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10. Alkyl Halides - Iran University of Science and...
Transcript of 10. Alkyl Halides - Iran University of Science and...
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, 6th edition
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What Is an Alkyl Halide?
An organic compound containing at least one
halogen attached to an sp3 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
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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
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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
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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
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Many Alkyl Halides That Are Widely Used Have Common Names
Chloroform CHCl3
Carbon tetrachloride CCl4
Methylene chloride CH2Cl2
Methyl iodide CH3I
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Properties of the Halomethanes
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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
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Reaction of Alkanes with Halogens
Alkane + Cl2 or Br2, 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
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Multiple Substitution
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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: 3o>2o>1o
The number of available hydrogens is a factor
Methyl hydrogens, for example, often
outnumber 2o or 3o hydrogens.
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Chlorination is unselective:
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Relative Reactivity
Based on quantitative analysis of reaction
products, relative reactivity is estimated
Order parallels stability of radicals
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Toward chlorination
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Bromination is much more selective
35% 65%
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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
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Reactivity of Alcohols with HX
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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 (SOCl2: ROH to RCl)
Phosphorus tribromide converts alcohols into
alkyl bromides (PBr3: ROH to RBr)
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Problem: Products?
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Prob.: Synthesize from alcohols?
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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
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François Auguste Victor Grignard
• Born in Cherbourg,
1871
• PhD, University of
Lyon, 1901
• Nobel Prize in
Chemistry, 1912
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Grignard Carbon as Nucleophile:
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Reactions of Grignard Reagents
Many useful reactions
RMgX behaves as R- (adds to C=O)
RMgX + H3O+ yields R-H
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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)
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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
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Lithium dialkylcopper reagents react with alkyl halides to give alkanes:
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Problem:
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Problem: Identify reagents
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Problem
a = BH3/THF followed by H2O2/OH-
b = PBr3/ether
c = (CH3)2CuLi/ether
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Chapter 7-1, Questions
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