carbocylic acids

Question 1

general formula

Answer 1

CnH2nO2

Question 2

functional group

Answer 2

carboxyl group
-C=O
l
OH

Question 3

Nomenclature

Answer 3

the carbon in the -CO2H group is always given the number 1

where two carboxyl groups are present, the suffix -dioic acid is used

Question 4

odour of carboxylic acids

Answer 4

sharp irritating odour

Question 5

why do carboxylic acids form stronger hydrogen bonds than alcohols?

Answer 5

their -OH group is more polarised due to the presence of the electron withdrawing -C=O group

Question 6

their capacity for hydrogen bonding makes short chain carboxylic acids…..

Answer 6

miscible with water, though as with all homologous seriers, solubility decreases with increasing molecular size

Question 7

explain higher boiling point of ethanoic acid, compared to an alkene with similar relative molecular mass

Answer 7

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

Question 8

how are nitriles hydrolysed

Answer 8

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

Question 9

acid hydrolysis of propan nitile

Answer 9

CH3CH2CN +2H2O +HCL = CH3CH2COOH +NH4CL

(HEAT UNDER REFLUX)

Question 10

ALKALINE HYDROLYSIS

Answer 10

CH3CH2CN +H2O +NaOH = CH3CH2COO-Na+ +NH3

(HEAT UNDER REFLUX)

Question 11

how is the free acid obtained after alkaline hydrolysis?

Answer 11

acidification of the product

CH3CH2COO-Na+ + HCL + CH3CH2COOH + NaCL

Question 12

ACIDIITY OF ALCOHOLS

Answer 12

acidity of the -oh group

alcohols are even less acidic than water

Question 13

acidity of carboxylic acids?

Answer 13

also contain -oh group
more acidic than water and alcohols.
weak acids compared to inorganic ones like hydrochloric acid (mineral acids)

Question 14

halogen atoms are…

Answer 14

electron withdrawing groups

Question 15

alkyl groups are ….

Answer 15

electron donating groups

Question 16

ethanoic acid 4.17 pKa
methanoic acid 3.75 pKa
monochloroethanoic 2.86 pKa
dichloroethanoic 0.65 pKa

Answer 16

increasing acid strength ( the lower the value for pKa, the stronger the acid)

Question 17

acid hydrolysis of sters (reverse esterification)

Answer 17

ethylethanoate + water = ethanoic acid + ethanol
(reversible arrow and H+aq)
heat the mixture under reflux with conc sulfuric acid catalyst

Question 18

alkaline hydrolysis

Answer 18

esthers, R-C=O
l
OR’
are hydrolysed to alcohols and carboxylic acid salts when they are heated under reflux with aqueous alkali

Question 19

how is carboxylic acid obtained after alkaline hydrolysis of esters?

Answer 19

acidifying the salt with a mineral acid (hcl)

RCO2-Na+ + HCL + RCO2H +NaCl

Question 20

diffreence between acid hydrolyisis and alkaline hydrolysis of esther?

Answer 20

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)

Question 21

reaction of carboxylic acids with metal

Answer 21

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

Question 22

reaction with base

Answer 22

sharp irritating odour of carboxylix acid disappreas
test tube warms up
solution emains colourless

Question 23

reaaction with carbonates and hydrogen carbonates

Answer 23

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

Question 24

the carboxylate salts reat with excess mineral acids to form….

Answer 24

the original acid, in common with other slats of weak acids

Question 25

reaction with PCL5

Answer 25

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.

Question 26

what is used to test for the presence of an -OH group?

Answer 26

PCL5

Question 27

HOW TO CARRY OUT TEST FOR -oh group

Answer 27

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

Question 28

reduction

Answer 28

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)

Question 29

what is it not possinle to do when reducing a CA?

Answer 29

produce an aldehyde directly

Question 30

esterification

Answer 30

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

Question 31

NAMING ESTERS

Answer 31

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

Question 32

esters uses

Answer 32

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