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Chapter 4 Alcohols and Alkyl Halides

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Title: Chapter 4 Alcohols and Alkyl Halides


1
Chapter 4Alcohols and Alkyl Halides
2
Overview of Chapter
This chapter introduces chemical reactions and
their mechanisms by focusing on two
reactionsthat yield alkyl halides.
  • (1) alcohol hydrogen halide
  • ROH HX ? RX H2O
  • (2) alkane halogen
  • RH X2 ? RX HX
  • Both are substitution reactions

3
Functional Group
  • a structural unit in a molecule responsible for
    itscharacteristic behavior under a particular
    set ofreaction conditions

4
Families of organic compoundsand their
functional groups
  • Alcohol ROH
  • Alkyl halide RX (X F, Cl, Br, I)
  • Amine primary amine RNH2
  • secondary amine R2NH
  • tertiary amine R3N

5
Families of organic compoundsand their
functional groups
Epoxide
  • Ether ROR'
  • Nitrile RCN
  • Nitroalkane RNO2
  • Sulfide RSR'
  • Thiol RSH

6
IUPAC Nomenclature
There are several kinds of IUPAC nomenclature.
  • The two that are most widely used
    are functional class nomenclature substitutive
    nomenclature
  • Both types can be applied to alcohols andalkyl
    halides.

7
Functional Class Nomenclature of Alkyl Halides
  • Name the alkyl group and the halogen asseparate
    words (alkyl halide)

CH3F
CH3CH2CH2CH2CH2Cl
8
Functional Class Nomenclature of Alkyl Halides
  • Name the alkyl group and the halogen asseparate
    words (alkyl halide)

CH3F
CH3CH2CH2CH2CH2Cl
Methyl fluoride
Pentyl chloride
1-Ethylhexyl bromide
Cyclohexyl iodide
9
Substitutive Nomenclature of Alkyl Halides
  • Name as halo-substituted alkanes.
  • Number the longest chain containing thehalogen
    in the direction that gives the lowestnumber to
    the substituted carbon.

CH3CH2CH2CH2CH2F
10
Substitutive Nomenclature of Alkyl Halides
  • Name as halo-substituted alkanes.
  • Number the longest chain containing thehalogen
    in the direction that gives the lowestnumber to
    the substituted carbon.

CH3CH2CH2CH2CH2F
1-Fluoropentane
2-Bromopentane
3-Iodopentane
11
Substitutive Nomenclature of Alkyl Halides
  • Halogen and alkyl groupsare of equal rank when
    it comes to numberingthe chain.
  • Number the chain in thedirection that gives the
    lowest number to thegroup (halogen or
    alkyl)that appears first.

12
Substitutive Nomenclature of Alkyl Halides
5-Chloro-2-methylheptane
2-Chloro-5-methylheptane
13
Functional Class Nomenclature of Alcohols
  • Name the alkyl group and add "alcohol" as
    aseparate word.

CH3CH2OH
14
Functional Class Nomenclature of Alcohols
  • Name the alkyl group and add "alcohol" as
    aseparate word.

CH3CH2OH
Ethyl alcohol
1,1-Dimethylbutylalcohol
1-Methylpentyl alcohol
15
Substitutive Nomenclature of Alcohols
  • Name as "alkanols." Replace -e ending of
    alkanename by -ol.
  • Number chain in direction that gives lowest
    numberto the carbon that bears the OH group.

CH3CH2OH
16
Substitutive Nomenclature of Alcohols
  • Name as "alkanols." Replace -e ending of
    alkanename by -ol.
  • Number chain in direction that gives lowest
    numberto the carbon that bears the OH group.

CH3CH2OH
Ethanol
2-Methyl-2-pentanol
2-Hexanol
17
Substitutive Nomenclature of Alcohols
  • Hydroxyl groups outrank alkyl groups when it
    comes to numberingthe chain.
  • Number the chain in thedirection that gives the
    lowest number to thecarbon that bears theOH
    group

18
Substitutive Nomenclature of Alcohols
6-Methyl-3-heptanol
5-Methyl-2-heptanol
19
Classification
  • Alcohols and alkyl halides are classified
    as primary secondary tertiaryaccording to
    their "degree of substitution."
  • Degree of substitution is determined by
    countingthe number of carbon atoms directly
    attached tothe carbon that bears the halogen or
    hydroxyl group.

