Transcript 4.5: Bonding in Alcohols and Alkyl Halides

Ch. 4: Alcohols and Alkyl Halides
C X
C OH
alcohol
alkyl halide (X= F, Cl, Br, I)
4.1: Functional Groups - >11 million organic compounds which are
classified into families according to structure and reactivity.
Functional Group (FG): a group of atoms, which are part of a larger molecule,
that have characteristic chemical behavior. FG’s behave similarly in every
molecule they are part of.
The chemistry of organic molecules is defined by the function groups it
contains
H
H
C C
H
H
H3C
H3C
H
H
HO
Br
Br
C
C
H
H
H
H
Br2
H3C
H
H3C
Br2
H
H
H
HO
Br
Br
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C C
Alkanes
Carbon - Carbon Multiple Bonds
Carbon-heteroatom single bonds
basic
C C
C N
C X
Alkenes
X= F, Cl, Br, I
Alkyl Halide
C C
H
C O
O
C N
O
amines
nitro alkane
C O C
O
C C
ethers
epoxide
Alkynes
alcohols
H
H
C C
C H
C
C C
H
H
H
acidic
C S C
C S S C
sulfides
(thioethers)
disulfide
C S
thiols
Arenes
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Carbonyl-oxygen double bonds (carbonyls)
Carbon-nitrogen multiple bonds
acidic
O
H
C
aldehyde
O
O
C
O
C
imine
(Schiff base)
acid chloride
O
O
C
C
ester
ketones
Cl
C
O
C
N
H
carboxylic acid
O
C
basic
O
O
C C N
C
anhydrides
O
C
N
amide
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4.2: IUPAC Nomenclature of Alkyl Halides (please read)
Use the systematic nomenclature of alkanes; treat the halogen as
a substituent of the alkane.
F- fluoro, Cl- chloro, Br- bromo, I- iodo
4.3: IUPAC Nomenclature of Alcohols
1. In general, alcohols are named in the same manner as
alkanes; replace the -ane suffix for alkanes with an -ol for
alcohols
CH3CH2CH2CH3
CH3CH2CH2CH2OH
OH
butane
1-butanol
2-butanol
2. Number the carbon chain so that the hydroxyl group gets the
lowest number
3. Number the substituents and write the name listing the
substituents in alphabetical order.
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Many alcohols are named using non-systematic nomenclature
OH
H3C
OH
OH
C OH
H3C
H3C
benzyl alcohol
(phenylmethanol)
allyl alcohol
(2-propen-1-ol)
HO
tert-butyl alcohol
(2-methyl-2-propanol)
OH
ethylene glycol
(1,2-ethanediol)
HO
OH
glycerol
(1,2,3-propanetriol)
4.4: Classes of Alcohols and Alkyl Halides - Alcohols and
alkyl halides are classified as according to the degree of
substitution of the carbon bearing the halogen or -OH group
OH
primary (1°) : one alkyl substituent
secondary (2°) : two alkyl substituents
tertiary (3°) : three alkyl substituents 2-methyl-2-pentanol
H
H
methanol
H
H
H
R
3-phenyl-2-butanol
R
C O
C O
C O
H
R
R
H
OH
H
C O
H
1¡ carbon
2¡ carbon
primary
secondary
R
R
H
3¡ carbon
tertiary
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4.5: Bonding in Alcohols and Alkyl Halides - the C-X bond
of alkyl halides and C-OH bond of alcohols has a significant
dipole moment.
H
+
C
H
-
Cl
H
+
+ 
C
H
O
H
H
 = 1.9
H
 = 1.7
4.6: Physical Properties of Alcohols and Alkyl Halides:
Intermolecular Forces
H2O
CH3CH2CH2CH3
MW=18
MW=58
bp= 100° C
bp= -0° C
C
CH3CH2CH2CH2Cl
MW=92.5
bp= 77° C
CH3CH2Cl
MW= 64.5
bp= 12° C
CH3CH2CH2CH2OH
MW=74
bp= 116°
CH3CH2OH
MW=60
bp= 78° C
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Like water, alcohols can form hydrogen bonds: a non-covalent
interaction between a hydrogen atom (+) involved in a
polar covalent bond, with the lone pair of a heteroatom
(usually O or N), which is also involved in a polar covalent
bond (-)
H
O
H
O
H
O
H
H
H
O
H
O
H
O
H
H
O
N H
C O
- +
O H
- +
N H
+ C O
H
H
O
H
O H
H
H
O
H
O
H
H
O
H
H
H
H
O
H
H

