Purpose: Differentiate primary and secondary alcohols from tertiary alcohols through oxidation.
Method: Redox reaction using acidified potassium dichromate (K₂Cr₂O₇).
Experiment Setup
Alcohol Samples Used
Primary alcohol: Propan-1-ol
Secondary alcohol: Propan-2-ol (position isomer of propan-1-ol)
Tertiary alcohol: 2-methylpropan-2-ol
Reagents
Acidified potassium dichromate (orange crystals in water and dilute sulfuric acid)
Apparatus
Hot water bath
Rubber bungs to shake solutions and avoid contamination
Chemical Principles
Oxidation-Reduction Reaction
Dichromate ions (Cr₂O₇²⁻) can oxidize alcohols, changing from a +6 (orange) to a +3 (dark green) oxidation state upon reduction.
Alcohols must have a carbon directly attached to at least one hydrogen atom to undergo oxidation.
Observations and Reactions
Primary Alcohol: Propan-1-ol
Structure: Carbon attached to OH group also has two H atoms.
Reaction: Undergoes oxidation, reduces dichromate ions from orange to dark green.
Secondary Alcohol: Propan-2-ol
Structure: Key carbon with OH group attached to one H atom.
Reaction: Also undergoes oxidation, reducing dichromate ions similarly to primary alcohol.
Tertiary Alcohol: 2-methylpropan-2-ol
Structure: Key carbon with OH group attached to no H atoms.
Reaction: Does not undergo oxidation; dichromate ions remain orange.
Conclusion
Positive Test: Both primary and secondary alcohols reduce dichromate ions, changing color.
Tertiary Alcohol: Does not change the color of dichromate, remaining orange.
Distinction: Test cannot differentiate between primary and secondary alcohols (both give dark green color), but is effective for identifying tertiary alcohols (remain orange).
Summary
The oxidation test using acidified potassium dichromate is useful to distinguish tertiary alcohols from primary and secondary alcohols based on their ability (or lack thereof) to reduce dichromate ions.
The experiment demonstrates the reaction of alcohols with dichromate ions in a hot water bath, observing color changes for identification.