Transcript Alcohols

Chapter 14
Alcohols,
Ethers, and
Thiols
Alcohols
• Alcohol: an -OH (hydroxyl) group bonded
to a tetrahedral carbon
• Nomenclature
1.longest chain with the -OH group numbered
lowest possible (rings OH=1)
2.change the ending of the parent alkane from e to -ol
3.name and number substituents and list them
in alphabetical order
Nomenclature
OH
Ethan ol
(Ethyl alcohol)
OH
OH
1-Propan ol
(Prop yl alcohol)
2-Prop anol
(Is op ropyl alcohol)
OH
OH
1-Butan ol
(Bu tyl alcohol)
OH
2-Bu tanol
(sec-Bu tyl alcohol)
OH
2-Methyl-2-p ropan ol
(t ert -Butyl alcoh ol)
2-Meth yl-1-propanol
(Is ob utyl alcoh ol)
OH
Cycloh exanol
(Cycloh exyl alcoh ol)
Nomenclature
• Problem: write the IUPAC name for
each alcohol
OH
(a)
OH
(b)
OH
(c)
(d)
OH
Nomenclature
• Solution:
(a)
5
4
OH
1
OH
(b )
2
2
4-Methyl-2-pentan ol
t rans-2-Meth ylcycloh exanol
OH
(c)
2
2-Heptanol
7
(d)
3
2
1
OH
2,2-D imeth yl-1-p ropanol
Nomenclature
– in the IUPAC system, a compound containing
two hydroxyl groups is named as a diol, one
containing three hydroxyl groups as a triol,
and so on
– IUPAC names for diols, triols, and so on retain
the final "-e" in the name of the parent alkane
– we commonly refer to compounds containing
two hydroxyl groups on adjacent carbons as
glycols
CH2 CH2
OH OH
CH3 CHCH2
HO OH
1,2-Ethanediol
(Ethylene glycol)
1,2-Propanediol
(Propylene glycol)
CH2 CHCH2
OH OHOH
1,2,3-Propanetriol
(Glycerol, Glycerin)
Physical Properties
• Alcohols are polar molecules
– the C-O and O-H bonds are both polar
covalent
Physical Properties
– in the liquid state, alcohols associate by
hydrogen bonding
Physical Properties
– bp increases as MW increases
– solubility in water decreases as MW increases
Molecular
Weigh t
bp
(°C)
Solubility
in Water
Structu ral Formula
N ame
CH3 OH
CH3 CH3
methanol
ethan e
32
30
65
-89
infinite
ins olub le
CH3 CH2 OH
46
44
78
-42
infinite
CH3 CH2 CH3
ethan ol
propane
CH3 CH2 CH2 OH
1-propanol
bu tane
97
0
infinite
CH3 CH2 CH2 CH3
60
58
CH3 CH2 CH2 CH2 OH
1-bu tanol
74
117
8 g/100 g
CH3 CH2 CH2 CH2 CH3
pen tane
72
36
ins olub le
ins olub le
ins olub le
Acidity of Alcohols
• Alcohols have about the same pKa values
as water; aqueous solutions of alcohols
have the same pH as that of pure water
• phenols contain an OH group that is more
acidic
– phenols are weak acids and react with NaOH
and other strong bases to form water-soluble
salts
OH + NaOH
Phenol
H2 O
O- Na + + H2 O
S od ium phenoxide
(a w ater-soluble salt)
Dehydration
• Dehydration: elimination of a molecule of
water from adjacent carbon atoms gives
an alkene
– heating an alcohol with either 85% H3PO4 or
concentrated H2SO4
– 1° alcohols are the most difficult to dehydrate
– 2° alcohols undergo reaction at lower
temperatures
– 3° alcohols easily undergo acid-catalyzed
dehydration at temperatures only slightly
above room temperature
Dehydration
CH3 CH2 OH
Ethanol
OH
Cycloh exanol
H2 SO4
180°C
H2 SO4
140°C
CH2 =CH2 + H2 O
Ethylene
+ H2 O
Cyclohexene
CH3
CH3
H2 SO4
CH3 CCH3
CH3 C=CH2 + H2 O
50°C
OH
2-Meth yl-2-p ropanol
2-Methylpropene
(Isobutylene)
(t ert -Butyl alcoh ol)
Your Lab Work
BP = 160 oC
BP = 89 oC
Dehydration
– when isomeric alkenes can be obtained, the
alkene having the greater number of alkyl
groups on the double bond predominates
– examples:
OH
CH3 CH2 CHCH3
2-Butanol
H3 PO4
-H2 O
CH3 CH=CHCH3 + CH3 CH2 CH=CH2
2-Butene
1-Butene
(80%)
(20%)
CH3
CH3
CH3
H2 SO4
CH3 CHCHCH3
CH3 C=CHCH3 + CH3 CHCH=CH2
-H2 O
OH
3-Meth yl-2-b utanol
2-Methyl-2-b utene 3-Methyl-1-bu tene
(major prod uct)
Dehydration-Hydration
• competing reactions
– the following equilibrium exists
C C
+ H2 O
dehydration
h yd ration
An alk ene
C C
H OH
An alcoh ol
– in accordance with Le Chatelier's principle,
excess water favor alcohol formation, whereas
removal of water from the equilibrium mixture
favors alkene formation
Addition or Elimination
Here A is acting negative and Q is acting positive.
