ’CH, = CH, Lead acetate Propyne Propene ii) CH;-C=CH+H2 Pd/CaCO, Lead acetate CH3-CH= CH, Ethyne Ethene o From sodium or potassium salts of saturated dicarboxylic acids (Kolbe's electrolytic reaction) Electrolysis of sodium or potassium salts of saturated dicarboxylic acids gives alkenes. For example. ÇH) + 2H20 Hectolysis +2C0, + H +2KOH CH,COOK CH, Pot. succinate Ethylene PHYSICAL PROPERTIES OF ALKENES: a) Nature: Alkenes resemble alkanes in most of their physical properties. gases. Except ethylene, all other alkenes Ethylene, propylene and a-butylene are are colourless and odourless. faint sweet odour. Next fourteen Ethylene is colourless gas with members (Cs-C1s) are liquids and higher ones are solids, b) Solubility: Alkenes are insoluble in water but quite soluble in organic solvents like benzene. ether. c) Boiling point: chloroform, l Boiling point of alkenes rises with increase in each addedcarbon atom. molecular mass and a rise of 20 to 30 K is observed lo Where every student matters and every moment counts" 7 ABEISHEK ABAISHEK, VINAY & ARUN'S ein alkanes, SIRI 9833395472 in alkenes also VINAY sIR:CHEMISRYo8674425o4 PVT. TUTORIALS ARUN SIR: 98705T7035 Due to greater branching lowers boiling point. polarizability -clectrons, the boiling points of alkene is of corresponding alkanes. Due to greater dipole higher than those of alkene.] moment, the boiling point of cis-alkene is higher than corresponding trans REACTION OF ALKENES: Being unsaturated, alkenes Alkenes are the rich SOurce undergo electrophilic addition held pi(z) electrons,reaction which the electrophiles add ofon loosely due to which Some reagents also add by free to the carbon-carbon double bond to form they show addition reaction in the addition products. alkenes also undergo free radical radical mechanis1m. There are cases when under special substitution Oxidation and ozonolysis reactions are also reactions. conditions, quite prominent in alkenes. I) Formation of alkanes (Hydrogenation): Alkenes on catalytic alkanes. Raney nickelhydrogenation at about is used as catalyst. 473 Kto 573 Kunder pressure, give palladium at room Hydrogenation temperature. In this reaction, a corresponding may be carried out in presence of platinum or in alkenes. hydrogen molecule is added across the double bond e.g., i) CH =CH +H Raney Ni Ethene ’CH3-CH, Ethane i) CH-CH=CH + H, Raney Ni Propene ’CH-CH,-CH; Propane 2) Formation of vicinal dihalides Alkenes react with halogens to give(Halogenation): The order of reactivity of halogens ivicinal dihalides. s Ch> Brp >b. With iodine poor In case of bromine, the red-brown yield of di-iodides is obtained. colour of bromine vanishes as it is consumed decolourisation of bromine is therefore used to detect the presence of by the alkene. This Addition of halogens to alkenes is an example of unsaturation. electrophilic addition reaction. i) CH,=CH, +Ch ’ C- CH- CH, - CI Ethene Ethylene dichloride ii) CH, - CH= CH (1,2-Dichloroethane) Br CH; -ÇH- ÇH Br Br Propene Propylene dibromide 3) Formation of alkvl halides (Hvdrobalen Alkenes react with hydrogen halides to give The order of reactivity of halogen acids is HIcorresponding alkyl halides > HBr > HCI. +HX ’-C-f H X Alkenes Alkyl halides e.g. i)CH, = CH, + HBr ’ CH3 - CH,- Br Ethene Ethyl bromide (Bromoethane) *Where every student matters and every moment counts" 8  CHEMISTRY PVT. ii)ABHISHEK CH - CH ABHISHEK, VINAY VINAYSIR: SIRI 9@3330s4T2 CH-CH, - CH-CH; CH-CH, + HBr &ARUN'S 9867442504 ARUN TUTORIALS Br sec-Butylbromide But-2-ene (2-Bromobutane) In addition of bvdrogen halides leads to only one produes case of SVmmetrical alkenes unsymnmetrical alkenes givestwo products. addition hydrogen of hhalides to CH:-CH-CH: However Br Isopropyl bromide (Major product) CH;-CH -CH; +HBr CH; - CH-CH-Br n-Propyl bromide Markownikoffs rule: Markownikoff (Minor product) proposed arule, which helps to predict the major product, in additionWhen of anan unsymmetrical unsymmetrical alkenes. unsymmetrical reagent tois added to an unsymmetrical alkene, the negative part of the number of hydrogen atoms. Teagent gets attached to that carbon atom