Alcohols, Ethers and Thiols

Bettelheim, Brown, Campbell and Farrell

Chapter 14

Review of Carbon Classification

Type of C Attached to

1o (primary) C one other carbon

2o (secondary) C two other carbons

3o (tertiary) C three other carbons

4o (quaternary) C four other carbons

Alcohols• Alcohol:Alcohol: Has an -OHOH (hydroxyl) group(hydroxyl) group bonded

to a tetrahedral carbon– methanol, CH3OH, is the simplest alcohol

Classification of Alcohols

• Depends on the C which has the –OH group attached

CH2CH3 OH

ethanol

CH2CH3 OH

ethanol CH3CHCH3

OH

2-propanol

CH3CHCH3

OH

2-propanol

2-methyl-2-propanol

CH3CCH3

OH

CH32-methyl-2-propanol

CH3CCH3

OH

CH3

1o Alcohol—attached to one C

2o Alcohol—

attached to

two Cs

3o Alcohol--

attached to three Cs

Alcohol Nomenclature1.Find longest carbon chain that contains the -OH group

(parent chain)

2.Number chain from end that gives the -OH the lower number

3.Change the ending -ee to -olol

4.Use a number to show the location of the -OH group

For cyclic alcohols, the carbon with the -OH group is C-1

5.Name and number substituents and list them in alphabetical order

Nomenclature

Ethanol(Ethyl alcohol)

1-Propanol(Propyl alcohol)

2-Propanol(Isopropyl alcohol)

1-Butanol(Butyl alcohol)

OH

OH

OHOH

2-Butanol(sec-Butyl alcohol)

2-Methyl-1-propanol(Isobutyl alcohol)

2-Methyl-2-propanol(tert-Butyl alcohol)

OH

Cyclohexanol(Cyclohexyl alcohol)

OHOH

OH

Nomenclature• Write the IUPAC name for each alcohol

(a) (b)OH OH

(c) (d) OHOH

Nomenclature

(a) (b)OH OH

(c) (d) OHOH

1

22

2 2

4

trans-2-Methylcyclohexanol4-Methyl-2-pentanol

2-Heptanol

7

5

3

2,2-Dimethyl-1-propanol

1

Nomenclature– Alcohols containing more than one hydroxyl

groups are named as a dioldiol, a trioltriol, and so on– IUPAC names for diols, triols, and so on retain

the final "-e" in the name of the parent alkane– Compounds containing two hydroxyl groups

on adjacent carbons are commonly called glycolsglycols

CH3CHCH2HO OH

CH2CH2OH OH

CH2CHCH2

OH OHOH1,2-Ethanediol

(Ethylene glycol)1,2-Propanediol

(Propylene glycol)1,2,3-Propanetriol

(Glycerol, Glycerin)

Name the following

CH2CH2CH2

OHOHHO

OH

Name the following

CH2CH2CH2

OHOHHO

OH

1,3-propanediol trans-1,3-cyclohexanediol

Physical Properties• Alcohols are polar molecules

– the C-O and O-H bonds are both polar covalent

Physical Properties

Alcohol molecules can form hydrogen bonds with each other

Alcohols can also form hydrogen bonds with water

Physical Properties of Alcohols– Boiling point increases as MW increases– Solubility in water decreases as MW increases

CH3CH2CH2OH

CH3CH2CH2CH3

CH3OHCH3CH3

CH3CH2OH

CH3CH2CH3

CH3CH2CH2CH2OH

CH3CH2CH2CH2CH3

Structural Formula NameMolecularWeight

bp(°C)

Solubilityin Water

methanol 32 65 infiniteethane 30 -89 insoluble

ethanol 46 78 infinite

propane 44 -42 insoluble

1-propanol 60 97 infinite

butane 58 0 insoluble

8 g/100 g117741-butanol

pentane 72 36 insoluble

Chemical Properties of Alcohols

• Combustion Reactions (they burn in O2)

• Acid-Base Chemistry

• Dehydration

• Oxidation

Acidity of Alcohols

• Alcohols have about the same pKa values as water– Aqueous solutions of alcohols have the same

pH as that of pure water

– Phenols have lower pKa values than normal alcohols do (are more acidic)

Acidity of Alcohols

• Alcohols and phenols both contain an OH group– Phenols are weak acids and react with strong bases

such as NaOH to form water-soluble salts– pKa = 9.89 for phenol

– Alcohols are much weaker acids and do not react in this manner

OH NaOHH2O

O-Na+ H2O+

Phenol Sodium phenoxide(a water-soluble salt)

+

Why are phenols more acidic than alcohols?

Aromatic ring and O have delocalization of electrons. Less pull on H, so it is more acidic.

