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CHAPTER 5
ALKYL HALIDE
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Objective
Outcome
Ability to explain the relationship between the structure, physical andchemical properties of the different bonds and functional group in organiccompounds.(CO2)
Ability to explain each of functional group activity. (CO3)
The student should be able to: -
Name alkyl halides.
Explain alkyl halides properties.
Predict, draw and name the products of functional groups reactions.
Draw the mechanistic pathway.
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BACKGROUND
The functional group of alkyl halides is a carbon-halogen bond, the
common halogens being fluorine, chlorine, bromine and iodine. With the
exception of iodine, these halogens have electronegativities significantly
greater than carbon.
This functional group is polarized so that the carbon is electrophilic and
the halogen is nucleophilic, as shown in the drawing on the right.
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In alkyl halides this polarity causes the carbon to become activatedto substitution reactions with neucleophiles.
Carbon-halogen bonds get less polar, longer and weaker in goingfrom fluorine to iodine.
Classes of halides :-
i. Alkyl : Halogen, X, is directly bonded to sp3 carbon
ii. Vinyl : X is bonded to sp2 carbon of alkene.
iii. Aryl : X is bonded to sp2 carbon on benzene ring.
C
H
H
H
C
H
H
Br
alkyl halide
C C
H
H
H
Cl
vinyl halide
I
aryl halide
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Solvents very good for a variety of organic compounds.
Reagents precursors for a variety of syntheses. One bigapplication is the so-called Grignard reagents.
Freons used as refrigerator coolants, in sprays and as blowingagents. Because of ozone problem, their application is foreseen toterminate in about 10 years.
Pesticides very powerful, but tend to accumulate in naturebecause of low reactivity, causing lasting contamination. Many of
them now avoided, DDT banned.
Anesthetics widely used in the past (chloroform), but because oftoxicity now are generally avoided.
USESOF ALKYL HALIDES
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Halogens are more electronegative than C.
Carbon-halogen bond is polar, so carbon has partial positive charge.
Carbon can be attacked by a nucleophile.
Halogen can leave with the electron pair.
POLARITY AND REACTIVITY
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REVIEW
PREPARATIONOF ALKYL HALIDES
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IUPAC SYSTEM
An alkyl halide is named as an alkane with a halogen substituent-that is , as a
halo alkane.
To name a halogen substituent, change the ine ending of the name of the
halogen to the suffix o (chlorine -> chloro)
The halogen is treated as a substituent.
4
5
6 7
Cl 8r9
4-bromo-2-chloro-1-methylcyclohexane
1
2
3
4
5
6
7
8
Br9
2 bromo-5-methylheptane
1
2
3
4
5
6
7
8
Cl9
1-chloro-5,5-dimethylhexane
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Example
How to name an alkyl halide Using the IUPAC System
1. Give the IUPAC name of the following alkyl halide :
STEP 1 :- Find the parent carbon chain containing the halogen.
STEP 2 :- Apply all other rules of nomenclature.
a. Number the chain
b. Name and number the substituents
c. Alphabetize
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Common Names
Common names for alkyl halides are used only for simple alkylhalides. To assign a common name:
- Name all the carbon atoms of the molecule as a single
alkylgroup.
- Name the halogen bonded to the alkyl group. To name the
halogen,change the ine ending of the halogen name to
the suffix ide; for example, bromine -> bromide.
- Combine the names of the alkyl group and halide,
separating the words with a space.
tert-butyl iodide ethyl chloride
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Undergoes ionic reactions - Nucleophilic Substitution and Nucleophilic
Elimination Reactions.
1. Nucleophilic Substitution Reaction
2. Nucleophilic Elimination Reaction
REACTIONSOF ALKYL HALIDES
Nu:-
+
nucleop ile
:
:
alkyl ali e
R-Nu +:
:
:-
substratepro uct hali e ion
R-X X: :
C
X
C
H
C C(-HX)
alkyl hali e alkene
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What does the term "nucleophilic substitution" imply ?
