Unit 6 – Alcohols and Ethers

Unit 6 – Alcohols and EthersTypes of AlcoholsNomenclatureProperties of

AlcoholsSynthesis of AlcoholsReactions of Alcohols

Types of Alcohols Alcohol:

organic compound containing one or more hydroxyl (OH) groups.

Alcohols are often classified by the type of carbinol carbon atom present the carbon bonded to the OH group

CH3OH

CH3CH2OH

CH3CHCH3

OH

CH3OH

CH3CH2OH

CH3CHCH3

OH

CH3OH

CH3CH2OH

CH3CHCH3

OH

Methanolwood alcohol

Ethanoldrinking alcohol

Isopropyl alcoholrubbing alcohol

CH3OH

CH3CH2OH

CH3CHCH3

OHOH

CH3

H3C

CH3

CCH3OH

CH3CH2OH

CH3CHCH3

OH

Types of Alcohols Primary alcohol:

the carbinol carbon is attachedto one other carbon atom

Secondary alcohol: the carbinol carbon is attached

to two other carbon atoms

Tertiary alcohol: the carbinol carbon is attached

to three other carbon atoms

CH3OH

CH3CH2OH

CH3CHCH3

OH

Types of Alcohols Diol:

an alcohol with two OH groups

Glycol: a vicinal diol

OH groups on adjacent carbons

OHCH3

HO OH

OHCH3

HO OH

HO

OH OHCH3

OH

CH3

Ethylene glycolcar antifreeze

propylene glycolmedicine, food

Types of Alcohols Phenol:

a compound with a hydroxyl group bonded directly to an aromatic (benzene) ring

Thiols: an organic compound with an SH group

sulfur analog of alcoholsalso called mercaptans

CH3CH2CH2CH2SH

OHphenol

3-methyl-1-butanethiolCH3CHCH2CH2SH

CH3CH=CHCH2SH

OH

CH3

IR of Alcohols Alcohols typically exhibit a strong, broad,

rounded peak at about 3300 cm-1+ for the O-H bond.

SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, 11/1/09)

IR of Alcohols In the absence of hydrogen bonding, the

O – H peak can be relatively sharp and hard to distinguish from an N – H peak. The frequency for an O-H peak in the

absence of hydrogen bonding is typically somewhat higher (~3600 cm-

1).

The O-H peak for a carboxylic acid is much broader and typically spans the distance from ~2500 – 3500 cm-1.

IR of Alcohols

acid O-H acid O-H

alcohol O-H w/hydrogen bonding

alcohol O-H w/o hydrogen bonding

Nomenclature IUPAC Naming System for Alcohols:

Find the longest continuous chain that contains the carbinol carbon atom

Drop the “e” from the corresponding alkane name and add the suffix “ol”

a substituted pentanolOH

CH3

Nomenclature IUPAC Naming System for Alcohols:

Number the longest chain starting from the end closest to the OH group.OH takes priority over double and triple bonds

OH group is assumed to be on C #1 on a cycloalkane ring

Name and number all substituents as with an alkane or alkene.

(2R, 3R)-3-methyl-2-pentanol

Don’t forget R or S if appropriate.

OH

CH3

Nomenclature To name an alcohol that contains a

double bond: use “ol” suffix after the name of the

alkene (drop the last “e”) number the chain to give the carbinol

carbon the lowest possible number place the position number for the C=C

in front of the base name (or before the “en”) and the position number for the OH in front of the “ol” suffix

OHCH3

4-methylhex-5-en-1-ol4-methyl-5-hexen-1-ol

Nomenclature To name diols:

use suffix “diol” after the name of the alkane (keep the ending “e”)

indicate the position of each OH groupplace position of each OH group in front of the base name or in front of the suffix “diol”

OH

OH

2, 3-pentanediolor

pentane-2, 3-diol

Nomenclature To name thiols:

Use the same rules for naming alcoholsuse “thiol” suffix instead of “ol” suffix

SH2-pentanethiol

SH

CH3

3-methyl-2-pentanethiol

Nomenclature Common names are often used with

phenols:

SH

CH3

OHBr

NO2

OH

Cl

OH

SH

CH3

OHBr

NO2

OH

Cl

OH

SH

CH3

OHBr

NO2

OH

Cl

OH

SH

CH3

OHBr

NO2

OH

Cl

OH

OH

phenol

2-bromophenol or

ortho-bromophenol

3-nitrophenol or

meta-nitrophenol

4-chlorophenol or

para-chlorophenol

ortho = 1,2 meta = 1,3 para = 1,4

Nomenclature Common names are often used with

phenols:OH

CH3

OHOH

OH

OH

OH

OH

OHCH3

OHOH

OH

OH

OH

OH

OHCH3

OHOH

OH

OH

OH

OH

OHCH3

OHOH

OH

OH

OH

OH

2-methylphenolortho-cresol

Benzene-1,2-diolcatechol

Benzene-1,3-diolresorcinol

Benzene-1,4-diolhydroquinone

OH OHOH

OH

OH

OH

OH

CH3

4-methylphenolpara-cresol

NomenclatureExample: Name the following compounds:

OH

CH3

OH

CH3

SH

OH

CH3

SH

OH

OH

OH

CH3

SH

OH

OH

OHBr

NomenclatureExample: Draw the structures of the following compounds.

