Alcohols:Organic Chemistry MANIK

Alcohols: Organic Chemistry
Prepared By: Shadid Uz Zaman At Tadir; B.Pharm.; M.Pharm. ;DU Page 1
Arrangement: Md. Imran Nur Manik; B.Pharm. M.Pharm. ;RU
Alcohols
Definition: Alcohols are those organic compounds whose molecules contain Hydroxyl (-OH)
goup attached to a saturated carbon atom.
C OH
Saturated Carbon Functional group
Alcohols containing one –OH group are called Monohydric alcohols. Those with two, three or
many C-OH groups are known as Dihydric, Trihydric or Polyhydric alcohols respectively.
Ethyl alcohol Ethylene glycol Glycerol
(Monohydric alcohol) (Dihydric alcohol) (trihydric alcohol)
General formula of alcohol:
R-OH
Alkyl/substituted alkyl group
Classification of alcohols:
Alcohols
Monohydric
alcohols
Dihydric
alcohols
Trihydric
alcohols
Polyhydric
alcohols
Primary (1ο
)
alcohols.
RCH2-OH
Secondary (2ο
)
alcohols.
R2CH-OH
Tertiary (3ο
)
alcohols.
R3C-OH
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Difference between alcohols, Phenols & Enols:
* Alcohols
- Compounds that have hydroxyl group bonded to a saturated, sp3
hybridized carbon atom.
* Phenols
- Compounds that have hydroxyl groups bonded to aromatic rings.
* Enols
- Compounds that have a hydroxyl group bonded to a vinylic carbon.
Physical properties of alcohols:
Some physical properties of Alcohols are as follows:
1. Odor & taste: The lower alcohols are colorless volatile liquids having characteristic
“alcoholic odor” and burning taste. This becomes less pronounced with the increase of
molecular weight.
2. Narcotic action: Alcohols as a class have narcotic action. The narcotic action of alcohols
increases with chain branching. The toxic character of liquid alcohols increases with
increasing molecular weight.
Exemplary Ethyl alcohol has a stimulant, followed by depression action on the CNS.
3. Solubility: The lower alcohols are completely soluble in water but as the number of carbon
atoms increases solubility decreases and the border line of water solubility and insolubility
occurs at 4 to 5 carbon atom.
Alcohols Methyl Ethyl n-Propyl n-Butyl n-Pentyl n-Hexyl
Solubility miscible miscible miscible 7.9gm 2.7gm .59gm
Why are alcohols soluble in water?
The solubility of alcohols in water in attributed to association of the two substances by
intermolecular hydrogen bonding. Due to its high electronegativity and small atomic volume, the
O atom in alcohol (R-O-H) has concentrated negative charge thereby polarizing the C-O & O-H
bonds. As a result, the H of OH group carries a partial positive charge and the O atom has a partial
negative charge. Water molecule is also polarized similarly.
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Obviously, the positive H atom of alcohol is attracted by the negative O atom of water molecule, &
H atom of this water molecule is attracted by O atom of another alcohol molecule. Thus a weak
electrostatic Hydrogen bond (shown dotted) is produced which causes the solubility of alcohol in
water.
*R is higher alkyl group.
The difference in water solubility of alcohols can be explained as follows.
 The OH group of alcohol confers/refers to polarity and water solubility of the alcohol molecule.
On the other hand, alkyl group R refers to nonpolarity and water insolubility.
 In lower alcohol the OH group is able to make it soluble, but in higher alcohol the alkane
portion increases, so the solubilizing effect of the OH group is gradually overwhelmed.
 In a higher alcohol like n-decyl alcohol the OH group becomes completely ineffective in
dragging a large alkane portion of the alcohol molecule into water structure.
4. Boiling points:
a) Alcohols have much greater boiling points than those of alkanes or most other compounds
having approximately the same molecular weight.
Compounds Formula Molecular weight Boiling point
n-Pentane CH3-CH2-CH2-CH3 72 36
Diethyl ether CH3-CH2-O-CH2-CH3 74 35
n-Propyl chloride CH3-CH2-CH2-Cl 78 46
n-Butyraldehyde CH3-CH2-CH2-CH2-CHO 72 76
n-Butyl alcohol CH3-CH2-CH2-CH2-OH 74 118
Higher boiling point of alcohols is attributed to the hydrogen bonding ability of alcohols.
The alcohol molecules are joined to each other through hydrogen bond formation. Although the
strength of hydrogen bond is much less than conventional chemical bonds, it is significant.
