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A2 Chemistry 1 > 1.8 Carboxylic Acids > Flashcards

1.8 Carboxylic Acids Flashcards

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1
Q

Give the general formula of a carboxylic acid

A

CnH2nO2

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2
Q

give the functional group of a carboxylic acid, its name, and the name of the groups that constitute it

A 
-COOH = carboxyl
C=O = carbonyl
-OH = hydroxyl
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3
Q

name some derivatives of carboxylic acids and their general formulae

A

acyl chlorides RCOCl
esters RCOOR’
amides RCONHR’

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4
Q

give the trivial name for methanoic acid

A

formic acid

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5
Q

give the trivial name for ethanoic acid

A

acetic acid

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6
Q

give the trivial name for benzenecarboxylic acid

A

benzoic acid

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7
Q

Describe the order of boiling points wrt similar alkanes, alcohols and carboxylic acids

A
  • alkanes have lowest bps - only van der waals forces
  • alcohols higher than alkanes - hydrogen bonds and VdW
  • carboxylic acids highest b[ due to ability to dimerise
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8
Q

describe the process of dimerisation and how this can increase the boiling point of a carboxylic acid

A
  • In glacial (pure) carboxylic acids, intermolecular hydrogen bonding will occur between two molecules of an acid to form a dimer (ie Slightly +ve H in -OH bonds to slightly -ve O in C=O)
  • This doubles the size of the molecule, so increasing VdW’s force, resulting in a high bp
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9
Q

besides dimerisation, describe two other things that will affect the boiling point of a carboxylic acid

A
  • As size increases, boiling point increases (due to increased VdW)
  • Greater branching leads to a lower boiling point
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10
Q

Describe the solubility of a carboxylic acid

A
  • In the presence of water, carboxylic acids will NOT dimerise with each other.
  • Instead, hydrogen bonds will be formed between water molecules and individual molecules of acid.
  • Carboxylic acids with up to four carbon atoms will mix with water in any proportion.
  • Solubility decreases rapidly with size. This is because the larger hydrocarbon ‘tails’ get between water molecules and break hydrogen bonds. Therefore the broken H bonds will be replaced with weaker VdW forces.
  • When you mix the two, the energy released when the new H bonds form is the same as is needed to break the H bonds in the pure liquids.
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11
Q

give the acid dissociation equilibrium equation for a general carboxylic acid

A

Ka = [RCOO- ][H+ ]

RCOOH

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12
Q

Name some ways in which a carboxylic acid can be prepared

A
  • oxidation of a primary alcohol
  • oxidation of an aldehyde
  • acid and base catalysed ester hydrolysis
  • hydrolysis of a nitrile
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13
Q

describe the practical oxidation of a primary alcohol to form a carboxylic acid

A

RCH2OH + 2[O] -> RCOOH + H2O
Reagent: acidified K2Cr2O7 or KMnO4
Conditions: reflux under heat

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14
Q

describe the practical oxidation of an aldehyde to form a carboxylic acid

A

RCHO + [O] -> RCOOH
Reagent: acidified K2Cr2O7 or KMnO4
Conditions: reflux under heat

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15
Q

how are esters formed?

A

during the reaction between a carboxylic acid and an alcohol, in the presence of an acid catalyst.

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16
Q

Give the general reversible reaction for esterification

A

RCOOH + R’OH RCOOR’ + H2O

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17
Q

is it more efficient to use acid or base catalysed hydrolysis to form an ester?

A

Base catalysed hydrolysis is more efficient

Acid catalysed hydrolysis forms an equilibrium so the yield is low. Base catalysed hydrolysis is not reversible

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18
Q

give the overall equation for base catalysed hydrolysis of an ester

A

CH3COOC2H5 + NaOH -> CH3COONa -> CH3CH2OH

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19
Q

why is based catalysed hydrolysis of an ester not reversible?

A

the base catalyse reacts with the carboxylic acid, forming the salt of the weak acid (RCOONa + H2O), removing it as it is formed, therfore driving equilibrium to the right hand side

20
Q

how can the weak acid be liberated from the salt in base catalysed ester hydrolysis?

A

reaction with a strong acid such as HCL
salt of a weak acid + strong acid -> weak acid
CH3COONa + HCl -> CH3COOH + NaCl

21
Q

how does hydrolysis of a nitrile form a carboxylic acid?

