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Organic Chemistry Year 13 > 6.1.2 Carbonyl Compounds > Flashcards

6.1.2 Carbonyl Compounds Flashcards

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1
Q

what are the 2 carbonyls

A

aldehydes and ketones

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2
Q

what are carbonyls

A

contain the carbonyl functional group C=O

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3
Q

what are aldehydes

A
  • carbonyl functional group found at the end of the carbon chain
  • CHO
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4
Q

what are ketones

A
  • carbonyl functional group is joined 2 to other carbon atoms in the carbon chain
  • CO
  • start with propanone
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5
Q

how do you name aldehydes

A
  • add the suffix -al
  • don’t need to name aldehyde group, this is ALWAYS on carbon-1
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6
Q

how do you name ketones

A
  • add the suffix -one
  • only needs to be numbered past 5 carbon atoms
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7
Q

what are aldehydes oxidised into

A

carboxylic acids

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8
Q

what are the reagents required for the oxidation of aldehydes

A
  • acidified potassium dichromate ions
  • usually found with K2Cr2O7 and H2SO4
  • oxidation agent shown via [O]
  • under reflux
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9
Q

what is the equation of oxidation of butanal

A

butanal + [O] ===> butanoic acid

  • (add reagents onto the arrow)
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10
Q

what happens to ketones when oxidised

A

ketones do not undergo oxidation reactions

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11
Q

what is the reactivity of carbonyls influenced by

A

the C=O bond

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12
Q

how is the C=O bond structured in carbonyls

A
  • a sigma single bond between the C-O
  • and a pi-bond above and below the planes of the carbon and oxygen atoms
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13
Q

what is the difference between the C=C and C=O double bond

A
  • the C=C bond is non-polar (electrophilic addition)
  • the C=O bond is polar
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14
Q

explain the polar C=O bond

A
  • O is more electronegative than C
  • so the electron-density in the C=O bond lies closer to the O atoms
  • so C=δ+ and O=δ-
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15
Q

how does the polar C=O bond affect the reactivity of aldehydes

A
  • can react with NUCLEOPHILES
  • via nucleophilic addition
  • as can attract them with the Cδ+
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16
Q

what is the reducing agent for carbonyls

A

NaBH4
- sodium tetrahydridoborate (III)

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17
Q

how are carbonyls reduced

A
  • reduce to alcohols
  • usually using a warm carbonyl and NaBH4
  • in an aqueous solution
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18
Q

what are aldehydes reduced to

A

primary alcohols

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19
Q

what is the equation for reduction of aldehydes

A

aldehyde + 2[H] ===> primary alcohol

  • need to add NaBH4 and H2O onto the arrow
  • e.g. butanal would go to butan-1-ol
20
Q

what are ketones reduced to

A

secondary alcohol

21
Q

what is the equation for the reduction of ketones

A

ketone + 2[H] ===> secondary alcohol

  • need to add NaBH4 and H2O on the arrow
  • e.g. propanone would go to propan-2-ol
22
Q

what is HCN

A

hydrogen cyanide

23
Q

what are the properties of hydrogen cyanide

A
  • colourless, poisonous gas
  • boils slightly over room temperature
  • cannot be used safely in an open lab
24
Q

what is used to provide HCN in a lab

A
  • sodium cyanide
  • sulfuric acid
  • STILL hazardous
25
Q

why is the addition of HCN useful for carbonyls

A
  • add across the C=O bond
  • provide means of increasing the length of the carbon chain
26
Q

what does HCN and carboxyls turn into

A

hydroxynitrile
- contains a nitrile C≡N and hydroxyl O-H group

27
Q

whats the equation of HCN and carboxyls

A

carbonyl + HCN ===> hydroxynitrile

  • need to add H2SO4 and NaCN onto the arrow
  • e.g. propanal + HCN ===> hydroxynitrile
  • the C≡N and O-H are on each side of the C atom that =O was attached to
28
Q

what is the basis of the mechanism for the nucleophilic addition of carbonyls

A
  • the carbon atom in the C=O is electron deficient
  • so attracts nucleophiles
29
Q

explain diagrammatically the mechanism for the nucleophilic addition of carbonyls and NaBH4

A
  • the NaBH4 can be considered as donating the hydride ion :H-

1) line goes from the lone pair on H:- to the carbon atom in the C=O
2) need to draw C as δ+ and O as δ-
3) need to draw curly arrow from C=O double bond to the O in the bond
4) next draw the intermediate
5) where the H is attached to the C and there is only a single bond between C-O
6) draw a line from the :O- to a H on a water molecule
7) water molecule should have δ+ H and δ- O
8) draw the alcohol, as the H attached to the O and forms OH
9) and OH- as another product

