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AN INTRODUCTION TO THE CHEMISTRY OF ALCOHOLS. KNOCKHARDY PUBLISHING. KNOCKHARDY PUBLISHING. THE CHEMISTRY OF ALCOHOLS. INTRODUCTION

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slide1

AN INTRODUCTION TO

THE CHEMISTRY

OF ALCOHOLS

KNOCKHARDY PUBLISHING

slide2

KNOCKHARDY PUBLISHING

THE CHEMISTRY OF ALCOHOLS

INTRODUCTION

This Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards.

Individual students may use the material at home for revision purposes or it may be used for classroom teaching if an interactive white board is available.

Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at...

www.knockhardy.org.uk/sci.htm

Navigation is achieved by...

either clicking on the grey arrows at the foot of each page

or using the left and right arrow keys on the keyboard

slide3

CLASSIFICATION OF ALCOHOLS

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

• the OH replaces an H in a basic hydrocarbon skeleton

slide4

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 behaves as a typical aliphatic alcohol

The first two compounds are

classified as aromatic alcohols

(phenols) because the OH group

is attached directly to the ring.

slide5

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 or phenols, the OH is attached directly to the ring

• an OH on a side chain of a ring behaves as a typical aliphatic alcohol

The first two compounds are

classified as 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

PRIMARY 1° SECONDARY 2° TERTIARY 3°

slide6

THE CHEMISTRY OF ALCOHOLS

  • CONTENTS
  • Structure of alcohols
  • Nomenclature
  • Isomerism
  • Physical properties
  • Chemical properties of alcohols
  • Identification using infra-red spectroscopy
  • Industrial preparation and uses of ethanol
  • Revision check list
slide7

SOLVENT PROPERTIES OF ALCOHOLS

SolubilityLow molecular mass alcohols are miscible with water

Due to hydrogen bonding between the two molecules

Heavier alcohols are less miscible

Solvent

propertiesAlcohols are themselves very good solvents

They dissolve a large number of organic molecules

slide8

BOILING POINTS OF ALCOHOLS

Increases with molecular size due to increased van der Waals’ forces.

Alcohols have higher boiling points than

similar molecular mass alkanes

This is due to the added presence of

inter-molecular hydrogen bonding.

More energy is required to separate the molecules.

Mr bp / °C

propane C3H8 44 -42 just van der Waals’ forces

ethanol C2H5OH 46 +78 van der Waals’ forces+ hydrogen bonding

slide9

BOILING POINTS OF ALCOHOLS

Increases with molecular size due to increased van der Waals’ forces.

Alcohols have higher boiling points than

similar molecular mass alkanes

This is due to the added presence of

inter-molecular hydrogen bonding.

More energy is required to separate the molecules.

Mr bp / °C

propane C3H8 44 -42 just van der Waals’ forces

ethanol C2H5OH 46 +78 van der Waals’ forces+ hydrogen bonding

Boiling point is higher for “straight” chain isomers.

bp / °C

butan-1-ol CH3CH2CH2CH2OH 118

butan-2-ol CH3CH2CH(OH)CH3 100

2-methylpropan-2-ol (CH3)3COH 83

Greater branching =

lower inter-molecular forces

slide10

BOILING POINTS OF ALCOHOLS

Increases with molecular size due to increased van der Waals’ forces.

Alcohols have higher boiling points than

similar molecular mass alkanes

This is due to the added presence of

inter-molecular hydrogen bonding.

More energy is required to separate the molecules.

Mr bp / °C

propane C3H8 44 -42 just van der Waals’ forces

ethanol C2H5OH 46 +78 van der Waals’ forces+ hydrogen bonding

Boiling point is higher for “straight” chain isomers.

bp / °C

butan-1-ol CH3CH2CH2CH2OH 118

butan-2-ol CH3CH2CH(OH)CH3 100

2-methylpropan-2-ol (CH3)3COH 83

Greater branching =

lower inter-molecular forces

slide11

INDUSTRIAL PREPARATION OF ALCOHOLS

FERMENTATION

Reagent(s) GLUCOSE - produced by the hydrolysis of starch

Conditions yeast

warm, but no higher than 37°C

EquationC6H12O6 ——> 2 C2H5OH + 2 CO2

Advantages LOW ENERGY PROCESS

USES RENEWABLE RESOURCES - PLANT MATERIAL

SIMPLE EQUIPMENT

Disadvantages SLOW

PRODUCES IMPURE ETHANOL

BATCH PROCESS

slide12

INDUSTRIAL PREPARATION OF ALCOHOLS

HYDRATION OF ETHENE

Reagent(s) ETHENE - from cracking of fractions from distilled crude oil

Conditions catalyst - phosphoric acid

high temperature and pressure

EquationC2H4+ H2O ——> C2H5OH

slide13

INDUSTRIAL PREPARATION OF ALCOHOLS

HYDRATION OF ETHENE

Reagent(s) ETHENE - from cracking of fractions from distilled crude oil

Conditions catalyst - phosphoric acid

high temperature and pressure

Equation C2H4 + H2O ——> C2H5OH

Advantages FAST

PURE ETHANOL PRODUCED

CONTINUOUS PROCESS

Disadvantages HIGH ENERGY PROCESS

EXPENSIVE PLANT REQUIRED

USES NON-RENEWABLE FOSSIL FUELS TO MAKE ETHENE

Uses of ethanol ALCOHOLIC DRINKS

SOLVENT - industrial alcohol / methylated spirits

FUEL - petrol substitute in countries with limited oil reserves

slide14

USES OF ALCOHOLS

ETHANOL

DRINKS

SOLVENT industrial alcohol / methylated spirits (methanol is added)

