Ch14Alcohols, Ethers and Thiols

8/10/2019 Ch14Alcohols, Ethers and Thiols

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ALCOHOLSAlcohol:Has an -OH (hydroxyl) group

bonded to a tetrahedral carbon

methanol, CH3OH, is the simplest alcohol

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CLASSIFICATION OF ALCOHOLS

Aliphatic general formula CnH2n+1OH - provided there are no rings

the OH replaces an H in a basic hydrocarbon skeleton

Aromatic in aromatic alcohols (or phenols)

the OH is attached directly to the ring

an OH on a side chain of a ring behavesas a typical aliphatic alcohol

The first two compounds are

classified as aromatic alcohols

(phenols) because the OH group

is attached directly to the ring.

Structural differences alcohols are classified according to the environment of the OH group

chemical behaviour, eg oxidation, often depends on the structural type


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CLASSIFICATION OF ALCOHOLS

Depends on the C which has theOHgroup attached

CH2CH3 OHethanol CH3CHCH3

OH

2-propanol

2-methyl-2-propanol

CH3CCH3

OH

CH31oAlcoholattached to

one C

2oAlcohol

attached to

two Cs

3oAlcohol--attached to

three Cs

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ALCOHOL NOMENCLATURE

1. 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 -e to -ol

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

group

5. For cyclic alcohols, the carbon with the -OH

group is C-1

6. Name and number substituents and list them

in alphabetical order 6


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NOMENCLATURE

Ethanol(Ethyl alcohol)

1-Propanol(Propyl alcohol)

2-Propanol(Isopropyl alcohol)

1-Butanol(Butyl alcohol)

OH

OH

OH

OH

2-Butanol(sec-Butyl alcohol)

2-Methyl-1-propanol(Isobutyl alcohol)

2-Methyl-2-propanol

(t er t -Butyl alcohol)

OH

Cyclohexanol

(Cyclohexyl alcohol)

OH

OH

OH

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NOMENCLATURE

Write the IUPAC name for each alcohol

(a) (b)OH OH

(c) (d) OH

OH

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NOMENCLATURE

(a) (b)OH OH

(c) (d) OH

OH

1

22

2 2

4

t rans-2-Methylcyclohexanol4-Methyl-2-pentanol

2-Heptanol

7

5

3

2,2-Dimethyl-1-propanol

1

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NOMENCLATURE

Alcohols containing more than one hydroxyl

groups are named as a diol, a triol, 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

glycols

CH3

CHCH2

HO OH

CH2 CH2

OH OH

CH2 CHCH2

OH OHOH

1,2-Ethanediol(Ethylene glycol)

1,2-Propanediol(Propylene glycol)

1,2,3-Propanetriol(Glycerol, Glycerin)

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NAME THE FOLLOWING

CH2CH2CH2

OHOHHO

OH

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NAME THESE:

CH3 CH

CH3

CH2OH

CH3 C

CH3

CH3

OH

CH3 CH

OH

CH2CH3

2-methyl-1-propanol

2-methyl-2-propanol

2-butanol

OH

Br CH3

3-bromo-3-methylcyclohexanol

=>


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PHYSICAL PROPERTIES

Alcohols are polar molecules

the C - O and O - bonds are both polarcovalent

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Alcohols can also formhydrogen bonds with water

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PHYSICAL PROPERTIES OF ALCOHOLS

Boiling point increasesas MW increases

Solubility in water decreasesas MW increases

CH3 CH2 CH2 OH

CH3 CH2 CH2 CH3

CH3 OH

CH3

CH3

CH3 CH2 OH

CH3

CH2

CH3

CH3 CH2 CH2 CH2 OH

CH3 CH2 CH2 CH2 CH3

Structural Formula NameMolecular

Weight

bp(C)

Solubilityin Water

methanol 32 65 infinite

ethane 30 -89 insolub le

ethanol 46 78 infinite

propane 44 -42 insolub le

1-propanol 60 97 infinite

butane 58 0 insolub le

8 g/100 g117741-butanol

pentane 72 36 insolub le16


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CHEMICAL PROPERTIES OF ALCOHOLS

Acid-Base Chemistry

Dehydration

Oxidation

Combustion Reactions (they burn in O2)