Thus the boiling points of alcohols are higher compared to those of alkanes and other compounds of
comparable molecular weight, due to the extra energy required to break the hydrogen bond.
b) The boiling point of alcohols increases regularly with the increase of carbon number.
Alcohols Methyl alcohol Ethyl alcohol n-Propyl alcohol n-Butyl alcohol
Boiling points 65 78 97 118
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The boiling points of alcohols increase with increase of molecular weight as heavier molecules
will fly off the liquid surface at a high temperature.
c) The boiling points of alcohols decrease with the increase of carbon branching.
Alcohols n-Butyl alcohol Isobutyl alcohol Sec-butyl alcohol Tert-Butyl alcohol
Boiling point 118 108 100 83
The polarity of O-H bond and hence the hydrogen bonding ability of alcohols is in the order
1ο
>2ο
>3ο
. That is why the B.P of alcohols decreases with branching.
Preparation of alcohols:
Alcohols can be prepared by the following general methods.
1. Hydration of Alkenes
2. Reduction of Carboxylic acids and esters
3. reduction of Carbonyl compounds
4. Hydrolysis of Alkyl halides.
1. Hydration of alkenes: Alcohols can be conventionally made from compounds containing
Carbon-Carbon double bonds in two ways.
(a) Oxymercuration-Demercuration /Oxymercuration-Reduction
(b) Hydroboration-Oxidation
In these processes, the addition of water to the double bond follows opposite orientation and hence,
the two methods neatly complement to each other.
a.) Oxymercuration-Demercuration/Oxymercuration-Reduction:
This process gives alcohols to Markovnikov addition of water to the carbon-carbon double
bond. Alkenes react with mercuric acetate in the presence of water to give mercurial compounds
which on reduction yields alcohols.
Oxymercuration involves the addition of –OH and –HgOAc followed by demercuration, in
which the –HgOAc is replaced by –H in the presence of NaBH4.
The reaction proceeds via the formation of a cyclic mercuranium ion.
The mercuranium ion is opened by the attack of water to complete the oxymercuration.
Demercuration is effected by a reduction using Sodium borohydride, NaBH4.
The reaction is highly regioselective and gives alcohols corresponding to markovnikov addition of
water to the –C=C- double bond.
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Mechanism for reaction of alkenes with Hg(OAc)2 &
H2O/Oxymercuration-demercuration reaction:
Step 1: The π electrons act as the Nucleophile with the electrophile
Hg. An acetate ion is lost as a leaving group, thus mercurinium ion.
Step 2: Water functions as a Nucleophile and attacks one of the
Mercury substituted carbons resulting in cleavage of the C-Hg bond.
Step 3: The acetate ion functions as a base deprotonating the
oxonium ion to give the alcohols. This completes the
oxymercuration part of the reaction.
Step 4: Hg is reduced off by the Hydride (NaBH4), creating a C-H
while breaking the C-Hg bond. This is the demercuration part of
the process.
Hydroboration-Oxidation of alkenes:
Alkenes undergo hydroboration with the reagent Borane
(BH3)/Diborane (B2H6) to yield alkylboranes which on oxidation
give alcohols.
Hydroboration involves the addition of BH3 to the double bond,
with the hydrogen becoming attached to one double bonded carbon
and Boron (BH2) to the other. The alkylborane then undergo
oxidation, in which the borane is replaced by –OH.
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 Borane reacts with alkenes via a concerted mechanism because of simultaneous making of
C-B and C-H bonds as C=C and B-H break.
 Electrophilic B atom adds at the least substituted end of the alkene.
 Regioselectivity: Anti-Markovnikov, since the B is the electrophile.
Mechanism of hydroboration:
The addition of BH3 to the alkene is a concerted reaction, with the multiple bond formation and
breaking occurring simultaneously, that is simultaneous making of C-B and C-H bonds as C=C
and B-H break.
In the transition state the carbon that is losing the π electrons becomes increasingly acidic and
begins to take that hydrogen with its electron pair, while boron as it gains the π electrons, is
increasingly willing to release that hydrogen. Boron and Hydrogen both add to the double bonded
carbon in the same transition state as follows:
H
│
CH3-CH=CH2 + B─H CH3-CHδ+
CH2 CH3-CHδ+
CH2
│
H
H─Bδ+
─H H----------Bδ+
─H
│ │
H H
Why boron adds to the lesser substituted carbon?
