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52 Chapter 3 REACTIONS OF ALKANES A complex mixture is obtained. Cl₂ is more selective than F₂, but still prefers 3° to 1° positions by a factor of only about 5 to 1. CH₃ CH₃ (e) CH₃ CH₂ CH₃ Br₂ is very selective for 3° C-H bonds. Br CH₃ 28 Relative yield = number of hydrogens of a given type in the starting alkane X relative reactivity relative yield of one product X 100% = % yield of that product sum of relative yields of all products Hydrogen Number of Relative Relative % Product type hydrogens reactivity yield yield (b) 2° 2 1.2 2.4 29 CH₃CH₂CH₂F 1° 6 1 6 71 (d) 3° 1 5 5 18 1° 6 1 6 21 2° 2 4 8 29 1° 9 1 9 32 (c) and (e) 3° substitution by Br₂ is about 73% selective in (c) and 91% in (e). 29. Only the bromination reactions (c) and (e) are really acceptable as synthetic methods. The other reactions, giving several products in comparable yields, are not synthetically useful. The fluorination (b) might look good on paper, but use of as a reagent in practice is very difficult. 30. (a) Pentane has two kinds of hydrogens-primary and secondary-but they are distributed in three distinct groups, because three different monobromopentanes are possible, depending on the site of radical bromination. Replacement of any one of the six primary hydrogens at and C5 (group 'a' in the structure below) gives 1-bromopentane. We know that primary hydrogens are 1/80th as reactive as secondary, so this should be the product formed in the lowest yield. The six secondary hydrogens on C2, C3, and C4 are all about equally reactive. Replacement of either of the two on C3 (group 'b') gives 3-bromopentane. Replacement of any one of the four on C2 and C4 (group 'c') gives 2-bromopentane. This will be the major product, because it can form from replacement of the greatest number of the most reactive hydrogens in the starting molecule. c H₂ H₂ C C a a H₃C C CH₃ H₂ b (b) and (c) First draw the molecule in a conventional formula and identify the bond of rotation: Br H H₂ C C 3 2 H₃C C CH₃ H₂ Then choose a sighting direction for preparing the Newman projections. For example, C2 in front and C3 behind. Begin with any arbitrary staggered conformation and proceed with 120° rotations of one carbon (say,