REACTIONS OF ALKENES: ADDITION REACTIONS 129 (f) The less substituted carbon of the double bond in 3-ethyl-1-pentene is at the end of the chain It is this carbon that bears the hydroxyl group in the product of hydroboration-oxidation H,C=CHCH(CH,CH HOCH, CH, CH(CH- CH3)2 2. oxidation Ethy l-1-pentene 3-Ethyl-1-pentano 6.13 The bottom face of the double bond of a-pinene is less hindered than the top face Methyl group lds top face.H3C H,C HO Hydroboration o This h comes from this directi from BH Syn addition of H and Oh takes place and with a regioselectivity opposite to that of Markovnikov's rule 6.14 Bromine adds anti to the double bond of l-bromocyclohexene to give 1, 1. 2-tribromocyclohexane. The radioactive bromines(Br) are vicinal and trans to each other. 1, 1, 2-Tribromocyclohexar 6. 15 Alkyl substituents on the double bond increase the reactivity of the alkene toward addition of 2-Methyl-2-butene 2-Methy l-l-butene 3-Methyl-1-butene ( trisubstituted double bond: most reactive) (disubstituted double bond) (monosubstituted double bond; least reactive) 6.16 (b) Bromine becomes bonded to the less highly substituted carbon of the double bond, the highly substituted CHD,C=CHCH (CH,),CCHCI (CH,), CHCHECI (CH3),CHCHCH, Br 3-Methyl-l-butene Bromo-3-methyl-2-butanol Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
( f ) The less substituted carbon of the double bond in 3-ethyl-1-pentene is at the end of the chain. It is this carbon that bears the hydroxyl group in the product of hydroboration–oxidation. 6.13 The bottom face of the double bond of -pinene is less hindered than the top face. Syn addition of H and OH takes place and with a regioselectivity opposite to that of Markovnikov’s rule. 6.14 Bromine adds anti to the double bond of 1-bromocyclohexene to give 1,1,2-tribromocyclohexane. The radioactive bromines (82Br) are vicinal and trans to each other. 6.15 Alkyl substituents on the double bond increase the reactivity of the alkene toward addition of bromine. 6.16 (b) Bromine becomes bonded to the less highly substituted carbon of the double bond, the hydroxyl group to the more highly substituted one. (c) (CH3)2CHCH CH2 Br2 H2O (CH3)2CHCHCH2Br OH 3-Methyl-1-butene 1-Bromo-3-methyl-2-butanol (CH3)2C CHCH3 Br2 H2O (CH3)2CCHCH3 HO Br 2-Methyl-2-butene 3-Bromo-2-methyl-2-butanol H 2-Methyl-2-butene (trisubstituted double bond; most reactive) H H H 3-Methyl-1-butene (monosubstituted double bond; least reactive) H H 2-Methyl-1-butene (disubstituted double bond) 82Br 82Br Br H 1,1,2-Tribromocyclohexane Br H 1-Bromocyclohexene Bromine 82Br 82Br H3C CH3 CH3 H 1. B2H6 2. H2O2, HO H3C CH3 CH3 H H HO Hydroboration occurs from this direction. Methyl group shields top face. This H comes from B2H6. 1. hydroboration 2. oxidation CHCH(CH2CH3) H2C 2 3-Ethyl-1-pentene 3-Ethyl-1-pentanol HOCH2CH2CH(CH2CH3)2 REACTIONS OF ALKENES: ADDITION REACTIONS 129 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
