Carbonyl - Aldehydes and Ketones Flashcards
What is a carbonyl functional group?
C=O
Why is C=O polar?
O is more electronegative than the C so it draws the electrons in the covalent bond closer to it so it has a partial negative charge on it while the C has a partial positive charge
What is the general formula of aldehydes?
CnH2nO
What is the general formula of ketones?
CnH2nO
What is the first member of the ketone group?
Propanone
What’s the first ketone to exhibit positional isomerism?
Pentanone
What is positional isomerism?
Movement of the placement of the functional group
Why do aldehydes have a higher boiling point than alkanes?
Aldehydes have additional permanent dipole - permanent dipole interactions between carbonyl groups
Explain how permanent dipole interactions work
Polar molecules have permanent dipole (caused by difference in electronegativity in covalent bond)
Pd interactions are electrostatic forces of attraction between polar molecules
Why do alcohols have a higher boiling point than aldehydes?
Because they have hydrogen bonds between the OH groups on neighbouring alcohol molecules
Put alcohol, alkane and aldehyde in order from highest to lowest boiling point
Alcohol, aldehyde, alkane
Why does boiling point increase with length of the carbon chain?
Greater Mr, more electrons, greater van der Waals forces
Why do ketones have a slightly higher boiling point than aldehydes?
Bc the carbonyl group is in the middle rather than the end so it’s stronger.
Why are branched aldehydes weaker than straight chain aldehydes?
They can’t pack as tightly together so there are weaker permanent dipole - permanent dipole interactions
Why do short chain aldehydes and ketones mix well with water?
C=O can form H bonds with the water
Write the equation to show the stages of oxidation starting with a primary alcohol
primary alcohol —-> aldehyde —–> carboxylic acid
Name an oxidising agent
Acidified potassium dichromate (VI)
What are the conditions for the oxidation of a primary alcohol?
Warm with acidified potassium dichromate
What is observed when a primary alcohol is oxidised?
Orange solution turns green
Explain the colour change when an aldehyde is formed using a half equation
Orange dichromate ion is reduced to a green chromium ion.
Cr2O7- + 14H+ + 6e- —-> 2Cr3+ + 7H2O
What conditions do you need to produce an aldehyde from a primary alcohol?
Warm with oxidising agent
Distil product immediately
What conditions do you need to produce a carboxylic acid from a primary alcohol
Heat under reflux with excess oxidising agent
What an equation to show the stages of oxidation of a secondary alcohol
Secondary alcohol —–> ketone (+ water)
Why don’t tertiary alcohols get oxidised?
Because the OH group is attached to a carbon which is attached to 3 alkyl groups so a hydrogen cannot be removed from the carbon so a C=O cannot be formed
What is Fehling’s solution?
A blue solution with copper (II) ion in alkaline solution
What tests can be used to distinguish aldehydes from ketones?
Fehling’s and Tollen’s
Explain how Fehling’s can be used to test for aldehydes
Add a few drops of the unknown solution to 1cm3 of freshly prepared Fehling’s and warm in a water bath. If there’s a red/orange precipitate then an aldehyde is made, if not then it’s a ketone (or something else)
Explain why aldehydes form a red/orange precipitate with Fehling’s using an equation
Cu2+ ions are reduced by aldehydes to form Cu2O bc the aldehyde is oxidised to carboxylic acid.
2Cu2+ + H2O + 2e- —-> Cu2O + 2H+
How is Tollen’s reagent made
Adding sodium hydroxide to silver nitrate solution then adding ammonia dropwise
What is Tollen’s reagent
Ammoniacal silver nitrate
Explain how to carry out a test for an aldehyde using Tollen’s reagent
Add a few drops of unknown solution to 2cm3 of Tollen’s reagent and warm in a water bath. If a silver mirror is formed then an aldehyde is present
Why do aldehydes form a silver precipitate when reacted with Tollen’s reagent?
Reduce Ag+ to Ag
Name a reducing agent
Sodium tetrahydridoborate (NaBH4)
Write an equation for the reduction of an aldehyde
Aldehyde —-> Primary alcohol
What are the conditions for the reduction of an aldehyde?
Heat under reflux with reducing agent in aqueous ethanol followed by acidification with sulfuric acid.
Write an equation for the reduction of ketones
Ketones —-> Secondary alcohol
What are the conditions for the reduction of a ketone?
Heat under reflux with reducing agent in aqueous solution followed by acidification with sulfuric acid
How can hydroxynitriles be formed?
Hydrogen cyanide + ketones or aldehydes
How can the risk of using HCN be reduced?
Using KCN instead bc it’s stored more safely despite also being toxic
Fume cupboard
Water bath/electric mantle rather than flame bc reactants are flammable
Why can C=O undergo addition reactions?
Bc its unsaturated
What is an addition reaction?
When a species is added across a double bond bc the pi bond is broken
Why is C=O attacked by nucleophiles?
Bc it’s polar
What is a nucleophile
Electron pair donor
