Alcohols - University of Texas at Austincolapret.cm.utexas.edu/courses/Chap 10.pdfAlcohols Chapter 10 2 ... Reaction with Metals 14 • Alcohols are also converted to metal alkoxides

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AlcoholsAlcohols

Chapter 10Chapter 10

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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

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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


1o 1o

1o

2o

2o 3o

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• 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)


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• 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

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•• 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.


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E.G. The association of ethanol molecules in the liquid state by hydrogen bonding.

Hydrogen BondingHydrogen Bonding

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• 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

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• 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.


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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

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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

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• 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

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• 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)

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– 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

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– 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

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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+

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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

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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

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• 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

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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

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• 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

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• 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

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• 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

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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)


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– 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


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• 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


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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+


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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


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• 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

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• 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

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• 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


(Chromic acid in acetone is(Chromic acid in acetone isthe Jones oxidation)the Jones oxidation)

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• 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


R R'

H2CrO4

R R'

O

or Na2Cr2O7 /H2SO4or CrO3/H2SO4

2o ROH Ketones

OH

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•• 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

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– PCC oxidizes a 1° alcohol to an aldehyde.

PCC

Geraniol Geranial

OH H

O


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– PCC oxidizes a 2° alcohol to a ketone.

OH PCC O

MenthoneMenthol


R R' R R'

O

2o ROH Ketones

OH

PCC or PDC

CH2Cl2

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Alcohols - University of Texas at Austincolapret.cm.utexas.edu/courses/Chap 10.pdfAlcohols Chapter 10 2 ... Reaction with Metals 14 • Alcohols are also converted to metal alkoxides