carbocylic acids Flashcards
general formula
CnH2nO2
functional group
carboxyl group
-C=O
l
OH
Nomenclature
the carbon in the -CO2H group is always given the number 1
where two carboxyl groups are present, the suffix -dioic acid is used
odour of carboxylic acids
sharp irritating odour
why do carboxylic acids form stronger hydrogen bonds than alcohols?
their -OH group is more polarised due to the presence of the electron withdrawing -C=O group
their capacity for hydrogen bonding makes short chain carboxylic acids…..
miscible with water, though as with all homologous seriers, solubility decreases with increasing molecular size
explain higher boiling point of ethanoic acid, compared to an alkene with similar relative molecular mass
only VDW forces exist between alkane molecules. As well as VDW forces, there are H-bonds between ethanoic acid molecules. the forces between the delta + H of the -OH group and delta - O
how are nitriles hydrolysed
by boiling with water but are hydrolysed more quickly if the hydrolysis is carried out with a dilute acid or alkali by boiling under reflux
acid hydrolysis of propan nitile
CH3CH2CN +2H2O +HCL = CH3CH2COOH +NH4CL
(HEAT UNDER REFLUX)
ALKALINE HYDROLYSIS
CH3CH2CN +H2O +NaOH = CH3CH2COO-Na+ +NH3
(HEAT UNDER REFLUX)
how is the free acid obtained after alkaline hydrolysis?
acidification of the product
CH3CH2COO-Na+ + HCL + CH3CH2COOH + NaCL
ACIDIITY OF ALCOHOLS
acidity of the -oh group
alcohols are even less acidic than water
acidity of carboxylic acids?
also contain -oh group
more acidic than water and alcohols.
weak acids compared to inorganic ones like hydrochloric acid (mineral acids)
halogen atoms are…
electron withdrawing groups
alkyl groups are ….
electron donating groups
ethanoic acid 4.17 pKa
methanoic acid 3.75 pKa
monochloroethanoic 2.86 pKa
dichloroethanoic 0.65 pKa
increasing acid strength ( the lower the value for pKa, the stronger the acid)
acid hydrolysis of sters (reverse esterification)
ethylethanoate + water = ethanoic acid + ethanol
(reversible arrow and H+aq)
heat the mixture under reflux with conc sulfuric acid catalyst
alkaline hydrolysis
esthers, R-C=O
l
OR’
are hydrolysed to alcohols and carboxylic acid salts when they are heated under reflux with aqueous alkali
how is carboxylic acid obtained after alkaline hydrolysis of esters?
acidifying the salt with a mineral acid (hcl)
RCO2-Na+ + HCL + RCO2H +NaCl
diffreence between acid hydrolyisis and alkaline hydrolysis of esther?
during acid hydrolysis, the acid is a catylyst and the reaction is reversible. therefore the yield of carboxylic acid and alcohol is low
the hydrolysis with alkali is not reversible, so ther is a good yield of the salt of the carboxylic acid and the alcohol( if the carboxylic acid is required, the solution is cooled and excess strong acid eg HCL or H2SO4 is added)
reaction of carboxylic acids with metal
effervesce whenadded to metal, eg maghnesium, due to the production of hydrogen gas.
smell of vinegar disappears, colourless gas burns with squeaky pop when lighted splint applied, tets tube warms up, coliulress solution remains
reaction with base
sharp irritating odour of carboxylix acid disappreas
test tube warms up
solution emains colourless
reaaction with carbonates and hydrogen carbonates
effervescence, white solid disappears, a colourless solution remains, the colourless gas evolved turns lime water milky when bubbled through it, test tube warms up, sharp irritating odour of CA disappears
the carboxylate salts reat with excess mineral acids to form….
the original acid, in common with other slats of weak acids
reaction with PCL5
form acyl chlorides (phosphorous trichloride oxide- POCL3 and hydrogen chloride gas)
misty steamy fumes of HCLg are produced on adding PCL5 to the carboxylic acid (or alcohol). the -OH group is replaced by a chlorine atom andclouds of misty fumes of HCL are given off. phosphorous trichloride remains in the vessel.
what is used to test for the presence of an -OH group?
PCL5
HOW TO CARRY OUT TEST FOR -oh group
1- to a few cm3 of liquid Y in a clean, dry test tube, carefully add some PLC5 (creamy powdery solid)
2- test any gas released with moist universal indiator paper/ damp blue litmus apaer
3- also test and gas released with a damp stopper froma bottle of conc ammonia solution
reduction
carboxylic acids are reduced by heating the carboxylix acid under reflux with lithium aluminium hydride (lithal) LiAlH4, in dry ethoxyethane to produce primary alcohol (and H2O)
what is it not possinle to do when reducing a CA?
produce an aldehyde directly
esterification
CA react with alcohols in the prescence of conc H2SO4 to form esters (and water).
acid and alcohol boiled under reflux with the conc H2SO4.
the ester as it is the most volatile component of the reaction mixture can be removed by distillation
(reversible arrow- equibibrium reaction that does not normally reach completion)
takes place mch faste rin presence of catalyst eg conc H2SO4
NAMING ESTERS
NAMED AS SUBSTITUTED ACIDS BY CHANGING THE SUFFIX FROM -OIC TO -OATE
alkyl grupis derived form alcohol
part of the molecule derivied from acid comes before part derived from alcohol
esters uses
characteristic sweet smells
used as food flavourings and perfumes.
tused widley as solvents
plasticisers added toplastics to make them sofe=ter and more flexible eg childrens toys