In borane, boron with only six electrons acts as an electrophile and should seek out the π
electrons of the doubl bond and began to attach itself to the carbon. The hydroborane will add to
the alkene so that boron always ends up on the lesser substituted carbon because
 In the transition state, carbon bears a partial positive charge (a partial carbocation).
 As a general rule, carbocations that are more substituted are more stable than lesser
substituted carbocations, which is electronically unfavourable.
 Hydroborane also tends to add to the lesser substituted carbon because it is sterically
hindered.
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Mechanism for reaction of alkenes with BH3/ Hydroboration-oxidation:
Step 1: It is a concerted reaction. The π electrons act as nucleophile with the electrophile B and the
H is transferred to the C.
Step 2: First step repeats twice more so that all of the B-H bonds react with C=C.
Step 3: Peoxide ion reacts as the nucleophile with the electrophilic B atom.
Step 4: Migration of C-B bond to form a C-O bond and displace hydroxide, stereochemistry of C
center is retained.
Step 5: Hydroxide attacks as a nucleophile with the electrophilic B displacing the alkoxide.
Step 6: An acid-base reaction occurs to form the alcohol.
2. Reduction of carboxylic acids & Esters: Carboxylic acids and esters are reduced to give primary
alcohols.
NaBH4 reduces esters slowly and does not reduce carboxylic acids; LiAlH4 reduces all carbonyl
groups.
3. Reduction of carbonyl compounds/Aldehydes & Ketones:
Aldehydes are reduced to give primary alcohols and Ketones are reduced to secondary alcohols.
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Example:
Sodium borohydride (NaBH4) is usually chosen as reagent for aldehyde and ketone reduction
because of its safety and ease of handling.
4. Hydrolysis of alkyl halides: Alkyl halides when treated with dilute aqueous solution of Sodium
or Potassium hydroxide, form alcohols.
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For example,
This method of preparing alcohols is not satisfactory because the alkyl halides may
simultaneously undergo dehydrohalogenation to produce alkenes.
To remove this difficulty, mild alkalis like moist Silver oxide or aqueous potassium carbonate is
used instead.
This method in used infrequently.
Grignard reagent: All three types of alcohols can be prepared from grignard reagent.
(i) Primary alcohols are obtained by the action of grignard reagent with
formaldehyde or with ethylene oxide.
(ii) Secondary alcohols are formed by treating the grignard reagent with aldehydes
other than formaldehyde.
(iii) Tertiary alcohols result by the reaction of grignard reagent with ketones.
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Fermentation of carbohydrates: Alcohols can be prepared by the fermentation of starches and
sugars under the influence of suitable microorganisms.
Reactions of alcohol:
The functional group of the alcohols is the hydroxyl group, -OH. This group has two reactive
covalent bonds, the C-O bond and the O-H bond. The electronegativity of oxygen is substantially
greater than that of carbon and hydrogen. Consequently, the covalent bonds of this functional
group are polarized so that oxygen is electron rich and both carbon and hydrogen are electrophilic.
 Unlike alkyl halides in which the halogen atom serves as a good leaving group, the OH
group in alcohols is a very poor leaving group.
 For an alcohol to undergo nucleophilic substitution, OH group must be converted into a
better eaving group. By using acid, OH
─
can be converted into H2O, a good leaving group.
Reaction of alcohols with Hydrogen halides:
When treated with HBr or HCl alcohols typically undergo a Nucleophilic substitution
reaction to generate alkyl halide and water.
Alcohol’s relative reactivity order: 30
>20
>10
>Methyl.
Hydrogen halide reactivity order: HI>HBr>HCl>HF (paralleling acidity order).
Reaction usually proceeds via an SN1 mechanism which proceeds via a carbocation
intermediate that can also undergo rearrangement.
Methanol and other primary alcohols will proceed via SN2 mechanism since these have
highly unfavorable carbocations.
Nucleophilic substitution reaction:
Nucleophilic substitution reaction is a fundamental class of substitution reaction in which an
“electron rich” nucleophile selectively bonds with or attacks the positive or partially positive
charge of an atom attached to a group or atom called the leaving group, the positive or partially
positive atom is referred to as an electrophile.
Nucleophilic substitution reactions occur when an electron rich species, nucleophile reacts
at/with an electrophilic saturated C atom attached to an electronegative group, the leaving group.