30
Q

explain the mechanism of NaBH4 and carbonyls

A

1) lone pair of e- from the :H- is attracted and donated to the δ+ C in the carbonyl C=O
2) a dative covalent bond forms between the C atom and the H- ion
3) the pi-bond in the C=O breaks down by heterolytic fission, forming a negatively charged intermediate
4) the O- atom of the intermediate donates a lone pair of electrons to a H atom in a water molecule, and the intermediate becomes protonated to form an alcohol

31
Q

explain diagrammatically the mechanism for the electrophilic addition between carbonyls and HCN (NaCN and H+)

A
  • the :C-N ion acts as a nucleophile (negative charge on the C not N)

1) draw a line from the lone pair of electrons on the :C-N ion to the C in the carbonyl
2) draw δ+ and δ- onto the C and O
3) draw a line from the C=O double bond to the δ- O
4) draw the intermediate
5) consists of CN attached FROM THE C onto the C, and the C=O turned into C-O, where the O is :O-
6) draw an arrow from the lone pair on the :O- to a H+ ion
7) draw the final product

32
Q

explain the mechanism for the nucleophilic addition of carbonyls with HCN

A
  • the cyanide ion :CN- attacks the electron-deficient carbon atom on the carbonyl

1) the lone pair of electrons on the :CN- is attracted and donated to δ+C in the C=O, forming a dative double bond
2) the pi-bond in the C=O breaks via heterolytic fission, forming a negatively charged intermediate
3) the intermediate is protonated by donating a lone pair of electrons from the :O- to a hydrogen ion, to form the product

  • (could also be done via water, as in NaBH4)
33
Q

how can the carbonyl functional group C=O be detected

A
  • use a solution of 2,4-dinitrophenylhydrazine
  • also known as 2,4-DNP/ 2,4-DNPH
  • also referred to as Brady’s solution
  • detects presence of C=O by forming a yellow precipitate called 2,4-dinitrophenylhydrazone
34
Q

how is Brady’s solution form

A
  • 2,4-DNP is usually dissolved in methanol and sulfuric acid
  • forms the pale orange Brady’s solution
35
Q

why is pure 2,4-DNP not really used when testing for C=O

A
  • in solid form can be hazardous
  • and friction or sudden blows can cause explosion
36
Q

what is the process of testing for the C=O functional group

A

1) add 5cm depth of 2,4-DNP to a clean test tube (this is in excess)
2) use a dropping pipette to add 3 drops of the unknown compound and leave to stand
3) if no crystals form, add a few drops of H2SO4
4) if you get a yellow/orange precipitate, shows presence of aldehyde/ketone

37
Q

how do you test for a ketone OR an aldehyde

A

use Tollen’s agent

38
Q

what is Tollen’s reagent

A
  • a solution of silver nitrate in aqueous ammonia
39
Q

what do you need to remember when using Tollen’s reagent

A
  • it has a short shelf-life
  • so should be made immediately before carrying out the test
40
Q

how does Tollen’s reagent show the presence of an aldehyde

A
  • if ALDEHYDE is present, should form a silver mirror
41
Q

how do you make Tollen’s reagent

A

1) in a clean test tube, add 3cm depth of AgNO3(aq)
2) add aqueous sodium hydroxide NaOH until a brown precipitate of silver oxide forms Ag2O
3) add dilute ammonia to the solution until brown dissolves to colourless = Tollen’s reagent

42
Q

how do you test for carbonyls using Tollen’s reagent

A

1) pour 2cm depth of unknown solution into clean test tube
2) add equal volumes of freshly prepared Tollen’s
3) leave the test tube in beaker of warm (50°C) water for 10-15 minutes
4) if a silver mirror forms, aldehyde is present, and if no reaction, then ketone is present

43
Q

how does Tollens reagent distinguish between ketones and aldehydes

A
  • Tollens’ contains silver (I) ions = Ag+ (aq)
  • can react as an oxidising agent in presence of ammonia
  • Ag+ gets reduced to Ag as the aldehyde is oxidised to a carboxylic acid
  • Ag+(aq) + e- ===> Ag(s)
  • causes silver mirror
  • whereas carbonyl + [O] goes to carboxylic acid
44
Q

what is a method for specifically identifying a ketone or aldehyde present

A
  • through melting points
  • where the orange precipitate formed from 2,4-DNP can be analysed further
45
Q

how would you compare the melting points of carbonyls to identify

A

1) impure yellow/orange solid is filtered to separate the solid precipitate from the solution
2) the solid is then recrystallised to form a pure sample of crystals
3) the melting point of the pure crystals (which at this point would be 2,4-dinitrophenylhydrazONE) is measured and recorded
4) the MP can be compared to a database or data table of melting points, to identify the compound

Organic Chemistry Year 13 (10 decks)

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