L 10 alcohols-structure_nomenclature_classification_etc_pch217_2013_2014

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Organic Chemistry

Course Number: PCH 1120-217

Lecture # 10

Sunday September 29, 2013

Alcohols: Structure, Nomenclacture, Classification, Physical Properties, Basicity &

Acidity and Preparation of Alcohols

Prof. Oludotun Phillips

Room # 2-81, 2nd Floor Pharmacy Building

Email: [email protected]

Tel: 4986070

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Alcohols: Learning Objectives

At the end of the class students should be able to:

discuss the structures and properties of alcohols.

classification of alcohols.

discuss the nomenclature (common and IUPAC names) of alcohols.

describe the physical properties of alcohols, basicity and acidity of alcohols.

discuss the different methods for the preparation of alcohols.

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• 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 also sp3 hybridized

– two sp3 hybrid orbitals form sigma bonds to carbon and hydrogen

– the remaining two sp3 hybrid orbitals each contain an unshared pair of electrons

Alcohols - Structure

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• IUPAC names

– the parent chain is the longest chain that contains the -OH group

– number the parent chain in the direction that gives the -OH group the lower number

– change the suffix -e to -ol

• Common names

– name the alkyl group bonded to oxygen followed by the word alcohol

Alcohols - Nomenclature

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• Examples:

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 of Alcohols

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• Problem: Write the IUPAC name of each alcohol

OH

CH3(CH2)6CH2OH

OH

(a)

(b)

(c)


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Polyhydric alcohols: compounds containing…. – two -OH groups are named as diols,– three -OH groups are named as triols, etc.

CH3CHCH2

HO OH

CH2CH2

OHOH

CH2CHCH2

OHHOHO

1,2-Ethanediol(Ethylene glycol)

1,2-Propanediol(Propylene glycol)

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


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Unsaturated alcohols– the double and triple bond are shown by the infix -en-

and -yn-– the hydroxyl group is shown by the suffix -ol– number the chain to give OH the lower number

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OH1346

trans-3-hexene-1-ol(E)-3-hexene-1-ol

IUPAC Nomenclature of Alcohols

HOH

5-Hexny-1-ol

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Alcohols are further classified as:

1. Methyl alcohol (CH3OH)

2. primary (1o)alcohol

2. secondary (2o)alcohol

3. tertiary (3o)alcohol

Classification of Alcohols

H OH

R

H

R' OH

R

H

R' OH

R

R"

(1o) (2o) (3o) primary secondary tertiary

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Alcohols are further classified as:

5. allylic alcohol

6. benzylic alcohol

Classification of Alcohols

CH3CH CHCH2OH

H

CH3

H

OH CH2OH C-CH3

OH

H

C-CH3

OH

CH3

Benzylic alcohols (1o) (2o) (3o)allylic alcohol

(1o primary)

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Alcohols are polar compounds both the C-O and O-H bonds are polar covalent

-

+

+O

HH

H

C

H

Alcohols – Physical Properties

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Alcohols associate in the liquid state by hydrogen bonding. Hydrogen bonding: the attractive force between a partial

positive charge on hydrogen and a partial negative charge on a nearby oxygen, nitrogen, or fluorine atom.

– the strength of hydrogen bonding in alcohols is approximately 2 to 5 kcal/mol.

– hydrogen bonds are considerably weaker than covalent bonds (for example, 110 kcal/mol for an O-H bond).

– hydrogen bonding can have a significant effect on physical properties.


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– association of ethanol molecules in the liquid state, with two of the three possible hydrogen bonds to the upper oxygen are shown as follows:.


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– alcohols have higher boiling points and are more soluble in water than hydrocarbons

CH3CH2CH2OHCH3CH2CH2CH3

CH3OHCH3CH3

CH3CH2OH

CH3CH2CH3

CH3CH2CH2CH2CH2OH

HOCH2CH2CH2CH2OHCH3CH2CH2CH2CH2CH3

Structural Formula Name

MolecularWeight(g/mol)

BoilingPoint(°C)

Solubilityin Water

methanol 32 65 infiniteethane 30 -89 insoluble

ethanol 46 78 infinitepropane 44 -42 insoluble

1-propanol 60 97 infinitebutane 58 0 insoluble

1-pentanol 88 138 2.3 g/100 g

1,4-butanediol 90 230 infinitehexane 86 69 insoluble


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Acidity Constants:

According to Bronsted-Lowry definitions:

an acid is a hydrogen ion donor.

a base is a hydrogen ion acceptor.

The strength of the acid is usually compared measuring their degree of ionization in water.

Relative Acidity of Alcohols

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Most alcohols are about the same or slightly weaker acids than water

– pKa = -logKa– aqueous solutions of alcohols have the same pH as

that of pure water– Alcohols are neutral compounds!

CH3O H O HH

[CH3O- ][H3O+]

[CH3OH]

CH3O H OH

H+

Ka =

+ +

= 3.2 x 10-16

pKa = 15.5

Acidity of Alcohols

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In the presence of strong acids, the oxygen atom of an alcohol can behave as a weak base – proton transfer from the strong acid forms an oxonium

ion

– thus, alcohols can function as both very weak acids and weak bases

– Alcohols can lose a proton to a very strong base, e.g.