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ACIDITY OF ALCOHOLS

Alcohols have about the same pKavaluesas water

Aqueous solutions of alcohols have the samepH as that of pure water

Phenols have lower pKavalues than normal

alcohols do (are more acidic)

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ACIDITY OF ALCOHOLS

Alcohols and phenols both contain an OH group

Phenolsare weak acids and react with strong basessuch as NaOH to form water-soluble salts

pKa= 9.89 for phenol 15 for alcohol

Alcohols are much weaker acids and do notreact inthis manner

OH NaOH H2 O

O-

Na+

H2 O+

Phenol Sodium phenoxide(a water-soluble salt)

+

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CH3 OH

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TABLE OF KaVALUES


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

O H

WHY ARE PHENOLS MORE ACIDIC THAN ALCOHOLS?

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DEHYDRATION

Dehydration :Removal of waterfromadjacent carbonsto form an alkene

most often occurs by heating an alcohol witheither 85% H3PO4or concentrated H2SO4

CH3 CH2 OHH2 SO4 CH2 = CH2 H2 O+

Ethanol Ethylene180C

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Dehydration essentially the reverse of

hydration (addition of water to doublebond)

Conditions required depend on kind of

alcohol

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ACID-CATALYZED DEHYDRATION

CH3

CH2

OHH2 SO4

CH2

= CH2

H2

O+

Ethanol Ethylene180C

OHH2 SO4 H2 O

140CCyclohexanol Cyclohexene

+

CH3

CCH3

OH

CH3

H2 SO4 CH3 C=CH2

CH3

H2 O50C

2-Methyl-2-propanol

(t er t-Butyl alcohol)

2-Methylpropene (Isobutylene)

+

1oca 180oC needed

Hard

2o lower

temperatures

Moderate

3o slightlyabove room

temperature

Easy

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DEHYDRATION Zaitsevs Rule: When two alkene products are

possible, the alkene having the greater numberof alkyl groups (i.e., C atoms) on the double

bond generally predominates (forms major

product)

Major product favored.

CH3CH

2CHCH

3

OHH3PO4

CH3CH=CHCH

3 CH

3CH

2CH=CH

2

1-Butene

(20%)

2-Butene

(80%)

2-Butanol

+-H2O

CH3 CHCHCH3

OH

CH3 H2 SO4CH3 C= CHCH3

CH3

CH3 CHCH= CH2

CH3

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

3-Methyl-2-butanol

+- H2 O

(minor product)26


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CH3 CHCHCH3

OH

CH3 H2 SO4CH3 C=CHCH3

CH3

CH3 CHCH= CH2

CH3

3-Methyl-1-butene2-Methyl-2-butene

(major prod uct)

3-Methyl-2-butanol

+- H2 O

CH3CH

2CHCH

3

OH

H3PO4 CH3CH=CHCH

3 CH

3CH

2CH=CH

2

1-Butene (20%)

2-Butene (80%)

2-Butanol

+-H2O

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DEHYDRATION-HYDRATION RELATED

Acid-catalyzed hydration of alkenes andacid-catalyzed dehydration of alcohols arecompeting 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 (LeChatelier's principle)

An alkene An alcohol

C C

H OH

+ H2 O dehydration

hydrationC C

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DEHYDRATION VS OXIDATION

Dehydration: RemoveOH andH

OH from one C : H from adjacent C

Oxidation : Remove twoH atomsOne H from OH group :

Other H from C that has theOH attached

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OXIDATION OF ALCOHOLS

Oxidation of a 1 alcohol gives an aldehydeor a carboxylic acid, depending on theexperimental conditions

Oxidation of a 1 alcohol to a carboxylic acidis carried out using an oxidizing agent such aspotassium dichromate, K2Cr2O7, in aqueoussulfuric acid

CH3 ( CH2 )6CH2OHK2Cr2O7

CH3 ( CH2 ) 6CH

O K2Cr2O7

H2SO4 H2SO4CH3 (CH2 )6COH

O

Octanal Octanoic acid1-Octanol 30


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OXIDATION OF ALCOHOLS

Can sometimes stop the oxidation at thealdehyde stage by distilling the mixture(Aldehydes usually have lower boilingpoints than either the 1 alcohol or thecarboxylic acid)

