Ch3 problems - 3mg four .11: bonds .r the 0V”- ; 57 keel...

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Unformatted text preview: 3mg four .11: bonds .r the 0V”- ; 57 keel I methane alTulI predict what ght’ the mllfll will bromine arm the iill'llll“ 1d eqaeili'lflfl- l r l 101 -1 mo not the I'esnlll _ ,thane with : :orresl“"“li1w' [I .- J 'r - ‘ II - IIII-:Illl “H” “W tnlUI'lthlUn = 3. Catalysts speed up the establishment of an equilibrium between starting materials and products. 4_ Alkanes react with halogens (except iodine) by a radi- cal chain mechanism to give haloalkanes. The mecha- nism consists of initiation to create a halogen atom, two propagation steps, and various termination steps. in the first propagation step, the slower of the two, a hy- .l.--~5.:.- atom is abstracted from the alkane chain, a reac— titm resulting in an alkyl radical and HX. Hence, reac- sirily increases from 12 to F2. Selectivity decreases along m,- krllllt: series, as well as with increasing temperature. "-J'I a, ' 11L- Hammond postulate states that fast, exothermic re- .u-Jions are typically characterized by early transition flutes. which are similar in structure to the starting ma- lta'ials. On the other hand, slow, endothermic processes usually have late (productlilce) transition states. 1', ['Ilc .15.?!" for a reaction may be calculated from the DE!" values of the bonds affected in the process as follows: I!" H" I- DHobonds broken _ E DH‘lbonds formed- thlrma 8. 9. 10. 127 The AH" for a radical halogenation process equals the sum of the AH“ values for the propagation steps. Problems The relative reactivities of the various types of alkane C— H bonds in halogenations can be estimated by factoring out statistical contributions. They are roughly constant un- der identical conditions and follow the order primary secondary tertiary CH4 < CH < CH < CH The reactivity differences between these types of CH bonds are greatest for bromination, maldng it the most selective radical halogenation process. Chlorination is much less selective, and fluorination shows very little selectivity. The AH° of the combustion of an alkane is called the heat of combustion, AHgomb. The heats of combustion of isomeric compounds provide an experimental mea- sure of their relative stabilities. Li; I :LIIcI 1hr. |]| luau 32, secondary, and tertiary hydrogens in each of the following rI-Iurloululs. CH3 CH3 lulL'll.IZI||.t'|Jn"I-I3 (b)CH3CHzCH2CH2CH3 (e) W (d) I®Vlmin I..'.IIt'l| of ill: following sets of alkyl radicals, name each radical; identify each as u l|l|u"I'-|ll llllill'll. secondary, or tertiary; rank in order of decreasing stability; and sketch llll min-ital pit-nu..- cl Ilu' most stable radical, showing the hyperconjugative lIlIr_|_.-Ir|Ir-IIr_i-.J on 1"“ a)“ ,t"! It'l I . :tutl CH3CH2CH2CH2- "a M m | ,u twat-11,. and (CH3CH2)2CCH3 m In I _ H .1_I-{._El,'r| (CH3)2CCHZCH3, and (CH3)2CHCH2CH2- . _Illj._ lII lht'il'iaur _ IIM'vri Ill-I." ill“.- .tinn 1hr tlurn m Tnlrlu: ‘- Jlilll llflllilt'li'll'uljlli‘ as from first step. IT. l'ululllulr Ml“ 1.1: Hull, - l |;. [tritium H - l-_ II "Mitt. I -vr'l,'[|_L:«.i:"l + HI. I Ill-III Em II Human-ml! ru Il‘ It . .flilll 1mm rm -:: __: hullfll tul.l'.lt|llll rut-ImMHL..,-_L,i i” P n u|1|.:- l'lllllLL!_'-;'hll1'llI”. H1 och molecule.) . I“ Il'ltlth'. lllt-I'immwl I..- II I,“ H . . _-. I.”