Download - ORGANIC CHEMISTRY CHM 207
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ORGANIC CHEMISTRY CHM 207
CHAPTER 5:ALKYL HALIDES
NOR AKMALAZURA JANI
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SUBTOPICS
• Nomenclature and structures of alkyl halides.• Classification of alkyl halides.• Physical properties of alkyl halides.• Reaction of alkyl halides.1) Formation of alkanes (Wurtz reaction)2) Nucleophilic substitution reaction:
i) Example: formation of alcohol, Williamson ether synthesis, amine synthesis, nitrile synthesisii) Mechanism of nucleophilic substitution reactions.iii)Types of nucleophilic substitution reaction: SN1 and SN2 reaction.
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3) Elimination reaction (dehydrohalogenation of alkyl halides).- E1 and E2 reactions.
• Uses of alkyl halides.
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ALKYL HALIDES
• General formula: CnH2n+1X where n = 1,2,… and X (halogen)
• Functional group: halogen, -X (X = F, Cl, Br, I)
• Naming alkyl halides: - same as nomenclature of alkanes
CH3 ICH3 CH2 CH
CICHCH3
CH3 C CH2 CH CH3
CH3
Br
CH3CH2CH3
iodomethane 3-chloro-2-methylpentane
12345 12345
4-bromo-2,4-dimethylhexane
6
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Classification of alkyl halides
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PHYSICAL PROPERTIES• BOILING POINTS
- molecules with higher molecular weight have higher boiling points.- reasons: the molecule is heavier, slower moving, have greater surface area, have larger London attractions, resulting higher boiling points.- example:
CH3F CH3Cl CH3Br CH3IRMM 34 50.5 95 142 bp (°C) -78 -24 4 42
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- compounds with branched have more spherical shapes, have smaller surface area, resulting lower boiling points.
CH3CH2CH2CH2Cl CH3CH2CHClCH3 CH3 C
CH3
CH3
Cl
bp 78 oC bp 67 oC bp 52 oC
- alkyl halides with more carbon atoms have higher boiling points.
CH3Cl CH3CH2CH2Clbp -24oC bp 12oC bp 47oC
CH3CH2Cl
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• DENSITIES
- alkyl fluoride and alkyl chlorides (with one Cl atom) are less dense than water.- alkyl chloride with two or more chlorine atoms are denser than water.- all alkyl bromides and alkyl iodides are denser than water.
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REACTIONS OF ALKYL HALIDES
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Formation of alkanes (Wurtz reaction)
• Equation:2R-X + 2Na → 2NaX + R-R
• Example:2CH3I + 2Na CH3-CH3 + 2NaI
• Most suitable for preparation of higher alkanes containing an even number of carbon atoms.
dry ether
reflux
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• Alkanes containing an odd number of carbon atoms can be prepared by using a mixture of two different alkyl halides.
• This reaction produces only low yields due to the formation of other alkanes as by-products.