20
Classification
H
CH3CH2CH2CH2CH2F
OH
primary alkyl halide
secondary alcohol
secondary alkyl halide
tertiary alcohol
21
Dipole Moments
  • alcohols and alkyl halides are polar

?

H
?
?
?
?
H
H
? 1.9 D
? 1.7 D
22
Dipole Moments
  • alcohols and alkyl halides are polar


? 1.9 D
? 1.7 D
23
Dipole-Dipole Attractive Forces
?? ?
?? ?
?? ?
?? ?
? ??
24
Effect of Structure on Boiling Point
CH3CH2CH3
CH3CH2OH
CH3CH2F
Molecularweight Boilingpoint,
C Dipolemoment, D
  • 44 48 46
  • -42 -32 78
  • 0 1.9 1.7

25
Figure 4.4 Hydrogen bonding in ethanol
?
?
?
?
26
Boiling point increases with increasingnumber of
halogens
Compound Boiling Point
  • CH3Cl -24C
  • CH2Cl2 40C
  • CHCl3 61C
  • CCl4 77C

Even though CCl4 is the only compound in this
list without a dipole moment, it has the highest
boiling point. Induced dipole-induced dipole
forces are greatest in CCl4 because it has the
greatest number of Cl atoms. Cl is more
polarizable than H.
27
But trend is not followed when halogenis fluorine
Compound Boiling Point
  • CH3CH2F -32C
  • CH3CHF2 -25C
  • CH3CF3 -47C
  • CF3CF3 -78C

28
But trend is not followed when halogenis fluorine
Compound Boiling Point
  • CH3CH2F -32C
  • CH3CHF2 -25C
  • CH3CF3 -47C
  • CF3CF3 -78C

Fluorine is not very polarizable and induced
dipole-induced dipole forces decrease with
increasing fluorine substitution.
29
Solubility in water
  • Alkyl halides are insoluble in water.
  • Methanol, ethanol, isopropyl alcohol
    arecompletely miscible with water.
  • The solubility of an alcohol in waterdecreases
    with increasing number of carbons (compound
    becomesmore hydrocarbon-like).

30
Figure 4.5 Hydrogen Bonding Between Ethanol and
Water
31
Density
  • Alkyl fluorides and alkyl chlorides areless
    dense than water.
  • Alkyl bromides and alkyl iodides are more dense
    than water.
  • All liquid alcohols have densities of about 0.8
    g/mL.

32
Reaction of Alcohols with Hydrogen Halides
ROH HX ? RX HOH
  • Hydrogen halide reactivity HF HCl HBr HI

33
Reaction of Alcohols with Hydrogen Halides
ROH HX ? RX HOH
  • Alcohol reactivityCH3OH RCH2OH R2CHOH
    R3COHMethanol Primary Secondary Tertiary

34
Preparation of Alkyl Halides
25C
(CH3)3CCl H2O
  • (CH3)3COH HCl

78-88
35
Preparation of Alkyl Halides
A mixture of sodium bromide and sulfuric acid may
be used in place of HBr.
36
4.8Mechanism of the Reaction of Alcohols with
Hydrogen Halides
37
About mechanisms
  • A mechanism describes how reactants
    areconverted to products.
  • Mechanisms are often written as a series
    ofchemical equations showing the elementary
    steps.
  • An elementary step is a reaction that
    proceedsby way of a single transition state.
  • Mechanisms can be shown likely to be
    correct,but cannot be proven correct.

38
About mechanisms
For the reaction
  • the generally accepted mechanism involves three
    elementary steps.
  • Step 1 is a Brønsted acid-base reaction.

39
Potential energy diagram for Step 1
(CH3)3COH HCl

40
Potential energy diagram for Step 2

41
Carbocation
  • The key intermediate in reaction of secondary
    and tertiary alcohols with hydrogen halides is
    a carbocation.
  • A carbocation is a cation in which carbon has6
    valence electrons and a positive charge.

