Alcohols.ppt

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Alcohols
• Alcohols contain a hydroxy group (OH) bonded to an sp3
hybridized carbon.
Introduction—Structure and Bonding
CH3 CH
CH3
CH2OH

Alcohols
• Compounds having a hydroxy group on a sp2 hybridized
carbon—enols and phenols—undergo different reactions
than alcohols.

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• The oxygen atom in alcohols, ethers and epoxides is sp3
hybridized. Alcohols and ethers have a bent shape like
that in H2O.
• The bond angle around the O atom in an alcohol or ether
is similar to the tetrahedral bond angle of 109.5°.
• Because the O atom is much more electronegative than
carbon or hydrogen, the C—O and O—H bonds are all
polar.
Alcohols

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Nomenclature of Alcohols
Alcohols

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• When an OH group is bonded to a ring, the ring is numbered
beginning with the OH group.
• Because the functional group is at C1, the 1 is usually omitted
from the name.
• The ring is then numbered in a clockwise or counterclockwise
fashion to give the next substituent the lowest number.
Figure 9.2
Examples: Naming
cyclic alcohols
Alcohols

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Alcohols
• Common names are often used for simple alcohols. To
assign a common name:
Name all the carbon atoms of the molecule as a single
alkyl group.
Add the word alcohol, separating the words with a
space.

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Alcohols
• Compounds with two hydroxy groups are called diols or
glycols. Compounds with three hydroxy groups are
called triols and so forth.

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Alcohols
• Alcohols have dipole-dipole interactions because they have a
bent structure with two polar bonds.
• Alcohols are capable of intermolecular hydrogen bonding. Thus,
alcohols are polar
Physical Properties
• Steric factors affect hydrogen bonding.

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Alcohols
Interesting Alcohols
Figure 9.3
Ethanol—The alcohol in
alcoholic beverages

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Alcohols
Interesting Alcohols
Figure 9.4
Some simple alcohols

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Alcohols
METHODS OF PREPARING ALCOHOLS
Alcohols can be prepared in two major ways. These are
industrial preparation which we shall refer to as Manufacture and
also Laboratory preparation. Manufacturing will be considered
first.
A. Manufacture of Alcohols
(i) From petroleum raw material - Methanol
Nowadays, petroleum sources provide the raw materials for the
manufacture
of well over 80% of these alcohols, the rest being manufactured
by the ancient methods of fermentation of starchy product and
the distillation of wood and coal.

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A. Manufacture of Alcohols (contd.)
The major process for the production of methanol involves
passing a mixture of two volumes of hydrogen and one volume
of carbon - monoxide (synthesis gas) over a zinc (II) oxide,
chromium oxide catalyst at about 300oc and about 300
atmospheres pressure.
Alcohols

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(ii) Manufacture of Ethanol by Fermentation
• Known to man from earliest times.
• the principal active ingredient of beers, wines
• fruits or starchy materials such as wheat, barley, cassava,
grapes, yam, apples, corn, rice, potatoes bananas and
plantains to ferment.
The fermentation is carried out by the addition of yeast to
a slurry of the raw material in water. Yeast contains
biological catalysts called enzymes such as maltase and
zymase.
• Ethanol produced by this method must be properly distilled
to remove primary alcohols and fused oil that are the bye-
products.

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(ii) Manufacture of Ethanol by Fermentation

Methods of Preparation
Alcohols
R OH
Alkyl Halides
Alkenes
HC CH2
R
R X
Ethers
R OR'
Ketones
Aldehydes
R C
O
R'
O
R
Epoxides
Carboxylic Acids
R C
O
OH
Acid chlorides
R C
O
Cl
Esters
R C
O
OR'
Alcohols can be formed from many, many different sources. More than 7
different functional groups can be converted to an alcohol.

METHODS OF PREPARATION
Hydrolysis of alkyl halides
 The term hydrolysis generally means the
introduction of a hydroxyl group
 into a molecule, usually in the place of some other
functional group. When
 an alkyl halide is treated with boiling aqueous alkali
or a suspension of silver
 oxide in boiling water, the halide functional group is
replaced by a hydroxyl group converting it to an
alcohol

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Alcohols
Preparation of Alcohols
Reduction of Aldehydes, Ketones, Esters and Acids
When aldehydes, ketones, esters and acid are treated with
appropriate reducing agents the corresponding alcohols are
formed.
• The traditional reducing agents include Na in ethanol or Na/Hg
in ether or alcohol and molecular H2 in the presence of a catalyst
such as Pt or Pd or Ni.
• However the complex hydrides of Group 1 and 3 metals e.g.,
lithum tetrahydro aluminate (LiAlH4) and sodium hydroborate
(NaBH4) are now commonly used. (in a dry solvent e.g.
Ether) excess of the hydride is destroyed by the drop wise
addition of cold water with vigorous stirring.

