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Chemistry Paper 2 > Carbonyls and Carboxylic Acids (Chapter 26) > Flashcards

Carbonyls and Carboxylic Acids (Chapter 26) Flashcards

1
Q

What is the carbonyl functional group?

A

C=O

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2
Q

Describe the aldehyde functional group

A
  • CHO
  • Carbonyl functional group found at the end of the carbon chain (the carbon is attached to 1 or 2 hydrogen atoms)
  • Ending: -al
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3
Q

Describe the ketone functional group

A
  • CO
  • Carbonyl functional group joined to 2 carbon atoms in a carbon chain (∴ simplest is propanone)
  • Ending: -one
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4
Q

What is the equation for the oxidation of aldehydes?

A

RCHO + [O] => RCOOH

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5
Q

What reactions do ketone not undergo?

A

Oxidation

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6
Q

What is the difference between the C=C bond and the C=O bond?

A

The C=C bond is non-polar and the C=O bond is polar (due to the electronegativity difference between carbon and oxygen)

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7
Q

What reaction can carbonyls undergo and why?

A

Nucleophilic addition

  • Due to the polarity of the C=O bond, aldehydes and ketones react with some nucleophiles
  • The nucleophile is attracted to and attacks the delta position carbon atom, resulting in addition across the C=O bond
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8
Q

How are carbonyls reduced?

A
  • They are reduced to alcohols in the presence of NaBH4 (reducing agent) and water
  • The carbonyl is warmed with NaBH4 in an aqueous solution
  • 2 [H] needed per 2 hydrogens wanted to be added
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9
Q

What is the full name of NaBH4?

A

Sodium tetrahydridoborate (III)

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10
Q

What is the equation for the reduction of aldehydes to primary alcohols?

A

RCHO + 2[H] => RCH2OH + OH-

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11
Q

What is the equation for the reduction of ketones to secondary alcohols?

A

RCOR + 2[H] => RCH(OH)R + OH-

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12
Q

What is the by product of reduction of carbonyls?

A

OH-

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13
Q

What are aldehydes reduced to?

A

Primary alcohols

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14
Q

What are ketones reduced to?

A

Secondary alcohols

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15
Q

Describe the reaction of carbonyl compounds with HCN (hydrogen cyanide)?

A
  • HCN adds across the C=O bond
  • NaCN and H2SO4 are used to provide HCN for the reaction
  • The reaction produces hydroxynitriles (2 functional groups: OH- and C=-N nitrile)
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16
Q

What are the characteristics of HCN that mean that NaCN and H2SO4 need to be used?

A

HCN is a colourless gas which is very poisonous

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17
Q

Why is the reaction of carbonyl compounds and HCN useful?

A

1) it provides a means of increasing the length of a carbon chain as you are making a C-C bond
2) it is the only way to synthesise a tertiary alcohol
3) can go from CN to COOH with H+ or H2O to make amino acids

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18
Q

What is the equation of an aldehyde and HCN?

A

RCHO + HCN => RCH(OH)(CN)

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19
Q

Describe the mechanism of nucleophilic addition in the context of reduction of carbonyls with NaBH4

A

1) the lone pair of electrons from the hydride ion (H-), the nucleophile, is attracted and donated to the delta positive carbon atom in the C=O bond
2) a dative covalent bond is formed between the hydride ion and the carbon atom of the C=O bond
3) the pi-bond in the C=O bond breaks by heterolytic fission, forming a negative 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 form an alcohol (quench step)

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20
Q

Describe the mechanism of nucleophilic addition in the context of reaction of carbonyls with NaBH4?

A

1) the lone pair of electrons from the cyanide ion (CN-), a nucleophile, is attracted and donated to the delta carbon atom in the C=O bond
2) a dative covalent bond is formed between carbon and the cyanide ion
3) the pi bond in the C=O bond breaks by heterolytic fission, forming negatively charged intermediate
4) the intermediate is protonated by donating a lone pair of electrons to the H+ ion, forming a hydroxynitrile

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21
Q

How do you test for the presence of a carbonyl group?

