Aldehydes and Ketones

Question 1

Functional group of aldehydes and ketones

Answer 1

C=O, carbonyl group

Question 2

How are aldehydes produced

Answer 2

By the oxidation of primary alcohols.

An excess of alcohol is warmed with acidified potassium dichromate solution

Question 3

Conditions for formation of aldehydes from primary alcohols

Answer 3

Heat and distil off the product as it forms

Question 4

How are ketones formed

Answer 4

By the oxidation of secondary alcohols

Question 5

Conditions for formation of ketones from secondary alcohols

Answer 5

Heat under reflux

Question 6

How to produce a carboxylic acid from a primary alcohol

Answer 6

Use excess acidified potassium dichromate instead of excess alcohol
Heat reacting mixture under reflux instead of distilling of the product

Question 7

Forces between alkane/alkene molecules

Answer 7

Van der Waals forces

Question 8

Forces between RX molecules

Answer 8

Permanent dipole-dipole interactions

Question 9

Forces between alcohol molecules

Answer 9

Hydrogen bonding

Question 10

What forces are found between molecules of an aldehyde/ketone

Answer 10

Permanent dipole-dipole interactions because they are polar molecules

Question 11

Why do aldehydes and ketones have higher BP’s than alkanes but lower than alcohols

Answer 11

There are Van der Waals forces between molecules of the alkane
There are permanent dipole-dipole interactions between molecules of aldehyde/ketones
There is hydrogen bonding between molecules of the alcohol.
Hydrogen bonds are the strongest intermolecular forces and Van der Waals are the weakest.
More energy is needed to break the stronger intermolecular forces in alcohols

Question 12

Why are the lower members of aldehydes and ketones soluble in water

Answer 12

They form hydrogen bonds with water molecules

Question 13

Why does the solubility of aldehydes and ketones decreases as the Mr increases

Answer 13

As the Mr increases, the non-polar hydrocarbon chain which is insoluble becomes larger. Therefore the solubility decreases.

Question 14

What are aldehydes readily oxidised to

Answer 14

Carboxylic acids

Question 15

How to distinguish between aldehydes and ketones using Tollen’s reagent

Answer 15

Aldehydes - oxidised to carboxylic acids. Forms a silver mirror

Ketones - No reaction. No colour change, remains colourless

Question 16

How to distinguish between aldehydes and ketones using Fehling’s solution

Answer 16

Aldehydes - Colour change from blue to red precipitate

Ketones - No reaction. No colour change, remains blue

Question 17

What happens when HCN is added to a cold solution of a carbonyl compound

Answer 17

It adds across the double bond.

Question 18

What is the reaction between a carbonyl compound and KCN with dilute H2SO4 (or HCN) called and what are the conditions

Answer 18

Nucleophilic Addition
pH6

Question 19

Why is the reaction between HCN and carbonyl compounds such as ethanal classified as nucleophillic addition

Answer 19

:CN- ions are nucleophiles as they donate a lone pair of electrons.
It is an addition because HCN adds across the C=O double bond to give a single saturated product

Question 20

Why is using cyanide ions useful in synthetic organic chem

Answer 20

Increases the length of the carbon chain by 1 carbon atom in the product

Question 21

What do aldehydes and unsymmetrical ketones form when they react with KCN followed by dilute acid

Answer 21

mixtures of enantiomers

Question 22

Why is the product of the reaction between aldehydes and ketones always optically inactive

Answer 22

:CN- could attack from above or below/either side of the planar C=O group, giving equal amounts of the two optical isomers. A racemic mixture is formed. A racemic mixture has no effect on plane polarised light because the rotations of the two isomers cancel each other out

Question 23

Describe the mechanism for the nucleophilic addition of HCN to ethanal

Answer 23

Draw the ethanal molecule with a delta positive charge on the C and delta negative charge on the O
Draw a :CN- molecule below the double bond with an arrow going from the lone pair to the C
Draw an arrow from the double bond to the O

This form a single saturated product with the CN now attached to the C and the O now has a electron pair and a negative charge

Draw another product (same as formed above) and with a H+ion next to it. There is an arrow from the electron pair to the H+ ion