130 REACTIONS OF ALKENES: ADDITION REACTIONS (d) Anti addition occur CH 1-Methylcyclopentene trans-2-Bromo. 1-methylcyclopentanol 6. 17 The structure of disparlure is as shown Its longest continuous chain contains 18 carbon atoms, and so it is named as an epoxy derivative of octadecane. Number the chain in the direction that gives the lowest number to the carbons that bear oxygen. Thus, disparlure is cis-2-methyl-7, 8-epoxyoctadecane 6.18 Disparlure can be prepared by epoxidation of the corresponding alkene. Cis all alkenes yi ides upon epoxidation. cis-2-Methyl peroxy .2-Methyl-7-octadecene 6.19 The products of ozonolysis are formaldehyde and 4, 4-dimethy l-2-pentanone H,C(CH3)3 ormaldehyde 4, 4-Dimethyl-2-pentanone The two carbons that were doubly bonded to each other in the alkene become the carbons that are doubly bonded to oxygen in the products of ozonolysis. Therefore, mentally remove the oxygens and connect these two carbons by a double bond to reveal the structure of the starting CH3 2,.4.4-Trimethyl-l-pentene 6.20 From the structural formula of the desired product, we see that it is a vicinal bromohydrin. Vicinal bromohydrin are made from alkenes by reaction with bromine in water. BrCH,C(CH3) is made from CH2-C(CH3)2 Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(d) Anti addition occurs. 6.17 The structure of disparlure is as shown. Its longest continuous chain contains 18 carbon atoms, and so it is named as an epoxy derivative of octadecane. Number the chain in the direction that gives the lowest number to the carbons that bear oxygen. Thus, disparlure is cis-2-methyl-7,8-epoxyoctadecane. 6.18 Disparlure can be prepared by epoxidation of the corresponding alkene. Cis alkenes yield cis epoxides upon epoxidation. cis-2-Methyl-7-octadecene is therefore the alkene chosen to prepare disparlure by epoxidation. 6.19 The products of ozonolysis are formaldehyde and 4,4-dimethyl-2-pentanone. The two carbons that were doubly bonded to each other in the alkene become the carbons that are doubly bonded to oxygen in the products of ozonolysis. Therefore, mentally remove the oxygens and connect these two carbons by a double bond to reveal the structure of the starting alkene. 6.20 From the structural formula of the desired product, we see that it is a vicinal bromohydrin. Vicinal bromohydrins are made from alkenes by reaction with bromine in water. BrCH2C(CH3)2 OH is made from C(CH3) CH2 2 C C H H CH3 CH2C(CH3)3 2,4,4-Trimethyl-1-pentene C O H H O C CH3 CH2C(CH3)3 Formaldehyde 4,4-Dimethyl-2-pentanone H H cis-2-Methyl-7-octadecene H H O Disparlure peroxy acid H H O H Br OH CH3 H CH3 H2O Br2 trans-2-Bromo- 1-methylcyclopentanol 1-Methylcyclopentene 130 REACTIONS OF ALKENES: ADDITION REACTIONS Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
REACTIONS OF ALKENES: ADDITION REACTIONS 131 Since the starting material given is tert-butyl bromide, a practical synthesis is (CH3)3 CBr CHCHOH ( CH3)2C=CH2 (CH3),CCH,Br tert-Butyl bromide 2-Methylpropene 1-Bromo-2-methyl-2-propanol 6.21 Catalytic hydrogenation of the double bond converts 2, 4, 4-trimethyl-l-pentene and 2, 4, 4-trimethyl 2-pentene to 2, 2, 4-trimethylpentane 分c-C H3 (CH3)2 CHCH,C(CH3)3 CH,C(CH3) H-O C(CH3) 2.4, 4-Trimethyl-l-pentene 2,4, 4-Trimethyl-2-pentene 2, 2, 4-Trimethylpentane 6.22 This problem illustrates the reactions of alkenes with various reagents and requires application of Markovnikov's rule to the addition of unsymmetrical electrophiles (a) Addition of hydrogen chloride to 1-pentene will give 2-chloropentane H,C=CHCH,CH, CH3 HCI CH CHCH CHCH