PTT 102
Organic Chemistry
Alcohol & Ether
Reaction of Alcohol and
Ethers
Mdm Khadijah Hanim Abdul Rahman
Course Outcome
CO2: Ability to EXPLAIN and DIFFERENTIATE the
chemical, physical properties and reactions of
alcohol, ether, aldehyde, ketone and
carboxylic acids
HOW ARE ALCOHOLS
CLASSIFIED AND
NAMED?
23.2
Alcohols
– An alcohol is an organic compound with an — OH group.
– The —OH functional group in alcohols is called a hydroxyl group
or hydroxy function.
23.2
Alcohols
23.2
Alcohols
23.2
Alcohols
1. Determine the longest hydrocarbon containing the functional
group:
2. The functional group suffix should get the lowest number:
8
3. When there is both a functional group suffix and a substituent, the
functional group suffix gets the lowest number:
4. The chain is numbered in the direction that gives a substituent the
lowest number:
9
5. The functional group substituent on a ring gets the number 1, but
the functional group is not numbered in the name:
CH3
HO
CH3
OH
CH3
CH3
OH
3-methylcyclohexanol
not
3-methylcyclohexan-1-ol
1-methylcyclohexanol
not
1-methylcyclohexan-1-ol
2, 2-dimethylcyclopentanol
not
2, 2-dimethylcyclopentan-1-ol
6. If there is more than one substituent, the substituents are cited in
alphabetical order:
10
exercise
Chemical and Physical Properties
• Alcohols have an odor that is often described as “biting” and as
“hanging” in the nasal passages.
• Alcohol has both non-polar alkyl groups and polar hydroxyl group.
• Is alcohol polar or non-polar molecules?
• Depends on the size of the alkyl group. As the alkyl group increases
in size, becoming a more significant fraction of the alcohol, the
compound become less and less soluble in water.
• Alcohol less than 4 carbons soluble in water.
• Because of hydrogen bonding, alcohols tend to have higher boiling
points than comparable hydrocarbons and ethers. The boiling point
of the alcohol ethanol is 78.29 °C.
• Alcohols can also undergo oxidation to give aldehydes, ketones, or
carboxylic acids, or they can be dehydrated to alkenes. They can
react to form ester compounds, and they can (if activated first)
undergo nucleophilic substitution reactions
23.2
Ethers
What is the general structure
of an ether and how are the
alkyl groups of an ether
named?
23.2
Ethers
– The general structure of an ether is R—O—R. The
alkyl groups attached to the ether linkage are
named in alphabetical order and are followed by
the word ether.
23.2
Ethers
• An ether is a compound in which oxygen is bonded to
two carbon groups.
Nomenclature of Ethers
As substituents:
16
Exercises
• CH3CH2CH2CH2CHCH2CH2CH3
OCH
• CH3CHOCH2CH2CHCH3
CH3
CH3
• 1- propoxy-butane
• 2-isopropoxyhexane
Physical and Chemical Properties
• Ether molecules cannot form hydrogen bonds
with each other, resulting in a relatively low
boiling points.
• Ethers are slightly polar.
• Ethers are more polar than alkenes but not as
polar as alcohols, esters, or amides of
comparable structure.
• Ethers in general are of low chemical reactivity,
but they are more reactive than alkanes
Reaction of Alcohol and Ethers
•
•
•
•
•
1. Alcohol Elimination Reaction
2. Oxidation of Alcohol
3. Ether substitution reaction
4. Epoxide Reaction
5.Crown Ethers Synthesis
1. Alcohol Elimination Reaction
• Alcohol can undergo elimination reaction by
losing an OH from one carbon and an H from
adjacent carbon.
• Overall, this amounts to the elimination of
WATER.
• Lost of water from a molecule is called
DEHYDRATION.
• The product of the reaction is an ALKENE.
• Dehydration of an alcohol requires an acid
catalyst and heat.
• Sulfuric acid (H2SO4) and Phosphoric acid
(H3PO4) are commonly used acid catalysts.
• The mechanism for acid-catalyzed dehydration
depends on the structure of the alcohol.