A

In the acid catalysed hydrolysis of a nitrile, a primary amide is formed, which in turn is hydrolysed to the ammonium salt and then the carboxylic acid.
This is formed through reflux with an acid catalyst

22
Q

give the overall reaction of forming a carboxylic acid through acid catalysed nitrile hydrolysis

A

RCN(l) + 2H2O(l) + HCl(aq) -> RCOOH(aq) + NH4Cl(aq)

23
Q

Give the equation for the base catalysed hydrolysis of a nitrile and subsequently how a carboxylic acid can be formed

A
  • RCN + H2O + NaOH -> RCOONa + NH3
  • Adding an acid such as HCl or H2SO4 to the sodium carboxylate would free the carboxylic acid: RCOONa + HCl -> RCOOH + NaCl
24
Q

Name some reactions that carboxylic acids can undergo

A
  • acid base reactions
  • reactions with reactive metals
  • esterification
  • reduction with lithal
  • reaction with PCl5
25
Q

name the ion formed by carboxylic acids in acid base reactions

A

RCOO- = the carboxylate ion

26
Q

give the equation for the reaction between ethanoic acid and sodium hydroxide

A

CH3COOH + NaOH -> CH3COONa + H2O

27
Q

give the equation for the reaction between ethanoic acid and sodium carbonate

A

2CH3COOH +Na2CO3 -> 2CH3COONa + H2O + CO2

28
Q

give the equation for the reaction between ethanoic acid and ammonia

A

CH3COOH + NH3 -> CH3COONH4

29
Q

give the general equation for reaction between a metal and acid

A

metal + acid -> salt + hydrogen

30
Q

give the equation for the reaction between magnesium and ethanoic acid

A

Mg + 2CH3COOH -> (CH3COO)2Mg + H2

31
Q

give the equation for the reaction between magnesium ethanoate and HCl

A

(CH3COO)2Mg + 2HCl -> 2CH3COOH + MgCl2

32
Q

give the conditions for esterification

A

Reflux with strong acid catalyst such as concentrated H2SO4

33
Q

why is a strong acid catalyst used in esterification?

A
  • provides an alternative energy pathway with lower activation energy
  • H+ is a drying agent - it will drive equilibrium to the right (as esterification is a condensation reaction)
34
Q

what molecules form the water molecule in esterification?

A

the -OH from the carboxylic acid and the H from the alcohol

35
Q

give the full name and formula for lithal, and its use

A

lithium aluminium hydride LiAlH4

a powerful reducing agent which will add hydrogen to a compound

36
Q

give the equation for full reduction of a carboxylic acid

A

RCOOH + 4[H] -> RCH2OH + H2O

37
Q

what are the conditions for reduction of a carboxylic acid to an alcohol?

A

reflux (not heat) in dry ether

38
Q

name some reagents that will change a carboxylic acid to an acyl chloride

A

PCl5 (phosphorus (V) chloride)

thionyl chloride/sulfur (IV) oxychloride

39
Q

give the general equation for formation of an acyl chloride from a carboxylic acid with PCl5

A

ROH + PCl5 -> RCl + HCl + POCl3

40
Q

what is the difference in reactivity between an acyl chloride and its derivative carboxylic acid?

A

the acyl chloride will be much more reactive than the carboxylic acid

41
Q

describe the procedure for preparing a carboxylic acid from an alcohol

A
  1. add 10cm3 of 1M sulfuric acid into a pear shaped flask
  2. add 5g of sodium dichromate and add to the flask, followed by 1 spatula of antibump granules. swirl the flask until all the oxidising agent has dissolved
  3. slowly add 2cm3 of conc sulfuric acid, cooling with a running tap of cold water if necessary. attach a condenser to the flask.
  4. add 1cm3 ethanol dropwise down the reflux condenser. allow for the reaction to subside after each addition before adding more.
  5. put the flask in a water bath and heat with an electrical hotplate.
  6. gently reflux the contents of the pearshaped flask for 15-20 minutes.
  7. allow the flask to cool, then rearrange the apparatus for distillation.
  8. distill off the liquid
42
Q

what could be observed from the distillate in the preparation of a carboxylic acid from an alcohol practical?

A

smells vinegary
universal indicator paper turns red
effervescence observed with sodium carbonate

43
Q

why should the conc sulfuric acid be added with care in the preparation of a carboxylic acid from an alcohol practical?

A

the addition of the conc sulfuric acid is exothermic

44
Q

how could the preparation of a carboxylic acid from an alcohol practical be adapted in order to obtain an aldehyde?

A

reaction mixture should be set up for distillation only and not refluxed. the receiving flask should be placed in an iced water bath so that the aldehyde condenses and is collected.

45
Q

write the equation for the reaction which occurs when carbon dioxide is bubbled through limewater

A

CO2 + Ca(OH)2 -> CaCO3 + H2O

46
Q

describe a chemical test which could be used to identify the presence of an -OH group in a carboxylic acid

A

add a spatula measure of phosphorus (V) pentachloride to the carboxylic acid - effervescence is observed along with steamy white fumes, which turn damp universal indicator paper red.

A2 Chemistry 1 (10 decks)

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