Unit 3 - Oxidation of Food Flashcards
Oxidation and Reduction in Organic Compounds
For carbon compounds:
* oxidation is an increase in the hydrogen ration (increase in O atoms & decrease in H atoms)
* reduction is a decrease in the oxygen to hydrogen ratio (decrease of O atom or increase in H atoms)
Oxidation of Alcohol
Primary Alcohols
primary alcohol -> aldehyde -> carboxylic acid
Oxidation of Alcohol
Secondary Alcohols
secondary alcohol -> ketone
Oxidation of Alcohol
Tertiary Alcohols
tertiary alcohol -> no oxidation
Oxidising Agents
- Acidified potassium dichromate solution: Orange dichromate ions reduced to green chromium(III) ions
- Hot copper(II) oxide: Black reduced to brown solid.
- Acidified potassium permangenate solution: Purple permangenate ions reduced to colourless manganese(II) ions
- Warm Fehling’s (Benedict’s) solution: blue copper (II) ions reduced to brick-red precipitate
- Warm Tollens’ reagent: colourless silver(I) ions reduced to grey solid (silver)
Acidified solutions - treated with acid to give it ions
Oxidation of Primary Alcohols
Primary alcohols are oxidised to aldehydes.
When oxidation takes place, two H atoms are removed from the alcohol, one from the -OH group and one from the hydroxyl bearing the C atom
Oxidation of Seondary Alcohols
Secondary alcohols are oxidised to ketones. Two H atoms are removed from the alcohol.
Oxidation of Tertiary Alcohols
Tertiary alcohols have no H atom directly attached to the hydroxyl bearing C atm therefore can’t undergo mild oxidation.
Carbonyl Compounds
Aldehydes and ketones contain a carbonyl function group.
In aldehydes, a H atom is always bonded to the carbonyl group i.e an aldehyde contains a carbonyl group at the end position of a saturated hydrocarbon chain.
In ketones, the carbonyl group has carbon atoms on either side i.e a ketone contains the carbonyl group within the saturated hydrocarbon chain.
Naming Aldehydes
The name for an aldehyde is based on the corresponding alkane with the ‘al’ ending.
General formula: CnH2nO
In branched chain aldehydes, the main chain is the longest chain which includes the carbonyl group. In aldehydes, the position of the carbonyl group doesn’t need to be indicated in the name.
Naming Ketones
The name for a ketone is based on the corresponding alkane with the ‘one’ endings.
General formula: CnH2nO
In straight chain ketones with 4+ C atoms, the position of the carbonyl group is indicated in the name. In branched chain ketones, the main chain is the longest chain which includes the carbonyl group.
Aldehydes and ketones with the same number of carbon atoms will be isomers.
Physical Properties of Carbonyl Compounds
As the length of the carbon chain increase, the bp increases due to increased number of electrons (increased LDFs)
The uneven distribution of the electrons makes the molecule polar.
The smaller aldehydes and ketones are soluble in water because they can form hydrogen bonds with water.
Oxidation of Carbonyl Compounds
Aldehydes undergo oxidation to form carboxylic acids but ketones don’t. this means oxidising reagents can be used to distinguish the two.
An O atom is inserted into the C-H bond attached to the carbonyl group when aldehydes are oxidised. Ketones don’t have a H atom attached to the carbonyl group so resists mild oxidation.
Oxidation of Food
Foods which contain edible oils will oxidise when exposed to oxygen from the air giving a rancid flavour and smell. For example, butter will spoil due to the formation of butanoic acid.
Many flavours and aroma molecules are aldehydes. These are oxidised to sour tasting carboxylic acids.
In some cases oxidation is part of the maturing process.
Antioxidants
Antioxidants are molecules that prevent unwanted oxidation reactions occuring by being oxidised in place of the compounds they have been added to protect. Antioxidants are therefore reducing agents.
Vitamin C (ascorbic acid) is one of the most common antioxidants added to foods.