3-isopropyl-2-methyl-2-hexanol

trans-2-bromocyclohexanol

(E)-2-chloro-2-buten-1-ol

Properties of Alcohols The physical properties of alcohols are

strongly influenced by the presence of the hydrophilic (“water loving”) OH group. The OH group is capable of forming

hydrogen bonds with other alcohol groups or with water.

Properties of Alcohols Due to hydrogen bonding, alcohols have

significantly higher boiling points than alkanes with comparable molecular weights.

CH3CH2OH MW = 46BP = 78oC

CH3CH2CH3 MW = 44 BP = - 42oC

Properties of Alcohols BP increases as the amount of hydrogen

bonding increases: 1-propanol BP = 97oC

1,2-propanediol BP = 188oC(propylene glycol)

1,2,3-propanetriol BP = 290oC(Glycerol)

Physical Properties

Alcohols with 1-3 carbons are soluble in water.

Properties of Alcohols Solubility decreases as the size of the

alkyl group increases. 1-hexanol is less soluble than ethanol

Solubility increases as the alkyl group becomes more compact/spherical. t-butyl alcohol is more soluble than 1-

butanol.

Solubility increases with increasing number of OH groups.

Acidity of Alcohols and Phenols The acidity of alcohols varies widely and

can be expressed using the acid dissociation constant, Ka

ROH + H2O RO- + H3O+

Ka = [RO-][H3O+] pKa = -log Ka[ROH]

Acidity increases as: Ka increases pKa decreases

Alkoxide ion

Acidity of Alcohols and Phenols Structural trends and acidity:

Acidity of H2O, CH3OH, and CH3CH2OH are similar. (pKa = 15.7, 15.5, and 15.9 respectively)

Acidity decreases as number of carbons in the R group increases.CH3OH pKa = 15.5t-butyl alcohol pKa = 18.0

Acidity of Alcohols and Phenols Structural trends and acidity:

Acidity increases with the addition of electron-withdrawing halogens

ClOH OH

pKa = 14.3 pKa = 15.9

C

OH

CH2OHClCl

Cl

pKa = 12.2

Acidity of Alcohols and Phenols Structural trends and acidity:

Phenols are more acidic than water or alcohols

C

OH

CH2OHClCl

Cl

OH

C

OH

CH2OHClCl

Cl

OH

OHpKa = 18.0 pKa = 10.0

O O

O

O

O O

O

O

O O

O

O

O O

O

O

Acidity of Alcohols and Phenols Phenols are stronger acids than alcohols.

React readily with aqueous NaOH

Phenols are soluble in aqueous strong base solutions but are insoluble in aqueous NaHCO3.

+ NaOH + H2O

NaOH O +

Sodium phenoxide

Acidity of Alcohols and Phenols Since alcohols are very weak acids, a very

strong base is used to remove the acidic proton.

Unless they are water soluble, alcohols are not soluble in aqueous base solution.

CH3CH2OH Na(s) CH3CH2O-Na+ H2 (g)+ 1/ 2+

OH + K (s) O-K+ + 1/ 2 H2 (g)

ROH + NaH THF RO-Na+ + H2 (g)

Synthesis of Alcohols Nucleophilic Substitution on an alkyl halide

SN2 conditions most usefulstrong nucleophilemethyl > 1o > 2o

inversion of configuration

C NaOHacetone

HOR

XH

HCH

H

R

Synthesis of Alcohols Acid Catalyzed Hydration (Hydrolysis) of

Alkenes

Markovnikov product formed Equilibrium process Less useful synthetically than other

methods rearrangements often occur

C NaOHacetone

HO

CH2SO4

H2O C

RX

HH

CH

H

R

C CR R

OH

Synthesis of Alcohols Oxymercuration-Demercuration of

Alkenes

Markovnikov product

Anti addition to double bond

C NaOHacetone

HO

C(1) Hg(OAc)2 (aq)

(2) NaBH4C

RX

HH

CH

H

R

C CR R

OH

H

Synthesis of Alcohols Hydroboration-Oxidation of Alkenes

Anti-Markovnikov product

Syn addition to double bond

C NaOHacetone

HO

C(1) BH3-THF

(2) H2O2, NaOH C

RX

HH

CH

H

R

C CR R

HOH

Synthesis of Alcohols Syn Hydroxylation of Alkenes

Syn addition to double bond

Reagents: OsO4, H2O2

cold, dilute KMnO4, OH-or

C NaOHacetone

HO

C COsO4, H2O2

RX

HH

CH

H

R

C CR R

OH OH

Synthesis of Alcohols Anti-Hydroxylation of Alkenes

epoxide intermediate

Anti addition to double bond Common peroxyacids:

CH3CO3H MCPBA

C NaOHacetone

HO

C C(1) RCO3H(2) H+/H2O

RX

HH

CH

H

R

C CR R

OH

OH