Overview of Alcohols: Classification, Preparation, and Properties
Anik Sinha
Department of Chemistry
S.B.S.Government collge, Hili
B.Sc 2
nd
Year(General)
Overview
Definition of alcohol.
Classification of alcohols.
Preparations of alcohols.
Reactions of alcohols.
Preparations of Ethers
Properties of Ethers.
Reference.
Definition of Alcohol:
An alcohol is a compound that contains one or more
hydroxyl
groups, i.e., alcohols are hydroxy derivatives of alkanes.
Classifications of alcohols
On the basis of type of chain
Aliphatic alcohols:
When the hydroxyl group (-OH group) of an alcohol is attached to an
aliphatic carbon chain (open chain), the alcohol is called an aliphatic
alcohol.
CH
3
OH
(Methanol)
Butane-2,3-diol
Example:
Aromatic alcohols:
When the hydroxyl group is attached to the carbon chain which is
attached to an aromatic ring, the alcohol is known as aromatic alcohol
.
Example:
Phenyl Methanol 2-Phenyl Methanol
On the basis of number of hydroxyl groups present
Monohydric
,
dihydric
,
trihydric
and
polyhydric
alcohols and Phenols contain 1, 2, 3
and more than three hydroxyl group respectively.
Monohydric Dihydric Trihydric Polyhydric
Alcohol Alcohol Alcohol Alcohol
Monohydric Dihydric Trihydric Polyhydric
Phenol Phenol Phenol Phenol
On the basis the hybridization of the carbon attached with hydroxyl group
(i) C
Sp3
-OH bonded alcohols:
(a) Primary (1
0
),Secondary (2
0
),tertiary (3
0
) alcohols:
If the hydroxyl group is attached to a primary carbon, it is called
primary alcohol
.
Similarly, secondary and tertiary alcohols are attached to secondary and tertiary
carbons respectively.
Primary (1
0
) Secondary (2
0
) tertiary (3
0
)
alcohol alcohol alcohol
(b)Allylic alcohol:
In theses type of alcohols –OH groups are attached with allylic carbon
atom.
Allylic group
Sp
2
Sp
2
Sp
3
(c) Benzylic alcohol:
In theses alcohol –OH group attached with Benzylic group.
(Sp
3
)
(ii) C
Sp2
-OH bonded alcohols:
In theses type of alcohols –OH groups are attached with doubly
bonded carbon atom. Theses are called
Vinyl alcohols
.
Vinyl alcohols
Identification of primary, secondary, tertiary alcohol by Victor-Meyar method
Gives
blood red
colour
with alkali
Gives
blue colour
with alkali
No reaction
with alkali
Lucas Method for identification of alcohols
Turbidity appears
instantly
Turbidity appears
after 5 minutes
No turbidity
appears
Primary alcohol
Secondary alcohol
Tertiary alcohol
Mixture of Anhydrous ZnCl
2
and HCl called Lewis Reagent.
Preparation of Alcohols
1
. From Haloalkanes:
Δ
Δ
2.From Alkenes:
Ease of preparation is tert>Sec>prim. This method is best for the preparation of
secondary alcohols.
3.By Hydroboration-Oxidation:
4.Oxymercuration-Demercuration of Alkenes:
5. From Carbonyl compounds:
R-CHO RCH
2
OH
H
2
/Ni
or Pt or Pd
R-CHO RCH
2
OH
Na/C
2
H
5
OH
R-CHO RCH
2
OH
NaBH
4
LiAlH
4
6. From carboxylic acid
:
R-COOH RCH
2
OH
LiAlH
4
/Ether
H
3
O
+
7.From Ester:
R-COO R’ RCH
2
OH+R’OH
Na/C
2
H
5
OH
CH
3
COO
CH
3
CH
3
CH
2
OH+
CH
3
OH
Na/C
2
H
5
OH
Bouveault–Blanc
reduction
8.From Grignard reagent:
(i)
Grignard Reagents
react with epoxides to
yield primary alcohols
containing two or
more carbon atoms
(ii)
With
aldehyde
Grignard regent gives
Secondary alcohol.
(iii
)
With
Ketones
,
Grignard reagents
gives tertiary
alcohols
Properties of Alcohols
1
.
Smell and Order:
The lower members are liquids and have a distinctive smell where
higher members are solids and almost odorless.
2. Boiling Points:
Because of hydrogen bonding, alcohols tend to have higher boiling
points than comparable hydrocarbons and ethers of similar molecular weight.
For example, the
boiling point of butyl alcohol is 118°C
whereas the
boiling point
of the isomeric diethyl ether is 36°C.
