L9 - alcohols

objectives

• investigate the structural formulae, properties and functional group including:
  • primary
  • secondary
  • tertiary alcohols
• investigate the products of the oxidation of primary and secondary alcohols
• explain the properties within and between the homologous series of alcohols with reference to the intermolecular and intramolecular bonding present

Alcohols Homologous Series

Alcohols are classified by the presence of an -OH group attached to the carbon chain.

This provides the suffix -anol to substances that contain it as their most reactive group

When the -OH group is not the most reactive group we call it the hydroxy group

Naming Alcohols

• 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

Structural Classification of Alcohols

Aliphatic Alcohols

Aromatic Alcohols

• in aromatic alcohols (or phenols) the OH is attached directly to a cyclo ring

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

In the above examples - The first two compounds are classified as aromatic alcohols (phenols) because the OH group is attached directly to the ring.

Primary, Secondary and Tertiary Aliphatic Alcohols

Much like the Haloalkanes from Lesson 3 -

When a Hydroxy group is bonded to a Carbon chain the carbon it is attached to can affect the chemistry of the compound.

Primary Alcohol:

• The Hydroxy group is bonded to a carbon that is only bonded to one other carbon
• These are usually found at the end of a carbon chain

Secondary Alcohol:

• The Hydroxy group is bonded to a carbon that is bonded to two other carbons
• These are usually found in the middle of a carbon chain

Tertiary Alcohol:

• The Hydroxy is bonded to a carbon that is bonded to three other carbons
• These are usually found joined to the middle carbon of a carbon chain that also has a methyl group coming off the same carbon - i.e. Methylpropane

You can see diagrams of the three types below - when they react you get different products.

Structural Isomers of Alcohols

Different structures are possible due to...

A Different positions for the OH group and

B Branching of the carbon chain

See the possible isomers of Butanol below:

Testing for Alcohols

Alcohols all react with sodium metal.

The observations of this reaction are:

• Effervescence of the Hydrogen gas
• A white Precipitate being formed from the sodium alkyloxide formed

The word and symbol equations for the reaction with ethanol are below:

sodium + ethanol → sodium ethoxide + hydrogen

2Na + 2C₂H₅OH → 2C₂H₅ONa + H₂

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) - we acidify it with Sulphuric Acid and gently heat it.

Primary: Easily oxidised to aldehydes and then to carboxylic acids.

CH₃CH₂CH₂CH₂OH + [O] → CH₃CH₂CH₂CHO + [O] → CH₃CH₂CH₂COOH + H₂O

Secondary: Easily oxidised to ketones

CH₃CH₂CH(OH)CH₃ + [O] → CH₃CH₂COCH₃ + H₂O

Tertiary: Not oxidised under normal conditions.

When the alcohols undergo oxidation the orange potassium dichromate turns Green. This does not happen for Tertiary Alcohols.

They do break down with very vigorous oxidation

Aldehydes, Ketones and Carboxylic Acids

So How do we Differentiate between aldehydes and carboxylic acids?

We can manipulate reaction conditions to produce the aldehyde over the carboxylic acid.

First Oxidation:

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

Further Oxidation:

CH₃CHO(l) + [O] ——> CH₃COOH(l)

Why can we oxidise Primary and Secondary Alcohols but not Tertiary?

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° alcohols but not in 3° alcohols.

Physical Properties of Alcohols

Boiling Point of Primary, Secondary and Tertiary Isomers

Solubility

• Low molecular mass alcohols are miscible with water Due to hydrogen bonding between the two molecules
• Heavier alcohols are less miscible

Solvent Properties

Alcohols are themselves very good solvents - They dissolve a large number of organic molecules