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Alcohols
Reduction of Aldehydes, Ketones, Esters and Acids
When aldehydes, ketones, esters and acid are treated with
appropriate reducing agents the corresponding alcohols are
formed.
• The traditional reducing agents include Na in ethanol or Na/Hg
in ether or alcohol and molecular H2 in the presence of a catalyst
such as Pt or Pd or Ni.
• However the complex hydrides of Group 1 and 3 metals e.g.,
lithum tetrahydro aluminate (LiAlH4) and sodium hydroborate
(NaBH4) are now commonly used. (in a dry solvent e.g.
Ether) excess of the hydride is destroyed by the drop wise
addition of cold water with vigorous stirring.

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• Lithium aluminum hydride LiAlH4 and and sodium borohydride
NaBH4reduce carbonyl groups without affecting carbon -
carbon double bonds,
• NaBH4 is a milder reducing agent and will not reduce acids and
esters.
• Aldehydes and acids give primary alcohol, e.g.

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Esters give two alcohols, one of which coming from the acid,
this portion is a primary alcohol while the other is from the
alcohol from which it was made,

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(iii) Using a Grignard Reagent
This method is used for the preparation of the three classes of
alcohols. In this method, reagents RMgX - Grignard reagents
are reacted with carbonyl compound such as aldehydes,
ketones, esters and epoxides to form organometallic
intermediates which are hydrolysed to alcohol.
SYNTHESIS OF 1° ALCOHOLS: Methanal gives primary
alcohol.
Grignard + formaldehyde yields a primary alcohol with one additional
carbon.
C O
H
H
C
CH3
H3C CH2 C MgBr
H
H
H
CH3 CH
CH3
CH2 CH2 C
H
H
O MgBr
HOH
CH3 CH
CH3
CH2 CH2 C
H
H
O H

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SYNTHESIS OF 2º ALCOHOLS : Alkanal gives secondary alcohol.
Grignard + aldehyde yields a secondary alcohol.
MgBr
CH3 CH
CH3
CH2 CH2 C
CH3
H
O
C
CH3
H3C CH2 C MgBr
H
H
H
C O
H
H3C
CH3 CH
CH3
CH2 CH2 C
CH3
H
O H
HOH

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SYNTHESIS OF 3º ALCOHOLS : Alkanal gives secondary alcohol.
Grignard + ketone yields a tertiary alcohol.
MgBr
CH3 CH
CH3
CH2 CH2 C
CH3
CH3
O
C
CH3
H3C CH2 C MgBr
H
H
H
C O
H3C
H3C
CH3 CH
CH3
CH2 CH2 C
CH3
CH3
O H
HOH

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SYNTHESIS OF 3º ALCOHOLS : Ester gives tertiary alcohol.
Esters will give tertiary alcohol with two molecules of the Grignard reagent.
The first molecule will convert the ester to a ketone and the second
molecule of Grignard reagent will react to give a tertiary alcohol.

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SYNTHESIS OF 1º ALCOHOLS : Epoxides gives tertiary alcohol.
Epoxides: Epoxides could also be made to react with Grignard reagent to
produce a primary alcohol with two carbon atoms added..

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(iv) Hydroboration / Oxidation of compounds containing
carbon - carbon double bond
This reaction occurs in two steps. Step one is the hydroboration of the carbon -
carbon and the second step involves the oxidation using hydrogen peroxide in
alkaline medium.
This reaction gives predominant products corresponding to anti-Markownikoff's
orientation - that is the hydroxyl group is directed to the carbon with more hydrogen
atoms attached to it.

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(iv) Hydroboration / Oxidation of compounds containing
carbon - carbon double bond
This reaction occurs in two steps. Step one is the hydroboration of the carbon -
carbon and the second step involves the oxidation using hydrogen peroxide in
alkaline medium.
This reaction gives predominant products corresponding to anti-Markownikoff's
orientation - that is the hydroxyl group is directed to the carbon with more hydrogen
atoms attached to it.

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PROPERTIES OF ALCOHOLS
A. Physical Properties of Alcohols

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B. Chemical Properties of Alcohols
The chemical properties of an alcohol are determined by its
functional group-OH.
Reactions of alcohol can involve breakage of either
(i) C - OH bond with removal of OH group
(ii) O - H bond with removal of H.
Both involve substitution of OH or elimination of H in which a
double bond is formed.
Alcohols show amphoteric behaviour i.e. act as acid and bases.

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The alcohols with general formula
(i) R - OH can be considered as derivatives of water H - OH:
Like water they show both acidic and basic characters.
An alcohol as an acid
• releases a proton plus RO- alkoxide ion (conjugate base)
Acidity depends on how well the conjugate base can
accommodate the negative charge i.e. any factor that
stabilizes the RO- more than it stabilizes R - OH should
increase acidity and vice - versa.
• Electron withdrawing substituents on R should disperse the
negative charge, stabilize RO and this increases acidity.
• Electron releasing substituent on R, should disperse the
negative charge, to - destabilize the anion and thus
decrease acidity.