O H

Dehydration

• Dehydration:Dehydration: Removal of water from adjacent carbons to form an alkene

– most often occurs by heating an alcohol with either 85% H3PO4 or concentrated H2SO4

CH3CH2OHH2SO4 CH2=CH2 H2O+

Ethanol Ethylene180°C

• Dehydration essentially the reverse of hydration (addition of water to double bond)

• Conditions required depend on kind of alcohol

Acid-Catalyzed DehydrationCH3CH2OH

H2SO4 CH2=CH2 H2O+Ethanol Ethylene

180°C

OHH2SO4 H2O140°C

Cyclohexanol Cyclohexene

+

CH3CCH3

OH

CH3H2SO4 CH3C=CH2

CH3

H2O50°C

2-Methyl-2-propanol (tert -Butyl alcohol)

2-Methylpropene (Isobutylene)

+

1o ca 180oC needed

Hard

2o lower temperatures

Moderate

3o slightly above room temperature

Easy

Dehydration– Zaitsev’s Rule: When two alkene products are

possible, the alkene having the greater number of alkyl groups (i.e., C atoms) on the double bond generally predominates (forms major product)

– Major product favored.

CH3CH2CHCH3

OH H3PO4 CH3CH=CHCH3 CH3CH2CH=CH2

1-Butene (20%)

2-Butene (80%)

2-Butanol

+-H2O

CH3CHCHCH3OH

CH3 H2SO4CH3C=CHCH3

CH3

CH3CHCH=CH2

CH3

3-Methyl-1-butene2-Methyl-2-butene (major product)

3-Methyl-2-butanol

+-H2O

(minor product)

CH3CHCHCH3OH

CH3 H2SO4CH3C=CHCH3

CH3

CH3CHCH=CH2

CH3

3-Methyl-1-butene2-Methyl-2-butene (major product)

3-Methyl-2-butanol

+-H2O

CH3CH2CHCH3

OH H3PO4 CH3CH=CHCH3 CH3CH2CH=CH2

1-Butene (20%)

2-Butene (80%)

2-Butanol

+-H2O

Dehydration-Hydration Related• Acid-catalyzed hydration of alkenes and acid-

catalyzed dehydration of alcohols are competing reactions

• Equilibrium between alkene and alcohol exists

– Large amounts of water favor alcohol formation, while removal of water from the equilibrium mixture favors alkene formation (Le Chatelier's principle)

An alkene An alcohol

C CH OH

+ H2Odehydration

hydrationC C

Dehydration vs Oxidation

• Dehydration: Remove –OH and –H– OH from one C: H from adjacent C

• Oxidation: Remove two –H atoms – One H from OH group: Other H from C that

has the –OH attached

Oxidation of Alcohols• Oxidation of a 1° alcohol gives an aldehyde or

a carboxylic acid, depending on the experimental conditions– Oxidation of a 1° alcohol to a carboxylic acid is

carried out using an oxidizing agent such as potassium dichromate, K2Cr2O7, in aqueous sulfuric acid

CH3(CH2)6CH2OHK2Cr2O7

CH3(CH2)6CHO K2Cr2O7

H2SO4 H2SO4CH3(CH2)6COH

O

Octanal Octanoic acid1-Octanol

Oxidation of Alcohols

– Can sometimes stop the oxidation at the aldehyde stage by distilling the mixture (Aldehydes usually have lower boiling points than either the 1° alcohol or the carboxylic acid)

CH3(CH2)6CH2OHK2Cr2O7

CH3(CH2)6CHO K2Cr2O7

H2SO4 H2SO4CH3(CH2)6COH

O

Octanal Octanoic acid1-Octanol

Carboxylic acidAldehydeAlcohol

Oxidation of Alcohol– Oxidation of a 2° alcohol gives a ketone

– Tertiary alcohols are resistant to oxidation

CH3

OH

K2Cr2O7

H2SO4

1-Methyl-cyclopentanol

(no oxidation)

OH

K2Cr2O7

H2SO4 O

2-Isopropyl-5-methyl-cyclohexanone(Menthone)

2-Isopropyl-5-methyl-cyclohexanol(Menthol)

Common Alcohols

• Ethanol– Ethyl alcohol, “grain alcohol”– Produced by fermentation– Natural fermentation of sugars and starches

in grains, corn, molasses, fruits, vegetables– Found in beer, wine, and other “spirits”

C6H12O6 → 2 CH3CH2OH + 2 CO2

glucose ethanol carbon dioxide

Common Alcohols

• Methanol – Methyl alcohol or “wood alcohol”– Distilled from wood pulp originally– Very toxic, even in small amounts– Reacts to make formaldehyde (a fixative)

CH3OH → CH2=O

methanol formaldehyde

Common Alcohols

• Isopropyl alcohol– 2-propanol “rubbing alcohol”– Evaporates quickly—used to reduce fever– Solvent in many cosmetics, perfumes, etc.

CH3CHCH3

OH

2-propanol

CH3CHCH3

OH

2-propanol

Common Alcohols

• 1,2-ethanediol– Ethylene glycol – Very soluble in water– Used as antifreeze

• Glycerol– 1,2,3-propanetriol – Very soluble in water– Sweet taste– Part of fats and oils

Preparation of Important Alcohols

Many alcohols (or their derivatives) can be formed by reacting smaller molecules.