This forms a molecule with both CN and OH. A hydroxynitrile

Question 24

Why is it necessary to use HCN AND KCN in nucleophilic addition

Answer 24

If no KCN present, [CN-] is very small because HCN is a weak acid and therefore step 1 will be very slow
Step 2 will be too slow if no HCN present

Question 25

What are aldehydes reduced to and using what

Answer 25

Primary alcohols, using NaBH4

Question 26

What are ketones reduced to

Answer 26

Secondary alcohols, using NaBH4

Question 27

What type of reaction is the reduction of aldehydes and ketones using NaBH4 an example of

Answer 27

Nucleophilic Addition

Question 28

C=O and C=C double bonds have the same electronic structure but their reactions occur by different mechanisms. Explain why

Answer 28

The C=O double bond is polar so is attacked by nucleophiles. The nucleophile attacks the delta positive C atom. The C=C double bond is not polar. The C=C bond has a high electron density so is attacked by electrophiles. The electrophile attacks the pi bond in C=C. The C=C bond also repels the nucleophile as they are negatively charged ions.

Question 29

What do nucleophilic additions reactions of carbonyl compounds with KCN, followed by dilute acid produce

Answer 29

hydroxynitriles

Question 30

Nucleophile

Answer 30

A species which donates a lone pair of electrons to form a dative covalent bond. It is attracted to an electron deficient centre

Question 31

How to go from a nitrile to a carboxylic acid

Answer 31

Hydrolysis Reagent : Dilute HCl Conditions : Heat under reflux

Question 32

What type of reaction is it when you go from a nitrile to carboxylic acid

Answer 32

Acid hydrolysis

Question 33

Why is the reaction of propanal with Tollens reagent a redox reaction

Answer 33

Propanal is oxidised to propanoic acid and silver ions are reduced to silver

Question 34

Why is the reaction of propanal with Fehlings solution a redox reaction

Answer 34

Propanal is oxidised to propanoic acid and Cu 2+ ions are reduced to Cu+ ions in Cu2O

Question 35

primary alcohol + [O] =

Answer 35

aldehyde + water

Question 36

secondary alcohol + [O] =

Answer 36

ketone + water

Question 37

RCHO + 2[H] =

Answer 37

RCH2OH

Question 38

propanal + 2[H] =

Answer 38

propan-1-ol

Question 39

How is NaBH4 used in reduction of aldehydes and ketones

Answer 39

In aqeuous solution

Question 40

Describe the mechanism for the reaction of an aldehyde/ketone with NaBH4 and water

Answer 40

1. Draw the displayed formula of the aldehyde/ketone and add the partial charges to the C=O bond. 2. There is a :H- ion and there is an arrow coming from the :H- to the C atom of the C=O double bond and an arrow from the middle of the C=O bond to the O. 3. This produces a single molecule where the H is now added and the double bond becomes a single bond. There is a negative charge and a lone pair of electrons on the O atom. 4. Draw the molecule produced above again and draw the structural formula for H20 besides it. 5. There is an arrow from the :O- to the H on the H2O and an arrow from the middle of the H-O bond to the O on the H2O. 6. This produces an alcohol + :OH-

Question 41

Explain why a racemic mixture and not a single optical isomer is obtained when butanone is reduced with NaBH4

Answer 41

:H- could attack from above or below the planar C=O group, giving equal amounts of the two optical isomers. A racemic mixture is formed. A racemic mixture has no effect on plane polarised light because the rotations of the 2 isomers cancel each other out.

Question 42

dangers of KCN

Answer 42

It is toxic

Question 43

Solution X reacts with liquid ketones to form a crystalline solid. This reaction can be used to identify a ketone if the crystalline solid is separated, purified by recrystallisation, and the melting point determined. Describe how the crystalline solid is separated and purified.

Answer 43

filter / decant in minimum vol of hot solvent cool / leave (to crystallise) AND filter (under reduced pressure) Wash with cold solvent/water, and dry (with method)

Question 44

Explain why NaBH4 reduces 2-methylbutanal but has no reaction with 2-methylbut-1-ene.

Answer 44

H– ion / nucleophile is attracted to δ+ C in 2-methylbutanal 2-methylbut-1-ene has an electron rich C=C H– ion / nucleophile is repelled by C=C C=C only attacked by/reacts with electrophiles