Transcript Ch-6-Alcohols and phenols - Home

Alcohols and Phenols
By
Dr. Nahed Nasser
THE CHEMISTRY OF ALCOHOLS
and PHENOLS
CONTENTS
• Structure of alcohols
•Types of alcohols
•Classification of alcohols
• Nomenclature of alcohols
• Hydrogen bonding and acidity )Physical properties)
• Structure of phenols
•Nomenclature of phenols
•Preparation of alcohols
•Preparation of phenols
• Reactions of alcohols
• Reactions of phenols
•Uses of alcohols and phenols
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STRUCTURE OF ALCOHOLS
• Alcohols are the family of compounds that contain one or more hydroxyl (-OH) groups. The OH
group is bound to a carbon atom.
•They can be considered both derivatives of hydrocarbons (by replacing a hydrogen atom with a
hydroxyl group -OH) and water (H2O) derivatives (the result of the substitution of a hydrogen atom by
an organic radical).
TYPES OF ALCOHOLS
•Two types of alcohols: mono hydroxyls (for example ethanol C2H5OH) and poly hydroxyls
(for example: propantriol CH2OH-CHOH-CH2OH).
•The alcohols containing 2 hydroxyl groups connected by different carbon atoms are called glycols,
for example: Ethylene glycol CH2OH-CH2OH.
Classification of Monohydroxyl Alcohols
The mono hydroxyl alcohols can be classified into three types according to the type of the carbon
atom connected to the hydroxyl group:
- primary alcohols
- secondary alcohols
- tertiary alcohols
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PRIMARY 1°
SECONDARY 2°
TERTIARY 3°
R'
H3C
Carbinol
OH
R
CH2
OH
Primary alcohol
R
CH
R''
OH
Secondary alcohol
R
C
OH
R'
Tertiary alcohol
For example, methanol CH3OH, ethanol CH3-CH2-OH, allyl acohol
CH2=CHCH2OH are primary alcohols;
2-propanol CH3-CH(OH)-CH3 is a secondary alcohol and 2-methyl2-propanol CH3-C(CH3)OH-CH3 is a tertiary alcohol.
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NOMENCLATURE OF ALCOHOLS
COMMON NOMENCLATURE OF ALCOHOLS
• Alcohols are named as alkyl alcohols i.e. name the alkyl group and follow it by the word
alcohol
CH3OH
CH3CH2OH
CH2=CHCH2OH
Common names
Methyl alcohol
Ethyl alcohol
Allyl alcohol
OH
OH
OH
H3C
CH3
CH CH3
Isopropyl alcohol
Cyclopentyl alcohol
Methylcyclohexyl alcohol
• The alcohols containing 2 hydroxyl groups (diols) connected to two different carbons
are called glycols for example: CH2OH-CH2OH is called ethylene gylcol
CH3-CH(OH)-CH2(OH) is called propylene gylcol
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IUPAC NOMENCLATURE OF ALCOHOLS
• Find the longest chain of C atoms containing the O-H group; to obtain the name
of the parent alkane
• Replace the e ending by ol suffix in the basic name
• Number the chain starting from the end nearer to the O-H group and add a locator
number for OH group just before the ol suffix or before the full name
e.g.
CH3CH(OH)CH2CH3 is named 2 -butanol or butan-2-ol
• Identify the substituents, allocate them numbers, then list them in alphabetical
order.
Examples:
CH3 - CH(CH3) - CH2 - CH2 - CH(OH) - CH3 is named 5-methylhexan-2-ol
6
5
4
3
2
1
or 5-methyl-2-hexanol
OH
H3C
Cl
OH
H3C
CH3
H3C
CH3
4-Ethyl-2-hexanol
OH
5-Chloro-3-methyl-1-heptanol
CH3
3-Methyl-1-cyclohexanol
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• If a molecule contains both an OH group and a c=c or c=c bond, the name should
include suffixes indicate presence of both OH group and the unsaturated groups. The
OH group takes preference before the double or triple bonds in getting the lower
number.
OH
H2C
OH
HC
CH3
OH
4-Pentyn-1-ol
3-buten-2-ol
(CH3)2C=CHCH(OH)CH3
5 4 3
2
H3C
CH3
CH2
5-Ethyl-5-hexen-3-ol
is 4-methyl-3-penten-2-ol.