CO

CO2H2 CH3OH

O2 CH2O

CH3COOHCoalor

methane Carbonmonoxide

Methanol

Acetic acid

Formaldehydeoxidation

catalyst

Preparation of Important Alcohols

H2O, H2SO4

CH3CH=CH2

CH3CHCH3

OH

HOCH2CHCH2OHOHPropene

2-Propanol

Glycerin, glycerol

several steps

CH2=CH2

O2

H2O, H2SO4

OH2C CH2

CH3CH2OH

H2O, H2SO4

H2SO4

HOCH2CH2OH

CH3CH2OCH2CH3

Ethylene

Ethanol

Ethylene glycol

180°C Diethyl ether

catalystEthylene

oxide

Ethers

• The functional group of an etherether is an oxygen atom bonded to two carbon atoms – Simplest ether is dimethyl ether– Most common ether is diethyl ether

• Often called just “ether”

CH3-O-CH3 CH3CH2-O-CH2CH3

Dimethyl ether Diethyl ether

Ether Nomenclature• Generally use common names for small (low-

molecular-weight) ethers– common names are derived by listing the alkyl

groups bonded to oxygen in alphabetical order and adding the word "ether”

– alternatively, name one of the groups on oxygen as an alkoxy group

OCH3CH3CH2OCH2CH3

Cyclohexyl methyl ether(Methoxycyclohexane)

Diethyl ether

Ether Nomenclature• Cyclic ether:Cyclic ether: an ether in which one of the atoms

in a ring is oxygen– Cyclic ethers also known by their common names– ethylene oxide is an important building block for the

organic chemical industry; it is also used as a fumigant in foodstuffs and textiles, and in hospitals to sterilize surgical instruments

– tetrahydrofuran is a useful laboratory and industrial solvent

Ethylene oxide Tetrahydrofuran (THF)

OO

Physical Properties of Ethers• Ethers are polar compounds

– O has a partial negative charge – Each C bonded to it has a partial positive charge

• Only weak forces of attraction exist between ether molecules in the pure liquid

• Boiling points of ethers are close to those of hydrocarbons of similar molecular weight

• Have lower boiling points than alcohols of the same molecular formula

CH3CH2OH CH3OCH3

bp -24°C

Ethanol

bp 78°C

Dimethyl ether

Reactions of Ethers• Ethers not very reactive (like hydrocarbons)

– Do not react with oxidizing agents such as potassium dichromate

– Do not react with reducing agents such as H2 in the presence of a transition metal catalyst

– Not affected by most acids or bases at moderate temperatures

• Good solvents for carrying out organic reactions, because of their general inertness and good solvent properties– Diethyl ether and THF (tetrahydrofuran)

Thiols

• Thiol:Thiol: a compound containing an -SHSH (sulfhydryl)(sulfhydryl) groupgroup– Also called mercaptans– Low-molecular-weight thiols STINK– Responsible for smells such as those from rotten

eggs and sewage– the scent of skunks is due primarily to these two

thiols

CH3CH=CHCH2SH

CH3

CH3CHCH2CH2SH3-Methyl-1-butanethiol2-Butene-1-thiol

Thiols - Nomenclature• IUPAC names are derived in the same manner

as are the names of alcohols– Retain the final -e-e of the parent alkane and add the

suffix -thiol-thiol – Common names for simple thiols are derived by

naming the alkyl group bonded to -SH and adding the word "mercaptanmercaptan"

CH3CH2SH

CH3

CH3CHCH2SHEthanethiol

(Ethyl mercaptan)2-Methyl-1-propanethiol

(Isobutyl mercaptan)

Thiol Physical Properties• S-H bonds are nonpolar covalent

– Small difference in electronegativity between sulfur and hydrogen (2.5 - 2.1 = 0.4)

– No hydrogen bonding– Lower boiling points than comparable alcohols– Less soluble in water and other polar solvents than

comparable alcohols

11778

65

1-butanolethanol

methanol

9835

6

1-butanethiolethanethiol

methanethiol

bp (°C)Alcoholbp (°C)Thiol

Reactions of Thiols

• Thiols are weak acids (pKa~10), and are comparable in strength to phenols– thiols react with strong bases such as NaOH

to form water-soluble thiolate salts

CH3CH2SH NaOHH2O

CH3CH2S-Na+ H2O+Ethanethiol

(pKa 10)

+Sodium

ethanethiolate

Reactions of Thiols• The most important thiol reaction in biological

systems is their oxidation to disulfides – Functional group is a disulfidedisulfide (-S-S-)(-S-S-) bondbond

• Disulfide bonds important in protein structure

– Thiols are readily oxidized to disulfides by O2

– -SH group very susceptible to oxidation and must be protected from contact with air during storage

– disulfides, in turn, are easily reduced to thiols by several reducing agents.

Reactions of Thiols

Oxidation of –SH groups to form S-S bondsReduction of S-S bonds to form –SH groups

2HOCH2CH2SH HOCH2CH2S-SCH2CH2OHA disulfide

oxidationreductionA thiol