1559T_ch03_38-5410/22/052:59Page43 ⊕ EQA Solutions to Problems.43 Cbond participating (b)In naming these.remember that the radical carbon is always Cl (just like the"point of CH-CHz CH3-CH2 CH-CH-CH-CH CH-CH-C-CH 2-Ethylbutyl radical 1-Ethyl-1-mthypropyl radica Primary,less stable ⊕ (c)Left to right:1.2-dimethylpropyl radical,secondary.intermediate in stability:1.1-dimethylpropyl radical,tertiary,most stable;3-methylbutyl radical,primary,least stable Hyperconjugation in the 1.1-dimethylpropyl radical is the same as in 1-ethyl-1-methylpropyl [(b)above]: in your picture.an H should replace one of the end CH groups. aCH,C,CH,一CH,cH,·+·CH,c-Cond c Then there are three possible recombinations: (d)CH3 CH3CH2-CHaCH2CH3 (Reverse of first step) Propane
Solutions to Problems • 43 COH bonds overlapping with the radical p orbital, and one with a COC bond participating instead of a COH: (b) In naming these, remember that the radical carbon is always C1 (just like the “point of attachment” carbon in alkyl groups). The parent name is based on the longest carbon chain beginning at C1, and all appendages are named as substituents: (c) Left to right: 1,2-dimethylpropyl radical, secondary, intermediate in stability; 1,1-dimethylpropyl radical, tertiary, most stable; 3-methylbutyl radical, primary, least stable. Hyperconjugation in the 1,1-dimethylpropyl radical is the same as in 1-ethyl-1-methylpropyl [(b) above]; in your picture, an H should replace one of the end CH3 groups. 15. Work problems like this “mechanistically”: Proceed via general reaction steps as you have previously seen illustrated in the text until you reach stable molecules. Pyrolysis of propane starts as follows: (a) Then there are three possible recombinations: (b) (c) (d) Propane CH3 CH3CH2 CH3CH2CH3 (Reverse of first step) Butane 2 CH3CH2 CH3CH2CH2CH3 Ethane 2 CH3 CH3CH3 CH3CH2 CH3 CH3CH2 CH3 C—C bond cleavage H H H C H2C CHCH3 CH3HC and H H H H C C H2C CH3 H H H H C C H2C CH3 1559T_ch03_38-54 10/22/05 2:59 Page 43
1559r_ch03.38-5410/2/052:59Page4d EQA 44.Chapter 3 REACTIONS OF ALKANES Two possible hydrogen abstractions can occur: H aa+an一+atcw 气H Abstraction only occurs from the carbon next to a radical carbon.Methyl radical.CH.doesn't have another carbon next to its radical center.so it cannot give up a hydrogen in an abstraction.It can still acking of propane:methane,ethane.butane,and ethen 16.(a)The weakest bond in butane is the C2-C3 bond.DH=88 kcal mol-!(Table 3-2).Pyrolysis should therefore proceed as follows: ①CH,c闻间6H,CH,一2CH,C:c-Cbd deag (2)2CH,CH2·一CHCH2CH,CH Reverse of(1) Ethane (b)The weakest bonds are the three equivalent C-Cbonds.D=88 kcal molTherefore. 0(aH.726一(CH.CH+·cH,cme @2a一 (6)2(CH2CH ④CH,CHCH一(CH,CH Revene of ()bi 句cH,fYam6ch一GL+cH=ca,lyop nc (6)(CHCH.HCHCHCH,-CH,CH.CH +CH:-CHCH, Propane Propene 17.The D data are readily found (Tables 3-1 and 3-4).Values in kcal mol- (a)104+38-2136)=-130 (b)104+58-2103)=-44 (c)104+46-2(87)=-24 (d104+36-2(71)=-2 (e)96.5+38-(110+136)=-1115 f)96.5+58-(85+103)=-335 (g)96.5+46-(71+87)=-15.5 h)96.5+36-(55+71)=+6.5
Two possible hydrogen abstractions can occur: (e) (f ) Abstraction only occurs from the carbon next to a radical carbon. Methyl radical, jCH3, doesn’t have another carbon next to its radical center, so it cannot give up a hydrogen in an abstraction. It can still accept a hydrogen, however [reaction (e), above]. So there are four new products formed from cracking of propane: methane, ethane, butane, and ethene (ethylene). 16. (a) The weakest bond in butane is the C2OC3 bond, DH° 88 kcal mol1 (Table 3-2). Pyrolysis should therefore proceed as follows: (1) (2) (3) (b) The weakest bonds are the three equivalent COC bonds, DH° 88 kcal mol1 . Therefore, (1) (2) (3) (4) (5) (6) 17. The DH° data are readily found (Tables 3-1 and 3-4). Values in kcal mol1 . (a) 104 38 2(136) 130 (b) 104 58 2(103) 44 (c) 104 46 2(87) 24 (d) 104 36 2(71) 2 (e) 96.5 38 (110 136) 111.5 (f ) 96.5 58 (85 103) 33.5 (g) 96.5 46 (71 87) 15.5 (h) 96.5 36 (55 71) 6.5 (CH3)2CH Propane Propene H CH2 CHCH3 CH3CH2CH3 CH2 CHCH3 CH3 Hydrogen abstractions Methane Propene H CH2 CHCH3 CH4 CH2 CHCH3 CH3 CH(CH3)2 (CH3)3CH Reverse of (1); recombinations 2,3-Dimethylbutane 2 (CH3)2CH (CH3)2CHCH(CH3)2 Ethane 2 CH3 CH3CH3 (CH3)2CH (CH CH3 3)2CH CH 3 Cleavage Hydrogen abstraction Ethane Ethene CH3CH2 H CH CH2 CH2 3CH3 CH2 CH2 2 CH3CH2 CH3CH2CH2CH3 Reverse of (1) CH3CH2 CH2CH3 2 CH3CH2 C—C bond cleavage Ethane Ethene CH3CH2 CH3CH3 CH2 CH2 H CH2 CH2 Methane Ethene CH3 CH4 CH2 CH2 H CH2 CH2 44 • Chapter 3 REACTIONS OF ALKANES 1559T_ch03_38-54 10/22/05 2:59 Page 44