.. ill-"I'll; “‘ “l-l'“ I‘I‘Hllliris as you can think of that might result from the pyrolytic crack- dikllnw that the only initial process is C—C bond cleavage. I‘Uhrzl in Problem 15 for (a) butane and (b) 2-methylpropane. Use '3 I“ defer-mine the bond most likely to cleave homolytically, and use Ines l'm' Li :c following reactions. (a) H2 + F2 _) 2 HF; . _I Elm-1'1“ Fl'.+ Br2_)2HBI-; (d) H2 +12_)2m; Ht .L ' 15"“! -f-‘F + HF; (f) (CH3)3CH + c12 —> .. lb} this .11: I W ms “1H + 3'2 “9 (CH3)3CBr + HBr; all 13, determine how many constitutional isomers can hit: Identify all groups of hydrogens that reside in dis- "l "H in Stations 3-6 and 3—7, write the products of the radi- and (ii) 3-methylpentane. (b) For each, estimate the 128 Chapter} REACTIONS OF ALKANES bond~strength d ata from Table 3-1 determine the AH" values of the pl'opagdh. , 1 I. mill-“"1 for the chlorination of 3-methylpentane at C3. What is the overall AH" value ‘1 flu?“ m if”) reaction? 5' I” will a"'l iml (ZED/Write In full the mechanism for monobromination of methane. Be sure to menus. M ” tion, propagation, and termination steps 'I' I '5! """[II .t. m: 1. 21. Wnte a mechanism for the radical bromrnation of the hydrocarbon benze 3, II f“ I,” ' . a 11% structure, see Section 2—3) Use propagation steps similar to those In the halogen”th I" lei-Hi I1' alkanes, as presented in Sections 3—4 through 3-6. Calculate AH“ values for each u, I. .I and for the reaction as a whole How does this reaction compare thermodynamicmh 1" It, .{I With the bromination of other hydrocarbons? Data: DH“ (CGHSHH) = 112 kcal mol -. “Jam.” DH” (gurus = 81 kcal marl. W, m. @} Write the major organic product(s), if any, of each of the following reactions. 1‘,“ flfluni'fl CH3 | | I A I . (a) CH3CH3 + I2 ——> (b) CH3CH2CH3 + F2 ——> (c) + 3,, I ‘ CH3 (EH3 (EH3 CH3 II 311’ in; CHIUHJLI‘ + C12 _—> (e) CHZL‘CCHg ‘i‘ BI2 _—) I IJHHHI'H! éHg (JZH3 will RUE: , 23 Calculate product ratios in each of the reactions in Problem 22. Use relative reactiviij "H" :1? E: .‘ data for F2 and Ci2 at 25°C and for Br2 at 150°C (Table 3-6). ”‘ me“ : . i 3 Which, if any, of the reactions in Problem 22 give the major product with reasonable :2} 1:! _l_ I! selectivity (i e , are useful “synthetic methods”)? I ‘- , H. 'I I. r 25. (a) What would be the major organic product of monobromination of pentane at 125"t".' “:2 1'; (b) Draw Newman projections of all possrble staggered conformations arismg from rori'l- 11.“: 3 tion about the C2—C3 bond for this product molecule. (0) Draw a qualitative graph of Ill-Wm,“ potential energy versus torsional angle for C2—C3 rotation in this molecule. (Note: ' A bromine atom is considerably smaller, steiically, than is a 26. (a); ketch a potentia is. aunt .Irrhins-ua pentane to give the major product (Problem 25). USE II DH" information from this chapter (Tables 3~l 3 2, and 3 4, as appropriate). (11) Indi- mm mm J cate the locations of the transition states and whether each is early or late. (c) Sketch a similar graph for reaction of pentane with 12. How does it differ fr romination? SIG. 'liypit'al ll} lulu] Ilii‘t't' dibromoethane exists as an equilibrium ' I'iiJ t'ult'ul , mixture in which 89% .nlmc M: s are in an anti conformation and 11% are gauche. The comparable ratio mlsihiiih. for butane under the same circumstances IS 72% am: and 28% gauche. Suggest an ex- finmw 1'“: planatipn for the difference, bearing in mind that Br is sterically smaller than CH3 (see mm“ rm. E Problem 25) (Hint: Consider the polarity of a C—Br bond and consequent electrostatic mm 65;:ij effects) .Il'lliTlll the f @. Write balanced equations for the combustion of each of the following substances (mo— a 7' m“ gimme lecular