FUEL used as a petrol substitute in countries with limited oil reserves

METHANOL

PETROL ADDITIVE improves combustion properties of unleaded petrol

SOLVENT

RAW MATERIAL used as a feedstock for important industrial processes

FUEL

Health warning Methanol is highly toxic

slide15

STRUCTURAL ISOMERISM IN ALCOHOLS

Different structures are possible due to...

A Different positions for the OH group and

B Branching of the carbon chain

butan-1-ol

butan-2-ol

2-methylpropan-2-ol

2-methylpropan-1-ol

slide16

NAMING ALCOHOLS

Alcohols are named according to standard IUPAC rules

• select the longest chain of C atoms containing the O-H group;

• remove the e and add ol after the basic name

• number the chain starting from the end nearer the O-H group

• the number is placed after the an and before the ol ... e.g butan-2-ol

• as in alkanes, prefix with alkyl substituents

• side chain positions are based on the number allocated to the O-H group

e.g. CH3 - CH(CH3) - CH2 - CH2 - CH(OH) - CH3 is called 5-methylhexan-2-ol

slide17

THE CHEMISTRY OF ALCOHOLS

  • Before you start it would be helpful to…
  • BE AWAKE!!!!!!!
slide18

REACTIONS INVOLVING BREAKING THE O-H BOND

1. SODIUM

.

Conditionsroom temperature

Product sodium alkoxide and hydrogen

Equation2CH3CH2OH(l) + 2Na(s) ——> 2CH3CH2O¯ Na + + H2(g) sodium ethoxide

Notes alcohols are organic chemistry’s equivalent of water

water 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+

slide19

2. ESTERIFICATION OF ALCOHOLS

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

Conditions reflux

Product ester

Equatione.g. CH3CH2OH(l) + CH3COOH(l) CH3COOC2H5(l) + H2O(l)

ethanol ethanoic acid ethyl ethanoate

NotesConcentratedH2SO4 is a dehydrating agent - it removes water

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

slide20

ESTERIFICATION OF ALCOHOLS

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

Conditions reflux

Product ester

Equation e.g. CH3CH2OH(l) + CH3COOH(l) CH3COOC2H5(l) + H2O(l)

ethanol ethanoic acid ethyl ethanoate

Notes Concentrated H2SO4 is 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 doesn’t make them useless

Used as flavourings

Naming esters Named from the alcohol and carboxylic acid which made them...

CH3OH + CH3COOHCH3COOCH3+ H2O

fromethanoic acidCH3COOCH3from methanol

METHYLETHANOATE

slide21

3. BROMINATION OF ALCOHOLS

Reagent(s)sodium (or potassium) bromide and concentrated sulphuric acid

Conditions reflux

Product haloalkane

EquationC2H5OH(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

slide22

OXIDATION OF ALCOHOLS

All alcohols can be oxidised depending on the conditions

Oxidation is used to differentiate between primary, secondary and tertiary alcohols

The usual reagent is acidified potassium dichromate(VI)

PrimaryEasily oxidisedto aldehydes and then to carboxylic acids.

SecondaryEasily oxidised to ketones

TertiaryNot oxidised under normal conditions.

They do break down with very vigorous oxidation

PRIMARY 1° SECONDARY 2° TERTIARY 3°

slide23

OXIDATION OF PRIMARY ALCOHOLS

Primary alcohols are easily oxidised to aldehydes

e.g. CH3CH2OH(l) + [O] ——> CH3CHO(l) + H2O(l)

ethanol ethanal

it is essential to distil off the aldehyde before it gets oxidised to the acid

CH3CHO(l) + [O] ——> CH3COOH(l)

ethanal ethanoic acid

slide24

OXIDATION OF PRIMARY ALCOHOLS

  • Primary alcohols are easily oxidised to aldehydes
  • e.g. CH3CH2OH(l) + [O] ——> CH3CHO(l) + H2O(l)
  • ethanol ethanal
  • it is essential to distil off the aldehyde before it gets oxidised to the acid
  • CH3CHO(l) + [O] ——> CH3COOH(l)
  • ethanal ethanoic acid
  • Practical details
  • the acidified K2Cr2O7 is dripped into the warm alcohol
  • aldehydes have low boiling points - no hydrogen bonding - they distil off immediately
  • if it didn’t distil off it would be oxidised to the equivalent carboxylic acid
  • to oxidise an alcohol straight to the acid, reflux the mixture
  • compound formula intermolecular bonding boiling point
  • ETHANOL C2H5OH HYDROGEN BONDING 78°C
  • ETHANAL CH3CHO DIPOLE-DIPOLE 23°C
  • ETHANOIC ACID CH3COOH HYDROGEN BONDING 118°C
slide25