Cl I-Pentene 2-Chloropentane (b) Electrophilic addition of hydrogen bromide will give 2-bromopentane CHCH CHCH+ HBr CH,CHCH,CH, CH3 B 2-Bromopentane (c) The presence of peroxides will cause free-radical addition of hydrogen bromide, and regiose lective addition opposite to Markovnikov's rule will be observed. H,C=CHCH, CH, CH3 HBr BrCH, CH,CH2CHCH3 1-Bromopentane (d) Hydrogen iodide will add according to Markovnikov's rule H,C=CHCH, CH, CH3 HI- CH3 CHCH,CH,CH3 2-lodopentane (e) Dilute sulfuric acid will cause hydration of the double bond with regioselectivity in accord with markovnikoy's rule HSO, H,C=CHCH,CH, CH3 H,O CH3 CHCH, CH,CH3 2-Pentanol Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
Since the starting material given is tert-butyl bromide, a practical synthesis is: 6.21 Catalytic hydrogenation of the double bond converts 2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl- 2-pentene to 2,2,4-trimethylpentane. 6.22 This problem illustrates the reactions of alkenes with various reagents and requires application of Markovnikov’s rule to the addition of unsymmetrical electrophiles. (a) Addition of hydrogen chloride to 1-pentene will give 2-chloropentane. (b) Electrophilic addition of hydrogen bromide will give 2-bromopentane. (c) The presence of peroxides will cause free-radical addition of hydrogen bromide, and regioselective addition opposite to Markovnikov’s rule will be observed. (d) Hydrogen iodide will add according to Markovnikov’s rule. (e) Dilute sulfuric acid will cause hydration of the double bond with regioselectivity in accord with Markovnikov’s rule. H2C CHCH2CH2CH3 H2O CH3CHCH2CH2CH3 2-Pentanol H2SO4 OH H CHCH2CH2CH3 HI 2C CH3CHCH2CH2CH3 2-Iodopentane I H CHCH2CH2CH3 HBr 2C BrCH2CH2CH2CH2CH3 1-Bromopentane peroxides H CHCH2CH2CH3 HBr 2C CH3CHCH2CH2CH3 Br 2-Bromopentane H CHCH2CH2CH3 HCl 2C CH3CHCH2CH2CH3 Cl 1-Pentene 2-Chloropentane C or H H CH2C(CH3)3 (CH3)2CHCH2C(CH3)3 CH3 C 2,4,4-Trimethyl-1-pentene 2,2,4-Trimethylpentane C C(CH3)3 H3C H H3C C 2,4,4-Trimethyl-2-pentene H2, Pt (CH3)3CBr (CH3)2CCH2Br OH (CH3)2C CH2 NaOCH2CH3 CH3CH2OH heat Br2 H2O tert-Butyl bromide 2-Methylpropene 1-Bromo-2-methyl-2-propanol REACTIONS OF ALKENES: ADDITION REACTIONS 131 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
132 REACTIONS OF ALKENES: ADDITION REACTIONS (f) Hydroboration-oxidation of an alkene brings about hydration of the double bond opposite to Markovnikov's rule; 1-pentanol will be the product. H_C=CHCH_CHCH3 2. H.O. Ho HOCH- CH2CH_CH2CH I-Pentanol (g) Bromine adds across the double bond to give a vicinal dibromide H,C=CHCH, CH, CH3 Br2 BrCH, CHCH, CH,CH (h) Vicinal bromohydrin are formed when bromine in water adds to alkenes. Br adds to the less substituted carbon. oH to the more substituted one H2C=CHCH, CH, CH, +Br. Ho, BrCH_ CHCH_ CH_ CH3 Bromo-2-pentanol (i) Epoxidation of the alkene occurs on treatment with peroxy acids H,C=CHCH, CH, CH3 CH,CO,OH H,C--CHCH, CH, CH3 CH3 CO,H 1, 2-Epoxypentane ( Ozone reacts with alkenes to give ozonides H,C=CHCH,CH,CH3+ O3- CHCH, CH,CH3 () When the ozonide in part (j)is hydrolyzed in the presence of zinc, formaldehyde and butanal are formed H,C CHCH,CH,CH HCH HCCH, CH, CH3 Formaldehyd Butanal 6.23 When we compare the reactions of 2-methyl-2-butene with the analogous reactions of 1-pentene, we find that the reactions proceed in a similar manner. (a)(CH,)2C=CHCH3+ HCI -(CH3),CCH, CH3 2-Methyl-2-butene 2-Chloro-2-methylbutane (b)(CH)2C=CHCH3 HBr -(CH3)2, CH3 2-Bromo-2-methylbutane (c)(CH),C=CHCH HB (CH3)2CHCHCH Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
