Organic Chemistry M. R. Naimi-Jamal Faculty of Chemistry Iran University of Science & Technology
Chapter 7-1. Alkyl Halides Based on McMurry s Organic Chemistry, 6 th edition
What Is an Alkyl Halide? An organic compound containing at least one halogen attached to an sp 3 hybridized carbon X (F, Cl, Br, I) replaces H Can contain many C-X bonds Properties and some uses Fire-resistant solvents Refrigerants Pharmaceuticals and precursors 3
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Naming Alkyl Halides Name is based on longest carbon chain (Contains double or triple bond if present) Number from end nearest any substituent (alkyl or halogen) Halogens have same priority as alkyl groups 5
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Naming with Multiple Halides If more than one of the same kind of halogen is present, use prefix di, tri, tetra If there are several different halogens, number them and list them in alphabetical order 7
Naming if Two Halides or Alkyl Are Equally Distant from Ends of Chain Begin at the end nearer the substituent whose name comes first in the alphabet 8
Many Alkyl Halides That Are Widely Used Have Common Names Chloroform CHCl 3 Carbon tetrachloride CCl 4 Methylene chloride CH 2 Cl 2 Methyl iodide CH 3 I 9
Properties of the Halomethanes 10
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Preparing Alkyl Halides Alkyl halide is formed by the addition of HCl, HBr, HI to alkenes to give the Markovnikov product (see Alkenes chapter) Alkyl geminal dihalide from anti addition of bromine or chlorine 12
Reaction of Alkanes with Halogens Alkane + Cl 2 or Br 2, initiated by heat or light; replaces C-H with C-X Hard to control for monosubstitution Reacts via a free radical mechanism It is usually not a good idea to plan a synthesis that uses this method 13
Multiple Substitution 14
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Radical Halogenation of Alkanes If there is more than one type of hydrogen in an alkane, the reaction favors replacing the hydrogen at the most highly substituted carbons: 3 o >2 o >1 o The number of available hydrogens is a factor Methyl hydrogens, for example, often outnumber 2 o or 3 o hydrogens. 16
Chlorination is unselective: 17
Relative Reactivity Based on quantitative analysis of reaction products, relative reactivity is estimated Order parallels stability of radicals 18
Toward chlorination 19
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Bromination is much more selective 35% 65% 21
Preparing Alkyl Halides from Alcohols Reaction of tertiary C-OH with HX is fast and effective Add HCl or HBr gas into ether solution of tertiary alcohol Primary and secondary alcohols react very slowly and often rearrange, so alternative methods are used 22
Reactivity of Alcohols with HX 23
Preparation of Alkyl Halides from Primary and Secondary Alcohols Specific reagents avoid acid and rearrangements of carbon skeleton Thionyl chloride converts alcohols into alkyl chlorides (SOCl 2 : ROH to RCl) Phosphorus tribromide converts alcohols into alkyl bromides (PBr 3 : ROH to RBr) 24
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Problem: Products? 26
Prob.: Synthesize from alcohols? 27
Reactions of Alkyl Halides: Grignard Reagents Reaction of RX with Mg in ether or THF Product is RMgX an organometallic compound (alkyl-metal bond) R is alkyl 1, 2, 3, aryl, alkenyl X = Cl, Br, I 28
François Auguste Victor Grignard Born in Cherbourg, 1871 PhD, University of Lyon, 1901 Nobel Prize in Chemistry, 1912 29
Grignard Carbon as Nucleophile: 30
Reactions of Grignard Reagents Many useful reactions RMgX behaves as R - (adds to C=O) RMgX + H 3 O + yields R-H 31
Organometallic Coupling Reactions Alkyllithium (RLi) forms from RBr and Li metal RLi reacts with copper iodide to give lithium dialkylcopper (Gilman reagents) Henry Gilman (1893-1986) 32
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Utility of Organometallic Coupling in Synthesis Coupling of two organometallic molecules produces larger molecules of defined structure Aryl and vinyl organometallics also effective 34
Lithium dialkylcopper reagents react with alkyl halides to give alkanes: 35
Problem: 36
Problem: Identify reagents 37
Problem a = BH 3 /THF followed by H 2 O 2 /OH - b = PBr 3 /ether c = (CH 3 ) 2 CuLi/ether 38
Chapter 7-1, Questions 22, 23, 24, 26, 28 39