1 Chapter 9 Alcohols, Ethers and Epoxides. 2 Alcohols contain a hydroxy group (OH) bonded to an sp 3...
-
Upload
joan-white -
Category
Documents
-
view
222 -
download
0
Transcript of 1 Chapter 9 Alcohols, Ethers and Epoxides. 2 Alcohols contain a hydroxy group (OH) bonded to an sp 3...
1
Chapter 9
Alcohols, Ethers and Epoxides
2
Alcohols, Ethers and Epoxides
• Alcohols contain a hydroxy group (OH) bonded to an sp3 hybridized carbon.
Introduction—Structure and Bonding
3
Alcohols, Ethers and Epoxides
• enols and phenols Compounds having a hydroxy group on a sp2 hybridized carbon undergo different reactions than alcohols.
Introduction—Structure and Bonding
• Will be discussed in chapter 11, 19
4
Alcohols, Ethers and Epoxides
Introduction—Structure and Bonding
• Ethers have two alkyl groups bonded to an oxygen atom.
5
Alcohols, Ethers and Epoxides
• Epoxides are ethers having the oxygen atom in a three-membered ring.
Epoxides are also called oxiranes.
Introduction—Structure and Bonding
• The C—O—C bond angle for an epoxide must be 60°, a considerable deviation from the tetrahedral bond angle of 109.5°. Thus, epoxides have angle strain, making them much more reactive than other ethers.
6
Alcohols, Ethers and Epoxides
• 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.
Introduction—Structure and Bonding
7
Alcohols, Ethers and Epoxides
Nomenclature of Alcohols : -- ol
8
Alcohols, Ethers and Epoxides
• 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.
For cyclic compounds
9
Alcohols, Ethers and Epoxides
• Common names are often used for simple alcohols.
Name all the carbon atoms of the molecule as a single alkyl group.
Add the word alcohol, separating the words with a space.
Nomenclature of Alcohols
10
Alcohols, Ethers and Epoxides
• Compounds with two hydroxy groups : diols or glycols. • Compounds with three hydroxy groups : triols
and so forth.
Nomenclature of Alcohols
11
Alcohols, Ethers and Epoxides
• Simple ethers are usually assigned common names. To do so:
Name both alkyl groups bonded to the oxygen, arrange these names alphabetically, and add the word ether.
For symmetrical ethers, name the alkyl group and add the prefix “di-”.
Nomenclature of Ethers
12
Alcohols, Ethers and Epoxides
• More complex ethers are named using the IUPAC system.
One alkyl group is named as a hydrocarbon chain, and the other is named as part of a substituent bonded to that chain:
Name the simpler alkyl group as an alkoxy substituent by changing the –yl ending of the alkyl group to –oxy.
Name the remaining alkyl group as an alkane, with the alkoxy group as a substituent bonded to this chain.
13
Alcohols, Ethers and Epoxides
• Cyclic ethers have an O atom in the ring. A common example is tetrahydrofuran (THF).
This name has a different origin
14
• Epoxides can be named in three different ways—As epoxyalkanes, oxiranes, or alkene oxides.
• To name an epoxide as an epoxyalkane, first name the alkane chain or ring to which the O atom is attached,
• and use the prefix “epoxy” to name the epoxide as a substituent.
• Use two numbers to designate the location of the atoms to which the O’s are bonded.
Nomenclature of Epoxides
15
• Epoxides bonded to a chain of carbon atoms can also be named as derivatives of oxirane, the simplest epoxide having two carbons and one oxygen atom in a ring.
• The oxirane ring is numbered to put the O atom at position one, and the first substituent at position two.
• No number is used for a substituent in a monosubstituted oxirane.
• Epoxides can be named in three different ways—As epoxyalkanes, oxiranes, or alkene oxides.
16
• Epoxides are also named as alkene oxides, since they are often prepared by adding an O atom to an alkene. To name an epoxide in this way:
Mentally replace the epoxide oxygen with a double bond.
Name the alkene.
Add the word oxide.
