Carbonyl - Aldehydes and Ketones

Question 1

What is a carbonyl functional group?

Answer 1

C=O

Question 2

Why is C=O polar?

Answer 2

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

Question 3

What is the general formula of aldehydes?

Answer 3

CnH2nO

Question 4

What is the general formula of ketones?

Answer 4

CnH2nO

Question 5

What is the first member of the ketone group?

Answer 5

Propanone

Question 6

What’s the first ketone to exhibit positional isomerism?

Answer 6

Pentanone

Question 7

What is positional isomerism?

Answer 7

Movement of the placement of the functional group

Question 8

Why do aldehydes have a higher boiling point than alkanes?

Answer 8

Aldehydes have additional permanent dipole - permanent dipole interactions between carbonyl groups

Question 9

Explain how permanent dipole interactions work

Answer 9

Polar molecules have permanent dipole (caused by difference in electronegativity in covalent bond)
Pd interactions are electrostatic forces of attraction between polar molecules

Question 10

Why do alcohols have a higher boiling point than aldehydes?

Answer 10

Because they have hydrogen bonds between the OH groups on neighbouring alcohol molecules

Question 11

Put alcohol, alkane and aldehyde in order from highest to lowest boiling point

Answer 11

Alcohol, aldehyde, alkane

Question 12

Why does boiling point increase with length of the carbon chain?

Answer 12

Greater Mr, more electrons, greater van der Waals forces

Question 13

Why do ketones have a slightly higher boiling point than aldehydes?

Answer 13

Bc the carbonyl group is in the middle rather than the end so it’s stronger.

Question 14

Why are branched aldehydes weaker than straight chain aldehydes?

Answer 14

They can’t pack as tightly together so there are weaker permanent dipole - permanent dipole interactions

Question 15

Why do short chain aldehydes and ketones mix well with water?

Answer 15

C=O can form H bonds with the water

Question 16

Write the equation to show the stages of oxidation starting with a primary alcohol

Answer 16

primary alcohol —-> aldehyde —–> carboxylic acid

Question 17

Name an oxidising agent

Answer 17

Acidified potassium dichromate (VI)

Question 18

What are the conditions for the oxidation of a primary alcohol?

Answer 18

Warm with acidified potassium dichromate

Question 19

What is observed when a primary alcohol is oxidised?

Answer 19

Orange solution turns green

Question 20

Explain the colour change when an aldehyde is formed using a half equation

Answer 20

Orange dichromate ion is reduced to a green chromium ion.

Cr2O7- + 14H+ + 6e- —-> 2Cr3+ + 7H2O

Question 21

What conditions do you need to produce an aldehyde from a primary alcohol?

Answer 21

Warm with oxidising agent

Distil product immediately

Question 22

What conditions do you need to produce a carboxylic acid from a primary alcohol

Answer 22

Heat under reflux with excess oxidising agent

Question 23

What an equation to show the stages of oxidation of a secondary alcohol

Answer 23

Secondary alcohol —–> ketone (+ water)

Question 24

Why don’t tertiary alcohols get oxidised?

Answer 24

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

Question 25

What is Fehling's solution?

Answer 25

A blue solution with copper (II) ion in alkaline solution

Question 26

What tests can be used to distinguish aldehydes from ketones?

Answer 26

Fehling's and Tollen's

Question 27

Explain how Fehling's can be used to test for aldehydes

Answer 27

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)

Question 28

Explain why aldehydes form a red/orange precipitate with Fehling's using an equation

Answer 28

Cu2+ ions are reduced by aldehydes to form Cu2O bc the aldehyde is oxidised to carboxylic acid. 2Cu2+ + H2O + 2e- ----> Cu2O + 2H+

Question 29

How is Tollen's reagent made

Answer 29

Adding sodium hydroxide to silver nitrate solution then adding ammonia dropwise

Question 30

What is Tollen's reagent

Answer 30

Ammoniacal silver nitrate

Question 31

Explain how to carry out a test for an aldehyde using Tollen's reagent

Answer 31

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

Question 32

Why do aldehydes form a silver precipitate when reacted with Tollen's reagent?

Answer 32

Reduce Ag+ to Ag

Question 33

Name a reducing agent

Answer 33

Sodium tetrahydridoborate (NaBH4)

Question 34

Write an equation for the reduction of an aldehyde

Answer 34

Aldehyde ----> Primary alcohol

Question 35

What are the conditions for the reduction of an aldehyde?

Answer 35

Heat under reflux with reducing agent in aqueous ethanol followed by acidification with sulfuric acid.

Question 36

Write an equation for the reduction of ketones

Answer 36

Ketones ----> Secondary alcohol

Question 37

What are the conditions for the reduction of a ketone?

Answer 37

Heat under reflux with reducing agent in aqueous solution followed by acidification with sulfuric acid

Question 38

How can hydroxynitriles be formed?

Answer 38

Hydrogen cyanide + ketones or aldehydes

Question 39

How can the risk of using HCN be reduced?

Answer 39

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

Question 40

Why can C=O undergo addition reactions?

Answer 40

Bc its unsaturated

Question 41

What is an addition reaction?

Answer 41

When a species is added across a double bond bc the pi bond is broken

Question 42

Why is C=O attacked by nucleophiles?

Answer 42

Bc it's polar

Question 43

What is a nucleophile

Answer 43

Electron pair donor