Alcohols, Aldehydes, Ketones + Optical Isomerism

What are optical isomers?

Molecules with same molecular formula but a different arrangement of atoms in space

What is a chiral centre?

A carbon atom with 4 different groups bonded around it

What is required for optical isomers?

A chiral centre

What are 2 different optical isomers called?

Enantiomers

What effect do enantiomers have on light?

Rotate plane polarised light (by 90° in opposite directions)

What is a racemic mixture?

Formed when optical isomers are produced as a pair of enantiomers in a 1:1 ratio

Why is the overall effect of racemic mixtures on polarised light zero?

Opposite rotational effects cancel out

By what mechanisms are racemic mixtures formed by?

Nucleophilic addition

Why does nucleophilic addition of carbonyls produce optical isomers?

Nucleophilic are able to attack a molecule with a carbonyl group from above OR below C=O bond

What are the 2 ways that alcohols can be produced?

Hydration, fermentation

General formula for alcohols

CnH2n+1OH

What catalyst and temperature is needed for hydration to make alcohols?

Phosphoric acid (H3PO4), 300°C

Equation for production of ethanol by hydration

ethene + water —> ethanol

How is ethanol made by fermentation?

Enzymes break down starch from crops into sugar which can then be fermented to for ethanol

Why is the fermentation method to make ethanol carbon neutral?

The CO2 given out when it is burned is taken in by plants during photosynthesis (net carbon output is 0)

Equation for production of ethanol by fermentation

C6H12O6 —> C2H5OH + CO2

What enzyme is needed to convert glucose to ethanol?

Yeast

What are disadvantages of production of ethanol by fermentation?

Slower and lower percentage yield

Why is fermentation cheaper than hydration?

It takes place at lower temperatures

What types of alcohols can be oxidised?

1° and 2°

Why can 3° alcohols not be oxidised?

C-C bond would have to be broken rather than C-H bond

What oxidising agent is needed for oxidation of primary alcohols?

Acidified potassium dichromate (Cr2O7 2-)

Equation for partial oxidation of 1° alcohols (example propanol)

Propan-1-ol + [O] —> propanal + H2O

Equation for full oxidation of primary alcohols (example propanol)

Propan-1-ol + 2[O] —> propanoic acid + H2O

What colour change is observed when oxidising alcohols using acidified potassium dichromate?

Orange to green

What is produced when you oxidise a secondary alcohol?

Ketone

what are the conditions needed for oxidation of secondary alcohols?

Potassium dichromate, heat under reflux

Equation for oxidation of secondary alcohols (example propanol)

Propan-2-ol + [O] —> Propanone + H2O

Why should you never seal the end of the condenser during reflux?

Build up of gas pressure can cause the apparatus to explode

Why are anti-bumping granules added to the flask in reflux and distillation?

To prevent vigorous, uneven boiling by making small bubbles form instead of large bubbles

Aldehyde functional group

-CHO (C=O)

How are aldehydes produced?

From oxidation of primary alcohols

Can aldehydes oxidise further?

Yes, into carboxylic acids

How are aldehydes tested for?

Using Tollen’s Reagent or Fehling’s Solution

Ketone functional group

C=O

How are ketones produced?

From oxidation of secondary alcohols

What type of mechanism is used to reduce carbonyls to alcohols?

Nucleophilic addition

Conditions required for nucleophilic addition of carbonyls

Reagent sodium borohydride (NaBH4), aqueous solution

Equation for reduction of aldehydes by nucleophilic addition (ethanal as example)

Ethanal + 2[H] —> ethanol

Where are the first and second [H] in reduction of carbonyls produced from?

H- from NaBH4, H+ from aqueous solution

Equation for reduction of ketones by nucleophilic addition

Propanone + 2[H] —> propan-2-ol

What product is formed when aldehydes undergo nucleophilic addition by HCN?

Hydroxy nitriles

What can hydroxy nitriles form using H+/H2O?

Carboxylic acids

What is the basis for the tests for aldehydes and ketones?

Aldehydes can be further oxidised to carboxylic acids whereas ketones cannot

What is Tollen’s Reagent made up of?

A mixture of aqueous ammonia and silver nitrate [Ag(NH3)2]+

Observations when aldehydes and ketones are tested using Tollen’s Reagent

Aldehydes - a silver mirror forms coating the inside of test tube

Ketones - no visible change

Why does a silver precipitate form when aldehydes are tested with Tollen’s?

Silver atoms are reduced to silver atoms

Test for presence of carboxylic acid

Sodium carbonate - will fizz/produce CO2

What ions does Fehling’s solution contain?

Blue Cu2+ ions

Observations when aldehydes and ketones are tested with Fehling’s solution

Aldehydes - red precipitate (of Cu2O)

Ketones - no reaction