• Epoxides can be named in three different ways—As epoxyalkanes, oxiranes, or alkene oxides.
17
Alcohols, Ethers and Epoxides
• Alcohols, ethers and epoxides exhibit dipole-dipole interactions because they have a bent structure with two polar bonds.
• Alcohols are capable of intermolecular hydrogen bonding. Thus, alcohols are more polar than ethers and epoxides.
Physical Properties
• Steric factors affect hydrogen bonding.
18
Alcohols, Ethers and Epoxides
19
Alcohols, Ethers and EpoxidesInteresting Alcohols
glycerol
Sugars, cellulose, starch etc.
20
Alcohols, Ethers and Epoxides
Interesting Ethers
Diethyl ether is the best-known ether, first used as a general anesthetic in the 1830s. Because of its high flammability and side-effects on some patients, its place as an anesthetic has been taken by other substances.
Cyclic polyether : crown ether
21
Alcohols, Ethers and Epoxides
Interesting Ethers• The ability of crown ethers to complex cations can be
exploited in nucleophilic substitution reactions, as shown in Figure 9.5.
22
Alcohols, Ethers and Epoxides
Interesting Ethers : nature’s crown ether
• Brevetoxin B is a naturally occurring polyether that interferes with Na+ ion transport across cell membranes.
“Red Tide”
23
Interesting Epoxides : epoxides are reactive!
• Ethylene oxide : precursor of ethylene glycol, polyethylene glycol
O OH
HO
*
OO
n
• Natural epoxides
24
Preparation of Alcohols, Ethers, and Epoxides
• Alcohols and ethers are both common products of nucleophilic substitution of alkyl halides.
Williamson ether synthesis.
25
Preparation of Alcohols, Ethers, and Epoxides
26
• SN2 reaction
Strong Nucleophile (An alkoxide salt) is needed to make an ether.
Williamson ether synthesis.
• Cyclic ethers are formed through intramolecular reaction.
nHO
BrNaH
O
n
When n= -2, epoxides
27
Preparation of Epoxides
• Organic compounds that contain both a hydroxy group and a halogen atom on adjacent carbons are called halohydrins.
28
Reactions of Alcohols
Substitution, elimination, oxidation
• The OH group in alcohols is a very poor leaving group.
• For an alcohol to undergo nucleophilic substitution, OH must be converted into a better leaving group (activated).
• By using acid, ¯OH can be converted into H2O, a good leaving group.
• By converting to sulfonates, ¯OH can be converted into ¯OSO2R, a good leaving group.
29
Reactions of Alcohols
OH OH2
+HBr (for SN)or H2SO4
(for E) SN
E
Br
Halide
Alkene
30
Reactions of Ethers and Epoxides
Ethers do not have a good leaving group and are difficult to activate.
Epoxides are highly reactive due to high angle strain.
O
C C
Nu:_
C
O
C
Nu
_relief of strainRing-opening results in
31
Reactions of Alcohols—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.
32
Reactions of Alcohols—Dehydration
• Typical acids used for alcohol dehydration : H2SO4 or p-toluenesulfonic acid (TsOH).
• More substituted alcohols dehydrate more easily, giving rise to the following order of reactivity.
strong organic acid
33
Alcohols, Ethers and Epoxides
Reactions of Alcohols—Dehydration• Dehydration follows the Zaitsev rule.
The more substituted alkene is the major product when a mixture of constitutional isomers is possible.
34
Alcohols, Ethers and Epoxides
Reactions of Alcohols—Dehydration• Secondary and 3° alcohols react by an E1 mechanism
35
Alcohols, Ethers and Epoxides
Reactions of Alcohols—Dehydration• The E1 dehydration of 20 and 30 alcohols with acid gives clean elimination
products without any by-products formed from an SN1 reaction.
• Clean elimination takes place because the reaction mixture contains no good nucleophile to react with the intermediate carbocation, so no competing SN1 reaction occurs.
• This makes the E1 dehydration of alcohols much more synthetically useful than the E1 dehydrohalogenation of alkyl halides.
