PTT 102
Organic Chemistry
Alcohol & Ether
Reaction of Alcohol and
Ethers
MISS NOORULNAJWA DIYANA YAACOB
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
Chemical and Physical Properties
• Alcohols have an odor that is often described as “biting”
and as “hanging” in the nasal passages.
• In general, the hydroxyl group makes the alcohol molecule
polar.
• 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:
15
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.
Mechanism of Alcohol Dehydration
The general form of alcohol dehydrations is as
follows:
The first step involves the protonation of the
alcohol by an acid, followed by loss of water to
give a carbocation.
Elimination occurs when the acid conjugate
base plucks off a hydrogen. Alcohol
dehydrations generally go by the E1
mechanism.
Example
• The mechanism for acid-catalyzed dehydration
depends on the structure of the alcohol.
• Dehydration of secondary and tertiary alcohol
are E1 reactions.
What is E1 Reaction??
B: = base
X = leaving group
• 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 removes a proton from a β carbon, forming an alkene
24
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:
25
Carbocation Rearrangement
•
•
Dehydration of 2° and 3° alcohols involves the formation of carbocation
intermediate, so be sure to check the structure of the carbocation .
Crbocation will rearrange if rearrangement produces a more stable carbocation.
26
Carbocation Stabilities
Alkyl groups decrease the concentration of positive
charge in the carbocation
27
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
Primary Alcohols Undergo Dehydration by
an E2 Pathway
29
• We can summarized what we have learned
about the mechanisms by which alcohol
undergo substitution and elimination reaction:
2° & 3° Alcohol: Undergo SN1 and E1 reaction
1° Alcohol: Undergo SN2 and E2 reaction
2. Oxidation of Alcohol
• Primary alcohols can be oxidized to aldehydes
or further to carboxylic acids
• Secondary alcohols can be oxidised to ketones
but no further
• Tertiary alcohols cannot be oxidized
Oxidation by Chromium (VI)
32
Oxidation of Alcohols
Oxidation by chromic acid:
Secondary alcohols are oxidized to ketones
33
Primary alcohols are oxidized to aldehydes and eventually carboxylic acids:
The oxidation of primary alcohol will stop at aldehyde if pyridinium chlorochromate (PCC) is
used as the oxidizing agent in a solvent such as dichloromethane (CH2Cl2).
In the absence of water, the oxidation stops at the aldehyde:
No water present
34
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
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
36
3. Ether substitution reaction
• 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:
What happenster the ether is protonated depend on the structure of ether.
If departure of ROH creates arelatively atable carbocation, an SN1 reaction occurs
38
Ether cleavage: an SN1 reaction:
Protonation converts the very basic RO- leaving group into the less basic ROH leaving group.
The leaving group departs
The halide ion combines with carbocation
Ether cleavage: an SN2 reaction:
39
4. Epoxide Reaction
• Alkene can be converted into epoxide by a
peroxyacid
• Or by the addition of ClOH (by using Cl2 and
H2O) followed by HO-
• Epoxides ,like ors, undergo substitution
reaction withv 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.
43
• 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:
45
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
Therefore:
47
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
The ability of a host to bond only certain guests is an
example of molecular recognition
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
49