formulas may be obtained from Table 3-7) (a) methane; (b) propane; (c) cyclo- “I”, 351;“, hexane; ((1) ethanol (e) sucrose mum for I 0 0 309.7 kcz O H H the complel 29. Propanal (CH3CH2CH) and acetone (CH3CCH3) are isomers With the formula C3H50. T e heat of combustion of propanal is ‘434.l kcal mol—J moi 1. (a) Write a balanced e 3-7. compa: I l c I; either compound. (1)) what 3r mo e u and acetone? Whi ergy 3a ' ennodynamically stable, propanal or ace . (Hint: Draw a diagram similar to that in Fi . hydrogen: 0 primary and :ing the _ steps r this do initia- EHf, (for :nation at ch step I lically l1 moi; 1'. + BI‘2 reactivi ly easonablt' . me at 1E5'11' mg from 1'L1lil' re graph “1- 5. (NM-t”- | -) I [opagariull .lem 2-5?- “5’3 tie). f-‘Jt W" _ (c) Sketch u graph 1'1” i1..- 1.‘-‘-'-‘ 30. Sulfuryl chloride (802C12, see margin for structure) is a liquid reagent that may be used .1]. .. for chlorinations of alkanes as a substitute for gaseous elemental chlorine. Propose a mechanism for chlorination of CH; using sulfuryl chloride. (Hint: Follow the usual model for a radical chain process, substituting SOZCIZ for C12 where appropriate.) Use the Arrhenius equation (Section 2-1) to estimate the ratio of the rate constants k for the reactions of a C—H bond in methane with a chlorine atom and with a bromine atom at 25°C. Assume that the A values for the two processes are equal, and use Ea = 19 real mol’l for the reaction between Br- and CH4. When an alkane with different types of CeH bonds, such as propane, re- ;lgrr. with an equimolar mixture of Br2 and C12, the selectivity in the formation of the itl‘filllllliliCd products is much worse than that observed when reaction is carried out yvith Hr: alone. (In fact, it is very similar to the selectivity for chlorination.) Explain. @ Hl-mni anti-1n of l-bronlopropane gives the following results: CH3CH2CH2Br CH3CH2CHBr2 + CH3CHBrCHzBr + BrCH2CH2CH2Br 90% 8.5 95: 1.5% |.':lI-::n|.-nt- the relative reactivities of the hydrogens on each of the three carbons toward hmmine atoms. Compare these results with those from a simple alkane such as propane, “nil suggchi explanations for any differences. 5.]. a Il-_'|.']kfl§2lt'llL'<ll alternative mechanism for the haiogenation of methane has the following pmp'ngfltirln steps. a] .‘ir : on: —> CH3X+H- tiltil: I-X; ———>HX +X- [in “sing Hf!” values from appropriate tables, calculate AH” for these steps for any our ul' Ihc halogens. (b) Compare your AH“ values with those for the accepted mechanism t I'ahle- 3-5: Do you expect this alternative mechanism to compete successfully with the natural-LI one" (Hint: Consider activation energies.) 1": 'I'll.- addition of certain materials called radical inhibitors to halogenation re— .li'i. 1'I.|I|t"I| llytlltlt': :'_ I “I'll -|'.'“_'I.'|||-\,I'|'}. "trima- canscs lll'.‘ reactions to come to a virtually complete stop. An example is the inhibi- tmn in. | . ul' “1-: chlorination of methane. Explain how this inhibition might come about. 'Illlill {Tiliutllnlu AH” values for possible reactions of the various species present in the sys— In” with |~. :ulul Lulu:ch the possible further reactivity of the products of these 12 reactions.) _ :ri‘mn fuels (e.g., 2,2,4-trilnethylpentane, a common component of gaso- llllf‘i '1'“ t' levy sililiiru' heats of combustion when calculated in kilocalories per gram. “ll L':I|~ Illnlc llL'illh of combustion per gram for several representative hydrocarbons in Mill: -" - 11” Make the same calculation for ethanol (Table 3-7). (c) In evaluating