A

1) add a solution of 2,4-DNP/Brady’s reagent to the test solution
2) in the presence of C=O, a yellow/orange ppt is formed
3) purify the 2,4-DNP derivatives (precipitates) using recrystallisation and heat them to find their melting points and compare to the known values of the melting points in a data table to identify the original carbonyl compound

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22
Q

How does the Tollen’s reagent test work?

A

1) Tollen’s reagent (a solution of AgNO3 in aqueous ammonia) contains Ag+ ions, which act as a reducing agent in the presence of ammonia
2) Ag+ ions are reduced to Ag as the aldehyde is oxidised to a carboxylic acid - ketones do not react as they cannot be oxidised
Equations: 1) Ag+ + e- => Ag (silver mirror)
2) RCHO + [O] => RCOOH

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23
Q

How do you test for the presence of aldehydes (distinguish between aldehydes and ketones)?

A

1) add Tollen’s reagent
2) in the presence of an aldehyde group, a silver mirror is produced
3) no reaction takes place in the presence of ketones

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24
Q

Describe the characteristics of carboxylic acids

A
  • Carboxylic acids are organic acids which contain the carboxyl functional group (carbonyl and hydroxyl groups)
  • To name: add the suffix -oic acid to the stem of the longest carbon chain
  • The carboxyl carbon atom is always designated as carbon 1
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25
What is the carboxyl group?
A carbonyl and hydroxyl group
26
Describe the solubility of carboxylic acids
1) the C=O and OH bonds in carboxylic acids are polar, allowing them to form hydrogen bonds with water molecules 2) carboxylic acids up to 4 carbon atoms are soluble in water - as the number of carbon atoms increases, the solubility decreases as non-polar carbon chain has a greater effect on the overall polarity of the molecules 3) dicarboxylic acids have 2 polar carboxyl groups to form hydrogen bonds ∴ they dissolve readily in water and are solids at room temperature
27
Describe the strength of carboxylic acids as acids
- They are classified as weak acids - When dissolved in water, carboxylic acids partially dissociate - RCOOH <=> H+ + RCOO-
28
Describe the redox reactions of carboxylic acids with metals and bases
- Carboxylic acids form carboxylate salts | - Carboxylate ion in salt = oate
29
Describe the redox reactions of carboxylic acids with metals incl the equation
- Aqueous solutions of carboxylic acids react with metals in a redox reaction to form hydrogen gas and carboxylate salt - Observation: metal disappearing and effervescence - Equation: 2RCOOH + Mg => (RCOO-)2Ca2+ + H2
30
What is the equation for the reaction of carboxylic acids with calcium oxide?
2RCOOH + CaO => (RCOO-)2Mg2+ + H2
31
Describe the reaction of carboxylic acids with alkalis (NaOH)
- RCOOH + NaOH => RCOO-Na+ + H2O | - May not see any reaction as the two solutions react together to form an aqueous salt solution
32
Describe the reaction of carboxylic acids with sodium carbonate
- 2RCOOH + Na2CO3 => 2RCOO-Na+ + H2O + CO2 | - If carboxylic acid is in excess, solid carbonate will appear
33
What is the test for carboxyl groups and why?
- Adding sodium carbonate - Carboxylic acids are the only common organic compounds sufficiently acidic to react with carbonates - It is especially useful for distinguishing between carboxylic acids and phenols (which are not acidic enough)
34
What is true about all carboxylic acid derivatives?
All have an acyl group in the structure
35
What are 3 examples of carboxylic acid derivatives?
1) esters 2) acyl chlorides 3) acid anhydrides
36
How do you name carboxylic acid derivatives?
1) esters: (alcohol prefix)yl (carboxylic acid prefix)oate 2) acyl chlorides: (carboxylic acid prefix)oyl chloride 3) acid anhydrides: -oic anhydride
37
How is an acid anhydride formed?
