Wk 12 C18 Alcohol ,Amines1

8/9/2019 Wk 12 C18 Alcohol ,Amines1

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1818 Alcohols,Alcohols,amines andamines andrelatedrelated

compoundscompoundsPrepared by

Emma Bartle

University of Western Australia


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Alcohols play a central role in the interconversion of organic

functional groups.


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18.1 Alcohols18.1 Alcohols

Alcohols contain the functional group

OH, a hydroxyl group

This hydroxyl group is bonded to an sp3

hybridised carbon atom

The oxygen atom is also sp3 hybridised

Two sp3 hybrid orbitals form bonds to

carbon and hydrogen The remaining two sp3 hybrid orbitals each

contain an unshared pair of electrons


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Alcohols are named the same way as

alkanes, with the following differences

The parent alkane is the longest chain ofcarbon atoms containing the OH group

Change the suffix of the parent alkane

from e to ol, and use a number to

show the location of the OH group

Name and number substituents and list

them in alphabetical order


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To derive common names for alcohols,

we name the alkyl group bonded to the

OH group and add the word alcohol

Ethanol(Ethyl alcoh ol)

1-Propanol(Propy l alcoho l)

2-Propanol(Is opropy l alcoho l)

1-Butanol(Butyl alcoho l)

OH

OH

OH

OH

2-Butanol

(s ec-Bu tyl alcoho l)2-Methyl-1-propanol

(Is obutyl alcoh ol)

2-Methyl-2-propanol(t ert-Butyl alcohol )

OH

Cyclohexanol(Cyclohexyl alcoho l)

OH

OH

OH


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Write the IUPAC name for each of the following alcohols.

a) is derived from an unbranched alkane so we need to identify only the

alkane and the position of the OH group. Compound

(b) includes a branch on the second last carbon, and the OH group ison the second carbon. Compound (c) is a little more complex. It is a

cyclic alcohol, so we start numbering from the OH group. There is also

a methyl group on the second carbon which is trans- to the OH group

a)

b)

c)

a) octan-1-olb) 4-methylpentan-2-ol

c) trans-2-methylcyclohexanol or 2-methylcyclohexanolstereochemistry,


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Compounds containing 2 OH groups

are named as diols

Compounds containing 3 OH groups

are named as triols

CH3

CHCH2

HO OH

CH2 CH2

OHOH

CH2 CHCH2

OHHOHO

1,2-Ethanedio lEthylene glycol

1,2-PropanediolPropylene glycol

1,2,3-Propanetrio lGlycerol, Glycerin)


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Compounds containing two hydroxyl groups on adjacent carbons are often

referred to as glycols. Ethylene glycol and propylene glycol are synthesised

from ethylene and propylene respectively,

We classify alcohols as primary (1),

secondary (2) or tertiary (3),Depending on whether the OH

group is on a primary, secondary

or tertiary carbon atom.


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Physical properties

Both the C-O and O-H bonds of an

alcohol are polar covalent

Alcohols are polar molecules

This is demonstrated for methanol below

H-

H+

H+O

HH

H

C

H


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Alcohols have higher boiling points and

water solubility than hydrocarbons

CH3 CH2 CH2OH

CH3 CH2 CH2 CH3

CH3OH

CH3 CH3

CH3 CH2 OH

CH3 CH2 CH3

CH3 CH2 CH2 CH2 CH2OH

HOCH2 CH2 CH2 CH2 OH

CH3 CH2 CH2 CH2 CH2 CH3

Structural Formula Name

Molecular

Wei ht(g/mol)

Boiling

Point(C)

Solubilityin Water

methanol 32 65 infinite

ethane 30 -89 insoluble

ethanol 46 78 infinite

propane 44 -42 insoluble

1-propanol 60 97 infinite

butane 58 0 insoluble

1-pentanol 88 138 2.3 g/100 g

1,4-butanediol 90 230 infinite

hexane 86 69 insoluble


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Alcohols associate in the liquid state by

hydrogen bonding



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Because ofhydrogen bonding between alcohol molecules in the liquid state,

extra energy is required to separate each alcohol molecule from its neighbours,

high boiling points of alcohols compared with alkanes.

Additional hydroxyl groups in a molecule further increase the extent of

hydrogen bonding:

the boiling points of pentan-1-ol (138 C) and butane-1,4-diol (230C),

which have approximately the same molar mass.


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Preparation of alcohols

Alcohols can be prepared from many

functional groups

Discussed here are:

Preparation from alkenes

Preparation from haloalkanes

Reduction of carbonyl compounds Addition of Grignard reagents to carbonyl

compounds


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Preparation from alkenes Acid catalysed hydration of alkenes


Obeys Markovnikov's rule (the hydrogen adds to the carbon of the double

bond with the most hydrogen atoms).


