Upload
callum-biggs
View
231
Download
3
Tags:
Embed Size (px)
DESCRIPTION
vreah
Citation preview
Chemistry 1103Charlie BondMCS Rm 4.16/[email protected]
What is Organic Chemistry?Organic Reactions I IIAlkanes (Ch 21)Conformational Analysis (Ch 21)Stereochemistry I II III (Ch 22)
S
Alkyl Halides I IIII (Ch 24)Alcohols and Ether I II (Ch 24)
2Alcohols - Nomenclature
IUPAC names the parent chain is the longest chain that
contains the -OH group number the parent chain in the direction that
gives the -OH group the lower number change the suffix -ee to -olol
Common names name the alkyl group bonded to oxygen
followed by the word alcoholalcohol
3Alcohols - Nomenclature
Ethanol(Ethylalcohol)
1Propanol(Propylalcohol)
2Propanol(Isopropylalcohol)
1Butanol(Butylalcohol)
OH
OHOH
OH
2Butanol(secButylalcohol)
2Methyl1propanol(Isobutylalcohol)
2Methyl2propanol(tertButylalcohol)
OH
Cyclohexanol(Cyclohexylalcohol)
OHOH
OH
4Alcohols - Nomenclature Compounds containing
two -OH groups are named as diols, three -OH groups are named as triols, etc.
CH3CHCH2HO OH
CH2CH2OHOH
CH2CHCH2OHHOHO
1,2Ethanediol(Ethyleneglycol)
1,2Propanediol(Propyleneglycol)
1,2,3Propanetriol(Glycerol,Glycerin)
5Polyols and sugarsLarger alcohol-containing molecules are of massive importance in biochemistry as they are used by cells as fuel, starting materials for making essential molecules (e.g. ribose in DNA), and to store information.
6Ignose and Godnose
Albert Szent-Gyorgyi isolated ascorbic acid and wanted to call the compound Ignose in his publication (as it resembles the sugars glucose and fructose). The journal editor refused and so he tried to call it Godnose instead.
Eventually they settled on hexuronic acid, until Charles King discovered that it was the same compound as vitamin C isolated from lemons.
7Physical Properties
Alcohols are polar compounds both the C-O and O-H bonds are polar
covalent
-
+
+O
HH H
C
H
8Hydrogen Bonding Association of ethanol molecules in the liquid
state (only two of the three possible hydrogen bonds to the upper oxygen are shown here).
9Boiling Points alcohols have higher boiling points and are
more soluble in water than hydrocarbons
CH3CH2CH2OHCH3CH2CH2CH3
CH3OHCH3CH3CH3CH2OHCH3CH2CH3
CH3CH2CH2CH2CH2OHHOCH2CH2CH2CH2OHCH3CH2CH2CH2CH2CH3
StructuralFormula Name
MolecularWeight(g/mol)
BoilingPoint(C)
SolubilityinWater
methanol 32 65 infiniteethane 30 89 insoluble
ethanol 46 78 infinitepropane 44 42 insoluble
1propanol 60 97 infinitebutane 58 0 insoluble
1pentanol 88 138 2.3g/100g1,4butanediol 90 230 infinitehexane 86 69 insoluble
10
Acidity of Alcohols
Most alcohols are about the same or slightly weaker acids than water
aqueous solutions of alcohols have the same pH as that of pure water
CH3O H O HH
[CH3 O- ][H3O+][CH3OH]
CH3O H OH
H+
Ka=
+ +
= 3.2x1016
pKa=15.5
11
Acidity of Alcohols pKa values for several low-molecular-weight alcohols
(CH3 )3COH
(CH3 )2CHOHCH3CH2OHH2OCH3OH
CH3COOHHCl
Compound pKa
7
15.515.715.91718
4.8hydrogenchloride
aceticacid
methanol
water
ethanol
2propanol2methyl2propanol
StructuralFormula
Strongeracid
Weakeracid
*AlsogivenforcomparisonarepKavaluesforwater,aceticacid,andhydrogenchloride.
12
Acidity of Phenols
Phenols are significantly more acidic than alcohols
pKa=9.95OH O-
Phenol Phenoxideion
+ H2O + H3O+
CH3CH2OH H2O CH3CH2O- H3O+ pKa=15.9
Ethanol Ethoxideion+ +
13
Acidity of Phenols the greater acidity of phenols compared with
alcohols is the result of the greater stability of the phenoxide ion relative to an alkoxide ion
ThesethreeKekulstructuresareequivalent
Thesethreecontributingstructuresdelocalizethenegativechargeonto
thecarbonsofthering
O O O O O:::::::
: : : : :
:
:
:
14
Rule of Thumb for OH pKa
Compound pKa Alkylsulfonates 0 Carboxylic acids 5 Phenols 10 Alcohols 15
15
Basicity of Alcohols In the presence of strong acids, the oxygen atom of an
alcohol behaves as a weak base proton transfer from the strong acid forms an oxonium
ion
thus, alcohols can function as both weak acids and weak bases
CH3CH2-O-H H OH
H OH
HH2SO4
CH3CH2-O HH
CH3CH2-O HH HH
O H
Ethyloxoniumion(pKa2.4)
Hydroniumion(pKa1.7)
Ethanol
++
+++
+
++
16
Reaction with Active Metals Alcohols react with Li, Na, K, and other active metals to
liberate hydrogen gas and form metal alkoxides Na is oxidized to Na+ and H+ is reduced to H2
alkoxides are somewhat stronger bases that OH- alkoxides can be used as nucleophiles in nucleophilic
substitution reactions they can also be used as bases in -elimination
reactions
2CH3CH2OH 2Na 2CH3CH2O-Na+ H2+ +Sodiumethoxide
17
Conversion of ROH to RX
Conversion of an alcohol to an alkyl halide involves substitution of halogen for -OH at a saturated carbon the most common reagents for this purpose
are the halogen acids, HX, and thionyl chloride, SOCl2
Is -OH a good leaving group?
