csee CHAPTER 14 ○ RGANOMETALLIC COMP○UNDS SOLUTIONS TO TEXT PROBLEMS 14.1 (b) Magnesium bears a cyclohexyl substituent and a chlorine. Chlorine is named as an anion. The ompound is cyclohexylmagnesium chloride 14.2(b) The alkyl bromide precursor to sec-butyllithium must be sec-butyl bromide. CH3 CHCH, CH3 2Li CH,CHCH, CH3 LiBr 2-Bromobutane (sec-butyl bromide) 14.3(b) Allyl chloride is converted to allylmagnesium chloride on reaction with magnesium. H,C=CHCH,CI H,C-CHCH,MgCI Allyl chloride lylmagnesium chloride (c) The carbon-iodine bond of iodocyclobutane is replaced by a carbon-magnesium bond in the gnard reagent. lodocyclobutane 342 Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
CHAPTER 14 ORGANOMETALLIC COMPOUNDS SOLUTIONS TO TEXT PROBLEMS 14.1 (b) Magnesium bears a cyclohexyl substituent and a chlorine. Chlorine is named as an anion. The compound is cyclohexylmagnesium chloride. 14.2 (b) The alkyl bromide precursor to sec-butyllithium must be sec-butyl bromide. 14.3 (b) Allyl chloride is converted to allylmagnesium chloride on reaction with magnesium. (c) The carbon–iodine bond of iodocyclobutane is replaced by a carbon–magnesium bond in the Grignard reagent. Mg diethyl ether Iodocyclobutane I Cyclobutylmagnesium iodide MgI Allyl chloride H CHCH2Cl 2C Allylmagnesium chloride H CHCH2MgCl 2C Mg diethyl ether CH 2Li LiBr 3CHCH2CH3 Br 2-Bromobutane (sec-butyl bromide) CH3CHCH2CH3 Li 1-Methylpropyllithium (sec-butyllithium) 342 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
ORGANOMETALLIC COMPOUNDS 343 (d) Bromine is attached to an sp-hybridized carbon in 1-bromocyclohexene. The product of its reaction with magnesium has a carbon-magnesium bond in place of the carbon-bromine MgBr diethyl ether I-Bromocyclohexene 14.4(b) 1-Hexanol will protonate butyllithium because its hydroxyl group is a proton donor only slightly less acidic than water. This proton-transfer reaction could be used to prepare lithium I-hexanolate CHa CH, CH,CH,CH,CH,OH CHCH,CH,CH,Li CH3 CH,, CH3 CHCH,CH, CH,CH, CH,OLi 1-Hexanol Butyllithium Butane Lithium 1-hexanolate (c) The proton donor here is benzenethiol. C6HSSH CH,CHCH,CH,Li CH, CHCHCH ChSLi Benzenethiol Butyllithium Butane Lithium 14.5(b) Propylmagnesium bromide reacts with benzaldehyde by addition to the carbonyl group CHI CH2CH2T-MgBr CHCHCH diethyl ether C6H CTO C6HSC-OMg Br C6HSCHCH,CH,CH 1-Phenyl-1-but (c) Tertiary alcohols result from the reaction of Grignard reagents and ketones CHCHCI CHa CH,CH,Mg Br 1-Propylcyclohexanol (d) The starting material is a ketone and so reacts with a Grignard reagent to give a tertiary alcohol H,CH,CH2-MgE H C CH, CHCH diethyl ether CHaCH,CH,CO CHa CH, Propylmagnesium bromide 3-Methyl-3-hexanol Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(d) Bromine is attached to an sp2 -hybridized carbon in 1-bromocyclohexene. The product of its reaction with magnesium has a carbon–magnesium bond in place of the carbon–bromine bond. 14.4 (b) 1-Hexanol will protonate butyllithium because its hydroxyl group is a proton donor only slightly less acidic than water. This proton-transfer reaction could be used to prepare lithium 1-hexanolate. (c) The proton donor here is benzenethiol. 