which carries less Mechanism of electrophilic addition: The addition ofhydrogen halides on alkenes is an example of electrophilic addition reaction. Step () : P'rotonation of C =C double bond: It is slow step. HBr provides the electrophile H". H Br ’H+Br This proton attacks carbon-carbon double bond toform carbocation. CH, CH,CH, (1° carbocation) CH CH =CH2+ H + CH-CH CH; (2° carbocation) The 2° cartbocation is more stable than 1° carbocation, hence 2° carbocation predominates and is formed at faster rate. Step (I): Carbocation-anion combination: lt is fast step. Br ion attacks more stable 2° carbocation to form isopropylbromide as major product. CH CH CH, + Br CH ÇH CH Br - Peroxide effect: (Anti-Marknikoffs rule/ Kharasch effect): The addition of hydrogen bromide to an unsymmetrical alkene in presence of peroxides takes place contrary to Markownikoff's rule. This is known as peroxide effect or Kharasch effect or abnormal addition or anti-Markownikoff's addition. It is observed only in case of hydrogen bromide. CH3 HsC- C= CH2+ H- Br CH,CO), 0, Benzoyl peroxide H:C -C-CH2 - Br H 2-Methylprop-1-ene 1-Bromo-2-methylpropane LWhere every student matters and every moment countel!  ABHISHEK, VINAY &ARUN'S ABHISHEK SIR: 983339S472 VINAY SIR:CHEMISTRY PVT. TUTORIALS 9867442504 ARUN SIR: 987o577035 CH CHCHBr peroxide n-Propyl bromide (Major product) CH CH=CH; + HBr CH-ÇH-CH Br Isopropyl bromide (Minor product) IIBr ’A CH-CH:-CH=CH: Write A and B HBr peroxide B Free radical mechanism: The addition of hydrogen bromide to peroxides follows free radical mechanism. unsymmetrical alkenes in presence of Addition of HBr to propylene in presence of benzayl peroxide a) Chain initiating steps: i)Peroxide dissociates homolytically to form free radicals. Homolysis ’2C&H; 0 2CoHs +2C02 i) Free radicals abstracts hydrogen atom from hydrogen bromide to give 'CcHs +H Br’ CoH, +'Br bromine free radical. b) Chain propagating steps: i)Bromine free radical isadded across the double bond in propylene.It should be noted that t bond in propylene is homolytically broken. +CH ÇH CH Br CH CH=CH, +'Br (1° free radical) CH, CHCH; Br (2 free radical) 20 free radical is. more stable than 1° free radical ii) 2° free radial formed in above step abstracts hydrogen atom from hydrogen CH, CH CHBr+H Br- ’ CH-CH CH Br + Br bromide. The peroxide effect is not observed in case of HCl and HH.This is because H Clbond (bond enthalpy 430.5 kJ mol) is stronger than H-Br bond (bond enthalpy 367.7 kJ mo) and not broken by free radical while H Hbond (bond enthalpy 296.8 kJ mo) is weaker and iodine free radicals combine together to form iodine molecule instead of adding to double bond. 4) Hydroboration- oxidation of alkene Alkenes with diborane in tetrahydrofuran (THF) solvent undergo hydroboration to form trialkylborane, which on oxidation with alkaline peroxide forms primary alcohol. The overall reaction gives Anti-Markovnikov's product from unsymmetrical alkenes. "Where every student matters and every moment counts9 10  ABHISHIEK, VINAY&VINAYSIR: ABHISHEK SIR: 9833395472 -+B(OH); OH 6(8H)k Trialkylborane Alcohol Alkene Diborane HH (H THI ’ 2/CH,-CH:-);B 6H--C-H + H-B sohent Diborane Ethene (H,C -CH, N 6CH:-CH-OH +2B(0H); O0+ 2 H,C -CH,B NaOl H,C -CH, Ethanol Tricthylborane HH (H, TIIF ’ 2(CH,-CH,-CH;-);B 6 H-C-C-H+ H-B sohent H Propene Diborane (H,C -CH,- CH, N SIHO-OH+ 3CH-CH-CH-OH + B(OH)3 H,C -CH,- CH; B NaOH Propan-1-ol (H,C-CH,-CH, Tri-n-propylborane sulphates:(Addition of sulphuricacid) sulphate which 5) Formation of alkyl hydrogen: sulphuric acid to give alkyl hydrogen are water Alkenes react with coldconcentrated with water gives alcohol. sulphate on boiling Soluble compounds. These alkyl hydrogen The process is called hydration of alkenes. e.g., i) CH, = CH, + H-OSO:H CH-CH-OSO;H Ethene (Conc.) Ethyl hydrogen sulphate CH3-CH,-OS0;H+ H-OH A CH-CH-OH + H;SO4 Ethyl alcohol Addition of sulphuric acid takes place according to Markownikoff'srule. ii) CH, - CH=CH + H- 0S0:H M.R. ’ ÇH; -CH -CH; Propene (Conc.) ÓSO,H Isopropyl hydrogen sulphate CH3 - CH -CH; + H-OH A, CH; - CH- CH +H;SO4 ÖSO;H OH Isopropylalcohol