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Preparation from haloalkanesNucleophilic substitution with a hydroxide ion

Tertiary haloalkanes are readily converted to alcohols by water.

Secondary and primary alcohols are better prepared by

reaction with hydroxide ions.


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Reduction of carbonyl compounds Aldehydes are reduced to primary alcohols

Ketones are reduced to secondary alcohols

Carboxylic acids and esters can be reduced to form

primary alcohols



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18.2 Reactions of alcohols18.2 Reactions of alcohols

Reactions discussed here are:

Acidity of alcohols

Basicity of alcohols Reaction with active metals

Dehydration of alcohols to alkenes

Conversion of alcohols to haloalkanes

Oxidation of alcohols to aldehydes,

ketones and carboxylic acids


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Acidity of alcohols

Alcohols have similar acid ionisation

constants (pKa

) to water (15.7)

Aqueous solutions of alcohols have a pH

close to that of pure water

CH3 O H O H

H

[CH3 O-

][ H3 O+]

[ CH3 OH]

CH3 O H O

H

H+

Ka

=

+ +

= .

Ka= .


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The table below gives the pKa for several

low-molar-mass alcohols

( H3 ) 3 OH

( H3 ) 2 HOH

H3 H2OH

H2O

CH3 OH

CH3 COOH

HCl

Co o Ka

-7

15 5

15 7

15 9

17

18

4 8

y roge c lori e

aceticaci

me t a o l

water

et a o l

2- ro a ol

2-met yl-2- ro a ol

Str ct ral

FormulaStro ger

aci

Weaker aci

*A lso give forcom ariso are Ka

values for water,

aceticaci , a y roge c lori e


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Basicity of alcohols

In the presence of a strong acid, theoxygen atom of an alcohol is a weak base

It reacts with the acid by proton transfer toform an oxonium ion

Alcohols can function as both weak acidsand weak bases

CH3 CH2 -O-H H O

H

H O

H

H

H2 SO4CH3 CH2 -O H

H

CH3 CH2 -O H

H HH

O H

Ethylo xon ium ion(pK

a-2.4)

Hydron ium i on(pK

a-1.7)

Ethanol

++

+

++

++

+


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Reaction with active metals

Alcohols react with Li, Na, K and other

active metals to form metal alkoxides

Na is oxidised to Na+ and H+ is reduced to H2

Alkoxide ions are stronger bases than thehydroxide ion

Alkoxide ions can be used as nucleophiles in

substitution reactions

2CH3CH2OH 2 Na 2CH3CH2O-Na

+ H2+ +

So iu m et oxi e


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Conversion to Haloalkanes

Reaction with Hydrohalic Acids (HCl, HBr and HI)

Water-soluble tertiary alcohols react very rapidly with HCl, HBr and HI.

Low-molar-mass, water-soluble primary and secondary alcohols do not

react under these conditions.


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Water-insoluble tertiary alcohols are converted to tertiary halides by bubbling

gaseous HX through a solution of the alcohol dissolved in diethyl ether

or tetrahydrofuran (THF).

Primary and secondary alcohols are converted to bromoalkanes and

iodoalkanes by treatment with concentrated hydrobromic and hydroiodic

acids. For example, heating butan-1-ol with concentrated HBr

gives 1-bromobutane


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From observations of the relative ease of reaction of alcohols with HX

(3 > 2 > 1), it has been proposed that the conversion of tertiary and

secondary alcohols to haloalkanes by concentrated HX occurs by an SNl

mechanism and involves the formation of a carbocation intermediate.

This occurs through the three steps shown below:

Rapid and reversible proton transfer from the acid to the OH group gives an

oxonium ion. This proton transfer converts the leaving group from OH, a poor

leaving group, to H2O, a better leaving group.

Loss of water from the oxonium ion gives a 3 carbocation intermediate.

Step 1

Step 2


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Reaction of the 3 carbocation intermediate (an electrophile)

with a chloride ion (a nucleophile) gives the haloalkane product.

Step 3


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Primary alcohols react with HX by an SN2 mechanism. In

the rate-determining step, the halide ion displaces H2O from

the carbon atom bearing the oxonium ion. The displacement

of H2O and the formation of the CX bond are simultaneous.

Step 1: Rapid and reversible proton transfer to the OH group converts the

leaving group from OH to H2O, which is a better leaving group

Step2: The nucleophilic displacement of H2O by Br then gives the bromoalkane.


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Why do tertiary alcohols react with HX by formation of carbocation

intermediates, whereas primary alcohols react by direct displacement of

the OH2+ group (the protonated OH group)?

1. Electronic factors: Tertiary carbocations are the most stable and mostreadily formed, whereas primary carbocations are the least stable and

hardest to form

2. Steric factors: To form a new carbonhalogen bond, the halide ion must

approach the carbon atom bearing the leaving group from the side directly

opposite that group and begin to form a new covalent bond.


