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AQA A Level Chemistry > Aldehydes and Ketones > Flashcards

Aldehydes and Ketones Flashcards

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1
Q

Functional group of aldehydes and ketones

A

C=O, carbonyl group

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2
Q

How are aldehydes produced

A

By the oxidation of primary alcohols.

An excess of alcohol is warmed with acidified potassium dichromate solution

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3
Q

Conditions for formation of aldehydes from primary alcohols

A

Heat and distil off the product as it forms

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4
Q

How are ketones formed

A

By the oxidation of secondary alcohols

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5
Q

Conditions for formation of ketones from secondary alcohols

A

Heat under reflux

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6
Q

How to produce a carboxylic acid from a primary alcohol

A

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

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7
Q

Forces between alkane/alkene molecules

A

Van der Waals forces

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8
Q

Forces between RX molecules

A

Permanent dipole-dipole interactions

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9
Q

Forces between alcohol molecules

A

Hydrogen bonding

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10
Q

What forces are found between molecules of an aldehyde/ketone

A

Permanent dipole-dipole interactions because they are polar molecules

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11
Q

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

A

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

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12
Q

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

A

They form hydrogen bonds with water molecules

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13
Q

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

A

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

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14
Q

What are aldehydes readily oxidised to

A

Carboxylic acids

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15
Q

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

A

Aldehydes - oxidised to carboxylic acids. Forms a silver mirror

Ketones - No reaction. No colour change, remains colourless

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16
Q

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

A

Aldehydes - Colour change from blue to red precipitate

Ketones - No reaction. No colour change, remains blue

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17
Q

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

A

It adds across the double bond.

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18
Q

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

A

Nucleophilic Addition
pH6

19
Q

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

A

: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

20
Q

Why is using cyanide ions useful in synthetic organic chem

A

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

21
Q

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

A

mixtures of enantiomers

22
Q

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

A

: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

23
Q

Describe the mechanism for the nucleophilic addition of HCN to ethanal

A

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

24
Q

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

A

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

25
Q

What are aldehydes reduced to and using what

A

Primary alcohols, using NaBH4

26
Q

What are ketones reduced to

A

Secondary alcohols, using NaBH4

27
Q

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

A

Nucleophilic Addition

28
Q

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

A

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

29
Q

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

A

hydroxynitriles

30
Q

Nucleophile

A

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

31
Q

How to go from a nitrile to a carboxylic acid

A

Hydrolysis
Reagent : Dilute HCl
Conditions : Heat under reflux

32
Q

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

A

Acid hydrolysis

33
Q

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

A

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

34
Q

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

A

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

35
Q

primary alcohol + [O] =

A

aldehyde + water

36
Q

secondary alcohol + [O] =

A

ketone + water

37
Q

RCHO + 2[H] =

A

RCH2OH

38
Q

propanal + 2[H] =

A

propan-1-ol

39
Q

How is NaBH4 used in reduction of aldehydes and ketones

A

In aqeuous solution

40
Q

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

A
  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-
41
Q

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

A

: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.

42
Q

dangers of KCN

A

It is toxic

43
Q

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.

A

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)

44
Q

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

A

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

AQA A Level Chemistry (27 decks)

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