42
Figure 4.9 Structure of tert-Butyl Cation.
  • Positively charged carbon is sp2 hybridized.
  • All four carbons lie in same plane.
  • Unhybridized p orbital is perpendicular to plane
    of four carbons.

43
Potential energy diagram for Step 3

(CH3)3CCl
44
Potential Energy Diagram-Overall
  • The potential energy diagram for a multistep
    mechanism is simply a collection of the
    potential energy diagrams for the individual
    steps.
  • Consider the three-step mechanism for the
    reaction of tert-butyl alcohol with HCl.

45
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46
Mechanistic notation
  • The mechanism just described is an example of
    an SN1 process.
  • SN1 stands for substitution-nucleophilic-unimole
    cular.
  • The molecularity of the rate-determining step
    defines the molecularity of th overall reaction.

47
Mechanistic notation
  • The molecularity of the rate-determining step
    defines the molecularity of theoverall reaction.

Rate-determining step is unimoleculardissociation
of alkyloxonium ion.
48
Carbocations
  • Most carbocations are too unstable to
    beisolated, but occur as reactive intermediates
    ina number of reactions.
  • When R is an alkyl group, the carbocation
    isstabilized compared to R H.

49
Figure 4.15 Stabilization of carbocations via
the inductive effect
electrons in CCbonds are more polarizable than
thosein CH bonds therefore, alkyl
groupsstabilize carbocationsbetter than H.
??
??
  • Electronic effects transmitted through ??bonds
    are called "inductive effects."

50
Figure 4.16 Stabilization of carbocations via
hyperconjugation
electrons in this ?bond can be sharedby
positively chargedcarbon because the? orbital
can overlap with the empty 2porbital of
positivelycharged carbon
51

Slow step is
R
  • The more stable the carbocation, the fasterit
    is formed.
  • Tertiary carbocations are more stable
    thansecondary, which are more stable than
    primary,which are more stable than methyl.
  • Tertiary alcohols react faster than secondary,
    which react faster than primary, which react
    fasterthan methanol.

52
Preparation of Alkyl Halides
25C
(CH3)3CCl H2O
  • (CH3)3COH HCl

78-88
80-100C


HBr
H2O
73
120C
CH3(CH2)5CH2OH HBr
CH3(CH2)5CH2Br H2O
87-90
53
Preparation of Alkyl Halides
  • Primary carbocations are too high in energy to
    allow SN1 mechanism. Yet, primary alcohols are
    converted to alkyl halides.
  • Primary alcohols react by a mechanism called SN2
    (substitution-nucleophilic-bimolecular).

120C
CH3(CH2)5CH2OH HBr
CH3(CH2)5CH2Br H2O
87-90
54
The SN2 Mechanism
  • Two-step mechanism for conversion of alcohols to
    alkyl halides
  • (1) proton transfer to alcohol to form
    alkyloxonium ion
  • (2) bimolecular displacement of water from
    alkyloxonium ion by halide

55
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56
Other methods of converting Alcohols to Alkyl
Halides
  • Thionyl chloride
  • SOCl2 ROH ? RCl HCl SO2
  • Phosphorus tribromide
  • PBr3 3ROH ? 3RBr H3PO3

57
Examples
SOCl2
CH3CH(CH2)5CH3
CH3CH(CH2)5CH3
K2CO3
Cl
OH
(81)
(pyridine often used instead of K2CO3)
PBr3
(CH3)2CHCH2OH
(CH3)2CHCH2Br
(55-60)
58
4.14Halogenation of Alkanes
RH X2 ? RX HX
59
Energetics
  • RH X2 ? RX HX
  • explosive for F2
  • exothermic for Cl2 and Br2
  • endothermic for I2

60
Chlorination of Methane
  • carried out at high temperature (400 C)
  • CH4 Cl2 ? CH3Cl HCl
  • CH3Cl Cl2 ? CH2Cl2 HCl
  • CH2Cl2 Cl2 ? CHCl3 HCl
  • CHCl3 Cl2 ? CCl4 HCl

61
Free Radicals
  • contain unpaired electrons

Examples O2
NO
..
62
Alkyl Radicals
C
  • Most free radicals in which carbon bearsthe
    unpaired electron are too unstable to
    beisolated.
  • Alkyl radicals are classified as
    primary,secondary, or tertiary in the same way
    thatcarbocations are.