If you can remove A or Q from the reaction what happens?
If you can remove the alkene from the reaction what
happens?
If you can precipitate the product from this reaction what
happens?
Eliminations
This reaction is run is a solvent which the product salt is insoluble.
How would you make the chlorocyclohexane from the cyclohexane?
3 More electrophylic Additions
Oxidation
• Oxidation of a 1° alcohol gives an
aldehyde or a carboxylic acid
– 1° alcohol to acid is carried out using K2Cr2O7
in aqueous sulfuric acid
CH3 ( CH2 ) 6 CH2 OH
1-Octanol
K2 Cr2 O7
H2 SO4
O K Cr O
2
2 7
CH3 ( CH2 ) 6 CH
H2 SO4
Octanal
O
CH3 (CH2 ) 6 COH
Octan oic acid
– it may be possible to stop the oxidation at the
aldehyde stage by distilling the mixture; the
aldehyde usually has a lower boiling point than
either the 1° alcohol or the carboxylic acid
Oxidation
– oxidation of a 2° alcohol gives a ketone
K2 Cr2 O7
OH
H2 SO4
2-Is opropyl-5-methylcyclohexanol
(Menthol)
O
2-Isopropyl-5-methylcyclohexan on e
(Menth on e)
– tertiary alcohols are resistant to oxidation
CH3
OH
1-Meth ylcyclopen tanol
K2 Cr2 O7
H2 SO4
(no oxidation )
Ethers
• The functional group of an ether is
an oxygen atom bonded to two
carbon atoms
– the simplest ether is dimethyl ether
– the most common ether is diethyl ether
CH3 -O-CH3
D imethyl ether
CH3 CH2 -O-CH2 CH3
Dieth yl eth er
Nomenclature
• Although ethers can be named according to the
IUPAC system, chemists almost invariably use
common names for low-molecular-weight ethers
– common names are derived by listing the alkyl groups
bonded to oxygen in alphabetical order and adding the
word "ether”
– alternatively, name one of the groups on oxygen as an
alkoxy group
CH3 CH2 OCH2 CH3
Diethyl ether
OCH3
Cyclohexyl methyl ether
(Methoxycyclohexane)
Nomenclature
• Cyclic ether: an ether in which one of the atoms
in a ring is oxygen
– cyclic ethers are also known by their common names
– ethylene oxide is an important building block for the
organic chemical industry; it is also used as a fumigant
in foodstuffs and textiles, and in hospitals to sterilize
surgical instruments
– tetrahydrofuran is a useful laboratory and industrial
solvent
O
Eth ylene oxide
O
Tetrahydrofu ran
(THF)
Physical Properties
• Ethers are polar compounds in which oxygen
bears a partial negative charge and each carbon
bonded to it bears a partial positive charge
– however, only weak forces of attraction exist between
ether molecules in the pure liquid
– consequently, boiling points of ethers are close to those
of hydrocarbons of similar molecular weight
– ethers have lower boiling points than alcohols of the
same molecular formula
CH3 OCH3
CH3 CH2 OH
Ethanol
Dimethyl ether
bp 78°C
b p -24°C
Making an Ether
• Williamson Ether Synthesis
Symmetrical Ethers
Reactions of Ethers
• Ethers resemble hydrocarbons in their resistance
to chemical reaction
– they do not react with oxidizing agents such as
potassium dichromate
– they do not react with reducing agents such as H2 in the
presence of a transition metal catalyst
– they are not affected by most acids or bases at
moderate temperatures (but can eliminate)
• Because of their general inertness and good
solvent properties, ethers such as diethyl ether
and THF are excellent solvents in which to carry
out organic reactions
Thiols
• Thiol: a compound containing an -SH (sulfhydryl)
group
