Upload
lycong
View
215
Download
0
Embed Size (px)
Citation preview
1
AlcoholsAlcohols
Chapter 10Chapter 10
2
Structure of AlcoholsStructure of Alcohols
• The functional group of an alcohol is an -OH group bonded to an sp3
hybridized carbon.– Bond angles about the hydroxyl oxygen
atom are approximately 109.5°.
• Oxygen is sp3 hybridized.– Two sp3 hybrid orbitals form sigma
bonds to a carbon and a hydrogen.– The remaining two sp3 hybrid orbitals
each contain an unshared pair of electrons.
108.9°O
CH H
H
H
3
• IUPAC names– The parent chain is the longest chain that
contains the OH group.
– Number the parent chain to give the OH group the lowest possible number.
– Change the suffix --ee to --olol.
• Common names – Name the alkyl group bonded to oxygen
followed by the word alcoholalcohol.
Nomenclature of AlcoholsNomenclature of Alcohols
4
1-Pro p an o l(Pro p y l a l co h o l )
2-P ro p an ol(I so p rop yl a l c oh ol )
1-B u tan o l(B u tyl a l c oh ol )
OHOH
OH
2-B u tan o l(s ec- B u ty l a l co h o l )
2-M e th yl -1-p ro p an ol(I so b u ty l a l co h o l )
2-M e th yl -2-p ro p an ol(t e rt - B u ty l a l co h o l )
OHOH
OH
Nomenclature of AlcoholsNomenclature of Alcohols
1o 1o
1o
2o
2o 3o
5
• Compounds containing more than one OH group are named diols, triols, etc.
CH3 CHCH2
HO OH
CH2 CH2
OH OH
CH2 CHCH2
HO HO OH
1,2-Ethanediol(Ethylene glycol)
1,2-Propanediol(Propylene glycol)
1,2,3-Propanetriol(Glycerol, Glycerine)
Nomenclature of AlcoholsNomenclature of Alcohols
6
• Alcohols are polar compounds.
– They interact with themselves and with other polar compounds by dipole-dipole interactions.
•• DipoleDipole--dipole interaction:dipole interaction: The attraction between the positive end of one dipole and the negative end of another.
Physical PropertiesPhysical Properties
7
•• Hydrogen bondingHydrogen bonding: When the positive end of one dipole is an H bonded to F, O, or N(atoms of high electronegativity) and the other end is F, O, or N.– The strength of hydrogen bonding in water is
approximately 21 kJ (5 kcal)/mol.– Hydrogen bonds are considerably weaker
than covalent bonds.– Nonetheless, they can have a significant
effect on physical properties.
Physical PropertiesPhysical Properties
8
E.G. The association of ethanol molecules in the liquid state by hydrogen bonding.
Hydrogen BondingHydrogen Bonding
9
Physical PropertiesPhysical Properties
• Ethanol and dimethyl ether are constitutional isomers.
• Their boiling points are dramatically different– Ethanol forms intermolecular hydrogen bonds, which
are attractive forces between its molecules, resulting in a higher boiling point.
– There is no comparable attractive force between molecules of dimethyl ether.
bp -24°CEthanolbp 78°C
Dimethyl ether
CH3 CH2 OH CH3 OCH3
10
• In relation to alkanes of comparable size and molecular weight, alcohols:– have higher boiling points.
– are more soluble in water.
• The presence of additional -OH groups in a molecule further increases solubility in water (polar solvents) and boiling points.
Physical PropertiesPhysical Properties
11
In dilute aqueous solution, alcohols are weakly acidic.
CH3 O H : HO
H
[ CH3OH]
[ CH3O-] [H3O+]
CH3 O:–O
H
HH+
+
= 10- 15 .5
pKa = 1 5.5
Ka =
+
Acidity of AlcoholsAcidity of Alcohols
12
Acidity of AlcoholsAcidity of Alcohols
( CH3 ) 3COH
( CH3 ) 2CHOH
CH3 CH2 OH
H2 O
CH3 OH
CH3 COOH
HClHydrogen chloride
Acetic acid
M ethanol
Water
Ethanol
2-Propanol
2-Methyl-2-propanol
Structural Formula
Stronger acid
Weakeracid
Also given for comparison are pKa values for water, acetic acid, and hydrogen chloride.
Compound pKa
-7
15.5
15.7
15.9
17
18
4.8
13
• Alcohols react with Li, Na, K, and other active metals to liberate hydrogen gas and form metal alkoxides.