• Dehydration of secondary and tertiary alcohol
are E1 reactions.
What is the E1 reaction?
• E1 reaction stand for, E=elimination and 1= unimolecular.
• The E1 reaction has 2 steps:
• In the E1 mechanism, the the first step is the loss of the
leaving group, which leaves in a very slow step, resulting in
the formation of a carbocation.
• The base then attacks a neighboring hydrogen, forcing the
electrons from the hydrogen-carbon bond to make the double
bond.
Dehydration of Secondary and Tertiary
Alcohols by an E1 Pathway
• The acids protonates the most basic atom in the reactant.
• Protonation converts the very polar leaving group (OH) into a good leaving (H2O).
• Water departs, leaving behind a carbocation.
• A base in the reaction mixture (water is the base) removes a proton from a β
carbon, forming an alkene
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• When acid-catalyzed dehydration leads to
more than 1 elimination product, the major
product will be the more substituted alkenethe 1 obtained by removing a proton from the
β-carbon bonded to fewest H.
Because the rate-determining step in the dehydration reaction of 2° or 3° alcohol is a formation
Of a carbocation intermediate,
The rate of dehydration reflects the ease with which the
carbocation is formed:
• In order to undergo dehydration, tertiary alcohols must be heated to 50oC in 5%
H2SO4.
• secondary alcohols must be heated to about 100oC in 75% acid
• primary alcohol can be dehydrated in extreme conditions, 170oC in 95% acid
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Carbocation Rearrangement
•
•
Dehydration of 2° and 3° alcohols involves the formation of carbocation
intermediate, so be sure to check the structure of the carbocation .
Carbocation will rearrange if rearrangement produces a more stable carbocation.
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Primary Alcohols Undergo Dehydration
by an E2 Pathway
• Why??
• Because primary carbocation are too unstable to be
formed.
• E2 reaction :one-step process of elimination with a
single transition state.
• Any base (B: ) in the reaction mixture (ROH,ROR,
H2O.HSO4-) can remove the proton in the elimination
reaction
• An ether is also obtained: it is the product of
competing SN2 reaction since 1° alcohol are one most
likely to form substitution products in SN2/E2 reaction
The E2 reaction
• E= elimination; 2= bimolecular
• The base (OH- ion) removes a proton from a C that is
adjacent to the carbon that is bonded to the halogen.
• As the proton is removed, the electrons that it shared
with C move toward the adjacent carbon that is bonded
to the halogen.
• As these electrons move toward the C, the halogen
leaves, taking its bonding electrons with it.
Primary Alcohols Undergo Dehydration by
an E2 Pathway
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2. Oxidation of Alcohol
• Reagent used to oxidize alcohols : chromic
acid (H2CrO4)
• Secondary alcohols : oxidized to ketones.
- Recognized as oxidation because the no of C-H
bonds in the reactant decreases and the no. of
C-O bonds increases.
Oxidation by Chromium (VI)
32
Primary alcohols
• Initially oxidized to aldehydes.
• The reaction continue (further oxidized- no of
C-O bonds increases) to carboxylic acid.
• Oxidation of a primary alcohol will stop at
aldehyde if PCC (pyridinium chlorochromate) is
used as the oxidizing agent in solvent (CH2Cl).
• Oxidation of both 1o and 2o alcohol, a H is
removed from C that OH is attached.
• The carbon bearing OH group in a 3o alcohol is
not bonded to H, so its OH group cannot be
oxidized.
A tertiary alcohol cannot be oxidized and is converted to a stable chromate ester instead:
O
O Cr O
O
No hydrogen on
this carbon
Di-tert-Butyl Chromate
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Mechanism:
An oxygen of chromic acid is protonated in the acidic solution
The alcohol molecule displaces a molecule of water in an SN2
reaction on chromium
A base present in the reaction mixture (H2O, ROH) removes a
proton from the strongly acidic spesies
A base removes a proton from chromates ester in an E2 reaction,
thereby forming the carbonyl compound
Exercises
•
•
•
•
3-pentanol
1-pentanol
2-methyl-2-pentanol
Propose a mechanism for the chromic acid
oxidation of 1-propanol to propanal.