18
Conversion of ROH to RX water-soluble 3 alcohols react very rapidly
with HCl, HBr, and HI
low-molecular-weight 1 and 2 alcohols are unreactive under these conditions
CH3COHCH3
CH3HCl CH3CCl
CH3
CH3H2O
2Chloro2methylpropane
2Methyl2propanol
25C+ +
19
Conversion of ROH to RX water-insoluble 3 alcohols react by bubbling gaseous
HCl through a solution of the alcohol dissolved in diethyl ether or THF
1 and 2 alcohols require concentrated HBr and HI to form alkyl bromides and iodides
+ HCl +H2OOHCH3
0Cether
ClCH3
1Chloro1methylcyclohexane
1Methylcyclohexanol
OH + HBr + H2O1Butanol 1Bromobutane
(Butylbromide)
Br
20
Reaction of a 3 ROH with HX 3 Alcohols react with HX by an SN1 mechanism
Step 1:Step 1: a rapid, reversible acid-base reaction transfers a proton to the OH group
this proton-transfer converts the leaving group from OH-, a poor leaving group, to H2O, a better leaving group
CH3-CCH3
CH3O H O H
HH CH3-C
CH3
CH3O
H
HO HH
+
2Methyl2propanol(tertButylalcohol)
Anoxoniumion
+
rapidandreversible + +
21
Reaction of a 3 ROH with HX Step 2:Step 2: loss of H2O from the oxonium ion gives a 3
carbocation intermediate
Step 3:Step 3: reaction with halide ion completes the reaction
CH3-CCH3
CH3O
H
HCH3-C+
CH3
CH3O HH
slow,ratedetermining
Anoxoniumion
+ +
A3carbocationintermediate
SN1
CH3-C+CH3
CH3Cl CH3-C Cl
CH3
CH3
fast+
2Chloro2methylpropane(tertButylchloride)
22
Reaction of a 1 ROH with HX 1 alcohols react by an SN2 mechanism
Step 1:Step 1: proton transfer to OH converts the leaving group from OH-, a poor leaving group, to H2O a better leaving group
Step 2:Step 2: nucleophilic displacement of H2O by Br-
Br - OHH
slow,ratedetermining
+ +SN2
OH
HBr
HH
H O O HH
rapidandreversible
Anoxoniumion
++
OH
H+OH
23
Reaction of ROH with HX
Same as for alkyl halides: Reactions are governed by a combination of electronic and steric effects
Increasingrateofcarbocationformation
3alcohol2alcohol1alcohol
SN1
SN2IncreasingrateofdisplacementofH2O
governedbystericfactors
governedbyelectronicfactors
neverreactbySN2
neverreactbySN1
24
From MasteringChem What is wrong with this mechanism
25
Dehydration of Alcohols
An alcohol can be converted to an alkene by elimination of H and OH from adjacent carbons (a -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 only at or slightly above room temperature
26
Dehydration of Alcohols
140oCCyclohexanol Cyclohexene
OH+ H2O
H2SO4
180oCCH3CH2OH
H2SO4CH2=CH2 + H 2O
+ H2 OCH3COHCH3
CH350oC
H2 SO4 CH3C=CH2
CH3
2Methylpropene(Isobutylene)
27
Dehydration of Alcohols
When isomeric alkenes are obtained, the more stable alkene (the one with the greater number of substituents on the double bond) generally predominates (Zaitsevs ruleZaitsevs rule)
CH3CH2CHCH3OH 85% H3PO4 CH3CH=CHCH3 CH3CH2CH=CH2
1Butene(20%)
2Butene(80%)
2Butanol+heat
28
Dehydration of a 2 Alcohol
A three-step mechanism Step 1:Step 1: proton transfer from H3O+ to the -OH
group converts OH-, a poor leaving group, into H2O, a better leaving group
CH3CHCH2CH3HO
OH
H H OH
H
CH3CHCH2CH3 OH
H+Anoxoniumion
rapidandreversible
++
+
29
Dehydration of a 2 Alcohol Step 2:Step 2: loss of H2O gives a carbocation
intermediate
Step 3:Step 3: proton transfer from an adjacent carbon to H2O gives the alkene and regenerates the acid catalyst
H OH
CH3CHCH2CH3 CH3CHCH2CH3 H2O
+slow,rate
determining ++
A2carbocationintermediate
CH3-CH-CH-CH3H
OH
H CH3-CH=CH-CH3 OH
HHrapid+
++
+E1
30
Dehydration of a 1 Alcohol
A two-step mechanism Step 1:Step 1: proton transfer gives an oxonium ion
Step 2:Step 2: proton transfer to solvent and loss of H2O gives the alkene and regenerates the acid catalyst
CH3CH2-O-H H OH
H OH
HCH3CH2 O
HH
rapidandreversible
+ +
H OH H
CH OH
CH2
H
HH O
HH +
+ HC C
H
H H+ O
HH+
slow,ratedetermining
E2
31
Hydration-Dehydration Acid-catalyzed hydration of an alkene and dehydration of
an alcohol are competing processes
large amounts of water favor alcohol formation scarcity of water or experimental conditions where
water is removed favor alkene formation
Le Chateliers Principle
Analkene Analcohol
C CH OH
H2O
acidcatalyst
+C C
Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31