Sodium methoxide(MeO Na+)
+2 CH3 O Na+ H22 CH3 OH + 2 Na
Reaction with MetalsReaction with Metals
14
• Alcohols are also converted to metal alkoxides by reaction with bases stronger than the alkoxide ion.– One such base is sodium hydride.
Ethanol Sodium ethoxide
CH3 CH2 OH CH3 CH2 O Na++ + H2Na+ H
Sodiumhydride
Reaction with MetalsReaction with Metals
ether; THF or alkanes2
(reaction is irreversible)
15
– 3° alcohols react very rapidly with HCl, HBr, and HI.
OH + H2 O+ HCl25°C
Cl
2-Methyl-2-propanol
2-Chloro-2-methylpropane
Reaction with HXReaction with HX
16
– Low-molecular-weight 1° and 2° alcohols are unreactive under these conditions.
– 1° and 2° alcohols require concentrated HBr and HI to form alkyl bromides and iodides.
reflux1-Bromobutane1-Butanol
++ HBr H2 OH2 O
OH Br
Reaction with HXReaction with HX
R R (H)
HX (conc)
1o & 2o ROH
OH
HR R (H)
X
H
1o & 2o Halides
Heat
17
Reaction with HX Reaction with HX -- SSNN11
Step 1: Proton transfer to the OH group gives an oxonium ion.
Step 2: Loss of H2O gives a carbocationintermediate.
CH3
CH3
CH3 -C-OH
:
H
H
H O O
H
H
CH3 -C
CH3
CH3
: H
H
O+
+
rapid andreversib le+
+
O
H
H
CH3 -C
CH3
CH3 CH3
CH3
CH3-C+ :
H
H
O+
A 3° carbocation intermediate
s low, ratedetermining
SN1+
18
Step 3: Reaction of the carbocationintermediate (an electrophile) with halide ion (a nucleophile) gives the product.
CH3
CH3
CH3-C+ :Cl CH3-C-Cl
CH3
CH3
2-Chloro-2-methylpropane (t ert-Butyl chloride)
fast+
Reaction with HX Reaction with HX -- SSNN11
19
1°alcohols react with HX by an SN2 mechanism.
Step 1: Rapid and reversible proton transfer.
Step 2: Displacement of HOH by halide ion.
Br:- RCH2 -O
H
H
RCH2 -Br :
H
HO
++
SN2+
slow, ratedetermining
+ H
H
H O H
H
O
rapid andreversible
RCH2 - O
H
HRCH2 - OH +++
Reaction with HX Reaction with HX –– SSNN22
20
• An alternative method for the synthesis of 1° and 2° bromoalkanes is reaction of an alcohol with phosphorous tribromide.– This method gives less rearrangement than
with HBr.
Reaction with PBrReaction with PBr33 –– SSNN22
21
Step 1: Make a bond between a nucleophile and an electrophile and simultaneously beak a bond to give stable molecules or ions. Formation of a protonated dibromophosphiteconverts H2O, a poor leaving group, to a good leaving group.
Step 2: Make a bond between a nucleophile and an electrophile and simultaneously beak a bond to give stable molecules or ions.
Reaction with PBrReaction with PBr33 –– SSNN22
22
• Thionyl chloride is the most widely used reagent for the conversion of 1° and 2°alcohols to alkyl chlorides.– A base, most commonly pyridine or triethylamine,
is added to catalyze the reaction and to neutralize the HCl. ( NEt3 or )
OH SOCl2
Cl SO2 HCl
Thionylchloride
1-Heptanol
1-Chloroheptane
pyridine+
+ +
Reaction with SOClReaction with SOCl22
23
• Reaction of an alcohol with SOCl2 in the presence of a 3° amine is stereoselective.– It occurs with inversion of configuration.
OH
SOCl2
Cl
SO2 HCl+3° amine
+ +(S)-2-Octanol Thionyl
chloride(R)-2-Chlorooctane
Reaction with SOClReaction with SOCl22
N
R R (H)
SOCl2
1o & 2o ROH
OH
HR R (H)
Cl
H
1o & 2o Chloride
Pyridine or N(Et)3
24
• An alcohol can be converted to an alkene by acid-catalyzed dehydration (a type of β-elimination).– 1° alcohols must be heated at high temperature in the
presence of an acid catalyst, such as H2SO4 or H3PO4. 2° alcohols undergo dehydration at somewhat lower temperatures.
– 3° alcohols often require temperatures at or only slightly above room temperature.
