Chapter 4 Alcohols and Alkyl Halides

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Chapter 4Alcohols and Alkyl Halides
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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

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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

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Families of organic compoundsand their
functional groups
Epoxide

• Ether ROR'
• Nitrile RCN
• Nitroalkane RNO2
• Sulfide RSR'
• Thiol RSH

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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.

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Functional Class Nomenclature of Alkyl Halides

• Name the alkyl group and the halogen asseparate
  words (alkyl halide)

CH3F
CH3CH2CH2CH2CH2Cl
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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
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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
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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
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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.

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Substitutive Nomenclature of Alkyl Halides
5-Chloro-2-methylheptane
2-Chloro-5-methylheptane
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Functional Class Nomenclature of Alcohols

• Name the alkyl group and add "alcohol" as
  aseparate word.

CH3CH2OH
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Functional Class Nomenclature of Alcohols

• Name the alkyl group and add "alcohol" as
  aseparate word.

CH3CH2OH
Ethyl alcohol
1,1-Dimethylbutylalcohol
1-Methylpentyl alcohol
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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
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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
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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

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Substitutive Nomenclature of Alcohols
6-Methyl-3-heptanol
5-Methyl-2-heptanol
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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.

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Classification
H
CH3CH2CH2CH2CH2F
OH
primary alkyl halide
secondary alcohol
secondary alkyl halide
tertiary alcohol
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Dipole Moments

• alcohols and alkyl halides are polar

?

H
?
?
?
?
H
H
? 1.9 D
? 1.7 D
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Dipole Moments

• alcohols and alkyl halides are polar

? 1.9 D
? 1.7 D
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Dipole-Dipole Attractive Forces
?? ?
?? ?
?? ?
?? ?
? ??
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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

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Figure 4.4 Hydrogen bonding in ethanol
?
?
?
?
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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.
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But trend is not followed when halogenis fluorine
Compound Boiling Point

• CH3CH2F -32C
• CH3CHF2 -25C
• CH3CF3 -47C
• CF3CF3 -78C

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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.
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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).

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Figure 4.5 Hydrogen Bonding Between Ethanol and
Water
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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.

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Reaction of Alcohols with Hydrogen Halides
ROH HX ? RX HOH

• Hydrogen halide reactivity HF HCl HBr HI

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Reaction of Alcohols with Hydrogen Halides
ROH HX ? RX HOH

• Alcohol reactivityCH3OH RCH2OH R2CHOH
  R3COHMethanol Primary Secondary Tertiary

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Preparation of Alkyl Halides
25C
(CH3)3CCl H2O

• (CH3)3COH HCl

78-88
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Preparation of Alkyl Halides
A mixture of sodium bromide and sulfuric acid may
be used in place of HBr.
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4.8Mechanism of the Reaction of Alcohols with
Hydrogen Halides
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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.

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About mechanisms
For the reaction

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

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

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Potential energy diagram for Step 2

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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.

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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.

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Potential energy diagram for Step 3

(CH3)3CCl
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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.

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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.

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Mechanistic notation

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

Rate-determining step is unimoleculardissociation
of alkyloxonium ion.
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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.

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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."

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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
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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.

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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
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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
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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

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Other methods of converting Alcohols to Alkyl
Halides

• Thionyl chloride
• SOCl2 ROH ? RCl HCl SO2
• Phosphorus tribromide
• PBr3 3ROH ? 3RBr H3PO3

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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)
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4.14Halogenation of Alkanes
RH X2 ? RX HX
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Energetics

• RH X2 ? RX HX
• explosive for F2
• exothermic for Cl2 and Br2
• endothermic for I2

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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

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Free Radicals

• contain unpaired electrons

Examples O2
NO
..
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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.

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Alkyl Radicals
less stablethan
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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.

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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
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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.

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Measures of Free Radical Stability

• Bond-dissociation energy measurements tell us
  that isopropyl radical is 13 kJ/mol more stable
  than propyl.

CH3CH2CH3
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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
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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)
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Chlorination of Alkanes
Major limitation Chlorination gives every
possible monochloride derived from original
carbonskeleton. Not much difference in
reactivity ofdifferent hydrogens in molecule.
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Example

• Chlorination of butane gives a mixture
  of1-chlorobutane and 2-chlorobutane.

(28)
CH3CH2CH2CH2Cl
Cl2
CH3CH2CH2CH3
h?
CH3CHCH2CH3
(72)
Cl
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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
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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.
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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.

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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.