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Nu: + R-LG R-Nu + LG:
│ │
Nu
─
+ ─ Cδ+
─LGδ-
─ C ─Nu + LG
─
│ │
A nucleophile is an electron rich species that will react with an electron poor species.
A Leaving group, LG, is an atom (or a group of atoms) that is displaced as stable species taking
with it the bonding electrons.
SN1 reaction of alcohols (with HBr):
SN1 indicates a unimolecular, nucleophilic substitution reaction. In an SN1 reaction, there is a
loss of leaving group which generates an intermediate carbocation which then undergoes a rapid
reaction with the nucleophile.
SN1 reaction proceeds in two steps.
Step 1 is slow and therefore rate determining. In this step loss of leaving group generates a
carbocation intermediate.
In step 2 rapid attack of a nucleophile on the electrophilic carbocation occurs and a new σ bond
is formed.
In an SN1 reaction, the key step is the loss of the leaving group to form the intermediate
carbocation. The more stable the carbocation is the easier it is to formand the faster the SN1
reaction will be. The only event in the rate determining step of the SN1 is breaking of the C-LG
bond.
Reactivity order: (CH3)3C-> (CH3)2CH-> CH3CH2-> CH3-
Since the nucleophile is not involved in the rate determining step of an SN1 reaction, the nature
of the nucleophile is not important.
In the reactions of alcohols with HX, the reactivity trend of HI> HBr> HCl> HF is not due to the
nucleophilicity of the halide ion but the acidity of HX which is involved in generating the leaving
group prior to the rate determining step.
SN1 mechanism for reaction of alcohols (with HBr):
Step 1: Protontion of the alcoholic oxygen to make it a better leaving group. This step is very fast
and reversible. The lone pairs on the oxygen make it a lewis base.
Step 2: Cleavage of the C-O bond allows the loss of the good leaving group, a neutral water
molecule, to give a carbocation intermideate. This is the rate determining step (bond breaking is
endothermic).
Step 3: Attack of the nucleophile bromide ion on the electrophilic carbocation creates the alkyl
bromide.
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Fig-1: SN1 mechanism for reaction of alcohols
(with HBr)
Fig-2: SN1 mechanism for reaction of alcohols
(with HBr):
SN2 indicates a bimolecular substitution reaction. In an SN2 reaction, there is simultaneous
formation of the carbon-nucleophile bond and breaking of the carbon-leaving group bond, hence
the reaction proceeds via a transition state in which the central C is partially bonded to five groups.
Reactivity order: CH3-> CH3CH2-> (CH3)2CH-> (CH3)3C-
For alcohols reacting with HX, methyl and 10
systems are more likely to react via an SN2
reaction since the carbocations are too high energy for the SN1 pathway to occur.
In his nucleophilic substitution, a lone pair from a nucleophile attacks an electron deficient
electrophilic center and bonds to it, expelling another group called a leaving group.
Thus the incoming group replaces the leaving group in one step.
The nucleophile attacks the carbon at 1800
to the leaving group and the leaving group is then
pushed off the opposite side and the product is formed.
The rate of an SN2 reaction is second order, as the rate determining step depends on the
nucleophile concentration, [Nu-
] as well as the concentration of substrate [RX].
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Step 1: Protonation of the alcoholic oxygen to make it a better leaving group. This step is very fast
and reversible. The lone pairs on the oxygen make it a lewis base.
Step 2: Simultaneous formation of C-Br bond and cleavage of the C-O bond allows the loss of the
good leaving group, a neutral water molecule, to give the alkyl bromide. This is the rate
determining step.
Carbocation rearrangements:
When carbocation’s are intermediates, a less stable carbocation can rearrange to a more stable
carbocation by a shift of a hydrogen or an alkyl group. This is called a carbocation rearrangement.
The migrating group in a 1,2-shift moves with two bonding electrons, giving carbon a net
positive (+) charge.
Figure : Carbocation rearrangement
 Movement of a hydrogen atom is called a 1,2-hydride shift.
 Movement of an alkyl group is called a 1,2-alkyl shift.
A 1,2-shift converts a less stable carbocation to a more stable carbocation.
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Rearrangement product in the SN1 reaction of the secondary or tertiary alcohol:
Reaction of alcohols with other halogenating agents:
Halogenating agents besides hydrogen halides can convert alcohols into alkyl halides. Two
halogenating agents, Thionyl chloride (SOCl2) and Phosphorus bromide (PBr3) are commonly
used.
Primary and secondary alcohols can be converted to alkyl halides using SOCl2 and PBr3.