A nucleophile is an the electron rich species that will react with anelectron poor species.A substitution implies that one group replacesanother
There are two fundamental events in these substitution reactions:
i. formation of the new bond to the nucleophile
ii. breaking of the bond to the leaving group
Depending on the relative timing of these events, two differentmechanisms are possible:
1. Bond breaking to form a carbocation preceeds the formation of
the new bond : SN1 reaction
2. Simultaneous bond formation and bond breaking : SN2 reaction
NucleophilicSubstitution Reaction
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SN1 indicates a substitution,nucleophilic,unimolecularreaction,
described by the expression rate = k [R-LG]
This pathway is a multi-step process with the following characteristics:
step 1: rate determining (slow) loss of the leaving group, LG, to
generate a carbocation intermediate, thenstep 2: rapid attack of a nucleophile on the electrophilic carbocation to
form a new bond
SN1- SubstitutionNucleophilicUnimolecular
Multi-step reactions have intermediates and
several transition states (TS).
In an SN1 there is loss of the leaving groupgenerating an intermediate carbocation which then
undergoes a rapid reaction with the nucleophile.
The reaction profiles shown here are simplified
and do not include the equilibria for protonation of
the -OH.
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Two step reaction with carbocation intermediate.
Rate is first order in the alkyl halide, zero order in the nucleophile
.
SN1 Mechanism
(CH3)3C Br (CH3)3C+
+ Br-
STEP 1 :
(CH3)3C+
+ H O H (CH3)3C O H
H
STEP 2 :
(CH3)3C O H
H
H O H+ (CH3)3C O H + H3O+
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RatesofSN1 Reactions
3 > 2 > 1 >> CH3X
Order follows stability of carbocations (opposite to SN2)
More stable ion requires less energy to form
Better leaving group, faster reaction (like SN2)
Carbocations can rearrange to form a more stable
carbocation.
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SN2 indicates a substitution,nucleophilic,bimolecularreaction,
described by the expression rate = k [Nu][R-LG]
This pathway is a concerted process (single step) as shown by the
following reaction coordinate diagrams, where there is simultaneous
attack of the nucleophile and displacement of the leaving group.
SN2- SubstitutionNucleophilicBimolecular
Single step reactions have no intermediates
and single transition state (TS).
In an SN2 there is simultaneous formation of
the carbon-nucleophile bond and breaking of
the carbon-leaving group bond, hence thereaction proceeds via a TS in which the central
C is partially bonded to five groups.
The reaction profiles shown here are simplified
and do not include the equilibria for protonation
of the -OH.
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Rate is first order in each reactant. (one-step reaction with no
intermediate
Concerted reaction: new bond forming and old bond breaking atsame time.
.
SN2 Mechanism
CH
Br
HH
H O CHO Br
H
HH
CHOH
HH
+ Br-
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RatesofSN2 Reactions
Relative rates for SN2: CH3X > 1 > 2 >> 3
Tertiary halides do not react via the SN2 mechanism, due to steric
hindrance.
Must have a good leaving group
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SN2 vs SN1
SN2 SN1
Primary or methyl Tertiary
Strong nucleophile ( Strong Lewis
base)
Weak nucleophile (Weak Lewis
base)
Polar aprotic solvent
(DMF, DMSO)
Polar protic solvent, (alcohol and
water)
Rate = k[halide][Nuc] Rate = k[halide]
Inversion at chiral carbon Racemization of optically activecompound
No rearrangements Rearranged products
Leaving Group :- for both SN1 and SN2 ( the weaker the base the group
departs, the better the leaving group)
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An elimination reaction is a type oforganic reaction in which two substituentsare removed from a molecule in either a one or two-step mechanism
The two most important methods are: Dehydration (-H2O) of alcohols, andDehydrohalogenation (-HX) of alkyl halides.
There are three fundamental events in these elimination reactions:
i. removal of a proton
ii. formation of the CC p bond
iii. breaking of the bond to the leaving group
Depending on the relative timing of these events, different mechanisms arepossible:i. Loss of the LG to form a carbocation, removal of H and formation of
C=C bond : E1 reaction
ii Simultaneous H removal, C=C bond formation and loss of the LG :
E2 reaction
iii. Removal of H to form a carbanion, loss of the LG and formation of C=C
bond (E1reaction)
Nucleophilic Elimination Reaction
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E1 Reaction
Unimolecular elimination
Two groups lost (usually X- and H )
Nucleophile acts as base)
Also have SN1 products (mixture SN1 and E1 have common first step.
