Chapter06notes

Chapter06notes - 1. Alkyl Halides CX Bond Length () 1.39...

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1 1. Alkyl Halides C–X Bond Length (Å) 1.39 1.78 1.93 2.14 increase 1 C–X Bond Strength (kJ/mol) 472 350 293 239 decrease 1. Types of Alkyl Halides 2
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2 Alkyl halides are weakly polar molecules. They exhibit dipole-dipole interactions because of their polar C-X bond. Since the rest of the molecule contains only CCandC H 1. Polarity of Alkyl Halides Since the rest of the molecule contains only C-C and C-H bonds, they are incapable of intermolecular hydrogen bonding. 3 1. Physical Properties of Alkyl Halides Boiling Point ( Boiling Point (bp bp/ o C) C h l o r i d e G r o u p F l u o r i d e B r o m i d e I o d i d e -23.8 13.1 78.4 68 Me Et Bu s Bu -78.4 -37.7 32 3.6 38.4 101 91 2 42.5 72 130 120 4 69 51 i Bu t Bu - - 12 91.2 91 73.3 119 100(dec)
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3 1. Physical Properties of Alkyl Halides Density ( ρ ) C h l o r i d e G r o u p F l u o r i d e B r o m i d e I o d i d e 0.92 0.91(15) 0.89 0 87 Me Et Bu s Bu 0.84(-60) 0.72 0.78 1.73(0) 1.46 1.27 1 26 2.28 1.95 1.61 1 60 5 0.87 0.87 0.84 i Bu t Bu - - 0.75(12) 1.26 1.26 1.22 1.60 1.60 1.57(0) 1. Common Alkyl Halides 6
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4 2. Reaction Types for Alkyl Halides 7 2. Bond Breaking and Making in Nucleophilic Substitution Mechanisms But what is the order of bond making and bond breaking? Two mechanisms: 8 Bond making and breaking occur at the same time. Bond breaking occurs first.
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5 Three components are necessary in any substitution reaction. 2. Substitution Reactions 9 Nucleophiles are Lewis bases that can be negatively charged or neutral. Negatively charged nucleophiles like HO¯ and HS¯ are used as salts with L + Na + o K + counterions to balance the charge 3. Nucleophiles in Substitution Reactions as salts with Li , Na , or K counterions to balance the charge. Since the identity of the counterion is usually inconsequential, it is often omitted from the chemical equation. 10
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6 When a neutral nucleophile is used, the substitution product bears a positive charge. 3. Neutral Nucleophiles The substitution product’s positive charge is usually caused by a proton bonded to O or N. Tha proton is readily los from this in a B Ø nsted 11 That proton is readily lost from this in a Br nsted- Lowry acid-base reaction, forming a neutral product. 3. Common Nucleophiles 12
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7 In a nucleophilic substitution reaction of R-X, the C-X bond is heterolytically cleaved, and the leaving group departs with the electron pair in that bond, forming X:¯. The more stable the leaving group X:¯, the better able it is to 3. The Leaving Group The more stable the leaving group X: , the better able it is to accept an electron pair. 13 For example, H 2 O is a better leaving group than HO¯ because H 2 O is a weaker base. 3. Drawing Products of Nucleophilic Substitution Reactions The overall effect of any nucleophilic substitution is the replacement of the leaving group by the nucleophile. To draw any nucleophilic substitution product: Find the sp 3 hybridized carbon with the leaving group.
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Chapter06notes - 1. Alkyl Halides CX Bond Length () 1.39...

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