OXIDATION OF PRIMARY ALCOHOLS

Controlling the products

e.g. CH3CH2OH(l) + [O] ——> CH3CHO(l) + H2O(l)

then CH3CHO(l) + [O] ——> CH3COOH(l)

OXIDATION TO ALDEHYDES

DISTILLATION

OXIDATION TO CARBOXYLIC ACIDS

REFLUX

Aldehyde has a lower boiling point so distils off before being oxidised further

Aldehyde condenses back into the mixture and gets oxidised to the acid

slide26

OXIDATION OF SECONDARY ALCOHOLS

Secondary alcohols are easily oxidised to ketones

e.g. CH3CHOHCH3(l) + [O] ——> CH3COCH3(l) + H2O(l)

propan-2-ol propanone

The alcohol is refluxed with acidified K2Cr2O7. However, on prolonged treatment with a powerful oxidising agent they can be further oxidised to a mixture of acids with fewer carbon atoms than the original alcohol.

slide27

OXIDATION OF SECONDARY ALCOHOLS

Secondary alcohols are easily oxidised to ketones

e.g. CH3CHOHCH3(l) + [O] ——> CH3COCH3(l) + H2O(l)

propan-2-ol propanone

The alcohol is refluxed with acidified K2Cr2O7. However, on prolonged treatment with a powerful oxidising agent they can be further oxidised to a mixture of acids with fewer carbon atoms than the original alcohol.

OXIDATION OF TERTIARY ALCOHOLS

Tertiary alcohols are resistant to normal oxidation

slide28

OXIDATION OF ALCOHOLS

Why 1° and 2°alcohols are easily oxidised and 3° alcohols are not

For oxidation to take place easily you must have two hydrogen atoms on adjacent C and O atoms.

slide29

H H

R C O + [O]R C O + H2O

H H

OXIDATION OF ALCOHOLS

Why 1° and 2°alcohols are easily oxidised and 3° alcohols are not

For oxidation to take place easily you must have two hydrogen atoms on adjacent C and O atoms.

slide30

H H

R C O + [O]R C O + H2O

H H

H H

R C O + [O]R C O + H2O

RR

OXIDATION OF ALCOHOLS

Why 1° and 2°alcohols are easily oxidised and 3° alcohols are not

For oxidation to take place easily you must have two hydrogen atoms on adjacent C and O atoms.

slide31

H H

R C O + [O]R C O + H2O

H H

H H

R C O + [O]R C O + H2O

RR

OXIDATION OF ALCOHOLS

Why 1° and 2°alcohols are easily oxidised and 3° alcohols are not

For oxidation to take place easily you must have two hydrogen atoms on adjacent C and O atoms.

This is possible in 1° and 2° alcoholsbut not in 3° alcohols.

slide32

H H

R C O + [O]R C O + H2O

H H

H H

R C O + [O]R C O + H2O

RR

R H

R C O + [O]

R

OXIDATION OF ALCOHOLS

Why 1° and 2°alcohols are easily oxidised and 3° alcohols are not

For oxidation to take place easily you must have two hydrogen atoms on adjacent C and O atoms.

This is possible in 1° and 2°alcoholsbut not in 3° alcohols.

slide33

INFRA-RED SPECTROSCOPY

Chemical bonds vibrate at different frequencies. When infra red (IR) radiation is passed through a liquid sample of an organic molecule, some frequencies are absorbed. These correspond to the frequencies of the vibrating bonds.

Most spectra are very complex due to the large number of bonds present and each molecule produces a unique spectrum. However the presence of certain absorptions can be used to identify functional groups.

BOND COMPOUND ABSORBANCE RANGE

O-H alcohols broad 3200 cm-1 to 3600 cm-1

O-H carboxylic acids medium to broad 2500 cm-1 to 3500 cm-1

C=O ketones, aldehydes strong and sharp 1600 cm-1 to 1750 cm-1

esters and acids

slide34

INFRA-RED SPECTROSCOPY

IDENTIFYING ALCOHOLS USING INFRA RED SPECTROSCOPY

DifferentiationCompound O-H C=O

ALCOHOLYESNO

ALDEHYDE / KETONENOYES

CARBOXYLIC ACIDYESYES

ESTERNOYES

ALCOHOL CARBOXYLIC ACID ESTER

BUTAN-1-OL PROPANOIC ACID ETHYL ETHANOATE

O-H absorption O-H + C=O absorption C=O absorption

slide35

LABORATORY PREPARATION OF ALCOHOLS

from haloalkanes - reflux with aqueous sodium or potassium hydroxide

from aldehydes - reduction with sodium tetrahydridoborate(III) - NaBH4

from alkenes - acid catalysed hydration using concentrated sulphuric acid

Details of the reactions may be found in other sections.