H
R
O

H
C O
H
O
R

R
O

H
H
O
R

R
O

H
O
R
Hydrogen-bonds are broken when the
alcohol reaches its bp, which requires
additional energy
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4.7: Preparation of Alkyl Halides from Alcohols and
Hydrogen Halides
R-OH
+
H-X
R-X
+ HOH
Reactivity of the alcohol:
H
H C OH
H
Methyl
<<
H
R C OH
H
Primary (1°)
<
H
R C OH
R
<
R
R C OH
R
Secondary (2°) Tertiary (3°)
increasing reactivity
Reactivity of the H-X : parallels the acidity of HX
HF < HCl < HBr < HI
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4.8: Mechanism of the Reaction of Alcohols with Hydrogen Halides
Curved Arrow Convention
1.
Curved arrows show the movement (flow) of electron during bond breaking and/or
bond making processes. The foot of the arrow indicates where the electron or
electron pair originates, the head of the arrow shows where the electron or
electron pair ends up. .
A.
The movement of a single electron is denoted by a curved single headed
arrow (fishhook or hook).
double-headed
arrow
B.
2.
The movement of an electron pair is denoted by a curved double headed
arrow.
If an electron pair moves in on a new atom, another electron pair must leave so
that the atom does not exceed a full valance of eight electrons. There are two
common exceptions:
A.
B.
3.
single-headed
arrow
When an atom already has an incomplete valance (R3C+).
With second row (or below) elements the octet rule may be violated.
The arrows completely dictate the Lewis structure of the product.
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Curved Arrow Convention
Other Suggestions for Proper Arrow Pushing:
4.
The natural polarization of double bonds between unlike atoms is in the direction
of the more electronegative atom and this will be the important direction of
electron movement.
5. In drawing a mechanism, the formal charges of atoms in the reactants may change
in the product. Use your knowledge of Lewis structures and formal charge to
determine this.
6.
The first step in writing a mechanism is to identify the nucleophile (Lewis base)
and the electrophile (Lewis acid). The first arrow is always from the nucleophile
to the electrophile.
The generally accepted mechanism for the reaction of t-butyl
alcohol and HCl involves to give t-butyl chloride has three basic
steps:
25°C (CH3)3CCl + H2O
(CH3)3COH + HCl
tert-Butyl
tert-Butyl
chloride
alcohol
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Mechanism: (CH3)3COH + HCl
(CH3)3CCl + H2O
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4.9: Potential Energy Diagrams for Multistep Reactions:
The SN1 Mechanism
The rate of oxonium ion
formation is very fast
The rate of carbocation
formation (dissociation
of the oxonium ion) is
slow
The rate of reaction
between the carbocation and for X- is fast
The overall rate is
dependent of the
slowest step (rate
limiting step)
rate = k [oxonium ion] , where k is the rate constant
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4.10: Structure, Bonding, and Stability of Carbocations Carbocations are sp2 hybridized and have a trigonal planar geometry
Substituents stabilize a carbocation through:
a. Inductive Effects: shifting of electrons in a -bond in response to the
electronegativity of a nearby atom (or group).
Carbon is a good electron donor. Substitution can also stabilize
carbocations by donating electron density through the  -bond.
R
R
C
R
+
C
R
R
R
+
C
H
3°: three alkyl groups 2°: two alkyl groups
donating electrons
donating electrons
H
H
+
C
H
H
+
H
1°: one alkyl group methyl: no alkyl groups
donating electrons donating electrons
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b. Hyperconjugation: The C-H σ-bond on the neighboring
carbon lines up with the vacant p-orbital and can donate electron
density to the carbon cation. This is a “bonding” interaction and
is stabilizing. More substituted carbocations have more possible
hyperconjugation interactions.
vacant p-orbital
4.11: Effect of Alcohol Structure on Reaction Rate.
The order of reactivity for the reaction:
R3C-OH + H-X
R3C-X + HOH
where 3° alcohols are most reactive and 1° alcohols are least
reactive, reflects the stability of the intermediate carbocation.
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The rate of a reaction is dependent of the activation energy (Eact)
There is no formal relationship between Gact and G°