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The acidity of alcohol is shown by their reaction with active
metal to give hydrogen and metal alkoxide.
An alcohol as a base accepts protons using a lone pair of
electrons on oxygen.
It reacts most rapidly with HI and HCl requires presence of
ZnCl2 as catalyst for the reaction with primary and secondary
alcohol. This reaction can actually be used to identify
different classes of alcohol using conc. HCl
/ ZnCl2 - known as Lucas reagent.

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REACTION WITH HCL
Chloride is a weaker nucleophile than
bromide.
Add ZnCl2, which bonds strongly with
–OH, to promote the reaction.
The chloride product is insoluble.
Lucas test: ZnCl2 in concentrated HCl:
1° alcohols react slowly or not at all.
2 alcohols react in 1-5 minutes.
3 alcohols react in less than 1 minute.

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LUCAS REAGENT

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Reaction with inorganic halides:
The hydroxyl group of the alcohol may be replaced by a
halogen atom using an inorganic halide such
as phosphorus pentachloride or sulphur dichloride oxide
also called thionyl chloride. In the case of thionyl chloride
reaction, the two bye-products are gases leaving the product
alone in the flask.

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Reaction with inorganic halides:
The hydroxyl group of the alcohol may be replaced by a
halogen atom using an inorganic halide such
as phosphorus pentachloride or sulphur dichloride oxide
also called thionyl chloride. In the case of thionyl chloride
reaction, the two bye-products are gases leaving the product
alone in the flask.

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Esterification: when an alcohol is refluxed with a carboxylic acid,
an, ester is slowly formed.
This reaction is carried out in the presence of a catalyst
concentrated sulphuric acid or dry hydrogen chloride to remove
the water and pushes the equilibrium to the right.
Isotopic experiments have established that the overall reaction
involves loss of a hydrogen atom from the alcohol and not
the hydroxyl group.

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iv) Oxidation : This can be used to distinguish the three
classes of alcohols. Oxidants commonly used include acidic
potassuim permanganate, Jones reagent (Na2Cr2O7/H+),
Ca/300oC, Cr(VI)O3 in pyridine.
Primary alcohols are oxidised to aldehyde and prolonged
oxidation results in the formation of the carboxylic acid having
the same number of carbon atoms as the primary alcohol.
CH3CH2CH2CH2
OH N H CrO3Cl
CH3CH2CH2CH
O
1° alcohol to aldehyde to carboxylic acid
Difficult to stop at aldehyde
Use pyridinium chlorochromate (PCC) to limit the oxidation.

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Secondary alcohol is oxidised to ketone. Further oxidation leads
to oxidative degradation. As a result of this, it is converted to a
mixture of carboxylic acid containing fewer carbon atoms than
the original ketone

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Tertiary alcohols are generally resistant to oxidation especially
under neutral or alkaline conditions. In acidic solution, however
they also undergo oxidation degradation to give a mixture of acids
and ketones.

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Dehydration
• Dehydration, like dehydrohalogenation, is a  elimination
reaction in which the elements of OH and H are removed from
the  and  carbon atoms respectively.
• Dehydration is typically carried out using H2SO4 and other
strong acids, or phosphorus oxychloride (POCl3) in the
presence of an amine base.

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• Typical acids used for alcohol dehydration are H2SO4 or p-
toluenesulfonic acid (TsOH).
• More substituted alcohols dehydrate more easily, giving rise
to the following order of reactivity.

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• Secondary and 3° alcohols react by an E1 mechanism,
whereas 1° alcohols react by an E2 mechanism.

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Carbocation Rearrangements
• Often, when carbocations are intermediates, a less stable
carbocation will be converted into a more stable carbocation
by a shift of a hydrogen or an alkyl group. This is called a
rearrangement.
• Because the migrating group in a 1,2-shift moves with two
bonding electrons, the carbon it leaves behind now has only
three bonds (six electrons), giving it a net positive (+) charge.

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(vii) The lodoform / Haloform Reaction Alcohols which contain
the group
Such as ethanol and propan-2-ol give a pale yellow precipitate
of tri-iodomethane - iodoform (CHI3 when warmed with a
mixture of potassium iodide) and excess sodium chlorate
(sodium hypochlorite) solution or with iodine and sodium
hydroxide solution. This is because under the mild oxidising
condition this type of alcohol group is converted to
CH3C=O which can then undergo iodination and cleavage,
giving tri-iodomethane as a product.

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(vii) The lodoform / Haloform Reaction Alcohols which contain
the group

Alcohols.ppt

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