1
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• If the parent hydrocarbon contain two hydroxyl groups, the suffix
diol is added to the name; the suffix triol is added when there are
three OH groups. In each case the relative positions of OH groups
must be identified.
OH
OH
CH3
HO
OH
OH
IUPAC
Ethane-1,2-diol
Propane-1,2-diol
or 1,2-Propandiol
Common
Ethylene glycol
Propylene glycol
OH
OH
Propane-1,2,3-triol
or 1,2,3,-Propantriol
Glycerol or Glycerin
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STRUCTURE OF PHENOL
• Phenol’s chemical formula is C6H5OH, it has the OH group directly
attached to the benzene ring.
C6 H6 O
•Thus the following compounds are alcohols not phenols
OH
OH
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Benzyl alcohol
Cyclohexanol
Nomenclature of phenols
•
Phenols are generally named as derivatives of the simplest member of the family,
phenol. Either the ortho, meta, para system or the numbering system can be
employed. Numbering of the ring begins at the hydroxyl-substituted carbon and
proceeds in the direction of the next substituted carbon that possesses the lower
OH
OH
OH
OH
OH
number.
NO2
NH2
Phenol
•
4-Aminophenol
O2N
Br
NO2
NO2
4-Bromo-2-nitrophenol
Cl
Cl
Cl
Cl
Cl
2,4,6-Trinitrophenol 2,3,4,5,6-Pentachlorophenol
Some phenols have common names:
OH
OH
OH
CH3
CH3
CH3
IUPAC: 2-Methylphenol
Common: o-Cresol
Common names:
3-Methylphenol
m-Cresol
4-Methylphenol
p-Cresol
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Physical Properties of Alcohols
Boiling Points of Alcohols
•Alcohols have higher boiling points than alkanes of similar mass
This is due to the presence of inter-molecular hydrogen bonding that connect alcohol
molecules together thus more energy is required to separate the molecules.
hydrogen bonding
propane C3H8
ethanol C2H5OH
• The
Mr
44
46
bp / °C
-42
+78
boiling points increases with increase in molecular weights
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• Boiling point is higher for straight chain isomers i.e.
branching decrease the boiling points
butan-1-ol
CH3CH2CH2CH2OH
butan-2-ol
CH3CH2CH(OH)CH3
2-methylpropan-2-ol (CH3)3COH
bp / °C
118
100
Greater branching =
83
lower inter-molecular forces (H-B) =
lower boiling points
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Solubility Of Alcohols
• Low molecular mass alcohols are miscible with water
• Due to hydrogen bonding between the two molecules
• Heavier alcohols are less miscible i.e. As the number of carbon atoms
increase the miscibility decrease
• As the number of OH groups present in a molecule increase the miscibility
increase. Thus triols > diols > monohydroxyl alcohols in solubility in water
Acidic properties of alcohols:
Alcohols are very weak acids thus they don’t react with bases
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Physical Properties of Phenols
• Phenol is a crystalline solid, with a higher boiling point than alcohols, and it
is moderately soluble in water.
• It is slightly acidic: the phenol molecule has weak tendencies to lose the H+
ion from the hydroxyl group, resulting in the highly water-soluble phenolate
anion C6H5O−, called phenoxide anion. Compared to aliphatic alcohols,
phenol shows much higher acidity (about 1 million times more acidic)
-
OH
O Na
+
•
+
+
NaOH
H2O
One explanation for the increased acidity over alcohols is resonance stabilization
of the phenoxide anion by the aromatic ring. In this way, the negative charge on
oxygen is shared by the ortho and para carbon atoms.
OH
O
O
-H
+H
Resonance structures of phenoxide anion
O
O
• Introduction of electron withdrawing groups such as NO2 or CN
on the benzene ring increases the acidity of phenols, While
introduction of electron releasing groups (e.g. OR, R) decrease
the acidity of phenols compared to unsubstituted phenol.
