Organohalides

John E. McMurry
www.cengage.com/chemistry/mcmurry
Chapter 10
Organohalides
Paul D. Adams • University of Arkansas
What Is an Organohalide?



An organic compound containing at least one
carbon-halogen bond (C-X)
 X (F, Cl, Br, I) replaces H
Can contain many C-X bonds
Properties and some uses
 Fire-resistant solvents
 Refrigerants
 Pharmaceuticals and precursors
Why this Chapter?


Reactions involving organohalides are less
frequently encountered than other organic
compounds, but reactions such as
nucleophilic substitutions/eliminations that
they undergo will be encountered
Alkyl halide chemistry is model for
mechanistically similar but more complex
reactions
10.1 Naming Alkyl Halides

Find longest chain, name it as parent chain
 (Contains double or triple bond if present)
 Number from end nearest any substituent (alkyl
or halogen)
Naming if Two Halides or Alkyl Are
Equally Distant from Ends of Chain

Begin at the end nearer the substituent having its
name first in the alphabet
10.1 Structure of Alkyl Halides



C-X bond is longer as you go down periodic table
C-X bond is weaker as you go down periodic table
C-X bond is polarized with slight positive charge on
carbon and slight negative charge on halogen
10.2 Preparing Alkyl Halides from
Alkanes: Radical Halogenation

Alkyl halide from
addition of HCl,
HBr, HI to
alkanes
Preparing Alkyl Halides from
Alkanes: Radical Halogenation


Alkane + Cl2 or Br2, heat or light replaces C-H with C-X
but gives mixtures
 Hard to control
 Via free radical mechanism
It is usually not a good idea to plan a synthesis that uses
this method
Radical Halogenation of Alkanes

If there is more than one type of hydrogen in an alkane,
reactions favor replacing the hydrogen at the most highly
substituted carbons (not absolute)
Relative Reactivity



Based on quantitative analysis of reaction products,
relative reactivity is estimated
Order parallels stability of radicals
Reaction distinction is more selective with bromine than
chlorine
Chlorination vs. Bromination
10.3 Preparing Alkyl Halides from
Alkenes: Allylic Bromination



N-bromosuccinimide (NBS) selectively brominates allylic
positions (due to lower E resulting from resonance)
Requires light for activation
A source of dilute bromine atoms
Allylic Stabilization



Allyl radical is delocalized
More stable than typical alkyl radical by 40 kJ/mol (9
kcal/mol)
Allylic radical is more stable than tertiary alkyl radical
10.4 Stability of the Allyl Radical:
Resonance Revisited


Three electrons are delocalized over three carbons
Spin density surface shows single electron is dispersed
Effects of Resonance
Allylic bromination of unsymmetrical alkenes usually produces mixed
products.
• Rxn at less hindered primary is favored.
• Also, in general, more highly-substituted alkenes are more stable.
Br
NBS
+
Br
hv, CCl4
minor
major
10.5 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 3°alcohol
1°and 2°alcohols react very slowly and often
rearrange, so alternative methods are used (Ch. 11)
10.6 Organometallic Reagents
for Alcohol Synthesis
A covalent bond between carbon (C) and a metal (M) makes the C nucleophilic.
C
δ-
M
δ+
C
δ-
Li
δ+
C
δ-
Mg
δ+
Atoms
C
Al
Mg
Li
Na
K
EN
2.5
1.6
1.3
1.0
0.9
0.8
Types of Organometallic
Coupling Reagents/Rxns




Grignard Reagents
Alkyllithium Reagents
Gilman Reagents
Suzuki-Miyaura Reaction
10.6 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 (aromatic), alkenyl (vinylic)
 X = Cl < Br < I
Reagent Synthesis

Formation of Grignard Reagent:
R X
Br

+
Mg
+
Mg
Ether
R MgX
Ether
MgBr
Formation of Alkyllithium Reagent:
R X
Br
+
2 Li
+
2 Li
R Li
+
Li
Li+- X
+
Li+- Br
Organometallic reagent
mechanism

The metals in both Grignard reagents and alkyllithium
reagents turn the attached R group into a nucleophile,
that can then attack an electrophilic carbon (e.g.,
carbonyl)
Examples
*We will return to these reactions after discussing alcohols and carbonyls
Limitations/Scope of Grignard
and Alkyllithium Reagents

Both are good nucleophiles, but will act as bases
if H+ available in solution:
MgBr

H2O
+
HOMgBr
In the presence of multiple bonds with a strong
EN atom, will attack as nucleophile:

C=O, C=N, C≡N, S=O, N=O
10.7 Organometallic Coupling
Reactions


Alkyllithium (RLi) forms from RBr and Li metal
RLi (primary, secondary or tertiary alkyl, aryl or vinyl R
group) reacts with copper iodide to give lithium
dialkylcopper (Gilman reagents)
Utility of Organometallic Coupling
in Synthesis


Lithium dialkylcopper (Gilman) reagents react with alkyl
halides to give alkanes
Aryl and vinyl organometallics also effective
Suzuki-Miyaura Reaction


Coupling rxn of aromatic or vinyl substituted boronic acid
with aromatic or vinyl substituted organohalide in
presence of base and palladium catalyst.
Widely used today in pharmaceutical industry.
I
+
OH
CaCO3, THF
B
OH
I
OH
B
OH
Pd(PPh3)4
+
Pd(PPh3)4
CaCO3, THF
10.8 Oxidation and Reduction in
Organic Chemistry



In organic chemistry, we say that oxidation occurs when
a carbon or hydrogen that is connected to a carbon atom
in a structure is replaced by oxygen, nitrogen, or halogen
 Not defined as loss of electrons by an atom as in
inorganic chemistry
Oxidation is a reaction that results in loss of electron
density at carbon (as more electronegative atoms
replace hydrogen or carbon)
Oxidation: break C–H (or (C–C) and form C–O, C–N,
C–X
Reduction Reactions

Organic reduction is the opposite of oxidation
 Results in gain of electron density at carbon (replacement of
electronegative atoms by hydrogen or carbon)

Reduction: form C–H (or C–C) and break C–O, C–N, C–X