CH3I + 2Na + CH3CH2I CH3CH2CH3 + 2NaI + CH3CH2-CH2CH3 + CH3CH3
dry ether
by-products
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Reactions of alkyl halides• Two types of reactions:
i) substitution reactionsii) elimination reactions
CH
CX
CH
CX
B-H
CH
CNuc
C C
a) nucleophilic substitution
Nuc - X -
b) elimination
B - X -
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NUCLEOPHILIC SUBSTITUTION REACTIONS
R-X OH
CH3CH2 Br NaOH
R-X
CH3 I CH3CH2 O Na+
R-OH
R-O-R'
CH3CH2 OH
X
X
CH3 O CH2CH3
NaBr
Na+ I-
1) Formation of alcohol
example
ethyl bromide ethyl alcohol
2) Williamson ether synthesis
R'O
example
methyl iodide sodium ethoxide ethyl methyl ether
nucleophile
nucleophile
CH3CH2 O Na+
sodium ethoxide
CH3CH2 OH Na
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R-X
CH3CH2 Br H-NH2
R-NH2
CH3CH2 NH2 HBr
3) Amine synthesis
example
ethyl bromide ethylamine (primary amine)
NH3 HXnucleophile
HBr(g) + NH3 (g) NH4Br (s)
amine are also act as nucleophile (more reactive than ammonia)
C2H5Br H N
H
C2H5 (C2H5)2NH + HBrdiethylamine(secondary amine)
(C2H5)2NHC2H5Br (C2H5)3N + HBrtriethylamine(tertiary amine)
C2H5Br (C2H5)3N (C2H5)4N+ Br-
tetraethylammonium bromide(quaternary salt)
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R-X CN
(CH3)2CHCH2CH2-Cl NaCN
R-CN X
(CH3)2CHCH2CH2-CN NaCl
4) Nitrile synthesis
cyanide (nucleophile)
nitrile
example
1-chloro-3-methylbutane 4-methylpentanenitrile
R-CN
H2O/H+
R-COOH (hydrolysis)H2/Ni
180oCR-CH2NH2 (reduction)
(CH3)2CHCH2CH2-CN
H2O/H+
H2/Ni
180oC
(CH3)2CHCH2CH2-COOH
(CH3)2CHCH2CH2-CH2NH2
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Mechanism of nucleophilic substitution reactions
R CH
XH
Nu-
R CH
NuH
X-
EXAMPLE
CH3 CH
BrH
CH3 CH
OHH
OH-
Br-
formation of alcohol
δ-
δ-
δ+
δ+
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Type of nucleophilic substitution reactions: SN1
and SN2 reactions• S = substitution• N = nucleophilic• 1 = a first order (unimolecular) reaction• 2 = a second order (bimolecular) reaction
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SN1 (Substitution, Nucleophilic, unimolecular)
reactions• Unimolecular : only one molecule involved in the transition state
of the rate-limiting step.• Example: the reaction between aqueous NaOH and tertiary alkyl
halides.OH-
(CH3)3C Br
OH-
Br-
Br-(CH3)3C-Br + (CH3)3C-OH +slow
MECHANISM OF SN1 REACTION
STEP 1: FORMATION OF CARBOCATION
(CH3)3C+ +slowrate limiting step
STEP 2: NUCLEOPHILIC ATTACK
(CH3)3C+ + fast
very reactive
(CH3)3C-OH
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• The reaction is first order and the rate depends only on the concentration of the tertiary alkyl halides.
Rate = k[(CH3)3CBr]
• The concentration of OH- does not have any effect on the rate of reaction.
• OH- does not involved in the rate-limiting step.
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Carbocation rearrangement in SN1 reactions• Rearrangement of the carbon skeleton will take place if a more
stable carbocation can be formed in the process. • For example, hydrolysis of the secondary alkyl bromide, 2-
bromo-3-methylbutane, yields the tertiary alcohol, 2-methyl-2-butanol.
CH3 C C
CH3 H
BrH
CH3SN1
H2O
CH3 C C
CH3 H
CH3
H
CH3 C C
CH3 H
H
CH3
H-OH
Br-
CH3 C C
CH3 H
CH3
HOH
(2o carbocation)slow step
(3o carbocation)
shift of H
fast
2-methyl-2-butanol
2-bromo-3-methylbutane
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• Reactivity towards SN1 substitution mechanisms follows the stability of carbocations:
SN1 reactivity: 3o > 2o > 1o > CH3X
inversionretention
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SN2 (Substitution, Nucleophilic, bimolecular) reactions
• The processes of bond breaking and bond forming occur simultaneously (one bond is forming, one bond is breaking).
• The mechanism involves only one step.• For example, hydrolysis of iodomethane (primary alkyl halides)
C IH
HH
HOC I
H
HH
HO CH
HH
HO I-
iodomethane transition state methanol
δ-δ+δ+δ- δ-
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• A second order reaction• Rate equation = k[CH3I][OH-]• Both iodomethane and the OH- are involved in the
rate-limiting step.• SN2 reactivity: CH3 X > 1o
> 2o > neopentyl > 3o
• Factor that determines the order of reactivity in SN2 reactions is the steric effect.