14.5 (b) Propylmagnesium bromide reacts with benzaldehyde by addition to the carbonyl group. (c) Tertiary alcohols result from the reaction of Grignard reagents and ketones. (d) The starting material is a ketone and so reacts with a Grignard reagent to give a tertiary alcohol. diethyl ether H3O CH3CH2CH2 H3C C OMgBr CH3CH2 3-Methyl-3-hexanol CH3CH2CH2COH CH2CH3 CH3 CH3CH2CH2 MgBr O CH3CH2 C H3C Propylmagnesium bromide 2-butanone 1. diethyl ether 2. H3O CH3CH2CH2MgBr O CH2CH2CH3 OH 1-Propylcyclohexanol diethyl ether H3O C6H5C O H CH3CH2CH2 C6H5C OMgBr H 1-Phenyl-1-butanol C6H5CHCH2CH2CH3 OH CH3CH2CH2 MgBr C6H5SH Benzenethiol C6H5SLi Lithium benzenethiolate CH3CH2CH2CH2Li Butyllithium CH3CH2CH2CH3 Butane CH3CH2CH2CH2CH2CH2OH 1-Hexanol CH3CH2CH2CH2CH2CH2OLi Lithium 1-hexanolate CH3CH2CH2CH2Li Butyllithium CH3CH2CH2CH3 Butane Mg diethyl ether 1-Bromocyclohexene Br 1-Cyclohexenylmagnesium bromide MgBr ORGANOMETALLIC COMPOUNDS 343 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
344 ORGANOMETALLIC COMPOUNDS removed because it is the most acidic, having a pK of approximately 2s Vne. The proton at C-1 is 14.6 Ethyl anion reacts as a Bronsted base to remove a proton from the alky CH2CH2+H-C≡CCH2CH2CH2CH3—CH2CH3+:C≡ CCHCH2CH2CH I-Hexyne Ethane Conjugate base of 1-hexyne 14.7(b) The target alcohol is tertiary and so is prepared by addition of a grignard reagent to a ketone. The retrosynthetic transformations are: CH3-C--CH3 CH CHiCH Because two of the alkyl groups on the hydroxyl-bearing carbon are the same(methyl), only two, not three, different ketones are possible starting materials O CH,Mgl+ CCH 1. diethyl ether CCH 2.H3O CH Methylmagnesium diethyl ether MgBr CH,CCH -CCH Acetone 2-Phenyl-2-propand 14.8(b) Recall that the two identical groups bonded to the hydroxyl-bearing carbon of the alcohol arose from the Grignard reagent. That leads to the following retrosynthetic analysis (C6H5)2 COR t 2CHs MgX Thus, the two phenyl substituents arise by addition of a phenyl grignard reagent to an ester of cyclopropanecarboxylic acid. L diethyl ether 2CH-MgBr COCH 2. H,o+(CH3)2C ChOH Phenylmagnesium carboxylate diphenylmethanol 14.9(b) Of the three methyl groups of 1, 3, 3-trimethylcyclopentene, only the one connected to the double bond can be attached by way of an organocuprate reagent Attachment of either of Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
344 ORGANOMETALLIC COMPOUNDS 14.6 Ethyl anion reacts as a Brønsted base to remove a proton from the alkyne. The proton at C-1 is removed because it is the most acidic, having a pKa of approximately 25. 14.7 (b) The target alcohol is tertiary and so is prepared by addition of a Grignard reagent to a ketone. The retrosynthetic transformations are: Because two of the alkyl groups on the hydroxyl-bearing carbon are the same (methyl), only two, not three,different ketones are possible starting materials: 14.8 (b) Recall that the two identical groups bonded to the hydroxyl-bearing carbon of the alcohol arose from the Grignard reagent. That leads to the following retrosynthetic analysis: Thus, the two phenyl substituents arise by addition of a phenyl Grignard reagent to an ester of cyclopropanecarboxylic acid. 14.9 (b) Of the three methyl groups of 1,3,3-trimethylcyclopentene, only the one connected to the double bond can be attached by way of an organocuprate reagent. Attachment of either of 1. diethyl ether 2. H3O (C6H5)2C OH Cyclopropyldiphenylmethanol CH3OH Methanol O COCH3 Methyl cyclopropanecarboxylate 2C6H5MgBr Phenylmagnesium bromide (C6H5)2C OH O COR 2C 6H5MgX MgBr CH3CCH3 O Phenylmagnesium bromide Acetone CCH3 1. diethyl ether 2. H3O OH CH3 2-Phenyl-2-propanol CH3MgI CCH3 CCH3 1. diethyl ether 2. H3O O OH CH3 Methylmagnesium iodide Acetophenone 2-Phenyl-2-propanol CH3 C O CH3 C OH CH3 CH3 CH3CCH3 O CH3CH3 1-Hexyne Ethane H C CCH2CH2CH2CH3 Ethyl anion CH3CH2 Conjugate base of 1-hexyne C CCH2CH2CH2CH3 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
