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Chemistry A-Level > Carbonyls and Carboxylic Acids (Chapter 26) > Flashcards

Carbonyls and Carboxylic Acids (Chapter 26) Flashcards

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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
Q

What is the carboxyl group?

A

A carbonyl and hydroxyl group

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26
Q

Describe the solubility of carboxylic acids

A

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

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27
Q

Describe the strength of carboxylic acids as acids

A
  • They are classified as weak acids
  • When dissolved in water, carboxylic acids partially dissociate
  • RCOOH <=> H+ + RCOO-
28
Q

Describe the redox reactions of carboxylic acids with metals and bases

A
  • Carboxylic acids form carboxylate salts

- Carboxylate ion in salt = oate

29
Q

Describe the redox reactions of carboxylic acids with metals incl the equation

A
  • 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
Q

What is the equation for the reaction of carboxylic acids with calcium oxide?

A

2RCOOH + CaO => (RCOO-)2Mg2+ + H2

31
Q

Describe the reaction of carboxylic acids with alkalis (NaOH)

A
  • RCOOH + NaOH => RCOO-Na+ + H2O

- May not see any reaction as the two solutions react together to form an aqueous salt solution

32
Q

Describe the reaction of carboxylic acids with sodium carbonate

A
  • 2RCOOH + Na2CO3 => 2RCOO-Na+ + H2O + CO2

- If carboxylic acid is in excess, solid carbonate will appear

33
Q

What is the test for carboxyl groups and why?

A
  • 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
Q

What is true about all carboxylic acid derivatives?

A

All have an acyl group in the structure

35
Q

What are 3 examples of carboxylic acid derivatives?

A

1) esters
2) acyl chlorides
3) acid anhydrides

36
Q

How do you name carboxylic acid derivatives?

A

1) esters: (alcohol prefix)yl (carboxylic acid prefix)oate
2) acyl chlorides: (carboxylic acid prefix)oyl chloride
3) acid anhydrides: -oic anhydride

37
Q

How is an acid anhydride formed?

A
  • ethanoic acid + ethanoic acid => ethnic anhydride

- OH + COOH => CO-O-CO

38
Q

Describe the esterification

A
  • 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
Q

What is the equation for esterification?

A

RCOOH + ROH => RCOOR + H2O

40
Q

What is the equation for esterification of an acid anhydride?

A
  • acid anhydride + alcohol => ester + carboxylic acid
  • 2RCOOCOR + ROH => RCOOR + RCOOH
  • reflux
41
Q

What is hydrolysis?

A

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

42
Q

What can esters be hydrolysed by?

A

Aqueous acid or alkali

43
Q

Describe acid hydrolysis of esters

A
  • 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
Q

Describe the alkali hydrolysis of esters (saponification)

A
  • Ester is heated under reflux with aqueous OH- ions
  • It is irreversible (advantage)
  • RCOOR + OH- => RCOOH + ROH (first RCOONa)
45
Q

How are acyl chlorides prepared?

A

Directly with SOCl2 (thionyl chloride), producing the by products of SO2 and HCL which are evolved as gases

46
Q

What is the equation for the preparation of acyl chlorides?

A

RCOOH + SOCl2 => RCOCl + SO2 + HCl

47
Q

Why are acyl chlorides useful?

A

They are very reactive and can be easily converted into carboxylic acid derivatives (e.g. esters/amides) with good yields

48
Q

How do acyl chlorides react?

A

They react with nucleophiles by losing the Cl- ion whilst retaining the C=O bond

49
Q

What is the equation for the reaction of acyl chlorides with alcohols?

A

RCOCl + ROH => RCOOR + HCl (anhydrous conditions needed)

50
Q

How are phenyl esters produced?

A
  • 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
Q

What is the equation for the production of a phenyl ester?

A

RCOCl + phenol => phenylCOOR + HCl

52
Q

What happens when water is added to acyl chlorides?

A

A violent reaction takes place, forming carboxylic acid and dense, steamy HCl fumes

53
Q

What is the equation for the reaction of water and acyl chlorides?

A

RCOCl + H2O => RCOOH + HCl

54
Q

What is the important product to remember when reacting acyl chlorides?

A

HCl

55
Q

How can ammonia and amines act as nucleophiles?

A

By donating the lone pair of electrons on the nitrogen atom to an electron deficient species

56
Q

Describe the reaction of acyl chlorides and ammonia

A

Ammonia reacts with acyl chlorides to form a primary amide

57
Q

What is the equation for the reaction of acyl chlorides and ammonia?

A

RCOCl + NH3 => RCONH2 + NH4Cl

58
Q

Describe the reaction of acyl chlorides and amines

A

A primary amine reacts with acyl chlorides, forming a secondary amide

59
Q

What is the equation for the reaction of acyl chlorides and amines?

A

RCOCl + RNH2 => RCONHR + RNH3+Cl-

60
Q

Describe acid anhydrides

A
  • 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
Q

What is the prefix for carbonyl compounds?

A

oxo

62
Q

What is the bond angle in a carbonyl bond?

A

120 degrees

63
Q

Describe the bonding in carbonyls

A

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
Q

What is special about the carboxylate ion?

A

It resonates (negative charge switches between the two oxygens) - this means that the ion is more stable

65
Q

What kind of bonds can carboxylic acids make?

A

Hydrogen bonds

Chemistry A-Level (27 decks)

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