• A steric effect is one in which the rate of chemical reaction depends on the size or spatial arrangement of the groups attached to, or near to, the reaction site of the molecule.
HO CH3 I HO CH3 I HOCH3 I-
iodomethane transition state methanolrate-limitingstep
or
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Relative reactivities of primary, secondary, and
tertiary alkyl halides• The reactivity of alkyl halides towards nucleophilic
substitution depend on the halogen.
• The rate of reaction decrease in the order R-I > R-Br > R-Cl > R-F
(most reactive) (least reactive)
• Reason: C-X bond become stronger from I to F
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Comparison SN1 and SN2 reactions
SN1 SN2Rate of reaction First order Second order
Stereochemistry Racemic mixture (mixture of inversion and retention)
Complete inversion
Reactivity Benzyl > allyl > ~ 3o > 2o > 1o
CH3X > 1o > 2o
> 3o
Nucleophiles Weak nucleophiles Strong nucleophiles
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Elimination reactions-dehydrohalogenation of alkyl
halides• Elimination: loss of two atoms or groups from the
substrate to form a pi bonds.• Dehydrohalogenation (removal of hydrogen and a
halogen atom) of alkyl halide to form alkene.
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• The elimination reaction is occurred when the reaction used strong base for examples, t-butoxide ion ((CH3)3CO-) or OH- ion and heated at high temperature.
• Dehydrohalogenation will yield an alkene that has the larger number of alkyl groups as the main product (Saytzeff’s rule).
• Elimination reactions can be divided into two:i) E1 reactionii) E2 reaction
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E1(Elimination, unimolecular) reaction
• The rate-limiting state involves a single molecular than a collision between two molecules.
• A first order reaction.
• Rate equation: k[RX]
• E1 reactivity: Benzyl > allyl > 3o > 2o > 1o
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B
C CXH
C C
H
C CH
B-H
X
C C
MECHANISM OF E1 REACTION
STEP 1: FORMATION OF THE CARBOCATION (RATE LIMITING)
STEP 2: A BASE ABSTRACTS A PROTON (FAST)
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E2(Elimination, bimolecular) reaction
• A second order reaction.• Rate equation: k[RX][Base]• E2 reactivity: 3o > 2o > 1o • Mechanism:
B
C CX
HC C B-H X-
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Comparison E1 and E2 reactions
E1 E2Rate of reaction First order Second order
Reactivity 3o > 2o > 1o 3o > 2o > 1o
Base Do not need strong base
Strong base
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Alkyl halides Reactions
1O RCH2X RCH2NuNu-
SN2
R2CHX
R2CHNu + alkene
alkene
Nu- strongSN2 + E2
E2B- (strong)
2O
R3CX
R3CNu + alkene
alkene
Nu- (weak)SN1 + E1
E2B- (strong)
3O
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(CH3CH2)3PS-H
I
(CH3CH2)2NH
C N
H-O
CH3-O
(CH3CH2)2N
strong nucleophiles
CH3-S-CH3
Cl
NH3
Br
CH3C OO
moderate nucleophiles
F
weak nucleophiles
H-O-H
CH3 O H
SOME COMMON NUCLEOPHILES
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USES OF ALKYL HALIDES• Solvents
- industrial and household solvents.- carbon tetrachloride (CCl4) used for dry cleaning, spot removing.- methylene chloride (CH2Cl2) is used to dissolve the caffeine from coffee beans to produce decaffeinated coffee.
• Reagents- as starting materials for making complex molecules.- for example, the conversion of alkyl halides to organometallic reagents (compounds containing carbon-metal bonds) is important tool for organic synthesis.
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• Anesthetics- examples: chloroform (CHCl3) and ethyl chloride.
• Freons: Refrigerants and foaming agents- Freons (called chlorofluorocarbons, or CFCs) is used as a refrigerant gas.
• Pesticides- example: DDT (Dichloro Diphenyl-Trichloroethane) is used as insecticides.