H C
H
H
C
CH3
CH3
Br
C
H
H
H
C CH3
CH3
OH
H
C
H
H
H
C CH3
CH3
C C
H
CH3
CH3
H
+ H3O+
Halide ion leaves, forming carbocation
Base removes H from adjacent carbon
and pi bond forms
E1 Mechanism
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E2 Reaction
Bimolecular elimination
Requires a strong base
Halide leaving and proton abstraction happens simultaneously - no
intermediate
H C
H
H
C
CH3
CH3
Br
C C
H
CH3
CH3
H
O
H
+ H2O Br-
+
E2 Mechanism
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Primary (1) carbonsNormally react by an SN2 pathway. With good nucleophiles such asBr-, I-, CN-, RS-, orNH3 get only SN2 reactions. However, withstrong base (hydroxide or alkoxide) get some competition by E2reaction, though SN2 still predominates.
Secondary (2) carbonsGo by either SN2 orE2:
- With good nucleophiles get mostly SN2.
- With strong base E2 predominates.
Tertiary (3) carbonsGo by SN1, E1, or E2:
- SN1 and E1 are both favored by acid conditions, but acidic
nucleophiles such as HCl, HBr, and HI favor SN1, while sulfuric
acid favors E1
- E2 is favored by strong base again.
How dowe determinewhethera reactionwill goviaan
elimination orasubstitutionand whetheritwill befirst
orsecond order?
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Secondary alkyl halides, often react with simple basic nucleophiles
to give a mixture of products arising from both substitutionand
elimination.
Substitutionand Elimination
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Control of the reaction pathway between substitution and elimination isgenerally accomplished by careful choice of the reactants; strong,stericallyhindered basestend tofavorelimination,whileweak,unhinderednucleophilestend tofavorsubstitution. The choice for a "strong,hindered base" is generally tert-butoxide
Substitutionor Elimination
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TRY THIS..
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FormationofAlcohol
(CH3)3C Br (CH3)3C+
+ Br-
(CH3)3C+
+ H O H (CH3)3C O H
H
(CH3)3C O H
H
H O H+ (CH3)3C O H + H3O+
CH3 Br OH60oC
H2OCH3 O Br
C
H
Br
HH
H O CHO Br
H
HH
CHO
H
HH
+ Br-
Example1
Example2
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CH3CH2CH2OH NaH CH3CH2CH2ONa H H
Propyl alcohol Sodium propoxide
CH3CH2I
CH3CH2OCH2CH2CH3Ethyl propyl ether
Na I
FormationofEther ( Williamsonsynthesis)
RO Na R'L R O R' Na LGeneral reaction
Thisisa good routeforsynthesisofunsymetrical ethers.
Example
The alkoxide ion reacts with the substrate in an SN2 reaction, with resulting
formation of ether.The substrate must be unhindered and bear a good leaving
group.Typical substrates are 1o
and 2o
alkyl halide halides, alkyl sulfonate, anddialkyl sulfates.
The alkyl halide ( alkyl sulfonate) should be primary to avoid E2 reaction.
Substitution favored over elimination at low tempertaure.
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FormationofAminocompound
Using ammonia as a nucleophile in a reaction with an appropriate (methyl, primary, or secondary) alkylhalide in an SN2 reaction to prepare primary amines does work, but it requires a huge excess of
ammonia, because the productprimary amine is also reactive towardthe alkyl halide. This would produce
a secondary amine, and then even further reaction with alkyl halide would give a tertiary amine. Thus, a
mixture of primary, secondary, and tertiary amines would be generated unless ammonia is used in large
excess.
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Wurtz synthesis- Coupling of alkyl halide with organometallic compounds.
The alkane is prepared by the synthesis of metallic sodium and the haloalkane in a dry
etheral ( ethoxyethane) solution.
R-Na ( an intermediate compound) is so reactive that is attacks RX itself, thus the method
can only be used to prepare symmetrical alkane.
The reaction is limited in its preparation, giving only low yields with haloalkanes of low
relative molecular mass, although much better yields are obtained with those of higher
relative molecular mass. A more versatile coupling reaction of this type is the Corey-House
reaction involving the haloalkane and a lithium dialkylcopper.
2 X 2 Nadry
(CH2H5)2OR R 2 NaX
FormationofAlkane ( Wurtzsynthesis)
RX R2'CuLi R R'R'Cu LiX
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Suggest.
FormationofAlkenecompound