16 Alcohols

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16.1 Intro to Alcohols

Learning Objectives:

1. Know the general formula for alcohols.

2. Be able to name alcohols.

3. Describe the shape of alcohols.

4. Classify primary, secondary, and tertiary alcohols.

5. Describe the physical properties of alcohols.

General Formula

Alcohols have the functional group –OH.

The general formula is C n

H

2n+1

OH.

Example:

Ethanol = C

2

H

5

OH

Nomenclature

Use the suffix –ol.

Use a number to designate the location of the

–OH group.

Nomenclature – Multiple OH groups

Use numbers to designate the location of each hydroxyl group.

Use the correct prefix to describe how many hydroxyl groups there are.

Primary, Secondary, or Tertiary

Physical Properties

The hydroxyl group means that

hydrogen bonding

can occur between molecules.

This leads to

higher melting and boiling points

than alkanes with similar chain lengths.

This is why vodka does not freeze in the freezer!

Solubility

Hydrogen bonding also makes shorter chain alcohols

soluble in water

(hydrogen bonds form between OH and water).

16.3 Reactions with Alcohols

Learning Objectives:

1. Describe the oxidation of alcohols to aldehydes and ketones.

2. Describe the structure of aldehydes and ketones.

3. Describe the chemical tests for aldehydes and ketones.

4. Describe elimination reactions of alcohols

(dehydration).

Combustion

Alcohols will burn completely to carbon dioxide and water if enough oxygen present.

Ethanol is used as a fuel.

Oxidation

Alcohols can also be oxidised gently and in stages.

Aldehydes

Contains a

carbonyl

group (C=O).

Aldehyde has the carbonyl at the

end of the chain

(only one R group attached.

Named with the suffix

–al

(ie. methanal)

Ketones

Has a carbonyl group in the

middle of the carbon chain

(two R groups attached).

Named with the suffix

–one

(propanone).

Carboxylic Acids

Contains the functional group

-COOH

.

Named using the suffix

–oic acid

(ie. ethanoic acid).

Weak acids

, only partially dissociate in water.

Ethanol to Ethanal (aldehyde)

Dilute

acid

Less

potassium dichromate [O]

Heated

gently

Ethanal distils off

(BP 20 o

C), collected in iced container,

cannot react further

Ethanol (BP = 78 o

C) remains in reaction mix

Ethanol to Ethanoic Acid (Carboxylic Acid)

Conc.

sulphuric acid

Excess

potassium dichromate [O]

Reflux

, vapourised aldehyde or alcohol drips back into reaction mixture

Until

reaction goes to completion

Secondary Alcohols to Ketones

Secondary alcohols

can be oxidised to form

ketones

.

There is no further reaction.

Ketones

CANNOT

oxidise into carboxylic acids.

Tests for Aldehydes and Ketones

Since

aldehydes can be further oxidised

to form carboxylic acids and

ketones cannot

, there are simply chemical tests to differentiate between the two.

OR

Tollen’s Test (the Silver Mirror)

Add

silver nitrate in aqueous ammonia

(gentle [O])

Aldehyde is oxidised,

silver is reduced

Produces

solid silver metal

(silver mirror)

Fehling’s/Benedict’s Test

Cu

2+

+ e

-

Cu

+

Add either reagent containing

copper(II) complex ions (blue)

, gentle [O]

Aldehyde is oxidized,

copper is reduced

to copper(I)

Forms brick red

copper(I) oxide

Elimination (Dehydration)

Elimination reactions

involve a small molecule being eliminated from the organic compound.

In the case of alcohols

water is eliminated

so are called a

dehydration

reaction.

Dehydration Reaction

Products of Dehydration

A

mixture of isomers

is formed from dehydration reactions.

Butan-1-ene

Butan-2-ol

Z But-2-ene E But-2-ene

16.2 Ethanol Production

Learning Objectives:

1. Describe how ethanol is produced by fermentation.

2. Describe the pros and cons of fermentation.

3. Describe what biofuels are.

4. Explain what it means for a fuel to be carbon neutral.

Ethanol is an important industrial product

Making Ethanol from Crude Oil

Ethene is produced from cracking crude oil .

Ethene can then be

reacted with steam

, using phosphoric acid as a catalyst , to produce

ethanol

.

Fermentation

Ethanol can also be made from the

fermentation

of

carbohydrates

(from plants) are broken down into

sugars

and then converted into

ethanol

by enzyme action by

yeast

.

Anaerobic Respiration

C

6

H

12

O

6

2C

2

H

5

OH + 2CO

2

Rate affected by temperature (compromise

35 o

C)

Air kept out to prevent oxidation to ethanoic acid (acid in vinegar).

Once solution contains 15% ethanol, enzymes cannot function and fermentation stops.

Distillation

Ethanol as Biofuel

Biofuel

= fuel made from plant material (renewable)

Being researched as an alternative to petrol so that we are no longer so reliant on crude oil, which is a non-renewable resource.

Carbon Neutral

The

combustion

of ethanol produces the same amount of carbon dioxide as the plant absorbed while living, so ethanol is said to be a

carbon neutral

fuel.

This is seen as a positive because CO

2 is a greenhouse gas associated with

global warming

.

However, this does not take into account other energy costs such as transportation or processing.

Production of Ethene

Possible to produce ethene from ethanol made from fermentation.

Renewable way to produce ethene which is an important industrial chemical.

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