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Z A Level Chemistry 2 > Carbonyls > Flashcards

Carbonyls Flashcards

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1
Q

What is the carbonyl functional group?

A
  • Aldehydes and ketones both contain the carbonyl functional group C=O
  • Aldehydes: found at end of carbon chain (CHO)
  • Ketones: joined to two carbon atoms in the carbon chain (CO)
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2
Q

How do you name carbonyl compounds?

A
  • al for aldehyde (the carbon atom of the carbonyl group is ways designated carbon-1)
  • one for ketone (the carbonyl carbon atom needs to be numbered
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3
Q

Describe the oxidation of aldehydes and ketones

A
  • Aldehydes can be oxidised to carboxylic acids when refluxed with dichromate (VI) ions usually as a mixture of sodium or potassium dichromate and dilute sulphuric acid
  • Ketones do not undergo oxidation reactions
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4
Q

What is the reactivity of aldehydes and ketones influenced by?

A

-By the nature of the C=O double bond and the double bond is made up of both a sigma bond and a pi bond

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5
Q

Why do alkenes and carbonyl groups reactive differently even though they both have a double bond?

A
  • The C=C double bond is non-polar
  • The C=O double bond is polar
  • Oxygen is more electronegative than carbon and the electron denser in the double bond lies closer to the oxygen atoms than the carbon so the carbon end of the bond is slightly positive and the oxygen end is slightly negative
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6
Q

What do aldehydes and ketones react with and why?

A
  • Due to the polarity of the C=O double bond, aldehydes and ketones react with some nucleophiles
  • A nucleophile is attracted to and attacks the slightly positive carbon atom resulting in a addition across the C=O double bond
  • This is nucleophilic addition
  • This is very different from the non polar C=C bond in alkenes which reacts with electrophiles by electrophilic addition
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7
Q

What is the reaction of carbonyl compounds with NaBH4 like?

A
  1. Sodium tetrahydridoborate (III), NaBH4, is sued as a reducing agent to reduce aldehydes and ketone to alcohol
  2. The aldehyde or ketones is usually warmed with the NaBH4 reducing agent in aqueous solution
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8
Q

How is an aldehyde reduced?

A

-Aldehydes are reduced to primary alcohol by NaBH4

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9
Q

How is a ketone reduced?

A

-Ketones are reduced to secondary alcohol by NahBH4

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10
Q

What is the reaction of carbonyl compounds with HCN like?

A
  • Hydrogen cyanide, HCN adds across the C=O bond of aldehydes and ketones
  • Hydrogen cyanide is colourless, extremely poisonous liquid that boils slightly above room temperature, so it cannot be used slightly above room temperature, so it cannot be used safely in the lab
  • Sodium cyanide and sulphuric acid are used to provide the hydrogen cyanide in the reaction, but the reaction is still potentially very hazardous
  • This reaction is very useful as it provides a means of increasing the length of the carbon chain
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11
Q

Describe the reaction of propanol with hydrogen cyanide

A
  • The organic product formed from the reaction of an aldehyde with hydrogen cyanide contains two functional groups, a hydroxyl group, -OH and a nitrile group C=-N
  • These compounds are classed as hydroxynitriles
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12
Q

What is the mechanism for the reaction of carbonyl compound with NaBH4 (nucleophilic addition)

A
  • In the nucleophilic addition reaction, NaBH4 can be considered as containing the hydride ion :H-, which acts as the nucleophile
    1. The lone pair of electrons from the hydride ion, :H-, is attracted and donated to the delta+ carbon atom in the aldehyde or ketone C=O double bond
    2. A dative covalent bond is formed between the hydride ion and the carbon atom of the C=O double bond
    3. The pi bond in the C=O double bond breaks by heterolytic fission forming a negatively charged intermediate
    4. The oxygen atom of the intermediate donates a lone pair of electrons to. hydrogen atom in a water molecule. The intermediate has been protonated to from an alcohol
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13
Q

Why do carbonyl compounds undergo nucleophilic addition?

A
  • The carbon atom in the C=O double bond is electron deficient and attracts nucleophiles
  • Aldehydes and ketones both react by nucleophilic addition to form alcohols
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14
Q

Describe the mechanism for the reaction of carbonyl compound with NaCN/H+

A
  • In the first stage of the reaction of an aldehyde or ketones with NaCN/H+, the cynaide ion :CN-, attacks the electron deficient carbon atom in the aldehyde or ketone
    1. The lone pair of electrons from the cyanide ion, :CN-, is attracted and donated to the delta + carbon atom in the aldehyde or ketone C=O double bond. A dative covalent bond forms
    2. The pi bond in the C=O double bond breaks by heterolytic fission, forming. negatively charged intermediate
    3. The intermediate is protonated by donating a lone pair of electrons to a hydrogen ion, to form the product
    4. The product is hydroxynitrile
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15
Q

How do you detect carbonyl compounds?

