AcidBase Reactivity of Water and Alcohols

1
Acid-Base Reactivity of Water and Alcohols
pKa 15.7
Dissociation
pKa 16-17
The lower the pKa value, the more acidic
Amphoteric nature of water and alcohols - acts as
an acid and a base
2
Using pKa Values to Predict Acid-Base Reactions
NaOH CH3CH2OH
NaOCH2CH3 HOH
pKa 15.7
pKa 15.9
Reactions favor the side with the higher pKa
Sodium hydroxide will not deprotonate most
alcohols
H3O CH3CH2OH
H2O CH3CH2OH2
pKa -1.7
pKa -2.4
Vollhardt, p. 291
Maitland Jones, Organic Chemistry, p. 212
Alcohol not a good base.
3
The Four Main Types of Organic Reactions
Substitution
Elimination
Addition
Rearrangement
4
Ether Nomenclature
Ethers are alkoxyalkanes - the larger portion is
the alkane and the smaller portion is the alkoxy
1-ethoxypropane
2-ethoxypropane
ethoxycyclohexane
Ethers can also be named by the two groups
followed by ether
diethyl ether
methyl ethyl ether
5
Boiling Points of Ethers and Alcohols
Why is the boiling point of dimethyl ether much
less than the boiling point of ethanol?
Ethers cannot Hydrogen Bond!
6
Williamson Ether Synthesis
Preparation of ethers by SN2 reaction of primary
alkyl leaving group compounds and alkoxides.
1. Formation of alkoxides
Alcohols with very strong bases (LDA, NaH, or Na)
pKa 16
pKa 30
pKa 40
7
Williamson Ether Synthesis
2. Reaction of alkoxide with primary leaving group
Alkoxides are strong bases, anything other than
primary will lead to elimination Also, hindered
bases lead to elimination even with primary
leaving groups
Propose a synthesis of (any alcohol and alkyl
halide)
8
Ether Reactivities Under Highly Acidic Conditions
This reaction requires freshly distilled 57 HI
diethyl ether
SN2
This reaction requires H2SO4 at 130C
9
In General, Ethers are not Reactive Compounds
diethyl ether
1,2-dimethoxyethane, glycol dimethyl ether, glyme
tetrahydrofuran
These are excellent solvents for many organic
reactions - many organics have good
solubilities - the ether itself is not reactive
Epoxy polymers are chains of ethers, therefore
the polymer is very unreactive
The one class of ethers that are very reactive
are the epoxides, why?
Tremendous Ring-Strain!
10
Preparation and Reactions of Epoxides
Cis-addition
SN2 Reaction
Trans-product
This reaction is very powerful because a variety
of nucleophiles can be used and the reaction
produces specific stereochemistry - stereospecific
11
Ring Opening of Epoxides with Grignard Reagents
New Type of Nucleophile - Grignard Reagent
Can be perceived as a Carbanion
Carbanions are very strong bases, the pKa of
cyclohexane is 50! Therefore, carbanions will
deprotonate just about any acid.
This is a carbon nucleophile
12
Cis Addition of Hydrogen to Alkenes -
Hydrogenation
The flat surface of the catalyst delivers the
hydrogens to the same side of the alkene
Platinum or Palladium adsorbs hydrogen quite well
13
Electrophilic Addition to Alkenes
Electrophile - chemical species that is electron
deficient
electron loving
The hydrogen cation(H) is one of the best
electrophiles - just a proton
This is major product
electrophilic addition to alkenes is an example
of a reaction that is regiospecific - given a
choice of regions in a molecule, the atoms add
specifically and predictably to certain positions
This product is not Formed
14
E,Z Nomenclature of Alkenes
With disubstituted alkenes, cis or trans are
commonly used, but so can E or Z
2
Substituents are ranked 1 or 2 on each end of the
double bond
1
1
1s same side or 1s zame zide Z
2
1s opposite side so E
E from entgegen, German, opposite
Z from zusammen, German, together
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Cis and Trans Isomerization of Alkenes
For double bonds that are disubtituted (2 other
carbons attached) cis and trans isomerization is
possible
trans-3-heptene
cis-3-heptene
16
Melting Points of Cis and Trans Alkenes
The cis-structure of alkenes inhibits chains from
packing close together to form a solid. Much
colder temperatures are required to freeze.
In General, trans isomers have higher melting
points than cis isomers
The trans-structure of alkenes allows chains to
pack close together to form a solid
17
Some Oil and Fat Chains - note pages 110 -116 of
Pavia Lab Manual
Stearic Acid - Animal Lard
The hydrogenation changes the chain shape and
allows for closer packing and a higher melting
point - Frying Temperature instead of Below Room
Temperature
Oleic Acid - Corn and Olive Oil
18
Peroxides
We store ethers in tightly sealed containers in
the refrigerator to slow reaction with oxygen
Over time, ethers react with oxygen to produce
peroxides
Two highly electronegative atoms next to one
another makes for a highly unstable molecule
Decomposition generates 2 alkoxy free-radicals
This decomposition reaction can be quite
exothermic, even explosive!