– the most noteworthy physical property of low-molecularweight thiols is their stench
– they are responsible for scents such as those from
rotten eggs and sewage
– Skunk scent is due to a molecule which has two thiols
CH3 CH=CHCH2 SH
2-Bu ten e-1-thiol
CH3
CH3 CHCH2 CH2 SH
3-Methyl-1-bu tanethiol
Thiols - Nomenclature
• IUPAC names are derived in the same manner as
are the names of alcohols
– to show that the compound is a thiol, the final -e of the
parent alkane is retained and the suffix -thiol added
• Common names for simple thiols are derived by
naming the alkyl group bonded to -SH and
adding the word "mercaptan"
CH3 CH2 SH
Ethaneth iol
(Ethyl mercap tan)
CH3
CH3 CHCH2 SH
2-Methyl-1-propan ethiol
(Isobu tyl mercaptan)
Physical Properties
• Because of the small difference in electronegativity
between sulfur and hydrogen (2.5 - 2.1 = 0.4), an
S-H bond is nonpolar covalent
– thiols show little association by hydrogen bonding
– thiols have lower boiling points and are less soluble in
water and other polar solvents than alcohols of similar
molecular weight
Thiol
methanethiol
ethan ethiol
1-bu tanethiol
bp (°C)
6
Alcoh ol
methanol
35
98
ethan ol
1-bu tanol
78
117
bp (°C)
65
Reactions of Thiols
• Thiols are weak acids (pKa 10), and
are comparable in strength to
phenols
– thiols react with strong bases such as
NaOH to form water-soluble thiolate
salts
CH3 CH2 SH + NaOH
Ethan ethiol
(pKa 10)
H2 O
-
+
CH3 CH2 S Na + H2 O
S od ium
ethaneth iolate
Reactions of Thiols
• The most common reaction of thiols in biological
systems is their oxidation to disulfides, the
functional group of which is a disulfide (-S-S-)
bond
– thiols are readily oxidized to disulfides by O2
– they are so susceptible to oxidation that they must be
protected from contact with air during storage
– disulfides, in turn, are easily reduced to thiols by several
reducing agents.
2HOCH2 CH2 SH
A thiol
oxidation
reduction
HOCH2 CH2 S-SCH2 CH2 OH
A disulfide
Important Alcohols
Coal
or
meth ane
2 H2
CO
Carbon
monoxide
CO
catalys t
CH3 OH
Meth anol
H2 O, H2 SO4
CH2 =CH2
Ethylen e
O2
catalyst
CH3 CH2 OH
Ethanol
CH3 COOH
Acetic acid
O2
CH2 O
oxidation Formaldehyde
H2 SO4
180°C
O
H2 O, H2 SO4
H2 C CH2
Ethylene
oxide
CH3 CH2 OCH2 CH3
Dieth yl eth er
HOCH2 CH2 OH
Ethylene glycol
Important Alcohols
H2 O, H2 SO4
CH3 CH=CH2
Propene
several s teps
OH
CH3 CHCH3
2-Propan ol
OH
HOCH2 CHCH2 OH
Glycerin, glycerol
Acid Base Equilibrium and
Basic conditions What?
• Substitution or Elimination
– Leaving group must be weak base.
– Attaching group is stronger base which
is negatively charged or neutral with
lone pair electrons.
Acidic Conditions What?
• Substitution or Elimination
– Acid activates the leaving group so that
it is a weak base.
– Attaching group is a neutral lone pair
atom.
Alcohols, Ethers, and Thiols
End
Chapter 14
Review
• Alkanes – Reactions of only
– Combustion
– Radical Halogenation
Review
• Aromatic – Reactions of only
– Substitution (Electrophilic Aromatic
Substitution)
Review
• Alkenes – Reactions of
– Electrophilic Addition
Review
• Alkenes – Synthesis
– Elimination
• Basic
• Acidic
Review
• Alkynes
– Just do everything twice
• 2 eliminations
• Or
• Two additons
Review
• Alcohols
– Synthesis
Review
• Alcohols
– Reactions of
Review
• Thiols
– Synthesis
Review
• Thiols
– Reactions
Review
• Alkylhalides
– Synthesis
Review
• Alkylhalides
– Reactions of