ORGANOMETALLIC COMPOUNDS 345 the other methyls would involve a tertiary carbon, a process that does not occur very efficiently. CH iCu(CHs),+ Br H3 Lithiun 1.3, 3-Trimethylcyclopentene dimethylcyclope 14.10(b) Methylenecyclobutane is the appropriate precursor to the spirohexane shown CH,I Spiro[3.2 ]hexane(22%0) 14.11 Syn addition of dibromocarbene to cis-2-butene yields a cyclopropane derivative in which the methyl groups are cis C CHBr H, CI KoC(CHi) cis-2-Butene cis-l, l-Dibromo-2.3. Conversely, the methyl groups in the cyclopropane derivative of trans-2-butene are trans to one H3C CH trans-2-Butene trans-1, I-Dibromo-2.3. dimethylcyclopropane 14.12 Iron has an atomic number of 26 and an electron configuration of [Arl45-3do. Thus, it has 8 valence electrons and requires 10 more to satisfy the 18-electron rule. Five CO ligands, each providing two electrons, are therefore needed. The compound is Fe(co) 14.13 (a) Cyclopentyllithium is It has a carbon-lithium bond. It satisfies the requirement for classification as an organo- (b) Ethoxymagnesium chloride does not have a carbon-metal bond. It is not an organometallic CH, CHOMgC CH,CH,O Mg Cl (c) 2-Phenylethylmagnesium iodide is an example of a Grignard reagent. It is an organometallic Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
the other methyls would involve a tertiary carbon, a process that does not occur very efficiently. 14.10 (b) Methylenecyclobutane is the appropriate precursor to the spirohexane shown. 14.11 Syn addition of dibromocarbene to cis-2-butene yields a cyclopropane derivative in which the methyl groups are cis. Conversely, the methyl groups in the cyclopropane derivative of trans-2-butene are trans to one another. 14.12 Iron has an atomic number of 26 and an electron configuration of [Ar]4s 2 3d6 . Thus, it has 8 valence electrons and requires 10 more to satisfy the 18-electron rule. Five CO ligands, each providing two electrons, are therefore needed. The compound is Fe(CO)5. 14.13 (a) Cyclopentyllithium is It has a carbon–lithium bond. It satisfies the requirement for classification as an organometallic compound. (b) Ethoxymagnesium chloride does not have a carbon–metal bond. It is not an organometallic compound. (c) 2-Phenylethylmagnesium iodide is an example of a Grignard reagent. It is an organometallic compound. CH2CH2MgI CH3CH2OMgCl CH3CH2O Mg2 Cl or H Li C C H H3C CH3 H trans-2-Butene trans-1,1-Dibromo-2,3- dimethylcyclopropane CHBr3 KOC(CH3)3 H3C CH3 Br Br H H C C H H3C CH3 H cis-2-Butene cis-1,1-Dibromo-2,3- dimethylcyclopropane H3C CH3 Br Br H H CHBr3 KOC(CH3)3 CH2I2 Zn(Cu), ether CH2 Methylenecyclobutane Spiro[3.2]hexane (22%) LiCu(CH3)2 diethyl ether 1-Bromo-3,3- 1,3,3-Trimethylcyclopentene dimethylcyclopentene Lithium dimethylcuprate CH3 CH3 Br H3C CH3 CH3 ORGANOMETALLIC COMPOUNDS 345 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
346 (d) Lithium divinylcuprate has two vinyl groups bonded to copper. It is an organometallic Li(H,C=CH-Cu-CH=CH, (e) Sodium carbonate, Na,CO3 can be represented by the Lewis structure There is no carbon-metal bond, and sodium carbonate is not an org ff Benzylpotassium is represented as CHK or CH K It has a carbon-potassium bond and thus is an organometallic compound. 14.14 The two alkyl groups attached to aluminum in [(CH3),,JAlH are isobutyl groups. The hy drogen bonded to aluminum is named in a separate word as hydride. Thus, dibal"is a shortened form of the systematic name diisobutylaluminum hydride. 