Unit 4 – Alkyl Halides, Nucleophilic Substitution, and Elimination Reactions Nomenclature and Properties of Alkyl Halides Synthesis of Alkyl Halides Reactions of Alkyl Halides Mechanisms of S N 1, S N 2, E1, and E2 Reactions Nucleophilicity, Substrate, and Leaving Group Effects
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Unit 4 – Alkyl Halides, Nucleophilic Substitution, and Elimination Reactions
Unit 4 – Alkyl Halides, Nucleophilic Substitution, and Elimination Reactions. Nomenclature and Properties of Alkyl Halides Synthesis of Alkyl Halides Reactions of Alkyl Halides Mechanisms of S N 1, S N 2, E1, and E2 Reactions Nucleophilicity, Substrate, and Leaving Group Effects. - PowerPoint PPT Presentation
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Unit 4 – Alkyl Halides, Nucleophilic Substitution, and Elimination Reactions
Nomenclature and Properties of Alkyl Halides
Synthesis of Alkyl Halides
Reactions of Alkyl Halides
Mechanisms of SN1, SN2, E1, and E2 Reactions
Nucleophilicity, Substrate, and Leaving Group Effects
Alkyl Halides
Alkyl halide: a compound with a halogen atom
bonded to one of the sp3 hybridized carbon atoms of an alkyl group
Two types of names: IUPAC system Common names
Nomenclature
IUPAC System: Alkyl halides are named as an alkane
with a halo-substituent:Review the rules for naming alkanes covered in Unit 2
CH3CH2CH2Cl
BrCH3CH2CH2Cl
Br
1-chloropropane bromocyclohexane
Nomenclature
Common Names: alkyl group name + halide
CH3CH2CH2Cl
Br
CH3CH2CH2Cl
Br
n-propyl chloride Cyclohexyl bromide
Nomenclature
Special common names: CH2X2 = methylene halide CHX3 = haloform CX4 = carbon tetrahalide
CH2Cl2 CHCl3
CCl4
chloroformtrichloromethane
Methylene chloridedichloromethane
Carbon tetrachloridetetrachloromethane
Types of Alkyl Halides
Alkyl halides can be classified by the type of carbon atom the halogen is bonded to: primary halide (1o):
halogen attached to a 1o carbon
secondary halide (2o): halogen attached to a 2o carbon
tertiary halide (3o): halogen attached to a 3o carbonCH3CBr
CH3
CH3
CH3CHCH3
I
CH3CH2Cl
Types of Alkyl Halides
Geminal dihalide: 2 halogens bonded to the same carbon
atom
Vicinal dihalide: 2 halogens bonded to
adjacent carbon atoms
CH
H
Cl
Cl
Br
Br
Other Organic Halides Aryl halide:
halogen is attached directly to an aromatic ring
Benzylic halide halogen is attached to a carbon that is
attached to a benzene ring
C
C
CF
F
F
F
CCl
H
H
H
HO
I
I
I
I
CH2
CH
NH2
CO2H
thyroxine
CH2Cl
benzylic carbon
benzylic chloride
Other Organic Halides
Allylic halide: halogen is attached to a carbon that is
attached to a C=C
Allylic carbon
Allylic chloride
C CCH2Cl
H
H
H
Other Organic Halides
Vinyl Halide: halogen attached to a carbon that is
part of a C=C
C
C
CF
F
F
F
CCl
H
H
H
HO
I
I
I
I
CH2
CH
NH2
CO2H
C
C
CF
F
F
F
CCl
H
H
H
HO
I
I
I
I
CH2
CH
NH2
CO2H
Monomer for PVC Monomer for teflon
Uses of Alkyl Halides
Anesthetics: Chloroform (CHCl3)
toxiccarcinogenic (causes cancer)
Solvents: CCl4
formerly used in dry cleaning CH2Cl2
formerly used to decaffeinate coffeeliquid CO2 used now
Uses of Alkyl Halides
Freons: Freon-12: CF2Cl2
Freon-22: CHClF2
Freon-134a:
Pesticides:
Cl C
Cl
Cl
C
H
Cl
Cl
DDT
Cl C
Cl
Cl
C
H
Cl
Cl
Cl Cl
Cl
ClCl
Cl
Cl ClChlordane (termites)
C CF
F
F
F
H
H
Physical Properties
Boiling Point: Compounds with higher MW’s and
greater surface area (more linear) tend to have higher BP.
BP increases as size of halogen increases
F < Cl < Br < I
BP decreases as branching increases
Physical Properties
Density: Alkyl chlorides are common solvents for
organic reactions.
CH2Cl2
CHCl3
CCl4
More dense than water
Preparation of Alkyl Halides
Alkyl halides can be prepared from a variety of starting materials including alkanes, alkenes, alkynes, alcohols, and other alkyl halides.
You are responsible for knowing and applying the synthesis of R-X by: free radical halogenation reactions free radical allylic bromination
reactions
Preparation of Alkyl Halides
Free Radical Halogenation of Alkanes
alkane + X2 alkyl halide(s) + HX
Poor selectivity and moderate yields often limit usefulness. Bromination is more selective and gives
the product formed from the most stable free radical.
Chlorination is useful when only one type of reactive hydrogen is present
hor
Preparation of Alkyl Halides
Useful Examples:
+ Cl2 + HCl
CH3 CH3+ Br2
Cl
C
CH3
CH3
H C
CH3
CH3
Br
h
50 %
CH3CHCH3
CH3
+ Br2h
CH3CCH3
CH3
Br
+ HBr
90%
Preparation of Alkyl Halides
The following free radical halogenation is doomed to failure!
The following addition reaction occurs instead:
Br2
h
Br
Br
Br2
h Br
Preparation of Alkyl Halides
Free Radical Allylic Bromination:
where NBS = N-bromosuccinimide
C + NBS CCC
H
CC
Br
h
+ HBr N
O
O
Br N
O
O
H + Br2
NBS
Preparation of Alkyl Halides
NBS is used to generate low levels of Br2 in situ. Minimizes addition of bromine across
the C=C
Allylic bromination is highly selective and occurs in the allylic position due to resonance stabilization of the resulting free radical.H
H
H
HH
H
HH
H
HH
H
Preparation of Alkyl Halides
Examples:
+ NBS
Br
h
+ NBS
+ NBS
+ NBS
Br
Br
Br
h
Reactions of RX
Most reactions of alkyl halides involve breaking the C-X bond. Nucleophilic substitution Elimination
The halogen serves as a leaving group in these reactions. the halogen leaves as X-, taking the
bonding electrons with it
C X+ -
Reactions of RX
Nucleophilic substitution: reaction in which a nucleophile replaces a
leaving group
Nucleophile: electron pair donor
Leaving group: an atom or group of atoms that are lost
during a substitution or elimination reactionretains both electrons from the original bond
Reactions of RX
General Equation for Nucleophilic Substitution
The nucleophile can be neutral or negatively charged, but it must have at least one lone pair of electrons.
Example:Br
+ CH3O
OCH3
+ Br
C C X Nuc+ C C Nuc + X-
Reactions of RX
Elimination Reaction: two substituents are lost from adjacent
(usually) carbons, forming a new bond
Dehydrohalogenation: an elimination reaction in which H+ and
X- are lost, forming an alkene
C CC CH3
H
H
H CH3
Br
CH
H
CH3
CH3
C CC CH3
H
H
H CH3
Br
CH
H
CH3
CH3
CH3O-
Reactions of RX There are two common types of nucleophilic