CH3( CH

2)6CH

2OH

K2Cr2O7CH

3( CH

2)6CH

O K2Cr2O7

H2SO4 H2SO4CH

3(CH

2)6COH

O

Octanal Octanoic acid1-Octanol

Carboxylic acidAldehydeAlcohol

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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)

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OTHER REACTIONS OF ALCOHOLS

OXYGEN Alcohols make useful fuels

C2H5OH(l) + 3O2(g) > 2CO2(g) + 3H2O(l)

Advantages have high enthalpies of combustion

do not contain sulphur so there is less pollutioncan be obtained from renewable resources


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OTHER REACTIONS OF ALCOHOLS

OXYGEN Alcohols make useful fuels

C2H5OH(l) + 3O2(g) > 2CO2(g) + 3H2O(l)

Advantages have high enthalpies of combustion

do not contain sulphur so there is less pollutioncan be obtained from renewable resources

SODIUM

Conditions room temperature

Product sodium alkoxide and hydrogen

Equation 2CH3CH2OH(l) + 2Na(s) > 2CH3CH2O Na+ + H2(g)

sodium ethoxide

Notes alcohols are organic chemistrys equivalent of waterwater reacts with sodium to produce hydrogen and so do alcohols

the reaction is slower with alcohols than with water.

Alkoxides are white, ionic crystalline solids e.g. CH3CH2O Na+


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BROMINATION OF ALCOHOLS

Reagent(s) conc. hydrobromic acid HBr(aq) or

sodium (or potassium) bromide and concentrated sulphuric acid

Conditions reflux

Product haloalkane

Equation C2H5OH(l) + conc. HBr(aq) > C2H5Br(l) + H2O(l)

Mechanism The mechanism starts off similar to that involving dehydration(protonation of the alcohol and loss of water) but the carbocation

(carbonium ion) is attacked by a nucleophilic bromide ion in step 3

Step 1 protonation of the alcohol using a lone pair on oxygen

Step 2 loss of a water molecule to generate a carbocation (carbonium ion)

Step 3 a bromide ion behaves as a nucleophile and attacks the carbocation


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ESTERIFICATION OF ALCOHOLS

Reagent(s) carboxylic acid + strong acid catalyst (e.g conc. H2SO4)

Conditions reflux

Product esterEquation e.g. CH3CH2OH(l) + CH3COOH(l) CH3COOC2H5(l) + H2O(l)

ethanol ethanoic acid ethyl ethanoate

Notes ConcentratedH2SO4is a dehydrating agent- it removes water

causing the equilibrium to move to the right and increases the yield


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ESTERIFICATION OF ALCOHOLS

Reagent(s) carboxylic acid + strong acid catalyst (e.g conc. H2SO4)

Conditions refluxProduct ester

Equation e.g. CH3CH2OH(l) + CH3COOH(l) CH3COOC2H5(l) + H2O(l)ethanol ethanoic acid ethyl ethanoate

Notes Concentrated H2SO4is a dehydrating agent - it removes water

causing the equilibrium to move to the right and increases the yield

Uses of esters Esters are fairly unreactive but that doesnt make them uselessUsed as flavourings

Naming esters Named from the alcohol and carboxylic acidwhich made them...

CH3OH + CH3COOH CH3COOCH3 + H2O

fromethanoic acid CH3COOCH3 frommethanolMETHYLETHANOATE


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COMMON ALCOHOLS

EthanolEthyl alcohol, grain alcohol

Produced by fermentation

Natural fermentation of sugars andstarches in grains, corn, molasses, fruits,vegetables

Found in beer, wine, and other spirits

C6H12O6 2 CH3CH2OH + 2 CO2

glucose ethanol carbon dioxide39


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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

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COMMON ALCOHOLS

Isopropyl alcohol

2-propanol rubbing alcohol

Evaporates quicklyused to reduce fever

Solvent in many cosmetics, perfumes, etc.