SOCl2 (thionyl chloride) converts alcohols into alkyl chlorides.
PBr3 (phosphorus tribromide) converts alcohols into alkyl bromides.
The reaction is often run in the presence of Triethylamine [(C2H5)3N] or pyridine (C6H5N).
In these reactions, the reagents undergo reaction with alcohols to form intermediate
inorganic esters and the resulting inorganic ester groups are good leaving groups that can be
displaced by the halide ions.
In each case the –OH reacts first as a nucleophile, attacking the electrophilic center of the
halogenating agent. A displaced halide ion then completes the substitution displacing the
leaving group.
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Conversion of alcohols to alkyl halides with SOCl2:
When a 10
or 20
alohol is treated with SOCl2 pyridine, an alkyl chloride is formed with HCl and
SO2 as byproducts.
The mechanism of this reaction consists of two parts:
a. Conversion of the OH group into a better leaving group &
b. Nucleophilic cleavage by Cl
─
via an SN2 reaction.
Mechanism: Reaction of ROH with SOCl2 and Pyridine occurs via an SN2 mechanism.
In step (1) and (2) the OH group is converted into a good leaving group.
Reaction of the alcohol with SOCl2 forms an intermediate that loses a proton by reaction with
pyridine in step (2).
The two step process conerts the OH group into OSOCl, a good leaving group, an also generates
the nucleophile (Cl
─
) needed for step (3).
In step (3) the C-O bond is broken as the C-Cl bond is formed.
Nucleophilic attack of Cl
─
and loss of the leaving group (SO2+Cl
─
) occur in a single step.
Conversion of alcohols to alkyl halides with PBr3:
Treatment of a 10
or 20
alcohol with PBr3 forms an alkyl bromide.
The mechanism of this reaction consists of two parts:
a. Conversion of the OH group into a better leaving group &
b. Nucleophilic cleavage by Cl
─
via an SN2 reaction.
Mechanism: Reaction of ROH with PBr3 is an SN2 mechanism.
In step (1) the OH group is converted into good leaving group.
Reaction of the alcohol with PBr3 converts the OH group into a better leaving group, and also
generates the nucleophile (Br
─
) needed for step (2).
I
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n step (2) the C-O bond is broken as the C-Br bond is formed.
Nucleophilic attack of Br
─
and loss of the leaving group (HOPBr2) occur in a single step.
Reactions of alcohols with Tosylates:
Unless the –OH group is converted into a better leaving group, alcohols are poor substrates for
substitution reactions. Protonation to convert the leaving group into H2O has limited utility as not
all substrates or nucleophiles can be utilized under acidic conditions without unwanted side
reactions.
An alternative is to convert the alcohol into a Tosylate, which has a much better leaving group
and will react with nucleophiles without the need for the acid. The advantage of this method is that
the substitution reactions are not under the strongly acidic conditions.
So tosylates are good substrates for substitution reactions because tosylates have a much
better leaving group.
Tosylates are used mostly for 10
and 20
alcohols (via SN2 reaction).
The –OH reacts first as a nucleophile, attacking the electrophilic center of tosylate,
displacing a Cl.
An alkyl tosylate is composed of two parts: 1. The alkyl group R, derived from an alcohol 2. The
tosylate (short from p-toluenesulfonate), which is a good leaving group.
Alcohols are converted to tosylates by treatment with p-toluenesulfonyl chloride (TsCl) in the
presence of pyridine. This process a poor leaving group (
─
OH) into a good one (
─
OTs). Tosylate is
a god leaving group because it’s conjugate acid, p-toluenesulfonic acid (CH3C6H4SO3H, TsOH) is a
strong acid.
Elimination reactions/Dehydration of alcohols:
Alcohols heated with strong acids catalysts (most commonly H2SO4, H3PO4), typically undergo
a 1,2-elimination reactions to generate an alkene and water. This is known as dehydration reaction
since it involves the removal of a molecule of water.
Dehydration of alcohols is a β elimination (1,2-elimination) reaction in which the elements of
the OH and H are removed from the α and β carbon atoms respectively.
Alcohol’s relative reactivity order: 3ο
>2ο
>1ο
. Therefore, more substituted alcohols dehydrate more
easily, giving rise to the following order of reactivity.
The rate of dehydration reflects the ease with which the carbocation is formed.
Secondary and tertiary alcohols will proceed via an E1 mechanism which proceeds via a
carbocation intermediate, which can often undergo rearrangement.
