Chemistry 1103Charlie BondMCS Rm 4.16/4.27Charles.Bond@uwa.edu.au

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

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