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©2012 Gregory R Cook
Chapter 04Alcohols and Alkyl Halides
part 01
CHEM 341: Spring 2012
Prof. Greg Cook
cook.chem.ndsu.nodak.edu/chem341
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©2012 Gregory R Cook
Functional Groups
• A functional group is a structural feature in a molecule that has characteristic reactivity.
• A functional group has similar behavior in every molecule that contains it.
2
Carbon-Carbon Multiple Bonds
Alkenes - double bonds
CH2 CH2
Alkynes - triple bonds
C C
Arenes - aromatic rings
Carbon-Other atom Single Bonds
OH
Alcohols
Br
Hallides
NH2
Amines
OCH3
Ethers
SH
Thiols
SCH3
Sulfides
Carbon-Oxygen Double Bonds (Carbonyls)
H
AldehydeO
KetoneO
OH
AcidO
OCH3
EsterO
NH2
AmideO
Cl
Acid HalideO
H H
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©2012 Gregory R Cook
Functional Groups
• A functional group is a structural feature in a molecule that has characteristic reactivity.
• A functional group has similar behavior in every molecule that contains it.
2
Carbon-Carbon Multiple Bonds
Alkenes - double bonds
CH2 CH2
Alkynes - triple bonds
C C
Arenes - aromatic rings
Carbon-Other atom Single Bonds
OH
Alcohols
Br
Hallides
NH2
Amines
OCH3
Ethers
SH
Thiols
SCH3
Sulfides
Carbon-Oxygen Double Bonds (Carbonyls)
H
AldehydeO
KetoneO
OH
AcidO
OCH3
EsterO
NH2
AmideO
Cl
Acid HalideO
H HR OH R Xalcohols alkyl halides
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©2012 Gregory R Cook
Reactions of Functional Groups
3
H XR OH + H OHR X +
OHR X + R OH + X
X XR H + H XR X +
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©2012 Gregory R Cook
General Reaction Classes
• Addition Reactions - two or more molecules combine to form a new product
• Elimination Reactions - a molecule breaks apart into two or more molecules
• Substitution Reactions - part of a molecule is substituted for another group
• Rearrangement Reactions - A molecule does not gain or lose atoms, but reorganizes its bonds or groups. Also called isomerization.
4
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©2012 Gregory R Cook
General Reaction Classes
• Addition Reactions
• Elimination Reactions
• Substitution Reactions
• Rearrangement Reactions
5
A + B C C CH
H
H
H+ H2 cat.
C CH
HHH
H
H
CA B + BA C + H3C Cl + OH H3C OH + Cl
BA heatO O
A + BC cat.OH
H
+ HOH
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©2012 Gregory R Cook
• Organic halogen compounds are found in many places from common solvents for chemistry and industry, to refrigerants (largely banned for their ozone destruction properties) and natural products.
Alkyl Halides
6
CH2Cl2CHCl3CH2Cl-CH2Cl
commonsolvents
CFCs(chlorofluorocarbons)
CF2Cl2
natural products(epibatidine)
NH
N
Cl
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©2012 Gregory R Cook
• Alkyl halides are often named using common naming (functional class) considering the alkyl group as a substituent on the halide.
Alkyl Halide Nomenclature
7
methyl bromide
H3C BrI
cyclopentyl iodide 1-methylpropyl chloride
Cl
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©2012 Gregory R Cook
• IUPAC naming is best using the carbon chain as the parent and the halogen as a halo- substituent
Alkyl Halide Nomenclature
8
bromomethane
H3C BrI
iodocyclopentane 2-chlorobutane
Cl
Br Br
4-bromo-2-methylhexane 2-bromo-4-methylhexane
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©2012 Gregory R Cook
• Alcohols are often named using common naming (functional class) considering the alkyl group as a substituent on the OH functional group.