36
Alcohols, Ethers and Epoxides
Reactions of Alcohols—Dehydration• 1° alcohols undergo dehydration following an E2 mechanism.• 1° carbocations are highly unstable, their dehydration cannot occur
by an E1 mechanism involving a carbocation intermediate.
37
Reactions of Alcohols—Dehydration
While dehydrations have favorable entropy changes, the position of equilibrium favors reactants.
Operation of the Principle of Le Châtelier in Alcohol Dehydrations
• removing a product from a reaction mixture as it is formed drives the equilibrium to the right, forming more product.
• Thus, the alkene, which usually has a lower boiling point than the starting alcohol, can be removed by distillation as it is formed, thus driving the equilibrium to the right to favor production of more product.
38
Carbocation Rearrangements
CH3
H3C
CH3
H
OH
CH3
H2SO4
CH3
H3C
CH3
H
CH2
CH3
H3C
CH3
CH3
39
Alcohols, Ethers and Epoxides
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.
40
Carbocation Rearrangements• A 1,2-shift can convert a less stable carbocation into a
more stable carbocation.• Rearrangements can occur whenever a carbocation is
formed as a reactive intermediate.
1,2-hydride shift
41
42
Dehydration of Alcohols Using POCl3 and Pyridine
• Dehydration under basic condition• A common method uses phosphorus oxychloride (POCl3) and
pyridine (an amine base) in place of H2SO4 or TsOH.
Dehydration proceeds by an E2 mechanism.
43
Dehydration of Alcohols Using POCl3 and Pyridine
Mechanism : E2 mechanism
44
Dehydration of Alcohols in the natural product synthesis
45
Conversion of Alcohols to Alkyl Halides with HX
• Substitution reactions do not occur with alcohols unless ¯OH is converted into a good leaving group.
• The reaction of alcohols with HX (X = Cl, Br, I) is a general method to prepare 1°, 2°, and 3° alkyl halides.
46
Conversion of Alcohols to Alkyl Halides with HX
• More substituted alcohols usually react more rapidly with HX:
• The mechanism depends on the structure of the R group.
47
Conversion of Alcohols to Alkyl Halides with HX
1o alcohols and methanol react via an SN2 mechanism
C
D
OHCH3
HHX
1. Protonation
Primary alcohol
(S)
2. SN2 with inversion of configuration
+C
D
CH3H
OH2
+C
D
CH3H
OH2X_
+ OH2CCH3
H
X
D+
(R)
48
2o and 3o alcohols react via an SN1 mechanism
CPh
OH
CH3CH3CH2
1. Protonation
2. SN1 reaction with racemization
CPh
CH3CH3CH2
OH2
HX+ X
_+
+C
Ph
CH3CH3CH2
OH2
Tertiary alcohol
C
CH3CH2CH3
Ph
+ OH2+
Carbocation (planar)
(R)
(a) Dissociation
(b) Attack of nucleophile
+C
CH3CH2CH3
Ph
CPh
CH3CH3CH2
X
Attack fromright
X_
Attack from left
X_
Racemate
CCH2CH3
CH3
PhX
(R) (S)
49
Alcohols, Ethers and Epoxides
Conversion of Alcohols to Alkyl Halides with HX
• The reactivity of hydrogen halides increases with increasing acidity.
HIRCH2 OH RCH2 I
HClRCH2 OH too slow to detect products
50
Conversion of Alcohols to Alkyl Halides with HX
• 2o alcohols
51
Conversion of Alcohols to Alkyl Halides
HIRCH2 OH RCH2 I
52
Conversion of Alcohols to Alkyl Halides with SOCl2 and PBr3
• For primary and 2° alcohols• SOCl2 (thionyl chloride) converts alcohols into alkyl chlorides.
• PBr3 (phosphorus tribromide) converts alcohols into alkyl bromides.
via an SN2 reaction
53
54
Conversion of Alcohols to Alkyl Halides with SOCl2 and PBr3
• For primary and 2° alcohols• SOCl2 (thionyl chloride) converts alcohols into alkyl chlorides.