A
  1. Brady’s reagent is used to detect the presence of carbonyl functional group in aldehydes and ketone
  2. The 2,4 DNP int he presence of a carbonyl group, a yellow or orange precipitate called a 2,4-dinitrophenylhydrazone is produced
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16
Q

How is 2,4 DNP used?

A
  1. 2,4 DNP is normally used dissolved in methanol and sulphuric acid as a pale orange solution called Brady’s reagent
  2. Solid 2,4 DNP can be very hazardous because friction or a sudden blow can cause it to explode
  3. 5cm of 2,4 DNP and a few drops of unknown, if not crystals from add drops of sulphuric acid and if yellow/orange precipitate forms
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17
Q

How do you distinguish between aldehydes and ketones?

A
  • Tollen’s Reagans (solution of silver nitrate in aqueous ammonia)
  • In the presence of an aldehyde a silver mirror is produced
  • Made up by: AgNO3, snd NaOH so brown precipitate of AgO2 from and then dilute ammonia until the brown precipitate dissolves to from a clear colourless solution
  • 1:1 ration of unknown and pollens and leave rest tube to stand in warm water (50 degrees for 10-15 mins and observe if silver mirror formed)
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18
Q

How does Tollen’s reagent work?

A
  • Tollen’s regent contains silver (I) ions, Ag+ (aq) which act as an oxidising agent in the presence of ammonia
  • In the reaction, silver ions are reduced to silver as her aldehyde is oxidised to a carboxylic acid
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19
Q

What is a carboxylic acid?

A

An organic acid which contains the carboxyl function group (which contains both a carbonyl group and a hydroxyl group)

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20
Q

Where are carboxylic acids found?

A
  • Medicines, vinegar
  • Carboxylic acids and their derivatives are used in organic synthesis as starting material or intermediates e.g. in aspirin can be synthesised from salicylic acid
21
Q

How are carboxylic acids names?

A
  1. The carboxylic acid functional group is indicated by adding -oic acid to the stem of the longest carbon chain
  2. The carboxyl carbon chain atom is always designated as carbon-1
22
Q

Are carboxylic acids soluble?

A

-The C=O and O-H bonds in carboxylic acids are polar allowing carboxylic acids to from hydrogen bonds with water molecules

23
Q

What happens to solubility of carboxylic acids with carbon number?

A
  1. Carboxylic acids with up to four carbon atoms are soluble in water
  2. As the number of carbon atoms increases, the solubility decreases as the non-polar carbon chain has a greater effect on the overall polarity of the molecule
  3. Dicarboxylic acids have two polar carboxyl groups to from hydrogen bonds. They are solids at room temperature and dissolve readily in water
24
Q

What is the strength of carboxylic acids?

A
  1. Carboxylic acids are classified as weak acids
  2. Methanoic acid, HCOOH, is the simplest carboxylic acid containing only one carbon atom
  3. When dissolved in water, carboxylic acids partially dissociate
25
Q

What are the acid reactions of carboxylic acids?

A
  • Carboxylic acids rate pale in redox and neutralisation reactions:
    1. Redox reactions with metals
    2. Neutralisation reactions with bases (alkalis, metal oxides, and carbonates)
  • In these reactions carboxylic acids from carboxylate salts
  • The carboxylate ion in the salt is named by casing the -ic acid ending of the carboxylic acid to -ate
26
Q

Describe the redox reactions of carboxylic acids with metals

A
  • Aqueous solutions of carboxylic acids react with metals in a redox reaction to from hydrogen gas and the carboxylate salt
  • In this reaction, you would observe the metal disappearing and effervesnce as hydrogen gas is evolved
27
Q

Describe neutralisation reaction of carboxylic acids with bases

A

Carboxylic acids react with all bases - metal oxides, alkalis and carbonates and these reactions are all examples of neutralisation

28
Q

Describe the reaction of carboxylic acids with metal oxides (neutralisation)

A

-Form a salt and water

29
Q

Describe the reaction of carboxylic acids with alkalis (neutralisation)

A
  • Form a salt and water

- In this reaction you may not see any reaction as two solutions react together to form an aqueous solution of the salt

30
Q

Describe the reaction of carboxylic acids with carbonates (neutralisation)

A
  • Carbon dioxide gas is evolved

- If the carboxylic acid is in excess a solid carbonate would disappear

31
Q

How do you test for the carboxyl group?

A
  • The neutralisation reaction of carboxylic acids with carbonates, for example sodium carbonate
  • Carboxylic acids are the only common organic compounds sufficiently acidic to react with carbonates
  • This is especially useful for distinguishing carboxylic acids from phenols and phenol are not acidic enough to react with carbonates
32
Q

What is a derivative of a carboxylic acid?