Alcohols
9
methyl alcohol
H3C OHOH
cyclopentyl alcohol isopropyl alcohol
OH
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©2012 Gregory R Cook
• IUPAC naming is best using the carbon chain as the parent and replacing the -ane ending with -anol
• note: alcohols outrank alkyl substituents
Alcohol Nomenclature
10
methanol
H3C OHOH
cyclopentanol 2-propanol
OH
OH OH
5-methyl-3-hexanol 4-methyl-2-hexanol
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©2012 Gregory R Cook
• Alcohols and alkyl halides are classified according to the degree of alkyl substituion on the carbon the functional group is attached
Classes of Alkyl Halides and Alcohols
11
OH
1-propanola primary alcohol
2-chloro-2-methylbutanea tertiary halide (chloride)
Cl OH
trans-3-methylcyclopentanola secondary alcohol
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©2012 Gregory R Cook
• Reactivity is dominated by the polarity of the C-X bond
Properties of Alkyl Halides
12
fluoromethane CH3-F 1.85 1.39 108
chloromethane CH3-Cl 1.87 1.78 84
bromomethane CH3-Br 1.81 1.93 70
iodomethane CH3-I 1.62 2.14 56
diopole moment(Debyes)
bond length(Å)
bond strength(kcal/mol)
µ = 1.9 D
C Cl
H
H
H
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©2012 Gregory R Cook
• Acohols are polar but have other reactive features
Properties of Alcohol
13
weakly acidic
RO
H
weakly basic
µ = 1.7 D
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©2012 Gregory R Cook
Boiling Points of Halides and Alcohol
14
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©2012 Gregory R Cook
Intermolecular Forces
15
• Induced dipole/induced dipole (London forces, van der Waals attraction)
• Dipole/induced dipole forces
• Diplole/Dipole forces
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©2012 Gregory R Cook
Intermolecular Forces
16
bp -24 °C
RO
H
µ = 1.7 Dµ = 1.9 D
C Cl
H
H
H
bp 65 °C
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©2012 Gregory R Cook
• At about 5 kcal/mol, H-bonds are weaker than covalent bonds but stronger than many other intermolecular forces
Hydrogen Bonding
17
RO
H
δ−
δ+
R
O Hδ− δ+
hydrogenbond
R
OHδ−δ+
hydrogenbond
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©2012 Gregory R Cook
• Polarizability - the ability to distort (polarize) electrons in a bond
Polarizability
18
chloromethanebp -24 °C
C Cl
HH
H
C Cl
ClH
H
C Cl
ClCl
H
C Cl
ClCl
Cl
dichloromethanebp 40 °C
trichloromethanebp 61 °C
tetrachloromethanebp 77 °C
fluroethanebp -32 °C
C F
HH
C
1,1-difluoroethanebp -25 °C
1,1,1-trifluoroethanebp -47 °C
hexafluoroethanebp -77 °C
HH
H
C F
FH
CH
H
H
C F
FF
CH
H
H
C F
FF
CF
F
F
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©2012 Gregory R Cook
• Alcohols are similar to water but the solubility decreases as the hydrocarbon chain increases
Alcohol Solubility
19
CH3 OH
AlcoholSolubility in Water(g/100 g H2O)
CH3CH2 OH
OH
OH
OH
OH
OH
OH
OH
miscible
miscible
miscible
9
2.7
0.6
0.18
0.054
insoluble
Carbons
1
2
3
4
5
6
7
8
10
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©2012 Gregory R Cook
• Alkyl halides are hydrophobic and insoluble in water.