- ethanoic acid + ethanoic acid => ethnic anhydride | - OH + COOH => CO-O-CO
38
Describe the esterification
- Reaction of alcohol and carboxylic acid (acid anhydride) to form an ester - An alcohol is warmed with a carboxylic acid and small amount of concentrated H2SO4, which acts a catalyst
39
What is the equation for esterification?
RCOOH + ROH => RCOOR + H2O
40
What is the equation for esterification of an acid anhydride?
- acid anhydride + alcohol => ester + carboxylic acid - 2RCOOCOR + ROH => RCOOR + RCOOH - reflux
41
What is hydrolysis?
The chemical breakdown of a compound in the presence of water or in an aqueous solution
42
What can esters be hydrolysed by?
Aqueous acid or alkali
43
Describe acid hydrolysis of esters
- Ester is heated under reflux with dilute aqueous acid - Ester is broken down by water (acid acts as a catalyst) - It is the reverse of esterification - It is reversible (disadvantage) - RCOOR + H2O => RCOOH + ROH
44
Describe the alkali hydrolysis of esters (saponification)
- Ester is heated under reflux with aqueous OH- ions - It is irreversible (advantage) - RCOOR + OH- => RCOOH + ROH (first RCOONa)
45
How are acyl chlorides prepared?
Directly with SOCl2 (thionyl chloride), producing the by products of SO2 and HCL which are evolved as gases
46
What is the equation for the preparation of acyl chlorides?
RCOOH + SOCl2 => RCOCl + SO2 + HCl
47
Why are acyl chlorides useful?
They are very reactive and can be easily converted into carboxylic acid derivatives (e.g. esters/amides) with good yields
48
How do acyl chlorides react?
They react with nucleophiles by losing the Cl- ion whilst retaining the C=O bond
49
What is the equation for the reaction of acyl chlorides with alcohols?
RCOCl + ROH => RCOOR + HCl (anhydrous conditions needed)
50
How are phenyl esters produced?
- By reacting phenol with acyl chlorides or acid anhydrides without an acid catalyst - Carboxylic acids are not reactive enough to form esters with phenols (the other two are more reactive) - Phenol is a poor nucleophile - The reaction is irreversible
51
What is the equation for the production of a phenyl ester?
RCOCl + phenol => phenylCOOR + HCl
52
What happens when water is added to acyl chlorides?
A violent reaction takes place, forming carboxylic acid and dense, steamy HCl fumes
53
What is the equation for the reaction of water and acyl chlorides?
RCOCl + H2O => RCOOH + HCl
54
What is the important product to remember when reacting acyl chlorides?
HCl
55
How can ammonia and amines act as nucleophiles?
By donating the lone pair of electrons on the nitrogen atom to an electron deficient species
56
Describe the reaction of acyl chlorides and ammonia
Ammonia reacts with acyl chlorides to form a primary amide
57
What is the equation for the reaction of acyl chlorides and ammonia?
RCOCl + NH3 => RCONH2 + NH4Cl
58
Describe the reaction of acyl chlorides and amines
A primary amine reacts with acyl chlorides, forming a secondary amide
59
What is the equation for the reaction of acyl chlorides and amines?
RCOCl + RNH2 => RCONHR + RNH3+Cl-
60
Describe acid anhydrides
- They react in a similar way to acyl chlorides with alcohols, 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
61
What is the prefix for carbonyl compounds?
oxo
62
What is the bond angle in a carbonyl bond?
120 degrees
63
Describe the bonding in carbonyls
1) three of carbon's four electrons are involved in sigma bonds to oxygen and two hydrogens 2) the remaining electron is in a 2p orbital 3) the oxygen also has an electron in a 2p orbital (one in sigma bond and two lone pairs as well) 4) 2p orbitals overlap to form the pi bond
64
What is special about the carboxylate ion?
It resonates (negative charge switches between the two oxygens) - this means that the ion is more stable
65
What kind of bonds can carboxylic acids make?
Hydrogen bonds

Chemistry Paper 2 (11 decks)

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