( f ) Hydroboration–oxidation of an alkene brings about hydration of the double bond opposite to Markovnikov’s rule; 1-pentanol will be the product. (g) Bromine adds across the double bond to give a vicinal dibromide. (h) Vicinal bromohydrins are formed when bromine in water adds to alkenes. Br adds to the less substituted carbon, OH to the more substituted one. (i) Epoxidation of the alkene occurs on treatment with peroxy acids. ( j) Ozone reacts with alkenes to give ozonides. (k) When the ozonide in part ( j) is hydrolyzed in the presence of zinc, formaldehyde and butanal are formed. 6.23 When we compare the reactions of 2-methyl-2-butene with the analogous reactions of 1-pentene, we find that the reactions proceed in a similar manner. (a) (b) (c) (CH CHCH3 HBr 3)2C (CH3)2CHCHCH3 2-Bromo-3-methylbutane Br peroxides (CH CHCH3 HBr 3)2C (CH3)2CCH2CH3 2-Bromo-2-methylbutane Br (CH CHCH3 HCl 3)2C (CH3)2CCH2CH3 2-Methyl-2-butene 2-Chloro-2-methylbutane Cl HCH O Formaldehyde HCCH2CH2CH3 O Butanal H2O H2 Zn C O O CHCH2CH2CH3 O H2C CHCH2CH2CH3 O3 Ozonide H2C O O CHCH2CH2CH3 O 1,2-Epoxypentane Acetic acid H2C CHCH2CH2CH3 CH3CO2OH H2C CHCH2CH2CH3 CH3CO2H O CHCH2CH2CH3 H2O H2C 1-Bromo-2-pentanol Br2 BrCH2CHCH2CH2CH3 OH CHCH2CH2CH3 CCl4 H2C 1,2-Dibromopentane Br2 BrCH2CHCH2CH2CH3 Br CHCH2CH2CH3 1. B2H6 2. H2O2, HO H2C 1-Pentanol HOCH2CH2CH2CH2CH3 132 REACTIONS OF ALKENES: ADDITION REACTIONS Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
REACTIONS OF ALKENES ION REACTIONS (d)(CHa), C=CHCH3 HI (CH3),CCH (e)(CH3),C=CHCH3 H,O (CH3),CCH,CH oH 2-Methyl-2-butanol L.B.H ((CH)C=CHCH3 2HOHo"+(CH3)2CHCHCH3 3-Methyl-2-butanol Br (R)(CH, ),C=CHCH,+ Br,CC.(CH)CCHCH 2.3-Dibromo-2-methylbutane (h)(CH3)2C=CHCH3 Br2 (CH3)2CCHCH 3-Bromo-2-methyl-2-butanol (i)(CH),C=CHCH3+ CH; CO,OH (CH3),C-CHCH3 CH,CO,H 2-Methyl-2, 3-epoxybutane HC ()(CH3)2C=CHCH3 t O3 H C H ( Hc CHa CCH3 HCCH Acetone 6.24 Cycloalkenes undergo the same kinds of reactions as do noncyclic alkenes CH +ia一O 1-Methylcyclohexene 1-Chloro-1-methylcyclohexane CH HB Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(d) (e) . ( f ) (g) (h) (i) ( j) (k) 6.24 Cycloalkenes undergo the same kinds of reactions as do noncyclic alkenes. (a) (b) CH3 HBr 1-Bromo-1-methylcyclohexane CH3 Br CH3 HCl CH3 Cl 1-Methylcyclohexene 1-Chloro-1-methylcyclohexane CH3CCH3 HCCH3 H2O Zn O O Acetone Acetaldehyde O O O H3C H H3C CH3 CHCH3 O3 (CH3)2C O O O H3C H H3C CH3 Ozonide (CH3)2C CHCH3 CH3CO2OH (CH3)2C CHCH3 CH3CO2H O 2-Methyl-2,3-epoxybutane CHCH3 Br 2 (CH3)2C (CH3)2CCHCH3 3-Bromo-2-methyl-2-butanol OH Br H2O CHCH3 Br 2 (CH3)2C (CH3)2CCHCH3 2,3-Dibromo-2-methylbutane Br Br CCl4 CHCH3 (CH3)2C (CH3)2CHCHCH3 3-Methyl-2-butanol OH 1. B2H6 2. H2O2, HO (CH3)2C CHCH3 H2O (CH3)2CCH2CH3 2-Methyl-2-butanol OH H2SO4 (CH CHCH3 HI 3)2C (CH3)2CCH2CH3 2-Iodo-2-methylbutane I REACTIONS OF ALKENES: ADDITION REACTIONS 133 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website