• PBr3 (phosphorus tribromide) converts alcohols into alkyl bromides.
via an SN2 reaction
55
Alcohols, Ethers and Epoxides
Conversion of Alcohols to Alkyl Halides with SOCl2 and PBr3
56
57
Alcohols, Ethers and Epoxides
58
Tosylate—Another Good Leaving Group
• An alkyl tosylate is composed of two parts: the alkyl group R, derived from an alcohol; and the tosylate (short for p-toluenesulfonate), which is a good leaving group.
• A tosyl group, CH3C6H4SO2¯, is abbreviated Ts, so an alkyl tosylate becomes ROTs.
59
Tosylate—Another Good Leaving Group
• This process converts a poor leaving group (¯OH) into a good one (¯OTs).
• Tosylate is a good leaving group because its conjugate acid, p-toluenesulfonic acid (CH3C6H4SO3H, TsOH) is a strong acid (pKa = -7).
60
Tosylate—Another Good Leaving Group• Because alkyl tosylates have good leaving groups, they
undergo both nucleophilic substitution and elimination, exactly as alkyl halides do.
61
Tosylate—Another Good Leaving Group• Substitution occurs via an SN2 mechanism.
• Tosylates are equivalent to halides.
62
Example:
63
Alcohols, Ethers and Epoxides
Reaction of Ethers with Strong Acid
cleavage occurs either side of the ether oxygen atom to give two alkyl halide products, so ethers are not generally as useful as alcohols in synthesis.
64
Mechanism of
65
Mechanism of
66
Reactions of Epoxides
• Nucleophilic attack opens the strained three-membered ring, making it a favorable process even with a poor leaving group.
Reactions occur with strong nucleophiles (such as OH-, CH3S-, CH3O- or CN-), in the absence of acid and with weaker nucleophiles (such as Cl-) in the presence of strong acid (such as HCl).
67
Reactions of Epoxides
• Common nucleophiles that open the epoxide ring include ¯OH, ¯OR, ¯CN, ¯SR and NH3. With these strong nucleophiles.
• The reaction occurs by an SN2 mechanism.
68
Reactions of Epoxides : stereochemical consequences
Nucleophilic attack of ¯OCH3 occurs from the backside at either C—O bond, because both ends are similarly substituted. Since attack at either side occurs with equal probability, an equal amount of the two enantiomers (i.e., a racemic mixture) is formed.
69
Alcohols, Ethers and Epoxides
Reactions of Epoxides
Optically inactive starting materials give optically inactive products!
70
Alcohols, Ethers and EpoxidesReactions of Epoxides• Acids HZ that contain a nucleophile Z also open epoxide rings by
a two-step sequence.• HCl, HBr and HI, as well as H2O and ROH in the presence of acid,
all open an epoxide ring in this manner.
71
Reactions of Epoxides
72
Alcohols, Ethers and Epoxides
Reactions of Epoxides
73
Reactions of Epoxides
74
Leukotriene Synthesis and Asthma Drugs
Leukotriene C4Leukotriene A4
CO2H
C5H11
Arachidonic acid
C5H11
CO2H
OOH
Lipoxygenase
5-HPETE
CO2HO
C5H11
Sulfur nucleophile,RSH (glutathione)
CO2H
C5H11
OH
SR
..
..
Zileuton blocksthis step
S
N CONH2
HO
Zileuton
75
Alcohols, Ethers and EpoxidesImportant Epoxide Ring Opening Reactions
9.48, 9.49, 9.50, 9.52, 9.56, 9.58, 9.66, 9.67, 9.70, 9.71,
9.76
Homework
Write mechanisms for following reactions.
HIRCH2 OH RCH2 I
Preview of Chapter 10
Alkenes
Reactions of alkenes
Addition reaction --- What is Markovnikov’s rule?
- Which reactions follow markovnikov’s rule?Hydrohalogenation reaction, Hydration reactionHalogenation reaction, Halohydrin formationHydroboration reaction