63
Alkyl Radicals
less stablethan
64
Alkyl Radicals
  • The order of stability of free radicals can be
    determined by measuring bond strengths.
  • By "bond strength" we mean the energy required
    to break a covalent bond.
  • A chemical bond can be broken in two different
    waysheterolytically or homolytically.

65
Homolytic
  • In a homolytic bond cleavage, the two electrons
    inthe bond are divided equally between the two
    atoms.One electron goes with one atom, the
    second with the other atom.
  • In a heterolytic cleavage, one atom retains
    bothelectrons.



Heterolytic
66
Homolytic
  • The species formed by a homolytic bond
    cleavageof a neutral molecule are free radicals.
    Therefore, measure energy cost of homolytic
    bond cleavage to gain information about
    stability of free radicals.
  • The more stable the free-radical products, the
    weakerthe bond, and the lower the
    bond-dissociation energy.

67
Measures of Free Radical Stability
  • Bond-dissociation energy measurements tell us
    that isopropyl radical is 13 kJ/mol more stable
    than propyl.

CH3CH2CH3
68
Measures of Free Radical Stability
  • Bond-dissociation energy measurements tell us
    that tert-butyl radical is 30 kJ/mol more stable
    than isobutyl.

.
(CH3)2CHCH2 H
.
410
380
(CH3)3CH
69
Chlorination of Alkanes
  • can be used to prepare alkyl chlorides from
    alkanes in which all of the hydrogens are
    equivalent to one another

420C
CH3CH3 Cl2
CH3CH2Cl HCl
(78)
h?
Cl2
HCl
Cl
(73)
70
Chlorination of Alkanes
Major limitation Chlorination gives every
possible monochloride derived from original
carbonskeleton. Not much difference in
reactivity ofdifferent hydrogens in molecule.
71
Example
  • Chlorination of butane gives a mixture
    of1-chlorobutane and 2-chlorobutane.

(28)
CH3CH2CH2CH2Cl
Cl2
CH3CH2CH2CH3
h?
CH3CHCH2CH3
(72)
Cl
72
Percentage of product that results from
substitution of indicated hydrogen if every
collision with chlorine atoms is productive
10
73
Percentage of product that actually results from
replacement of indicated hydrogen
18
18
4.6
4.6
4.6
4.6
4.6
4.6
18
18
10
74
Relative rates of hydrogen atom abstraction
  • divide by 4.6

18
4.6
1
3.9
A secondary hydrogen is abstracted 3.9 times
faster than a primary hydrogen by a chlorine
atom.
75
  • Similarly, chlorination of 2-methylbutane gives
    a mixture of isobutyl chloride and tert-butyl
    chloride

76
Percentage of product that results from
replacement of indicated hydrogen
7.0
37
77
Relative rates of hydrogen atom abstraction
  • divide by 7

7.0
37
1
5.3
7
7
A tertiary hydrogen is abstracted 5.3 times
faster than a primary hydrogen by a chlorine
atom.
78
Selectivity of free-radical halogenation
  • R3CH gt R2CH2 gt RCH3
  • chlorination 5 4 1
  • bromination 1640 82 1
  • Chlorination of an alkane gives a mixture of
    every possible isomer having the same
    skeletonas the starting alkane. Useful for
    synthesis only when all hydrogens in a molecule
    are equivalent.
  • Bromination is highly regioselective for
    substitution of tertiary hydrogens. Major
    synthetic application is in synthesis of
    tertiary alkyl bromides.

79
Synthetic application of chlorination of an alkane
(64)
  • Chlorination is useful for synthesis only when
    all of the hydrogens in a molecule are
    equivalent.

80
Synthetic application of bromination of an alkane
Br2
h?
(76)
  • Bromination is highly selective for substitution
    of tertiary hydrogens.
  • Major synthetic application is in synthesis of
    tertiary alkyl bromides.
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