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A2 Chemistry > Carbonyl - Aldehydes and Ketones > Flashcards

Carbonyl - Aldehydes and Ketones Flashcards

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1
Q

What is a carbonyl functional group?

A

C=O

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2
Q

Why is C=O polar?

A

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

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3
Q

What is the general formula of aldehydes?

A

CnH2nO

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4
Q

What is the general formula of ketones?

A

CnH2nO

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5
Q

What is the first member of the ketone group?

A

Propanone

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6
Q

What’s the first ketone to exhibit positional isomerism?

A

Pentanone

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7
Q

What is positional isomerism?

A

Movement of the placement of the functional group

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8
Q

Why do aldehydes have a higher boiling point than alkanes?

A

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

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9
Q

Explain how permanent dipole interactions work

A

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

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10
Q

Why do alcohols have a higher boiling point than aldehydes?

A

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

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11
Q

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

A

Alcohol, aldehyde, alkane

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12
Q

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

A

Greater Mr, more electrons, greater van der Waals forces

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13
Q

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

A

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

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14
Q

Why are branched aldehydes weaker than straight chain aldehydes?

A

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

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15
Q

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

A

C=O can form H bonds with the water

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16
Q

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

A

primary alcohol —-> aldehyde —–> carboxylic acid

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17
Q

Name an oxidising agent

A

Acidified potassium dichromate (VI)

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18
Q

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

A

Warm with acidified potassium dichromate

19
Q

What is observed when a primary alcohol is oxidised?

A

Orange solution turns green

20
Q

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

A

Orange dichromate ion is reduced to a green chromium ion.

Cr2O7- + 14H+ + 6e- —-> 2Cr3+ + 7H2O

21
Q

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

A

Warm with oxidising agent

Distil product immediately

22
Q

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

A

Heat under reflux with excess oxidising agent

23
Q

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

A

Secondary alcohol —–> ketone (+ water)

24
Q

Why don’t tertiary alcohols get oxidised?

A

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

25
Q

What is Fehling’s solution?

A

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

26
Q

What tests can be used to distinguish aldehydes from ketones?

A

Fehling’s and Tollen’s

27
Q

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

A

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)

28
Q

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

A

Cu2+ ions are reduced by aldehydes to form Cu2O bc the aldehyde is oxidised to carboxylic acid.

2Cu2+ + H2O + 2e- —-> Cu2O + 2H+

29
Q

How is Tollen’s reagent made

A

Adding sodium hydroxide to silver nitrate solution then adding ammonia dropwise

30
Q

What is Tollen’s reagent

A

Ammoniacal silver nitrate

31
Q

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

A

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

32
Q

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

A

Reduce Ag+ to Ag

33
Q

Name a reducing agent

A

Sodium tetrahydridoborate (NaBH4)

34
Q

Write an equation for the reduction of an aldehyde

A

Aldehyde —-> Primary alcohol

35
Q

What are the conditions for the reduction of an aldehyde?

A

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

36
Q

Write an equation for the reduction of ketones

A

Ketones —-> Secondary alcohol

37
Q

What are the conditions for the reduction of a ketone?

A

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

38
Q

How can hydroxynitriles be formed?

A

Hydrogen cyanide + ketones or aldehydes

39
Q

How can the risk of using HCN be reduced?

A

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

40
Q

Why can C=O undergo addition reactions?

A

Bc its unsaturated

41
Q

What is an addition reaction?

A

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

42
Q

Why is C=O attacked by nucleophiles?

A

Bc it’s polar

43
Q

What is a nucleophile

A

Electron pair donor

A2 Chemistry (17 decks)

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