OH
OH
OH
OH
OH
thus:
<
<
<
<
OCH3
Cl
H3C
OH
O 2N
NO 2
OH
HO
NO 2
>
>
NO 2
OH
OH
O
NO 2
>
NO 2
>
H3C
• Phenols are less acidic than carboxylic acids
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Preparation of Alcohols
1- From alkenes
H3C
1) B2H6
CH2
2) H2O2 / NaOH
H3C
OH
Anti-Markovnikov's product
H
H2O / H2SO4
OH
CH3
Markovnikov's product
OH
KMnO4 / OH
Oxidation
cis-Diol
OH
1) RCO3H
2) H2O2 / NaOH
OH
trans-Diol
OH
2- From alkyl halide by nucleophilic substitution
OH
dil KOH
Cl
CH3
CH3
H2O
Br
HO
CH3
CH3
CH3
CH3
3- By reduction of aldehydes, ketones and carboxylic
acids using metal hydrides
O
Aldehyde R
1) LiAlH4 or NaBH4
C
H
R
CH2-OH
Primary alc.
2) H3O
O
R'
Ketone
R'
R
1) LiAlH4 or NaBH4
R
2) H3O
R
1) LiAlH4 or NaBH4
C
OH
2) H3O
Secondary alc.
H
O
Acid
C-OH
R
CH2-OH
Primary alc.
4- By nucleophilic addition of Grignard reagent
to aldehydes, ketones and esters
• Addition of RMgX to formaldehyde gives 1◦ alc.
• Addition of RMgX to any other aldehyde gives 2◦ alc.
• Addition of RMgX to ketones and esters give 3◦ alc.
R'
O
R
+
C
R'MgX
H
1) Dry ether
R
OH
HO
O
H3C
CH
2) H 2O
+
C
C2H5MgX
H
1) Dry ether
2) H 2O
H3C
CH
C2H5
R'
O
R
C
+
R'
R''MgX
1) Dry ether
R
+
R''
CH 3MgX
1) Dry ether
OH
2) H 2O
R''
O
+
C
2 R''MgX
1) Dry ether
R
2) H 2O
OR'
C
+
OC 2H5
2 CH 3MgX
1) Dry ether
2) H 2O
C
OH
R''
CH3
O
H3C
OH
CH3
O
R
C
2) H 2O
H3C
C
H3C
OH
Preparation of Phenols
1- Via hydrolysis of Diazonium salts
+
N2 Cl
OH
-
H2SO 4 / H2O
+
N2
Heat
2- Fusion of sodium with benzene-sulfonates:
-
O Na
SO3H
OH
+
H3 O
NaOH / 350
+
3- From alkyl halide:
-
O Na
Cl
NaOH / 350
300 atm
OH
+
H3 O
+
Typical Alcohol Reactions
Salt formation
Dehydration ( Alkene and ether formation)
Ester formation
Alkyl halide formation
Oxidation
Typical Phenol Reactions
Salt formation
Oxidation
Reactions of aromatic ring and side chain of phenols
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Reaction of Alcohols and Phenols
1- Salt Formation by reaction with active
metal
2R
OH
+ 2 Na
2R
Alcohol
H3C
OH
ONa
+ H2
Sodium alkoxide
2 CH3
+ 2 Na
ONa
+ H2
Sodium methoxide
Methanol
OH
ONa
+ 2 Na
or NaOH
Sodium phenoxide
2- ELIMINATION OF WATER (DEHYDRATION)
•
Reagent/catalyst
conc. sulphuric acid (H2SO4) or conc. phosphoric acid (H3PO4)
•
Conditions
reflux at 180°C
•
Product
alkene
H3C
CH3
OH
H2SO4 or H3PO4
- H2O
2-Butanol
+
1- Butene
2-Butene
Major
Minor
• While dehydration of alcohols at lower temperature will give ethers
H2SO 4
R-O-R + H 2O
2 ROH
140
+
2 CH 3OH
H
140
H3C
O
CH3
+
H2O
3- Ester formation
carboxylic acid + alcohol in presence of strong acid
catalyst (e.g conc. H2SO4 ) produces esters
O
R
O
+
+
C
H
R'OH
R
+
C
OH
H2O
OR'
O
COOH
C
+
+
CH 3OH
OCH 3
H
+
H2O
4- Alkyl halides formation
ZnCl2
R
X
+ H2O
R
X
+ SO2 + HCl
R
OH
+ HX
R
OH
+ SOX2
R
OH
+ PX3
R
X
+ HOPX2
R
OH + PX
5
R
X
+ HOPX4
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5- OXIDATION OF ALCOHOLS
Alcohols can be oxidised depending on their class
Oxidation is used to differentiate between primary, secondary and tertiary alcohols
The usual reagent is acidified potassium dichromate(VI)
Primary
Easily oxidised to aldehydes and then to carboxylic acids.
