Title: Chapter 4 Alcohols and Alkyl Halides
1Chapter 4Alcohols and Alkyl Halides
2Overview 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
3Functional Group
- a structural unit in a molecule responsible for
itscharacteristic behavior under a particular
set ofreaction conditions
4Families 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
5Families of organic compoundsand their
functional groups
Epoxide
- Ether ROR'
- Nitrile RCN
- Nitroalkane RNO2
- Sulfide RSR'
- Thiol RSH
6IUPAC 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.
7Functional Class Nomenclature of Alkyl Halides
- Name the alkyl group and the halogen asseparate
words (alkyl halide)
CH3F
CH3CH2CH2CH2CH2Cl
8Functional 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
9Substitutive 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
10Substitutive 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
11Substitutive 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.
12Substitutive Nomenclature of Alkyl Halides
5-Chloro-2-methylheptane
2-Chloro-5-methylheptane
13Functional Class Nomenclature of Alcohols
- Name the alkyl group and add "alcohol" as
aseparate word.
CH3CH2OH
14Functional Class Nomenclature of Alcohols
- Name the alkyl group and add "alcohol" as
aseparate word.
CH3CH2OH
Ethyl alcohol
1,1-Dimethylbutylalcohol
1-Methylpentyl alcohol
15Substitutive 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
16Substitutive 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
17Substitutive 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
18Substitutive Nomenclature of Alcohols
6-Methyl-3-heptanol
5-Methyl-2-heptanol
19Classification
- 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.
20Classification
H
CH3CH2CH2CH2CH2F
OH
primary alkyl halide
secondary alcohol
secondary alkyl halide
tertiary alcohol
21Dipole Moments
- alcohols and alkyl halides are polar
?
H
?
?
?
?
H
H
? 1.9 D
? 1.7 D
22Dipole Moments
- alcohols and alkyl halides are polar
? 1.9 D
? 1.7 D
23Dipole-Dipole Attractive Forces
?? ?
?? ?
?? ?
?? ?
? ??
24Effect of Structure on Boiling Point
CH3CH2CH3
CH3CH2OH
CH3CH2F
Molecularweight Boilingpoint,
C Dipolemoment, D
- 44 48 46
- -42 -32 78
- 0 1.9 1.7
25Figure 4.4 Hydrogen bonding in ethanol
?
?
?
?
26Boiling 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.
27But trend is not followed when halogenis fluorine
Compound Boiling Point
- CH3CH2F -32C
- CH3CHF2 -25C
- CH3CF3 -47C
- CF3CF3 -78C
28But 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.
29Solubility 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).
30Figure 4.5 Hydrogen Bonding Between Ethanol and
Water
31Density
- 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.
32Reaction of Alcohols with Hydrogen Halides
ROH HX ? RX HOH
- Hydrogen halide reactivity HF HCl HBr HI
33Reaction of Alcohols with Hydrogen Halides
ROH HX ? RX HOH
- Alcohol reactivityCH3OH RCH2OH R2CHOH
R3COHMethanol Primary Secondary Tertiary
34Preparation of Alkyl Halides
25C
(CH3)3CCl H2O
78-88
35Preparation of Alkyl Halides
A mixture of sodium bromide and sulfuric acid may
be used in place of HBr.
364.8Mechanism of the Reaction of Alcohols with
Hydrogen Halides
37About 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.
38About mechanisms
For the reaction
- the generally accepted mechanism involves three
elementary steps. - Step 1 is a Brønsted acid-base reaction.
39Potential energy diagram for Step 1
(CH3)3COH HCl
40Potential energy diagram for Step 2
41Carbocation
- 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.
42Figure 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.
43Potential energy diagram for Step 3
(CH3)3CCl
44Potential 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(No Transcript)
46Mechanistic 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.
47Mechanistic notation
- The molecularity of the rate-determining step
defines the molecularity of theoverall reaction.
Rate-determining step is unimoleculardissociation
of alkyloxonium ion.
48Carbocations
- 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.
49Figure 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."
50Figure 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
51Slow 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.
52Preparation of Alkyl Halides
25C
(CH3)3CCl H2O
78-88
80-100C
HBr
H2O
73
120C
CH3(CH2)5CH2OH HBr
CH3(CH2)5CH2Br H2O
87-90
53Preparation 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
54The 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(No Transcript)
56Other methods of converting Alcohols to Alkyl
Halides
- Thionyl chloride
- SOCl2 ROH ? RCl HCl SO2
- Phosphorus tribromide
- PBr3 3ROH ? 3RBr H3PO3
57Examples
SOCl2
CH3CH(CH2)5CH3
CH3CH(CH2)5CH3
K2CO3
Cl
OH
(81)
(pyridine often used instead of K2CO3)
PBr3
(CH3)2CHCH2OH
(CH3)2CHCH2Br
(55-60)
584.14Halogenation of Alkanes
RH X2 ? RX HX
59Energetics
- RH X2 ? RX HX
- explosive for F2
- exothermic for Cl2 and Br2
- endothermic for I2
60Chlorination 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
61Free Radicals
- contain unpaired electrons
Examples O2
NO
..
62Alkyl 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.
63Alkyl Radicals
less stablethan
64Alkyl 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.
65Homolytic
- 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
66Homolytic
- 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.
67Measures of Free Radical Stability
- Bond-dissociation energy measurements tell us
that isopropyl radical is 13 kJ/mol more stable
than propyl.
CH3CH2CH3
68Measures 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
69Chlorination 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)
70Chlorination of Alkanes
Major limitation Chlorination gives every
possible monochloride derived from original
carbonskeleton. Not much difference in
reactivity ofdifferent hydrogens in molecule.
71Example
- Chlorination of butane gives a mixture
of1-chlorobutane and 2-chlorobutane.
(28)
CH3CH2CH2CH2Cl
Cl2
CH3CH2CH2CH3
h?
CH3CHCH2CH3
(72)
Cl
72Percentage of product that results from
substitution of indicated hydrogen if every
collision with chlorine atoms is productive
10
73Percentage 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
74Relative rates of hydrogen atom abstraction
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
76Percentage of product that results from
replacement of indicated hydrogen
7.0
37
77Relative rates of hydrogen atom abstraction
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.
78Selectivity 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.
79Synthetic application of chlorination of an alkane
(64)
- Chlorination is useful for synthesis only when
all of the hydrogens in a molecule are
equivalent.
80Synthetic 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.