H-bonding
In Ethanol
3.Solublity:
Alcohols with shorter carbon chains are usually more soluble than those
with longer carbon chains because the increasing size of the non polar chain
disrupts the hydrogen bonding network. That is why alcohols are much
more soluble in water than their corresponding alkanes, aromatic
hydrocarbons, alkyl halides or aryl halides. Among isomeric alcohols, the
solubility increases with branching.
CH
3
CH
2
OH
CH
3
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
OH
Soluble in Water
Insoluble in Water
Home Task:
B.P
118
0
C 108
0
C 83
0
C
M.Wt 74 74 74
Compare the
boiling points
among
isomeric
alcohols?
4.Viscosity:
Viscosity of small alcohols are much higher than the viscosity of alkanes.
5.Density:
Generally alcohols are lighter than water, i.e., less dense than water. Density of
alcohols increases with molecular mass.
Chemical reactions of alcohols
1. Acylation of alcohols:
When alcohol reacts with acyl halide and anhydride substitution of
hydrogen atom by acyl group is known as acylation of alcohols.
2. Dehydration:
Alcohols undergo dehydration to form unsaturated hydrocarbon on treating with
a protic acid e.g., con. H2SO4or H3PO4, or catalysts such as anhydrous ZnCl2or Al2O3. In this
reaction the OH and an H groups removes from an adjacent carbons. Since water is removed
from the alcohol, this reaction is known as a
dehydration reaction.
For primary alcohols, the
conditions required are
conc. sulphuric acid and
temperature of 170
0
C.
Mechanism
Secondary alcohols dehydrate
under milder conditions than
primary alcohols.
Tertiary alcohols dehydrate
under even milder conditions.
The main function of the acid is to transform the poor leaving group –OH into the very
good leaving group -OH
2.
The order of the relative ease of dehydration of alcohols is:
3
0
>2
0
>1
0
Dehydration of secondary and tertiary alcohols containing more than three
carbon atoms will give a mixture of alkenes, the major product can be determined from
Saytzeff’s rule
.
Saytzeff’s rule:
When an alkene is
produced in an
elimination reaction,
the major product is
the one with the more
highly substituted
double bond
.
3.Halogenation:
Alcohols can be converted to alkyl halides using one of three
reactions.
Reactions with
Hydrogen Halide.
Reaction with
thionyl chloride
Reactions with
phosphorous
halides
4.Iodoform Reaction:
When Iodine and sodium hydroxide are added to alcohol that contain CH
3
CH(OH)-
Group a pale yellow precipitate of iodoform or tri iodomethane is formed. This is called
iodoform reactions of alcohol.
Ethanol is the only primary alcohol that gives iodoform test.
Home Task:
Differenciate with chemical reaction: 1. Ethanol and Methanol
2. 1-Propanol and 2-Propanol
3. 1-Phenyl Ethanol and 2-Phenyl Ethanol
5.Esterification of alcohol:
Conversion of Higher
alcohol to lower alcohol
Ethanol Methanol
Conversion of Lower
alcohol to Higher alcohol
Methanol Ethanol
Conversion:
Ethers
Preparations of Ethers
1.By Dehydration of Alcohols:
N.B.
at higher temperature
alkene are formed instead
of Ether.
2.By alkyl Halides:
2 AgBr
2 AgBr
3.Williamson synthesis of ethers:
This method involves treatment of an alkyl halide with sodium or potassium salt of
alcohol or phenol. This reaction involves a nucleophilic substitution of halide ion by
alkoxide ion(S
N
2 mechanism).
(
S
N
2 mechanism
)
If a tertiary alkyl halide is used
,an alkene is the only reaction
product because
alkoxide then
act as a strong base and
elimination favours over
substitution.
4. From Acyl chloride:
Friedel-Craft
Reaction
5.From Grignard reagent:
Properties of Ether
(i) Physical state:
The lower members are gases or volatile liquids and their vapours are
highly inflammable
(ii) Boiling points:
Boiling points of ethers are much less than isomeric alcohols due to
the absence of H-bonding.
(iii) Solubility:
All ethers are less dence than water in which they are not very soluble
but solubility increased with adding alcohol due to the formation of H-
bond.
Chemical reactions of Ethers
1. Reactions with Lewis acid:
Being Lewis base ethers
forms complexes with
lewis acid which are called
Etherates
.
2. Action of Hydroiodic acid:
(a)
With cold HI:
(b)
With Hot HI:
+
3. Action of PCl
5
:
4. Dehydration:
Reference
1.Organic Chemistry by I.L.Finar
2.Organic Chemistry by Morrison and Boyd.
3.Advance organic chemistry by bahl & bahl.
4.Organic Chemistry by Brucee.
5.Organic chemistry by R.L.Madan
6.A Guidebook to mechanism in Organic
chemistry by P.sykes.
