aldehydes and ketones

Question 1

what is the carbonyl group?

Answer 1

C=O

Question 2

where is the carbonyl carbon found in aldehydes?

Answer 2

at the end of a carbon chain (func. group = COH)

Question 3

what suffix is used to name aldehydes?

Answer 3

-anal (e.g CH3CH2CH2COH = butanal)

Question 4

aldehydes are…

Answer 4

easily oxidised to form carboxylic acids

Question 5

what is functional group for carboxylic acids?

Answer 5

-COOH

Question 6

what suffix is used for carboxylic acids?

Answer 6

-ioc acid (e.g. CH3CH2COOH = propanoic acid)

Question 7

where is the carbonyl carbon found in ketones?

Answer 7

in the middle of a carbon chain

Question 8

what suffix is used for ketones?

Answer 8

-one (e.g CH3C(O)CH3 = propanone)

Question 9

out of aldehydes and ketones, which is easier to oxidise?

Answer 9

aldehydes

Question 10

what two chemical tests are used to distinguish between aldehydes and ketones?

Answer 10

• Fehling’s solution
• Tollens reagent (silver mirror)

Question 11

both Tollen’s and Fehling’s are…

Answer 11

weak oxidising agents - therefore change is observed when there is an aldehyde, NOT A KETONE

Question 12

what is Fehling’s solution?

Answer 12

an alkaline solution of copper(II) sulfate - complex Cu2+ ions give it its blue colour

Question 13

what happens to the solution in the presence of Fehling’s solution?

Answer 13

Cu2+ is reduced (Cu2+ + e- → Cu ) to form a brick red precipitate of
copper(I) oxide which has the formula Cu2O

Question 14

what happens to the ALDEHYDE in the Fehling’s solution reaction?

Answer 14

it is oxidised to form a carboxylic acid

Question 15

what is the process of using Fehling’s solution?

Answer 15

• a few drops of the organic solution added to a test tube containing Fehling’s
• gently warmed in a hot water bath

Question 16

how do you prepare Tollens reagent?

Answer 16

• add a drop of sodium hydroxide to silver(I) nitrate solution to make a precipitate of silver(I) oxide
• dissolve this precipitate by adding dilute ammonia

Question 17

what complex ion does Tollen’s reagent contain?

Answer 17

[Ag(NH3)2]+

Question 18

what happens to the complex ion in Tollen’s reagent in the presence of an aldehyde?

Answer 18

[Ag(NH3)2]+ is reduced to form a precipitate of metallic silver (Ag+(aq) + e- → Ag(s)

Question 19

what happens to the ALDEHYDE in the presence of Tollen’s reagent?

Answer 19

it is oxidised to form a carboxylic acidw

Question 20

what forces exist in aldehydes and ketones?

Answer 20

• van der waals forces
• permanent dipole-dipole forces

Question 21

what is the effect of the forces on BP of aldehydes and ketones?

Answer 21

it means that the b.p. of aldehydes and ketones are higher than those of alkanes with a similar Mr

Question 22

bp of aldehydes/ketones: as the length of the carbon chain increases…

Answer 22

the b.p. of the aldehydes/ ketones increases too. This is due to a greater number of electrons,
and therefore stronger van der Waals’ forces

Question 23

as branching increases…

Answer 23

the b.p. of the aldehydes/ketones decreases.
* This is due to less effective permanent dipole-dipole forces.
* Think of it as though the branches are getting in the way of the Intermolecular forces, while they’re trying to pack the molecules
together.

Question 24

how soluble are aldehydes and ketones in water?

Answer 24

all short chain aldehydes and ketones are soluble in water because the
polar carbonyl groups are able to form H-bonds with water molecules.

Question 25

what is the effect of increasing chain length on solubility?

Answer 25

* As the chain length increases, the aldehydes and ketones become less soluble. * This is because the hydrocarbon chain that doesn’t have the carbonyl group on it is non-polar. * The longer this chain is, the less polar the molecule is so therefore the less soluble

Question 26

what kind of reaction do carbonyls undergo?

Answer 26

nucleophilic addition reactions

Question 27

why do carbonyls undergo nucleophilic addition reactions?

Answer 27

oxygen is more electronegative therefore attracts the electrons from the covalent bond - leaves the carbon with a partial positive charge

Question 28

what are some examples of nucleophilic addition reactions which aldehydes and ketones undergo?

Answer 28

- The reduction of aldehydes to primary alcohols, and ketones to secondary alcohols, using the reducing agent NaBH4 in aqueous solution. - The reaction of the -:CN ion with aldehydes and ketones to produce hydroxynitriles (KCN, followed by dilute acid)

Question 29

what is NaBH4?

Answer 29

sodium borohydride - it is a complex anion but behaves like a hydride ion (H-) So we draw it as a H- in the mechanism; that’s the species acting as the nucleophile

Question 30

what happens in the intermediate of the addition reaction with NaBH4?

Answer 30

gains a H+ from the acid

Question 31

what are the conditions for the reduction of aldehydes by NaBH4?

Answer 31

Reagent: Aqueous NaBH4 followed by H2SO4 Conditions: Room temp. Mechanism: Nucleophilic - H- = nucleophile Reaction type: Addition/Reduction Product: Primary alcohol

Question 32

what are the conditions for the reduction of ketones by NaBH4?

Answer 32

Reagent: Aqueous NaBH4 followed by H2SO4 Conditions: Room temp. Mechanism: Nucleophilic - H- = nucleophile Reaction type: Addition/Reduction Product: Secondary alcohol

Question 33

what is the overall equation for the reaction of aldehydes with sodium borohydride?

Answer 33

R-CO-H + 2[H] --> R-CH2(OH)

Question 34

what is the overall equation for the reaction of ketones with sodium borohydride?

Answer 34

R-CO-R + 2[H] --> R-CH(OH)-R

Question 35

what are the conditions for the reaction of carbonyls with hydrogen cyanide?

Answer 35

Reagent: HCN - formed by reaction with KCN followed by dilute H2SO4 Conditions: Room temp. Mechanism: Nucleophilic SO CN- is the nucleophile Reaction type: Addition Product: Hydroxynitrile

Question 36

what is the overall equation for the reaction of aldehydes with hydrogen cyanide?

Answer 36

R-CO-H + HCN --> RCH(OH)CN

Question 37

what is the overall equation for the reaction of aldehydes with hydrogen cyanide?

Answer 37

R-CO-R’ + HCN → RC(OH)R’CN

Question 38

what are the hazards of using KCN?

Answer 38

HCN gas is not used directly in the lab as it is an extremely toxic + flammable gas. Instead it is formed in situ in the reaction mixture by adding a dilute acid (H2SO4) to an aqueous solution of KCN. * KCN must be handled carefully as it is toxic when ingested + forms HCN when in contact w/ acid.

Question 39

what do aldehydes and unsymmetrical ketones form?

Answer 39

mixtures of enantiomers (when they react with KCN followed by dilute acid) - this is because the products of the reactions contain chiral carbon atoms

Question 40

nucleophillic addition reactions of KCN followed by dilute acid can produce...

Answer 40

a racemic mixture of enantiomers which is optically inactive because: 1. The C atom in the planar carbonyl group can be attacked by a nucleophile from above or below. 2. There is an equal chance of attack from above or below.