What is the structure of a transition state?
How can the structures of the reactants and products affect Gact
The Hammond Postulate provides an intuitive relationship
Between rate Gact) and product stability (G°).
Typical reaction coordinate
less common
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The Hammond Postulate: The structure of the transition state more closely
resembles the nearest stable species (i.e., the reactant, intermediate or
product)
For an endothermic reaction (G° > 0), the TS is nearer to the product. The
structure of the TS more closely resembles that of the product. Therefore,
factors that stabilize the product will also stabilize the TS leading to that product.
For an exothermic reaction (G° < 0), the TS is nearer to the reactant. The
structure of the TS more closely resembles that of the reactants.
G°= 0
TS is halfway between
reactant and products on
the reaction coordinate
G°> 0
TS lies closer to the
products than the
reactants on the
reaction coordinate
G°< 0
TS lies closer to the
reactants than the
products on the reaction
coordinate
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Fig. 4.16
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4.12: Reaction of Primary Alcohols with Hydrogen Halides.
The SN2 Mechanism:
Methyl and primary carbocations are the least stable, and they are
not likely to be intermediates in reaction mechanism
RH2C-OH
+
H-X
RH2C-X
+ HOH
SN2 (substitution-nucleophilic-bimolecular).
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4.13: Other Methods for Converting Alcohols to Alkyl Halides
Preparation of alkyl chlorides by the treatment of alcohols
with thionyl chloride (SOCl2)
R-OH
+
SOCl2 + base
R-Cl + SO2 + HCl
Preparation of alkyl bromides by the treatment of alcohols
with phosphorous tribromide (PBr3)
R-OH
+
PBr3
R-Br
+ P(OH)3
These methods work best on primary and secondary alcohols.
They do not work at all for tertiary alcohols
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4.14: Halogenation of Alkanes
R-H + X2
R-X
+
H-X
Reactivity: F2 >> Cl2 > Br2 >> I2
4.15: Chlorination of Methane
Mechanism of free radical halogenation has three distinct steps
1. Initiation
2. Propagation
3. Termination
Free radical chlorination is not very useful for making alkyl
chlorides: polychlorination, non-specific chlorination
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4.16: Structure and Stability of Free Radicals
Free radical: species that contain unpaired electrons
O O
Cl
N O
Organic (alkyl) radicals are usually highly reactive.
The stability and structure of alkyl radicals parallels those of
carbocations:
H
H C
H
<
H
R C
H
methyl
<
primary (1°)
<
R
R C
H
<
R
R C
R
< secondary (2°) < tertiary (3°)
Increasing stability
H° (KJ/mol)=
H
H C H
H
H
R C H
H
R
R C H
H
R
R C H
R
435
410
397
380
Radicals are also stabilized by hyperconjugation
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4.17: Mechanism of Chlorination of Methane
Free-radical chain mechanism:
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4.18: Halogenation of Higher Alkanes – free radical chlorination
of more substituted carbons is favored, reflecting the stability of
the intermediate radical.
H H H H
H C C C C H
H H H H
Cl2, h
H H H H
H C C C C Cl
H H H H
four 2° hydrogens
six 1° hydrogens
H
H C H
H
H
H C C C H
H H H
+
H H Cl H
H C C C C H
H H H H
(72 %)
(28 %)
Cl2, h
H
H C H
H
H
H C C C Cl
H H H
one 3° hydrogens
nine 1° hydrogens
+
H
H C H
H
H
H C C C H
H Cl H
(37 %)
(63 %)
Relative rates of free radical chlorination
H
R C H
H
Primary (1°)
hydrogens
1.0
H
R C H
R
<
Secondary (2°)
hydrogens
3.9
R
R C H
R
<
Tertiary (3°)
hydrogens
5.2
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Free radical bromination is highly selective
H
H C H
H
H
H C C C H
H H H
Br2, h
H
H C H
H
H
H C C C Br
H H H
H
H C H
H
H
H C C C H
H Br H
+
1 : 99
H
R C H
H
Primary (1°)
hydrogens
1.0
H
R C H
R
<
Secondary (2°)
hydrogens
82
R
R C H
R
<
Tertiary (3°)
hydrogens
1600
The propagation step for free radical bromination is endothermic,
Whereas chlorination which is exothermic. According to the
Hammond postulate the transition state for bromination should
resemble the product radical, and therefore be more selective for
the product going through the more stable radical intermediate
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