14.15 (a) Grignard reagents such as pentylmagnesium iodide are prepared by reaction of magnesium with the corresponding alkyl halide diethyl ethe CH CH, CH, CH,CH,I Mg CHa CH,CH,CH, CH,Mgl b) Acetylenic Grignard reagents are normally prepared by reaction of a terminal alkyne with a readily available Grignard reagent such as an ethylmagnesium halide. The reaction that takes place is an acid-base reaction in which the terminal alkyne acts as a proton donor diethyl ether CH2CHC≡CH+CHCH2MgI CH3CHC≡CMgI+CH3CH3 1-Butyne Ethylmagnesium 1-Butynylmagnesium Ethane (c) Alkyllithiums are formed by reaction of lithium with an alkyl halide. CHCH,CH,CH,CH,X 2Li CH, CH, CH,CH, Li Lix (X= Cl Br, or D (d) Lithium dialkylcuprates arise by the reaction of an alkyllithium with a Cu(r) salt. 2CH CHCH.CHCHL LiCu(CH, CH, CH,CHa CH3)2+ Lix Pentyllithium, from part(c) (X=Cl, Br. or D) Lithium dipentylcuprate 14.16 The polarity of a covalent bond increases with an increase in the electronegativity difference be- tween the connected atoms. Carbon has an electronegativity of 2.5 (Table 14. 1). Metals are less elec- tronegative than carbon. When comparing two metals, the less electronegative one therefore has the Po (a) Table 14.1 gives the electronegativity of lithium as 1.0, whereas that for aluminum is 1.5. The carbon-lithium bond in CH,CH,Li is more polar than the carbon-aluminum bond in (CH,,)3AL. Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(d) Lithium divinylcuprate has two vinyl groups bonded to copper. It is an organometallic compound. (e) Sodium carbonate, Na2CO3 can be represented by the Lewis structure. There is no carbon–metal bond, and sodium carbonate is not an organometallic compound. (f) Benzylpotassium is represented as It has a carbon–potassium bond and thus is an organometallic compound. 14.14 The two alkyl groups attached to aluminum in [(CH3)2CHCH2]2AlH are isobutyl groups. The hydrogen bonded to aluminum is named in a separate word as hydride. Thus, “dibal” is a shortened form of the systematic name diisobutylaluminum hydride. 14.15 (a) Grignard reagents such as pentylmagnesium iodide are prepared by reaction of magnesium with the corresponding alkyl halide. (b) Acetylenic Grignard reagents are normally prepared by reaction of a terminal alkyne with a readily available Grignard reagent such as an ethylmagnesium halide. The reaction that takes place is an acid–base reaction in which the terminal alkyne acts as a proton donor. (c) Alkyllithiums are formed by reaction of lithium with an alkyl halide. (d) Lithium dialkylcuprates arise by the reaction of an alkyllithium with a Cu(I) salt. 14.16 The polarity of a covalent bond increases with an increase in the electronegativity difference between the connected atoms. Carbon has an electronegativity of 2.5 (Table 14.1). Metals are less electronegative than carbon. When comparing two metals, the less electronegative one therefore has the more polar bond to carbon. (a) Table 14.1 gives the electronegativity of lithium as 1.0, whereas that for aluminum is 1.5. The carbon–lithium bond in CH3CH2Li is more polar than the carbon–aluminum bond in (CH3CH2)3Al. CuX Pentyllithium, from part (c) (X Cl, Br, or I) 2CH3CH2CH2CH2CH2Li LiX Lithium dipentylcuprate LiCu(CH2CH2CH2CH2CH3)2 2Li 1-Halopentane (X Cl, Br, or I) CH3CH2CH2CH2CH2X LiX Pentyllithium CH3CH2CH2CH2CH2Li diethyl ether CH3CH2MgI Ethylmagnesium iodide CH3CH3 1-Butyne Ethane CH3CH2C CH 1-Butynylmagnesium iodide CH3CH2C CMgI Mg diethyl ether CH3CH2CH2CH2CH2I 1-Iodopentane CH3CH2CH2CH2CH2MgI Pentylmagnesium iodide CH or 2K CH2 K Na O C O O Na Li(H2C CH CH2 Cu CH ) 346 ORGANOMETALLIC COMPOUNDS Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website