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140oC

Cyclohexanol Cyclohexene

OH

+ H 2 OH2 SO4

180oCCH3 CH2 OH

H2 SO4CH2 =CH 2 + H 2 O

+ H2 OCH3 COH

CH3

CH350oC

H2 SO4CH3 C=CH2

CH3

2- et yl rope e

(Isobutyle e)

Acid-catalysed Dehydration to Alkenes


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When isomeric alkenes are obtained, the

more stable alkene generally predominates

The more stable alkene has the greater

number of substituents on the double bond The acid-catalysed dehydration of alcohols

follows Zaitsevs rule

CH3 CH2 CHCH3

H8 5 %H3 P 4

CH3 CH= CHCH3 CH3 CH2 CH= CH2

1-Butene(20%)

2-Butene(80%)

2-Butanol+

heat


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Oxidation of 1 and 2 alcohols

Oxidation of a 1 alcohol gives an aldehyde or

carboxylic acid, depending on the experimental

conditions

2 alcohols are oxidised to ketones

3 alcohols are not easily oxidised


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The most common reagent used for the

oxidation of a 1 alcohol to carboxylic acid

and 2 alcohol to a ketone is chromic acid

CrO3 H2 OH2 SO4 H2 CrO4+

Chro ic acidChro i (VI)oxide

CH3 (CH2 )6 CH2 OHCrO3

H2 SO4 , H2 OCH3 (CH2 ) 6 CH

O

CH3 (CH2 )6 COH

O

Octa al( ot isolated)

Octa oic acid1-Octa ol


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The mild oxidising agent PCC is used to

oxidise 1 alcohols to aldehydes

CrO3 HClN N

H

CrO3 Cl

-

Py idinium chlo och omate(PCC)

Py idine

+ ++

OHPCC

CH2 Cl2H

O

Ge aniol Ge anial


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18.4 Ethers18.4 Ethers

Ethers contain an atom of oxygen bonded

to 2 carbon atoms

Oxygen is sp

3

hybridised with bondangles of approximately 109.5

H

H O

H

C H

H

H

C


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The OR group bonded to the parent

alkane is named as an alkoxy

Common names are derived by listingthe alkyl groups bonded to oxygen in

alphabetical order and adding ether

CH3

CH3

CH3 CCH3

Et 2

H

Et

E h h n

Di h h

h m h p p nm h t ert-bu h , TBE

t rans-2-E h h n


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Cyclic ethers contain the ether oxygen

as one of the atoms in the ring

Cyclic ethers are generally known bytheir common names

E hylene

oxi e

Tetrahydro-

furan,T F

Tetrahydro-

yran

1,4-Dioxane

O O

O

OO


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Physical properties

Ethers are moderately polar compounds

However, only weak forces of attraction exist

between ether molecules



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Ethers have lower boiling points than

alcohols of similar molar mass

CH3 CH2 OH

CH3 OCH3

CH3 CH2 CH2 CH2 CH2 OH

HOCH2 CH2 CH2 CH2 OH

CH3 CH2 CH2 CH2 OCH3

CH3 CH2 CH2 CH2 OH

CH3 CH2 OCH2 CH3

CH3 OCH2 CH2 OCH3 ethylene g lycoldimethyl ethe

90 84 infinite

8 g/100 g3574diethyl ethe

1-butanol 74 117 7.4 g/100 g

slight7188butyl methyl e the

infinite230901,4-butanediol

2.3 g/100 g138881-pentanol

7.8 g/100 g-2446dimethyl ethe

infinite7846ethanol

olubilityin Wate

Boiling

Point(C)

MoleculaWeightNamet uctu al Fo mu la


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The effect of hydrogen bonding is

illustrated by comparing the boiling

points of ethanol and dimethyl ether

The difference is due to the polar O-Hgroup in the alcohol

This increases the attractive force

between molecules of ethanol

CH3 CH2 OH CH3 OCH3

bp-24C

Ethanol

bp C

Dim thyl ther


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Reactions of ethers

Ethers resemble hydrocarbons in their

resistance to chemical reactions

They do not react readily with oxidising

agents or reducing agents

They are not affected by most acids or bases

at moderate temperature

Because of their good solvent properties

and general inertness to chemical

reaction, ethers are excellent solvents in

which to carry out organic reactions


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SummarySummary

Alcohols

Contain the functional group OH, the

hydroxyl group, bonded to an sp3

hybridised carbon atom

Classified as 1, 2 or 3

IUPAC names are formed by changing

the suffix of the parent alkane to ol Common names are derived by naming

the alkyl group followed by alcohol


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Wk 12 C18 Alcohol ,Amines1

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