7.Wikipidea.
Thank
You
Alcohols are compounds containing hydroxyl groups, classified based on chain type and number of hydroxyl groups. They can be aliphatic or aromatic, monohydric or polyhydric. Classification also includes primary, secondary, and tertiary alcohols based on carbon hybridization with the hydroxyl group. Different methods like Victor-Meyer test can identify their types. Understanding alcohol properties and reactions is essential in the study of chemistry.
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Presentation Transcript
B.Sc 2nd Year(General) Anik Sinha Department of Chemistry S.B.S.Governmentcollge, Hili
Overview Definition of alcohol. Classification of alcohols. Preparations of alcohols. Reactions of alcohols. Preparations of Ethers Properties of Ethers. Reference.
Definition of Alcohol: An alcohol is a compound that contains one or more hydroxyl groups, i.e., alcohols are hydroxy derivatives of alkanes. Classifications of alcohols On the basis of type of chain Aliphatic alcohols: When the hydroxyl group (-OH group) of an alcohol is attached to an aliphatic carbon chain (open chain), the alcohol is called an aliphatic alcohol. CH3 H C H C CH3 OH OH Example: CH3OH (Methanol) Butane-2,3-diol Aromatic alcohols:When the hydroxyl group is attached to the carbon chain which is attached to an aromatic ring, the alcohol is known as aromatic alcohol. H2 C H2C OH H2C OH Example: Phenyl Methanol 2-Phenyl Methanol
On the basis of number of hydroxyl groups present Monohydric, dihydric, trihydric and polyhydric alcohols and Phenols contain 1, 2, 3 and more than three hydroxyl group respectively. H C H C H C H C H2 C H2 C H C CH3 H2C CH2 H2 C HO OH H2 C HO OH OH OH OH OH OH OH OH OH Monohydric Dihydric Trihydric Polyhydric Alcohol Alcohol Alcohol Alcohol OH OH OH OH OH OH OH OH OH OH OH OH Monohydric Dihydric Trihydric Polyhydric Phenol Phenol Phenol Phenol
On the basis the hybridization of the carbon attached with hydroxyl group (i) CSp3-OH bonded alcohols: (a) Primary (10),Secondary (20),tertiary (30) alcohols: If the hydroxyl group is attached to a primary carbon, it is called primary alcohol. Similarly, secondary and tertiary alcohols are attached to secondary and tertiary carbons respectively. CH3 CH3 H C CH3 OH C H OH C H3C OH CH3 CH3 H Primary (10) Secondary (20) tertiary (30) alcohol alcohol alcohol (b)Allylic alcohol: In theses type of alcohols OH groups are attached with allylic carbon atom. H2 C C H Allylic group CH2 OH Sp2 Sp2 Sp3
(c) Benzylic alcohol: In theses alcohol OH group attached with Benzylic group. (Sp3) H2C OH (ii) CSp2-OH bonded alcohols:In theses type of alcohols OH groups are attached with doubly bonded carbon atom. Theses are called Vinyl alcohols. H Vinyl alcohols C C H OH H Identification of primary, secondary, tertiary alcohol by Victor-Meyar method No reaction with alkali Gives blood red colour with alkali Gives blue colour with alkali
Lucas Method for identification of alcohols No turbidity appears Primary alcohol Turbidity appears after 5 minutes Secondary alcohol Turbidity appears instantly Tertiary alcohol Mixture of Anhydrous ZnCl2 and HCl called Lewis Reagent.
Preparation of Alcohols 1. From Haloalkanes: 2.From Alkenes: Ease of preparation is tert>Sec>prim. This method is best for the preparation of secondary alcohols. 3.By Hydroboration-Oxidation:
4.Oxymercuration-Demercuration of Alkenes: 5. From Carbonyl compounds: H2/Ni R-CHO RCH2OH or Pt or Pd Na/C2H5OH R-CHO RCH2OH NaBH4 LiAlH4 R-CHO RCH2OH
6. From carboxylic acid: LiAlH4/Ether R-COOH RCH2OH H3O+ 7.From Ester: Na/C2H5OH Bouveault Blanc reduction R-COO R RCH2OH+R OH Na/C2H5OH CH3COOCH3CH3CH2OH+CH3OH 8.From Grignard reagent: Grignard react with epoxides to yield primary alcohols containing more carbon atoms Reagents (i) two or aldehyde With Grignard regent gives Secondary alcohol. (ii)
Ketones, With Grignard reagents gives alcohols (iii ) tertiary Properties of Alcohols 1. Smell and Order: The lower members are liquids and have a distinctive smell where higher members are solids and almost odorless. 2. Boiling Points: Because of hydrogen bonding, alcohols tend to have higher boiling points than comparable hydrocarbons and ethers of similar molecular weight. H-bonding In Ethanol For example, the boiling point of butyl alcohol is 118 C whereas the boiling point of the isomeric diethyl ether is 36 C.
