Transcript functional group

Page 1
Chapter 4 - “Alcohols and Alkyl
Halides”
1)
2)
3)
4)
Functional groups and nomenclature
Role of intermolecular forces on properties
Synthesis of alkyl halides from alcohols
Two reaction mechanisms: SN1 and SN2
cation stability and the Hammond Postulate
5) Additional ways to prepare RX from ROH
6) Halogenation of alkanes: reactivityselectivity principle, radical stability and
chain reactions
Minto - Lectures 7-8
All sections are covered.
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Functional Groups and Priorities
Nomenclature assembles names like trees: start with a parent
structure (e.g., an alkane), then add prefixes and a functional group
(FG) suffix.
FGs are the centers of reactivity within molecules and have
predictable chemical behavior.
In naming, FGs are usually higher priority than a substituent. In
more complicated names, FGs can be indicated by a prefix.
IUPAC naming of acyclic compounds
Double and triple bonds are incorporated into the parent chain, as much as
possible, resulting in a modified hydrocarbon name. Beyond their presence
in the parent chain, they are numbered in a prioritized fashion with any other
FGs that are present.
Minto - Lectures 7-8
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FG Priority List
High
Carboxylate
Carboxylic acid
Anhydride
Ester
Acid halide
Amide
Nitrile
Aldehyde
Ketone
Alcohol (phenol)
Amine
Ether
Alkene
Alkyne
Alkyl halide, phenyl, alkyl
Minto - Lectures 7-8
Low
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IUPAC Nomenclature of Alkyl Halides
CH3Cl
chloromethane
CH3CH2F
fluoroethane
CH3CHI
CH3CH2CHBr
CH3
CH3
2-iodopropane
2-bromobutane
1. In the formal IUPAC system, alkyl halides are named as
substituted alkanes: F, fluoro; Cl, chloro; Br, bromo; I, iodo. In
common naming, they are named as halides (e.g., methyl
chloride).
2. Halogens are treated as substituents of equal priority to alkyl
groups.
4-chloro-3,3-dimethyl-6-(1-methylethyl)nonane
or
4-chloro-6-isopropyl-3,3-dimethylnonane
Cl
Br
4-bromo-2-iodo-1-methylcyclohexane
I
Minto - Lectures 7-8
Classification of Alkyl Halides
1°
2°
Br
Br
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3°
Br
tert-butyl bromide (common)
n-butyl bromide (common)
Or
or
2-bromo-2-methylpropane (IUPAC)
1-bromobutane (IUPAC)
sec-butyl bromide (common)
or
2-bromobutane (IUPAC)
Minto - Lectures 7-8
Nomenclature of Alcohols
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In an alcohol, the OH is a high-priority functional group.
IUPAC NAME = substituent prefixes+parent+functional group
suffix(es). For alcohols, alkane suffix “e” changes to “ol”.
1. Determine the parent hydrocarbon containing the functional
group. On cyclic compounds, OH receives number “1”; only
show the “1” if multiple OH groups are present.
2. If necessary, the suffix is preceded by a positional identifier.
**
**
**both o.k. as formal names
Minto - Lectures 7-8
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Alcohols can be categorized as primary (1°), secondary (2°),
and tertiary (3°) by the number of carbons directly attached to
the hydroxyl-bearing carbon.
1°
OH
2°
Minto - Lectures 7-8
OH
3°
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Classification of amines is different than ROH or RX.
1-butanamine or butan-1-amine
NH2
A primary (1°) amine, one C attached to N
N-ethyl-3-heptanamine
NH
N
A secondary (2°) amine, two C attached to N
2-ethyl-N-methyl-N-propylcyclohexanamine
A tertiary (3°) amine, three C attached to N
Minto - Lectures 7-8
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Hybridization and Bonding in ROH
MeOH
MeCl
 = 1.7 D
Minto - Lectures 7-8
 = 1.9 D
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Recall:
A hydrogen bond is a very strong dipole–dipole interaction:
ONLY occurs when N-H, O-H, and F-H bonds (H-bond donors)
interact with N, O, and F atoms (H-bond acceptors).
Solubility of hydrogen-bondable molecules in water depends upon
their proportion of H-bond donor/acceptor atoms.
Minto - Lectures 7-8
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Effect of Structure on Boiling Point
CH3CH2CH3
CH3CH2F
CH3CH2OH
Molecular
weight
44
48
46
Boiling
point, °C
-42
-32
+78
0
1.9
1.7
Dipole
moment, D
Minto - Lectures 7-8
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Polarizability
Distortion of electron clouds is important to van der Waals
attractive interactions (induced dipole-induced dipole).
Increasing the atomic radii within a group raises the
boiling point.
Minto - Lectures 7-8
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Example: Boiling point increases with
increasing number of halogens
Compound
CH3Cl
CH2Cl2
CHCl3
CCl4
Boiling Point
-24°C
40°C
61°C
77°C
Zero dipole
moment
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.
Minto - Lectures 7-8
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???
Minto - Lectures 7-8
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Reaction Mechanisms
We will dissect the reactions of alcohols with HX to make alkyl
halides to learn about reaction mechanisms. There are two main
reaction types SN1 and SN2. We will define these names in a few
minutes.
Example: (CH3)3COH + HCl ------> (CH3)3CCl + HOH
This reaction is a substitution reaction, but the net reaction does
not give the details of the reaction mechanism, or path.
Ten new terms:
Elementary step
Transition state
Activation barrier
Concerted reaction
Molecularity
Minto - Lectures 7-8
Intermediate
Oxonium ion
Carbocation
Nucleophile
Electrophile
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Reaction Pathway
Energy
Barrier
2nd
intermediate
Intermediate
Minto - Lectures 7-8
Products
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tert-butanol + HCl, 1st elementary step
pKa -3.9
pKa -2
Intermediates
Minto - Lectures 7-8
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tert-butanol + HCl, 2nd elementary step
Hammond Postulate: If the structure
of a transition state is similar in energy
to the species immediately preceding
or following it, the transition state
structure will most resemble that
species.
Minto - Lectures 7-8
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tert-butanol + HCl, 3rd elementary step
Minto - Lectures 7-8
Alkyl
Chloride
Potential Energy Diagram of SN1
Reaction of an Alcohol with HX
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Rate = k[alkyloxonium ion]
Mechanisms can be supported or disproved, but cannot be proven correct.
Minto - Lectures 7-8