A
  • A compound that can be hydrolysed to from the parent carboxylic acid
  • Carboxylic acid derivatives have a common sequence of atoms in their structure, known as an acyl group
  • Esters, acyl chorines, acid anhydrides and amides are all derivatives of carboxylic acids
  • (OH removed)
33
Q

Describe the ester

A
  • An ester is named after the parent carboxylic acid from which it is derived
  • To name an ester, remove the -ioc acid suffix from the parent carboxylic acid and replace with -oate
  • The alkyl chain attached to the oxygen atom of the COO group is then added as the first word in the name
34
Q

Describe acyl chloride

A
  • An acyl chloride is named after the parent carboxylic acid from which it is derived
  • Remove -oic acid suffix from the parent carboxylic acid and replace with -oyl chloride
35
Q

Describe acid anhydrides

A

-An acid anhydride is formed by the removal of water from two carboxylic acid molecules

36
Q

What is esterification?

A
  1. The reaction of an alcohol with a carboxylic acid to from an ester
  2. An alcohol is warmed with a carboxylic acid with a small amount of concentrated sulphuric acid, which acts as a catalyst
  3. Esters are sweet-smelling liquids
37
Q

What is the hydrolysis of esters?

A

Esters can be hydrolysed by aqueous acid or alkali

38
Q

What is hydrolysis?

A

The chemical breakdown of a compound in the presence of water or in aqueous solution

39
Q

What is acid hydrolysis?

A
  • Acid hydrolysis of an ester is the reverse of esterification and to carry out hydrolysis
    1. The ester is heated under reflect with dilute aqueous acid
    2. The ester is broken down by water, with the acid aciting as a catalyst
  • Products are a carboxylic acid and an alcohol
40
Q

What is alkaline hydrolysis?

A
  • Also known as saponification and is irreversible

- The ester is heated under reflex with aqueous hydroxide ions

41
Q

How do you prepare acyl chlorides?

A
  1. Acyl chlorides can be prepared directly from their parent carboxylic acid by reaction with thionyl chloride, SOCL2 and the other products of this reaction, SO2 and HCL are evolved as gases, leaving just the acyl chloride (products harmful so carry out in a fume cupboard)
42
Q

What are the reaction fo acyl chlorides like?

A
  • Acyl chlorides are very reactive and are very useful in organic synthesis as they can be easily converted into carboxylic acid derivatives such as esters and amides with good yields
  • Acyl chlorides reaction with nucleophiles by losing the chloride ion whilst retaining the C=O double bond
43
Q

What does the reaction of acyl chloride with alcohol form?

A

Esters

44
Q

Describe the reaction of acyl chlorides with phenols?

A
  • Carboxylic acids are not reactive enough to from esters with phenols
  • Acyl chlorides and acid anhydrides are both much more reactive than carboxylic acids and react with phenols to produce phenyl esters and neither reaction needs an acid catalyst
45
Q

Describe the reaction fo acyl chlorides with water to from carboxylic acids

A
  • When water is added to an acyl chloride, a violent reaction makes place with the evolution of dense steamy hydrogen chloride fumes
  • A carboxylic acid is formed
46
Q

Describe the reaction of acyl chlorides with ammonia and amines to from amides

A
  1. Ammonia and amines can act as nucleophiles by donating the lone pair of electrons on the nitrogen atom to an electron-deficient species
    - Their reaction with acyl chlorides forms amides
  2. Ammonia reacts with acyl chlorides forming a primary amide
    - In the primary amide the nitrogen atom is attached to one carbon atom
  3. a primary amine reacts with acyl chloride in the same way as ammonia to form a secondary amide
    - In the secondary amide the nitrogen atom is attached to two carbon atoms
47
Q

Describe the reaction of acid anhydrides

A
  • Acid anhydrides react in a similar way to acyl chlorides with alcohol, phenols, water, ammonia and amines
  • They are less reactive than acyl chlorides and are useful for some laboratory preparations where acyl chlorides may be too reactive
48
Q

How do you synthesis ester using phenyethanal?

A
  1. React phenyethanal with H2SO4/K7Cr2O7 (reflux)
  2. To get phentylethanoic acid C6H5CH2COOH
  3. React phenyethanal with NaBH4
  4. To get 2-phenyletanol/C6H5CH2CH2OH
  5. React phentylethanoi acid with 2-phenylethanol (reflux)
  6. H2SO4 with alcohol and acid as catalyst
49
Q

How do you ensure contains carbonyl group?

A
  1. Add 2-4 DNP and orange precipitate formed
  2. Take melting point of crystals
  3. Compare to known values

Z A Level Chemistry 2 (17 decks)

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