• Fluorides and chlorides are less dense than water, bromides and iodides more dense
• But, if it has more than one chlorine, they are more dense
Alkyl Halide Solubility and Density
20
less dense
C Cl
HH
H
C Cl
ClH
H
C Cl
ClCl
H
C Cl
ClCl
Cl
more dense more dense more dense
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©2012 Gregory R Cook
• from alcohols by a substitution reaction
Preparation of Alkyl Halides
21
H XR OH + H OHR X +
Alkyl Alcohol Hydrogen Halide Alkyl Halide Water
reactivity
H F H Cl H Br H I< < <
reactivity
<H CH
HOH < R C
H
HOH R C
H
ROH R C
R
ROH< <
methyl primary secondary tertiary
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©2012 Gregory R Cook
Preparation of Alkyl Halides
22
OH
H Br
80-100°C
Br
OHH Br
25°C
Br
H Br
120°COH Br
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©2012 Gregory R Cook
Preparation of Alkyl Halides
23
• Can use a combination of halide salt and other Brønsted acids
heatOH Br
NaBr H2SO4
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©2012 Gregory R Cook
Preparation of Alkyl Halides
23
• Can use a combination of halide salt and other Brønsted acids
heatOH Br
NaBr H2SO4
• How does the reaction work?
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©2012 Gregory R Cook
Preparation of Alkyl Halides
23
• Can use a combination of halide salt and other Brønsted acids
heatOH Br
NaBr H2SO4
• How does the reaction work?
• A reaction mechanism is a detailed step by step description of a chemical transformation showing all elementary steps and intermediates
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©2012 Gregory R Cook
Thermodynamics
24
• The change in energy of a reaction from one side to the other
• If ΔG° is negative (energy released) the reaction is downhill (exothermic).
• ΔG° is positive (energy absorbed) the reaction is uphill (endothermic)
ΔG° = -RT ln Keq ΔG° = ΔH° - TΔS°
Gibb's Free Energytotal change in energy in the system from reactants to products
Enthalpy"Heat of Reaction" total change in the bonding energies of a reaction
EntropyAmount of freedom (or disorder). Greater entropy means more freedom, less entropy means more restricted
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©2012 Gregory R Cook
Thermodynamics
25
• The difference in energy is a reflection in the change of the bonds (enthalpy)
• Bond Dissociation Energy - the energy required to homolytically cleave a bond
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©2012 Gregory R Cook
Thermodynamics of Alcohol Substitution
26
• Thermodynamics only deals with the energy of reactans and products (equilibrium). It says nothing about the path between them.
Bonds Broken (energy put in)
C-O bond 358 kJ/mol366 kJ/molH-Br bond
total energyput in
ΔH° = 724 - 750 = - 26
H BrR OH + H OHR Br +
Bonds Formed (energy released)
C-Br bond 285 kJ/mol465 kJ/molH-O bond
total energyreleased
724 kJ/mol 750 kJ/mol
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©2012 Gregory R Cook
Kinetics
27
E
reaction progress
transition state ‡
ΔG°A
B
energybarrierto climb
• Kinetics deals with the pathway and describes the rate of reactions - higher energy paths are slower. We can visually map the energy on a reaction potential energy diagram.
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©2012 Gregory R Cook
Kinetics
28
E
reaction progress
transition state ‡
ΔG°
transition state ‡slowest stephighest energy hill
A
B
C
intermediate
• A multistep reaction path
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©2012 Gregory R Cook
• Activation Energy - the energy required to overcome the energy barrier
• Rate Determining Step - the slowest step in a reaction mechanism - the step with the highest energy barrier
• Transition State - the structure of molecules at the highest energy point of each energy barrier.
• The structure of the TS will most closely resemble the nearest intermediate - Hammond’s Postulate
Some Terms
29
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Mechanism for Alcohol reaction with HX
30
H XR OH + H OHR X +
Alkyl Alcohol Hydrogen Halide Alkyl Halide Water
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Mechanism for Alcohol reaction with HX
31
H BrR OH + R O
H
H
Br+
Step 1
fast
bimolecular reaction
R +
Step 2
slow
unimolecular reaction
R O
H
H
H OH
R Br
Step 3
fast
bimolecular reaction
Br+R
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Step 1 - Protonation of Alcohol
32
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Step 2 - Dissociation of Water
33
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Step 3 - Addition of Halide
34
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Overall Reaction Energy Diagram
35
• SN1 reaction mechanism
Substitution Nucleophilic Unimolecular
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