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Primary alcohols will proceed via an E2 mechanism since the primary carbocation is highly
unfavourable.
When an alcohol has two or three β carbons, dehydration is regioselective and follows the
Zaitsev’saytzeff’s/Saytsev’s rule. The more substituted alkene is the major product when a
mixture of isomers is possible.
Zaitsev’s/Saytzeff’s/Saytsev’s rule:
It is a rule named after Alexander Mikhailovich Zaitsev that states that if more than one alkene
can be formed by an elimination reaction, the more stable alkene is the major product.
When an alcohol has two or three β carbons, a mixture of isomers is possible and the more
substituted alkene is the major product because the compound that has a morew highly substituted
C=C double bond is more stable due to the electron donating properties of the alkyl group.
The major product will be the most highly substituted alkene, the product with the fewest H
substituents on the double bonded carbons.
Mechanism of dehydration reaction of 2ο
and 30
alcohols by an E1 mechanism:
Step (1): The O atom is protonated.
Protonation of the oxygen atom of the alcohol converts a poor leaving group (-OH) into a good
leaving group (H2O).
Step (2): The C-O bond is broken.
Heterolysis of the C-O bond forms a carbocation. This step is rate determining because it involves
only bond cleavage.
Step (3): A C-H bond is cleaved and the π bond is formed.
A base (such as HSO4
-
or H2O) removes a proton from a carbon adjacent to the carbocation (a β
carbon). The electron pair in the C-H bond is used to form the new π bond.
Mechanism of dehydration reaction of primary alcohols by an E2 mechanism:
Since 10
carbocations are highly unstable, their dehydration cannot occur by an E1 mechanism
involving a carbocation intermediate. Therefore, 10
alcohols undergo dehydration following an E2
mechanism.
Step (1): The O atom is protonated.
Protonation of the oxygen atom of the alcohol converts a poor leaving group (-OH) into a good
leaving group (H2O).
Step (2): The C-H and C-O bonds are broken and the π bond is formed.
Two bonds are broken and two bonds are formed in a single step. The base (HSO4
─
or H2O)
removes a proton from the β carbon. The electron pair in the β C-H bond forms the new π bond.
The leaving group (H2O) comes off with the electron pair in the C-O bond.
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Carbocation rearrangement during dehydration of alcohols:
Synthesis of Esters by reaction with carboxylic acids:
Alcohols react with carboxylic acid in the presence of an acid catalyst to form esters. This reaction
is also known as the Fischer esterification.
Alcohol reactivity order: CH3OH> 10
> 20
> 30
Esterification is the general name for a chemical reaction in which two chemicals (typically an
alcohol and an acid) form an ester as the reaction product.
Esters are obtained by refluxing the parent carboxylic acid with the appropriate alcohol with an
acid catalyst.
Mechanism:
Step 1: The Ethanoic acid takes a proton (a hydrogen ion) from the concentrated sulphuric acid.
The proton becomes attached to one of the lone pairs on the oxygen which is double-bonded to the
carbon.
Step 2: The positive charge on the carbon atom is attacked by one of the lone pairs on the oxygen of
the Ethanol molecule.
Step 3: Deprotonation of the alcoholic oxygen. A proton gets transferred to make the –OH leave, so
convert it into a good leaving group by protonation.
Step 4: A molecule of water is lost. The electrons of an adjacent oxygen help to “push out” the
leaving group, a water molecule.
Step 5: Proton is removed from the oxygen by reaction with the HSO4
─
(hydrogensulphate) ion
which was formed in the first step. The ester is formed, and the sulphuric acid catalyst is
regenerated.
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Oxidation of alcohols:
Oxidation reactions of alcohols depend on the substituents on the carbinol carbon. In order for
each oxidation step to occur, there must be a α H on the carbinol carbon.
i) Primary alcohols can be oxidized to aldehydes or further to carboxylic acids.
ii) Secondary alcohols can be oxidized to ketones but it can’t be oxidized further.
iii) Tertiary alcohols can not be oxidized (since there is no carbinol C-H/ no α H).
Chromium (Cr) oxidation of alcohols:
In the oxidation reaction, the alcohol reacts to form a Chromate ester. A base (here a water
molecule) abstracts a proton from the chromate ester. The C=O bond forms and a Cr species
leaves. This demonstrates the importance of the carbinol H to this mechanism.
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Alcohols:Organic Chemistry MANIK

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