19
Peroxides are Excellent Initiators for
Free-Radical Reactions
Anti-Markovnikov Addition
Taking stereochemistry into account, a total of 4
products are produced
20
Mechanism of Free-Radical addition of HBr to
Alkenes
Initiation Steps
Propagation Steps
Br adds to less substituted because more stable
free radical is formed
21
Free-Radical Polymerization of Alkenes -
Polystyrene
Initiation Steps
Styrene monomer
Propagation Steps
These propagation steps may repeat hundreds or
thousands of times to produce a styrene polymer!!!
22
Alkynes - Hydrocarbons with Triple bonds
In General, what ever alkenes do, alkynes do it
twice.
The pi bonds of alkynes are two independent pi
bonds.
Instead of one mole of hydrogen for alkenes,
alkynes react with two moles of hydrogen
The terminal H on alkynes, if present, is quite
acidic pKa 25.
Bases such as amides are strong enough to
deprotonate.
23
Alkynes as Nucleophiles
pKa 36
pKa 25
Alkynyl anion - a great nucleophile
Alkynyl anions react via a SN2 process with
primary alkyl halides (leaving group)
Acetylene has two terminal hydrogens so SN2
reactions could occur on either or both ends.
In making the alkynyl anion of acetylene, only 1
proton is removed at a time. It is very
difficult for any multi-acidic compound to give
up multiple protons at the same time.
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Almost all of the addition reactions performed on
alkenes can be done with alkynes - Twice!
In the case of hydration the reaction does not go
twice, but a rearrangement occurs
enol
tautomerization
25
Hydroxy Functional Group
The structure and characteristics resemble those
of water
d-
d-
Similar shape and bond polarities
d
d
d
This lead to similar characteristics such as
boiling points and solubilities
Hydrophilic - Water loving
Because alcohols are polar molecules like water,
they are considered hydrophilic compounds
Hydrophobic - Water hating
26
Boiling Point and Intermolecular Interactions
In the boiling process molecules pass from a
liquid state to a gaseous state
Boiling points are greatly affected by molecular
weight and intermolecular forces
Molecular Weight - more energy required to move a
molecule from the liquid state to the gaseous
state as molecular weight increases
Intermolecular Forces - more energy required to
break stronger forces holding molecules together
in liquid state
27
Characteristics of Alkanes and Alcohols
CH3CH2OH
CH3Cl
higher molecular weights have higher boiling
points
28
Hydrogen Bonding
Bonding that occurs between a hydrogen bonded to
a O,N,F and another O,N,F
The hydrogen bonds are quite strong, on the order
of 1/2 strength of a covalent bond. They must be
broken in the boiling process
d-
d-
d
d
d
d-
d-
d
d
d
Alcohols typically have higher boiling points
than expected for the molecular weight
29
Alcohol Solubility
Because alcohols are polar, they are soluble in
water. The OH group of the alcohol is able to
form hydrogen bonds with water.
30
Rationalizing Acidities
The acidity of a compound is determined by the
stability of the resulting Anion
RO-H
RO- H
The stability determines to what degree this
reaction will proceed
Reactions with stable anions proceed readily,
thus the original compound is considered acidic.
By stabilizing the negative charge on the anion,
the more acidic the original compound becomes
31
Stabilizing Negative Charge
Stabilizing charge is accomplished by spreading
the charge over several atoms Inductive,
Resonance, and Solvation Effects are important
Methanol more acidic than t-butanol because the
anion of methanol is better solvated, negative
charge spead to solvent molecules
pKa 18 (CH3)3COH
pKa 15.5 CH3OH
32
Inductive Effect
Inductive effect - transmission of positive or
negative charge through sigma bonds
Remember pKa is a logarithmic function, so a
difference of 2 is 100 times more acidic
pKa 15.9
pKa 14.3
The highly electronegative chlorine atom draws
negative charge toward itself, in the meantime,
negative charge is distributed over the other
atoms
Chlorine stablizes the anion, making the chloro
alcohol more acidic
33
Characteristics of Inductive Effect
Greater inductive effects are seen with a greater
number of electronegative atoms and with closer
proximity to the anion
CH3CH2OH
pKa 15.9
ClCH2CH2OH
pKa 14.3
pKa 12.4
CF3CH2OH
pKa 14.6
CF3CH2CH2OH
pKa 15.4
CF3CH2CH2CH2OH
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Using pKa Values to Predict Acid-Base Reactions
conjugate acid
conjugate base
base
acid
NaOH CH3CH2OH
NaOCH2CH3 HOH
pKa 15.7
pKa 15.9
Reactions favor the side with the highest pKa
Sodium hydroxide will not deprotonate most
alcohols
35
Prediction of Acid-Base Reaction
Will potassium ethoxide deprotonate
3,3,3-trifluoroethanol?
KOCH2CH3 CF3CH2OH
HOCH2CH3 CF3CH2OK
conjugate acid
acid
pKa 12.4
pKa 15.9
The higher pKa is on the right side of the
equation, therefore the reaction will proceed in
this direction So YES, potassium ethoxide will
deprotonate 3,3,3-trifluoroethanol.
Acid-Base reactions favor formation of weaker
acids and bases
36
E1 Elimination of Alcohols with Sulfuric Acid
H2SO4 (sulfuric acid) dehydrating acid - the acid
can be made to be nearly 100 pure, no water
also, the acid anions are poor nucleophiles
(HSO4- and SO4-2)
HBr is a strong acid, but the anion is also a
good nucleophile that can lead to substitution
products