3.3.8 Aldehydes and Ketones (A2)

Question 1

what is the functional group of an aldehyde?

Answer 1

C=O(H)
on the end of a molecule

Question 2

what is the functional group of ketones?

Answer 2

C=O in the central region of a molecule

Question 3

how are aldehydes made?

Answer 3

primary alcohol - one carbon bound to the carbon with -OH
oxidise - H+/K2Cr2O7 (sulfuric acid)
orange—>green
distillation

Question 4

what is the aldehyde called which includes a benzene ring?

Answer 4

benzanal

Question 5

how are ketones made?

Answer 5

oxidation of secondary alcohol
H+/K2Cr2O7 (potassium dichromate) orange —> green
under reflux

Question 6

what are the properties of aldehydes and ketones?

Answer 6

C=O bond is polar
dipole-dipole interactions between molecules
higher boiling points than their counterpart alkanes - more energy needed to overcome intermolecular forces

Question 7

what is the solubility like of aldehydes and ketones?

Answer 7

aldehydes and ketone with less than 4 carbons can dissolve in water
because carbonyl group can hydrogen bond to water.

Question 8

why can’t aldehydes and ketones with more than 4 carbons dissolve in water?

Answer 8

the larger the amount of carbons, the more non polar the molecule, this prevents molecule from dissolving

Question 9

what state is methanal at room temperature?

Answer 9

gas

Question 10

what state are short chain aldehydes and ketones at room temperature?

Answer 10

liquids (apart from methanal) with a characteristic smell

Question 11

what is an example of a ketone used in a household item?

Answer 11

propanone found in nail varnish remover

Question 12

what is benzaldehyde used for? why?

Answer 12

scenting soaps and flavouring foods
smells of almonds

Question 13

why do ketones have no fewer than 3 carbons?

Answer 13

the double bond oxygen has to be on neither end of the carbon chain

Question 14

what is the lowest number of carbons in a ketone?

Answer 14

3

Question 15

why is no numbering system needed for aldehydes?

Answer 15

R=O(H) has to be on either end of the chain

Question 16

why are there no H bonds between propanone molecules?

Answer 16

no oxygen bonded to hydrogen

Question 17

what reactions tend to happen with aldehydes/ketones?

Answer 17

REDOX
nucleophilic addition - the delta positive carbon is susceptible to nucleophilic attack, carbonyl group unsaturated

Question 18

why can’t we use the cyanide ion in the lab?

Answer 18

cyanide is toxic

Question 19

what group are the products of reactions with cyanide ions in?

Answer 19

nitrile

Question 20

why do we get a racemic mixture?

Answer 20

C=O bond is planar
nucleophile can attack from either above or below the plane
50:50 chance of either happening
so we get a 50:50 mixture of both enantiomers

Question 21

why do racemic mixtures show no optical activity?

Answer 21

the rotation on plane-polarised light cancels out

Question 22

what is oxidation?

Answer 22

loss of e-
gain of oxygen
loss of H

Question 23

what is reduction?

Answer 23

gain of e-
loss of oxygen
gain of H

Question 24

what does the oxidation of an aldehyde result in?

Answer 24

carboxylic acid

Question 25

can ketones be oxidised further? why?

Answer 25

no
C-C bond is too strong to be overcome

Question 26

what is Fehling’s A?

Answer 26

CuSO4 (copper sulphate)

Question 27

what is Fehling’s B?

Answer 27

alkali, complexing agent

Question 28

what does the Fehling’s test give for an aldehyde?

Answer 28

colour change from blue to orange/red precipitate

Question 29

what does the Fehling’s test give for a ketone?

Answer 29

no observable changes - remains blue

Question 30

what is going on during the Fehling’s test for an aldehyde?

Answer 30

Aldehyde being oxidised to a carboxylic acid
this reduces the Cu2+ ions (blue) to Cu+ ions (orange/red)

Question 31

why does the Fehling’s test show no results in a ketone?

Answer 31

ketone cannot be oxidised further

Question 32

what is the Fehling’s test for?

Answer 32

distinguish between aldehydes and ketones

Question 33

what chemical does the silver mirror test involve?

Answer 33

Tollens’ reagent

Question 34

what happens when we put Tollens’ reagent into an aldehyde?

Answer 34

silver precipitate formed, “mirror” layer on inside of test tube

Question 35

what is the result when we add Tollens’ reagent to a ketone?

Answer 35

no observable changes

Question 36

what two tests do we use to distinguish between aldehydes and ketones?

Answer 36

Fehlings’ test
Silver mirror test (tollens’ reagent)

Question 37

why do we get no observable changes when we add Tollens’ reagent to ketones?

Answer 37

ketones cannot be further oxidised

Question 38

what happens when Tollen’s reagent is added to an aldehyde?

Answer 38

Aldehyde is oxidised to a carboxylic acid
silver ion reduced from Ag+ to Ag2+

Question 39

what reducing agent is used to reduce an aldehyde / ketone?

Answer 39

sodium tetraborohydride

Question 40

what is released from reducing an aldehyde/ketone?

Answer 40

hydride ion (H-) with a lone pair

Question 41

what does reducing an aldehyde form?

Answer 41

primary alcohol

Question 42

what is the general equation for reduction of an aldehyde?

Answer 42

RCHO + 2H —> RCHOH

Question 43

what type of mechanism is the reduction of aldehydes?

Answer 43

nucleophilic addition

Question 44

what is formed after the reduction of a ketone?

Answer 44

secondary alcohol

Question 45

what is the general equation of reduction of a ketone?

Answer 45

RCOR’ + 2H —> RCH(OH)R’

Question 46

what type of mechanism is the reduction of ketones?

Answer 46

nucleophilic addition

Question 47

which isomer of an aldehyde does not contain a chiral carbon and therefore cannot show stereoisomerism

Answer 47

symmetrical ketone

Question 48

what is the usual reason for molecules not showing stereoisomerism? (optical)

Answer 48

no chiral carbon