CH3CHCH3

OH

2-propanol

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COMMON ALCOHOLS

1,2-ethanediol

Ethylene glycol

Very soluble in water

Used as antifreeze

Glycerol

1,2,3-propanetriolVery soluble in water

Sweet taste

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PREPARATION OF IMPORTANT ALCOHOLS

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

CO

CO2H

2 CH3 OHO2

CH2 O

CH3 COOHCoal

ormethane Carbon

monoxideMethanol

Acetic acid

Formaldehydeoxidation

catalyst

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PREPARATION OF IMPORTANT ALCOHOLS

H2 O, H2 SO4

CH3 CH=CH2

CH3 CHCH3

OH

HOCH2 CHCH2 OH

OHPropene

2-Propanol

Gly cerin, gly cerol

several steps

CH2 =CH2

O2

H2 O, H2 SO4

O

H2 C CH2

CH3 CH2 OH

H2 O, H2 SO4

H2 SO4

HOCH2 CH2 OH

CH3 CH2 OCH2 CH3

Ethylene

Ethanol

Ethylene glycol

180CDieth yl ether

catalystEthylene

oxide

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ETHERS


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ETHERS

The functional group of an etheris anoxygen atom bonded to two carbonatoms

Simplest ether is dimethyl etherMost common ether is diethyl ether

Often called just ether

CH3 -O-CH3 CH3 CH2 -O-CH2 CH3

Dimethyl ether Diethyl ether45

ETHER NOMENCLATURE


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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

OCH3CH3 CH2 OCH2 CH3

Cyclohexyl methyl ether

(Methoxycyclohexane)

Diethyl ether46

ETHER NOMENCLATURE


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ETHER NOMENCLATURE

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 blockfor 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

Eth ylene oxide Tetrahydrofuran(THF)

OO

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PHYSICAL PROPERTIES OF ETHERS


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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 ethermolecules in the pure liquid

Boiling points of ethers are close to those ofhydrocarbons of similar molecular weight

Have lower boiling points than alcohols of thesame molecular formula

CH3 CH2 OH CH3 OCH3

bp -24C

Ethanol

bp 78C

Dimethyl ether49

REACTIONS OF ETHERS


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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 catalystNot affected by most acids or bases at

moderate temperatures

Good solvents for carrying out organicreactions, because of their general inertnessand good solvent properties

Diethyl ether and THF (tetrahydrofuran)50

THIOLS


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THIOLS

Thiol:a compound containing an -SH(sulfhydryl) group

Also called mercaptans

Low-molecular-weight thiols STINKResponsible for smells such as those from

rotten eggs and sewage

the scent of skunks is due primarily to these

two thiols

CH3CH=CHCH2 SH

CH3

CH3 CHCH2 CH2 SH

3-Methyl-1-butanethiol2-Butene-1-thiol51


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THIOL PHYSICAL PROPERTIES


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THIOL PHYSICAL PROPERTIES

S-H bonds are nonpolar covalent

Small difference in electronegativity betweensulfur and hydrogen (2.5 - 2.1 = 0.4)

No hydrogen bonding

Lower boiling points than comparable alcoholsLess soluble in water and other polar solvents

than comparable alcohols

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65

1-butanol

ethanol

methanol

98

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6

1-butanethiol

ethanethiol

methanethiol

bp (C)Alcoholbp (C)Thiol

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REACTIONS OF THIOLS

Thiols are weak acids (pKa~10), and arecomparable in strength to phenols

thiols react with strong bases such as NaOH

to form water-soluble thiolate salts

CH3

CH2

SH NaOHH

2O

CH3

CH2

S-Na

+H

2

O+

Ethanethiol(pKa 10)

+

Sodiumethanethiolate

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REACTIONS OF THIOLS


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REACTIONS OF THIOLS

The most important thiol reaction inbiological systems is their oxidation todisulfides

Functional group is a disulfide (-S-S-) bond

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 airduring storage

disulfides, in turn, are easily reduced to thiols

by several reducing agents.56


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REACTIONS OF THIOLS

Oxidation ofSH groups to form S-S bondsReduction of S-S bonds to formSH groups

2 HOCH2 CH2 SH HOCH2 CH2 S-SCH2 CH2 OH

A disulfide

oxidation

reductionA thiol

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REACTIONS OF THIOLS


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REACTIONS OF THIOLS

Oxidation ofSH groups to form S-S bondsReduction of S-S bonds to formSH groups

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UNSATURATED ALCOHOLS

Hydroxyl group takes precedence. Assignthat carbon the lowest number.

Use alkene or alkyne name.

4-penten-2-ol (old)

pent-4-ene-2-ol(1997 revision of IUPAC rules)

=>

CH2 CHCH2CHCH3

OH

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Ch14Alcohols, Ethers and Thiols

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