the l"“"”-"'“’? "I ":-"-I'~'- -|l- Ii" (90% gasoline and 10% ethanol) as a motor fuel, it has been es- ""l""'"~"-l "14“ -||1 thlinllmiliiu running on pure ethanol would get approximately 40% fewer 3”?" U-t'lnm Ii-L'ul would an identical automobile running on standard gasoline. Is ML"; Hull-1'?"I“:”-M311ISIIH"W1lh the results in (a) and (b)? What can you say in general l" ' 4J'Ih'lfiilltms of oxygen-containing molecules relative to hydrocarbons? n' '. - . .. . " ml“ "'i'-'”"-' lllt'l-Jll-‘Jl-“s that have been used as fuel additives are methanol (CH3OH) “I”! m III WlimeIto'lpropanc [tart-butyl methyl ether, (CH3)3COCH3]. The AH‘éomb Rm 1 h I‘ L"ll-"E-‘Iliifihl': in the gas phase are -182.6 kcal mol—1 for methanol and “M " " 3'1’n--"flIE-F\:y-TE-methylpropane. (:1) Write balanced equations for ‘-'”'"“'I'-In of each of these molecules to C02 and H20. (b) Using Table ‘ Ll” .. . . . !:I "hung: In" Min Values for these compounds with those for alkanes with simi- J'hi' 1.'i‘III||1|.:-|.L._ 'l'l'l:||.'-|||}_ - f --.- .- . . . . . lthtmmm “I IEMIIIIIJH'” a: "‘ "' 'l‘-'I -.'~.the reactions of (31- With the primary and secondary Putt-n. mil uh I “l '_i"~'l'-~ a smular diagram comparing the reactions of Br° with the I n ....-. Intel-1139.1). of propane. (Hint: First obtain the necessary DH“ Problems :0: za—l—jcl: ll. Sulfuryl chloride (ELI). 69°C) 129 130 [haplerg REACTIONS OF AIKANES values from Table 3-1 and calculate AH" for both the primary and the secondary hydrogen abstraction reactions. Other data: Ea = 15 Read me];1 for Br- reacting with a primary C—H bond and EBl = 13 kcal mol" for Br. reacting with a secondary C—H bond.) to (c) consis—tent with the selectivity differe Br- reacting with propane? Explain. CI+03—>C10+02 C10+O—~—>Cl +02 Calculate AH" for each of these propagation steps. Use the following data: DHCI for 5' C10 = 64 kcal mol"; DH” for 02 = 120 kcal moi—1; DH" for an 0—02 bond in 03 = 26 kcal molil. Write the overall equation described by the combination of these steps amjc favorability of the process. n H and calculate its AH°. Comment on the thermodyn nation and monobromination products that are structurally isomeric. (c) Referring to Table 3-6, discuss which starting alkan e and which halogen will yield the least number of isomeric products. Preprofessional Problems i 41. The reaction CH4 + C1; —> (:ch1 + HCI is an example of (a) neutralization (b) an acidic reaction (c) an isomerization (d) an ionic reaction I (e) a radical chain reaction 42. CHZCI | CHZ—CHCHg CH3CH2CH2CHCHZCHZCH2CH3 II The sum of all the digits the following? (a) Five that appear in the (IUPAC) name for this compound is which of 43. In a competition reaction, equimolar amounts react with a limited amount of C12 at 300” pleted most from the mixture? (3) Pentane (b) 2-Methylpropane 44. The reaction of CH4 with C1 the values in the short table (a) +135 (b) _135 of the four alkanes shown were allowed to C. Which one of these alkanes would be de- (c) Butane (d) Propane 2 to yield CH3C1 and HCl is well known. 0n the basis of below, the enthalpy AH” (kcal moi—1) of this reaction is (c) +25 (d) —25 Bond Dissociation Energies DH° (kcal mol") MM H—Cl 103 H3C—Cl 85 ClaCl 53 H3C—H 105 Steiuiai have I1; ul‘ mflilily. TTUW airrhtced secrete _I.Lllmhydrrigesliir about steroids .tton'fi' How do All exatm entrants of Tilt: commercial 5 ...
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Ch3 problems - 3mg four .11: bonds .r the 0V”- ; 57 keel...

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