3.Solublity: Alcohols with shorter carbon chains are usually more soluble than those with longer carbon chains because the increasing size of the non polar chain disrupts the hydrogen bonding network. That is why alcohols are much more soluble in water than their corresponding alkanes, aromatic hydrocarbons, alkyl halides or aryl halides. Among isomeric alcohols, the solubility increases with branching. CH3CH2OH Soluble in Water CH3CH2CH2CH2CH2CH2CH2CH2OH Insoluble in Water Home Task: Compare the boiling points among isomeric alcohols? B.P 1180C 1080C 830C M.Wt 74 74 74 4.Viscosity: Viscosity of small alcohols are much higher than the viscosity of alkanes. 5.Density: Generally alcohols are lighter than water, i.e., less dense than water. Density of alcohols increases with molecular mass.
Chemical reactions of alcohols 1. Acylation of alcohols: When alcohol reacts with acyl halide and anhydride substitution of hydrogen atom by acyl group is known as acylation of alcohols. 2. Dehydration: Alcohols undergo dehydration to form unsaturated hydrocarbon on treating with a protic acid e.g., con. H2SO4or H3PO4, or catalysts such as anhydrous ZnCl2or Al2O3. In this reaction the OH and an H groups removes from an adjacent carbons. Since water is removed from the alcohol, this reaction is known as a dehydration reaction. Mechanism For primary alcohols, the conditions required conc. sulphuric acid and temperature of 1700C. are
Secondary alcohols dehydrate under milder conditions than primary alcohols. Tertiary under even milder conditions. alcohols dehydrate The main function of the acid is to transform the poor leaving group OH into the very good leaving group -OH2. The order of the relative ease of dehydration of alcohols is: 30>20>10 Dehydration of secondary and tertiary alcohols containing more than three carbon atoms will give a mixture of alkenes, the major product can be determined from Saytzeff s rule. Saytzeff s rule: When an alkene is produced elimination the major product is the one with the more highly double bond . in reaction, an substituted
3.Halogenation: Alcohols can be converted to alkyl halides using one of three reactions. Reactions with Hydrogen Halide. Reaction with thionyl chloride Reactions with phosphorous halides 4.Iodoform Reaction: When Iodine and sodium hydroxide are added to alcohol that contain CH3CH(OH)- Group a pale yellow precipitate of iodoform or tri iodomethane is formed. This is called iodoform reactions of alcohol. Ethanol is the only primary alcohol that gives iodoform test.
Home Task: Differenciate with chemical reaction: 1. Ethanol and Methanol 2. 1-Propanol and 2-Propanol 3. 1-Phenyl Ethanol and 2-Phenyl Ethanol 5.Esterification of alcohol: Conversion: Conversion alcohol to lower alcohol Ethanol Methanol of Higher Conversion alcohol to Higher alcohol Methanol Ethanol of Lower
Ethers Preparations of Ethers 1.By Dehydration of Alcohols: N.B. at higher temperature alkene are formed instead of Ether. 2.By alkyl Halides: 2 AgBr 2 AgBr 3.Williamson synthesis of ethers: This method involves treatment of an alkyl halide with sodium or potassium salt of alcohol or phenol. This reaction involves a nucleophilic substitution of halide ion by alkoxide ion(SN2 mechanism).
(SN2 mechanism) If a tertiary alkyl halide is used ,an alkene is the only reaction product because alkoxide then act as a strong base and elimination favours substitution. over 4. From Acyl chloride: Friedel-Craft Reaction
5.From Grignard reagent: Properties of Ether (i) Physical state: The lower members are gases or volatile liquids and their vapours are highly inflammable (ii) Boiling points: Boiling points of ethers are much less than isomeric alcohols due to the absence of H-bonding. (iii) Solubility: All ethers are less dence than water in which they are not very soluble but solubility increased with adding alcohol due to the formation of H- bond. Chemical reactions of Ethers 1. Reactions with Lewis acid: Being Lewis base ethers forms complexes lewis acid which are called Etherates. with
2. Action of Hydroiodic acid: (a) With cold HI: + (b) With Hot HI: 3. Action of PCl5: 4. Dehydration:
Reference 1.Organic Chemistry by I.L.Finar 2.Organic Chemistry by Morrison and Boyd. 3.Advance organic chemistry by bahl & bahl. 4.Organic Chemistry by Brucee. 5.Organic chemistry by R.L.Madan 6.A Guidebook to mechanism in Organic chemistry by P.sykes. 7.Wikipidea.
Thank You
