1.8 Carboxylic Acids Flashcards
Give the general formula of a carboxylic acid
CnH2nO2
give the functional group of a carboxylic acid, its name, and the name of the groups that constitute it
-COOH = carboxyl C=O = carbonyl -OH = hydroxyl
name some derivatives of carboxylic acids and their general formulae
acyl chlorides RCOCl
esters RCOOR’
amides RCONHR’
give the trivial name for methanoic acid
formic acid
give the trivial name for ethanoic acid
acetic acid
give the trivial name for benzenecarboxylic acid
benzoic acid
Describe the order of boiling points wrt similar alkanes, alcohols and carboxylic acids
- 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
describe the process of dimerisation and how this can increase the boiling point of a carboxylic acid
- 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
besides dimerisation, describe two other things that will affect the boiling point of a carboxylic acid
- As size increases, boiling point increases (due to increased VdW)
- Greater branching leads to a lower boiling point
Describe the solubility of a carboxylic acid
- 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.
give the acid dissociation equilibrium equation for a general carboxylic acid
Ka = [RCOO- ][H+ ]
RCOOH
Name some ways in which a carboxylic acid can be prepared
- oxidation of a primary alcohol
- oxidation of an aldehyde
- acid and base catalysed ester hydrolysis
- hydrolysis of a nitrile
describe the practical oxidation of a primary alcohol to form a carboxylic acid
RCH2OH + 2[O] -> RCOOH + H2O
Reagent: acidified K2Cr2O7 or KMnO4
Conditions: reflux under heat
describe the practical oxidation of an aldehyde to form a carboxylic acid
RCHO + [O] -> RCOOH
Reagent: acidified K2Cr2O7 or KMnO4
Conditions: reflux under heat
how are esters formed?
during the reaction between a carboxylic acid and an alcohol, in the presence of an acid catalyst.
Give the general reversible reaction for esterification
RCOOH + R’OH RCOOR’ + H2O
is it more efficient to use acid or base catalysed hydrolysis to form an ester?
Base catalysed hydrolysis is more efficient
Acid catalysed hydrolysis forms an equilibrium so the yield is low. Base catalysed hydrolysis is not reversible
give the overall equation for base catalysed hydrolysis of an ester
CH3COOC2H5 + NaOH -> CH3COONa -> CH3CH2OH
why is based catalysed hydrolysis of an ester not reversible?
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
how can the weak acid be liberated from the salt in base catalysed ester hydrolysis?
reaction with a strong acid such as HCL
salt of a weak acid + strong acid -> weak acid
CH3COONa + HCl -> CH3COOH + NaCl
how does hydrolysis of a nitrile form a carboxylic acid?
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
give the overall reaction of forming a carboxylic acid through acid catalysed nitrile hydrolysis
RCN(l) + 2H2O(l) + HCl(aq) -> RCOOH(aq) + NH4Cl(aq)
Give the equation for the base catalysed hydrolysis of a nitrile and subsequently how a carboxylic acid can be formed
- 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
Name some reactions that carboxylic acids can undergo
- acid base reactions
- reactions with reactive metals
- esterification
- reduction with lithal
- reaction with PCl5
name the ion formed by carboxylic acids in acid base reactions
RCOO- = the carboxylate ion
give the equation for the reaction between ethanoic acid and sodium hydroxide
CH3COOH + NaOH -> CH3COONa + H2O
give the equation for the reaction between ethanoic acid and sodium carbonate
2CH3COOH +Na2CO3 -> 2CH3COONa + H2O + CO2
give the equation for the reaction between ethanoic acid and ammonia
CH3COOH + NH3 -> CH3COONH4
give the general equation for reaction between a metal and acid
metal + acid -> salt + hydrogen
give the equation for the reaction between magnesium and ethanoic acid
Mg + 2CH3COOH -> (CH3COO)2Mg + H2
give the equation for the reaction between magnesium ethanoate and HCl
(CH3COO)2Mg + 2HCl -> 2CH3COOH + MgCl2
give the conditions for esterification
Reflux with strong acid catalyst such as concentrated H2SO4
why is a strong acid catalyst used in esterification?
- 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)
what molecules form the water molecule in esterification?
the -OH from the carboxylic acid and the H from the alcohol
give the full name and formula for lithal, and its use
lithium aluminium hydride LiAlH4
a powerful reducing agent which will add hydrogen to a compound
give the equation for full reduction of a carboxylic acid
RCOOH + 4[H] -> RCH2OH + H2O
what are the conditions for reduction of a carboxylic acid to an alcohol?
reflux (not heat) in dry ether
name some reagents that will change a carboxylic acid to an acyl chloride
PCl5 (phosphorus (V) chloride)
thionyl chloride/sulfur (IV) oxychloride
give the general equation for formation of an acyl chloride from a carboxylic acid with PCl5
ROH + PCl5 -> RCl + HCl + POCl3
what is the difference in reactivity between an acyl chloride and its derivative carboxylic acid?
the acyl chloride will be much more reactive than the carboxylic acid
describe the procedure for preparing a carboxylic acid from an alcohol
- add 10cm3 of 1M sulfuric acid into a pear shaped flask
- 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
- slowly add 2cm3 of conc sulfuric acid, cooling with a running tap of cold water if necessary. attach a condenser to the flask.
- add 1cm3 ethanol dropwise down the reflux condenser. allow for the reaction to subside after each addition before adding more.
- put the flask in a water bath and heat with an electrical hotplate.
- gently reflux the contents of the pearshaped flask for 15-20 minutes.
- allow the flask to cool, then rearrange the apparatus for distillation.
- distill off the liquid
what could be observed from the distillate in the preparation of a carboxylic acid from an alcohol practical?
smells vinegary
universal indicator paper turns red
effervescence observed with sodium carbonate
why should the conc sulfuric acid be added with care in the preparation of a carboxylic acid from an alcohol practical?
the addition of the conc sulfuric acid is exothermic
how could the preparation of a carboxylic acid from an alcohol practical be adapted in order to obtain an aldehyde?
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.
write the equation for the reaction which occurs when carbon dioxide is bubbled through limewater
CO2 + Ca(OH)2 -> CaCO3 + H2O
describe a chemical test which could be used to identify the presence of an -OH group in a carboxylic acid
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.