e.g. CH3CH2OH(l) + [O] ——> CH3CHO(l) + H2O(l)
ethanol
ethanal
it is essential to distil off the aldehyde before it gets oxidised to the acid
CH3CHO(l) + [O]
ethanal
——>
CH3COOH(l)
ethanoic acid
Secondary
Easily oxidised to ketones
Tertiary
Not oxidised under normal conditions.
They do break down with very vigorous oxidation
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PRIMARY 1°
SECONDARY 2°
TERTIARY 3°
Why 1° and 2° alcohols are easily oxidised and 3° alcohols are not
For oxidation to take place easily you must have two hydrogen atoms on adjacent
C and O atoms.
1°
R
H
H
C
O
+
[O]
R
H
2°
R
C
O
+
H 2O
O
+
H 2O
H
H
H
C
O
+
[O]
R
R
C
R
This is possible in 1° and 2° alcohols but not in 3° alcohols.
3°
R
R
H
C
O
R
+
[O]
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R
O
Cu or CrO 3 / pyridine
CH2 OH
heat
H
adehyde
Primary alcohol
H3C
weak oxidizing reagent
R
CH2 OH
O
Cu or CrO 3 / pyridine
H3C
heat
H
+
H2Cr 2O 7 or K 2Cr 2O7/ H
R
CH2 OH
O
R
or KMnO 4 / heat
OH
carboxylic acid
Primary alcohol
OH
O
H2Cr2O7 or Na2CrO7 / H
or KMnO4 / heat
Cyclohexanol
Secondary alcohol
OH
Cyclohexanone
Ketone
O
H2Cr2O7 or Na2CrO7 / H
OH
KMnO4 / heat
or KMnO4 / heat
O
Phenol
[1,4]Benzoquinone
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OH
Hydroquinone
Reactions of aromatic ring of phenols
OH
Br
Br
Br2 / H2O
HO
HO
Br
HO
Br2 / CCl 4
Br
+
Br
HO
HO
NO 2
dil HNO 3
+
OH
conc HNO 3
O 2N
O 2N
HO
conc H2SO4
NO 2
NO 2
OH
SO 3H
+
SO3H
USES OF ALCOHOLS AND PHENOLS
• Uses of Alcohols
•
•
•
•
•
Methanol and ethanol can be used as an alternative to fossil fuels as they burn very cleanly, producing only
carbon dioxide and water. Ethanol is considered a renewable fuel as it can be made from renewable sources
such a sugar cane and can be used as a fuel in its own right, or in mixtures with petrol (gasoline). It's really
useful for countries without an oil industry as it reduces their dependence upon imports of petrol.
As ethanol is the least toxic of the alcohols it is used in perfumes to stop the plant and animal extracts from
going off. The amount added depends on whether you are making a perfume, toilet water or cologne.
As ethanol is the safest of the alcohols it is often used to dissolve chemicals that are insoluble in water.
Examples include perfumes, cosmetics and vegetable essences such as vanilla extract.
Methylated spirits which is ethanol with a small quantity of methanol added (The methanol makes the mixture
highly poisonous ) is very good for cleaning paint brushes.
Glycerol used in midicine.
• Uses of phenol
•
•
•
•
•
It is used in an air freshener
Another use for phenol includes an intermediate stage in the process of producing caprolactam, which is used in
nylon and many other man-made fibres.
Phenol is also a powerful disinfectant and bacteria killer.
However the chemical is highly corrosive and moderately toxic. It effects humans by burning the skin and other
tissue that it comes into contact with. This gives severe skin burning and if inhaled serious internal corrosion.
The skin burning is not initially felt because the phenol has a local anaesthetic effect. It can affect the central
nervous system, which will at first lead to sweating, weakness, dizziness and twitching but with prolonged
exposure leads to nausea, vomiting and coma. If ingested even a small dose can lead to be fatal in humans and
therefore care must be taken at all times using phenol-containing products.
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Resorcinol is used in mouth wash