3. Nucleophilic substitution reactions
of ethers
• The OR group of an ether and the OH group of
an alcohol have nearly the same basicity.
• Both groups are strong bases, so both are very
poor leaving group.
• Consequently, ethers, like alcohols, needs to
be activated before they can undergo a
nucleophilic substitution reaction
Nucleophilic Substitution
Reactions of Ethers
• Ethers, like alcohols, can be activated by protonation.
• Ethers can undergo nucleophilic substitution reactions with HBr/HI
If departure of ROH creates a relatively stable carbocation, an SN1 reaction occurs
40
Mechanism for Ether cleavage: an SN1 reaction:
1. Protonation converts the very basic RO- leaving group into the less basic ROH leaving group.
2. The leaving group departs
3. The halide ion combines with carbocation
However if departure of the leaving group would create an unstable carbocation, the
leaving group cannot depart but has to be displaced by halide ion  SN2 reaction.
41
Ether cleavage: an SN2 reaction:
1. Protonation converts the very basic RO- leaving group into the less basic ROH
leaving group.
2. The halide ion preferentially attacks the less sterically hindered of the 2 alkyl
groups.
exercise
• Give the major product obtained from heating
the following ether with HI
• CH3CH=CH-O-CH2CH3
4. Nucleophilic substitution Reactions
of Epoxides
• Alkene can be converted into epoxide by a
peroxyacid
• Or by the addition of ClOH (by using Cl2 and
H2O) followed by HO-
• Epoxides undergo substitution reaction with
hydrogen halides.
• The mechanisms of the reaction depends on
whether it is carried out under acidic or
neutral/basic conditions.
• Nucleophilics Substitution: Acid
Conditions
Acid-Catalyzed Epoxide Ring Opening
HBr:
The acid protonates the oxygen atom of the epoxide
The protonated epoxide undergoes back-side attack by the halide ion
Protonated epoxides are so reactive that they can be opened by poor
nucleophiles, such as water and alcohols, where HB+ is any acid in the solution
and :B is any base.
47
• If different substituent are attached to the two
carbons of the protonated epoxide, and the
nucleophile is something other than H2O, the
product obtained from nucleophilic attack on the
2- position of the oxirane will be different from
that obtained from nucleophilic attack on the 3position .
• The major product is the one resulting from
nucleophilic attack on the more substituted
carbon
Reaction of an epoxide in the presence of methanol and acid
Regioselectivity:
Mechanism:
49
Nucleophilics
Substitution:
Basic/Neutral
Conditions
When a nucleophile attacks an unprotonated epoxide,
the reaction is a pure SN2 reaction:
The C-O bond does not begin to break until the carbon is attacked by the nucleophile.
The nucleophile is more likely to attack the less substituted carbon because it is less
sterically hindered
The alkoxide ion picks up a proton from the solvent or from acid added after the reaction is over.
Thus the site of nucleophilic attack on an unsymmetrical epoxide under neutral/basic conditions
is different from the site of nucleophilic attack under acidic conditions
Therefore:
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5.Crown Ethers Synthesis
• Crown ethers are cyclic compounds containing
several ether linkage around a central cavity
• A crown ether specifically binds certain metal
ions or organic molecules.
• The crown ether is called the “host” and the
species it binds is called the “guest”
• Crown ethers are named [X]-Crown-Y, where X is
the total no of atoms in the ring and Y is the no of
oxygen atoms in the ring.
• Binding of Na+ and crown ethers are thru
interaction of the positively charged ion with lone
pair electrons of oxygen atoms that point into the
cavity.
• Crown ethers are soluble in nonpolar solvents
because the outside of the crown is composed of
primarily C-H bonds.
Crown Ethers
The ability of a host to bond only certain guests is an
example of molecular recognition.
The recognition of 1 molecule for another as a result of
specific interactions.
Because the ether linkages
are chemically inert, the
crown ether can bind the
guest without reacting
with it.
The crown-guest complex
is called INCLUSION
COMPOUND
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