NaOH + ROH RO- Na+ + H2

CH3CH2-O-H H OH

H OH

HH2SO4

CH3CH2-O HH

CH3CH2-O HH HH

O H

Ethyloxonium ion(pKa -2.4)

••

Hydronium ion(pKa -1.7)

Ethanol

++

+++

••+

++

Basicity of Alcohols

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– pKa values for several low-molecular-weight alcohols

(CH3)3COH

(CH3)2CHOH

CH3CH2OH

H2O

CH3OH

CH3COOH

HCl

Compound pKa

-7

15.5

15.7

15.9

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18

4.8

hydrogen chloride

acetic acid

methanol

water

ethanol

2-propanol

2-methyl-2-propanol

Structural Formula

Stronger acid

Weaker acid

*Also given for comparison are pKa values for water, acetic acid, and hydrogen chloride.

Acidity of Alcohols

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Alcohols can be prepared by the following methods:

i. Fermentation of carbohydrates

ii. Hydration of alkenes

iii. Nucleophilic substitution reaction

iv. Reduction of aldehydes and ketones

Preparation of Alcohols

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Alcohols can be prepared by:

i. Fermentation of carbohydrates:

Fermentation is the decomposition of complex organic compounds into simpler compounds by the action of biological catalysts known as enzymes.

Fermentation of glucose give ethanol:…


C6H12O6 CH3CH2OHenzymes

Glucose ethanol(a sugar) (an alcohol)

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ii. Hydration of alkenes:

Treatment of alkenes with water in the presence of a strong acid as a catalytic results in the addition of water to the double bond (hydration reaction):….

+ H2OCH2 CH2 CH3 CH2OH


Cyclohexane cyclohexanol

H+

Strong acid (conc. H2SO4)

+ OHH2O

H+

Strong acid (conc. H2SO4)Ethylene Ethanol

b.

a.

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iii. Nucleophilic Substitution Reactions: Reaction of an alkyl halide in hydroxide ion is a

nucleophilic substitution reaction. Primary alcohols are prepared by heating primary

alkyl halides in aqueous sodium hydroxide (NaOH), this reacion proceed by SN2 mechanism:…

Since 2o and 3o alkyl halides are likely to give elimination reaction products, this method is not very useful for preparing alcohols.


CH3CH2CH2Br CH3CH2CH2OH + NaBr+ NaOHaqheat

1-Bromopropane 1-Propanol (a 1o alkyl halide) (a 1o alcohol)

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iv. Reduction of Aldehydes and Ketones:

a. Alcohols are prepared from carbonyl compounds by reduction reactions in which hydrogen atoms are added to the carbonyl group by NaBH4 or LiAlH4 followed by H2O or dilute acid hydrolysis:…..


R CH2CHO R CH2CH2OHNaBH4

OrLiAlH4An Aldehyde A 1o alcohol

R COCH3 R CHCH3

OHA Ketone A 2o alcohol

NaBH4

OrLiAlH4

Reagents: Sodium borohydride: (NaBH4) Lithium aluminum hydride (LiAlH4)

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b. By Catalytic Hydrogenation involves the addition of Hydrogen to aldehydes or ketones under pressure to produce 1o and 2o Alcohols respectively:…..


R CH2CHO R CH2CH2OH+ H2 Ni

PressureAn Aldehyde hydrogen A 1o alcohol

R COCH3R CHCH3

OH

+ H2

A Ketone hydrogen A 2o alcohol

Reagents: Nickle (Ni); Hydrogen gas (H2)

Ni

Pressure

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c. By Grignard addition to an Aldehyde or Ketone followed by H2O or dilute acid hydrolysis to produce 1o, 2o and 3o

Alcohols respectively.

Grignard Reagent is prepared by reacting Magnesium metal with an Alkyl halide in diethyl ether as solvent.


R X R Mg X+ Mgdiethyl ether

(as solvent)Alkyl halide Magnesium Grignard reagent

R = CH3; CH3CH2; or Ph;

X = Cl; Br; IIn this reaction, the “R” of Grignard reagent acts as Nucleophile

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Alcohols can be prepared by: Formaldehyde is reduced to 1o alcohol:..

Aldehydes are reduced to 2o alcohols:…

Ketones are reduced to 3o alcohols:…


H CH

O

H C-H

R"

O- +MgX+ R"MgX H C-H

R"

OH

Formaldehyde Grignard reagent Magnesium salt of alcohol 1 o alcohol

R COCH3 R C-CH3

R"

O- +MgX

+ R"MgX R C-CH3

R"

OH

R' CHO R' C-H

R"

O- +MgX

+ R"MgX R' C-H

R"

OH

H2O

H+

Aldehyde Grignard reagent Magnesium salt of alcohol 2 o alcohol

H2O

H+

H2O

H+

Ketone Grignard reagent Magnesium salt of alcohol 3 o alcohol

Technology

L 10 alcohols-structure_nomenclature_classification_etc_pch217_2013_2014