Dehydration of ROHDehydration of ROH
25
180°CCH3 CH2 OH
H2 SO4CH2 = CH2 + H2 O
140°CCyclohexanol Cyclohexene
OH
+ H2 OH2 SO4
CH3 COH
CH3
CH3
H2 SO4CH3 C= CH2
CH3
+ H2 O50°C
2-Methyl-2-propanol(tert- Butyl alcohol)
2-Methylpropene(Isobutylene)
Dehydration of ROHDehydration of ROH
26
– Where isomeric alkenes are possible, the alkene having the greater number of substituents on the double bond (the more stable alkene) usually predominates(Zaitsev rule).
1-Butene (20%)
2-Butene (80%)
2-Butanol
+
heat
8 5 % H3 PO4
CH3 CH= CHCH3
CH3 CH2 CHCH3
CH3 CH2 CH= CH2 + H2 O
OH
Dehydration of ROHDehydration of ROH
27
• Dehydration of 1° and 2° alcohols is often accompanied by rearrangement.
– Acid-catalyzed dehydration of 1-butanol gives a mixture of three alkenes.
OH
H2 SO4
140 - 170°C+
3,3-Dimethyl-2-butanol
2,3-Dimethyl-2-butene
(80%)
2,3-Dimethyl-1-butene
(20%)
H2 SO4
140 - 170°C1-Butanol
+
trans- 2-butene(56%)
cis- 2-butene(32%)
+
1-Butene(12%)
OH
Dehydration of ROHDehydration of ROH
28
Step 1: Proton transfer to the -OH group gives an oxonium ion.
Step 2: Loss of H2O gives a carbocation intermediate.
O
H O
H
H
O
O
H
H
H HH
+
+
rapid and
reversible
+
+
A 2° carbocation
intermediate
OH H+ slow, rate
determining
H2 O+
Dehydration of ROHDehydration of ROH
29
Step 3: Proton transfer from a carbon adjacent to the positively charged carbon to water. The sigma electrons of the C-H bond become the pi electrons of the carbon-carbon double bond.
rapid andreversible
O
H
H
HH
+ + O
H
++ H H
Dehydration of ROHDehydration of ROH
30
• Oxidation of a primary alcohol gives an aldehydeor a carboxylic acid, depending on the experimental conditions.
– oxidation to an aldehyde is a two-electron oxidation.
– oxidation to a carboxylic acid is a four-electron oxidation.
[O] [O]OH
H
HCH3 -C
A primary alcohol
An aldehyde A carboxylic acid
CH3 -C- H
O
CH3 -C- OH
O
Oxidation of 1Oxidation of 1oo ROHROH
31
• A common oxidizing agent for this purpose is chromic acid, prepared by dissolving chromium(VI) oxide or potassium dichromate in aqueous sulfuric acid.
Potassiumdichromate
Chromic acid
K2 Cr2 O7H2 SO4 H2 Cr2 O7
H2 O2 H2 CrO4
+
Chromic acidChromium(VI) oxide
CrO3 H2 O H2 CrO4H2 SO4
Oxidation of ROHOxidation of ROH
HH22CrOCrO44
32
• Oxidation of 1-octanol gives octanoic acid.– The aldehyde intermediate is not isolated.
OHH2 CrO4
H
O
OH
O
1-Hexanol Hexanal(not isolated)
Hexanoic acid
H2O, acetone
Oxidation of 1Oxidation of 1oo ROHROH
(Chromic acid in acetone is(Chromic acid in acetone isthe Jones oxidation)the Jones oxidation)
33
• A 2° alcohol is oxidized by chromic acid to a ketone.
2-Isopropyl-5-methyl-cyclohexanone
(Menthone)
2-Isopropyl-5-methyl-cyclohexanol
(Menthol)
acetone+ H2CrO4 + Cr
3 +OH O
Oxidation of 2Oxidation of 2oo ROHROH
R R'
H2CrO4
R R'
O
or Na2Cr2O7 /H2SO4or CrO3/H2SO4
2o ROH Ketones
OH
34
•• PyridiniumPyridinium chlorochromatechlorochromate (PCC):(PCC): A form of Cr(VI) prepared by dissolving CrO3 in aqueous HCl and adding pyridine to precipitate PCC as a solid.– PCC is selective for the oxidation of 1° alcohols to aldehydes; it
does not oxidize aldehydes further to carboxylic acids.
CrO3 HClN N
H
ClCrO3-
chlorochromate ion
pyrid inium ion
Pyridinium chlorochromate (PCC)
Pyridine
+ +
Oxidation of 1Oxidation of 1oo ROH to RCHOROH to RCHO
PCCPCC