carbonyls + carboxylic acids + esters

Question 1

carbonyl definition

Answer 1

an organic moleucle that contains a C=O group

Question 2

give 3 examples of carbonyl compounds

Answer 2

aldehydes, ketones, carboxylic acids

Question 3

outline the test for carbonyl groups

Answer 3

• react sample with 2,4-DNP, a pale orange solution
  POSITIVE= a condensation reaction will occur and a yellow-orange precipitate is formed

Question 4

after a positive test result with 2,4-DNP, what can be done to identify the carbonyl compund

Answer 4

the melting point of the precipitate formed can be found, then this can be compared to a database to identify the carbonyl compound

Question 5

outline a test for aldehydes

Answer 5

• prepare a solution of 3cm3 AgNO3(aq) + 1 drop of NaOH to produce AgO a brown precipitate
• add NH3 dropwise until precipitate redissolves- this is tollens reagent
• add tollens reagent to the sample in a water bath at around 50C-60C
  POSITIVE= a silver mirror will form as the aldehyde is oxidised to a carboxylic acid as Ag+(aq) + e-&raquo_space; Ag(s)

Question 6

why will ketones not react with tollens reagent

Answer 6

ketones cannot be oxidised, as there is no H group on the carbon with the C=O group to lose

Question 7

by what mechanism do ketones normally react

Answer 7

nucleophilic addition

Question 8

compare the reactivity of carbonyls and alkenes

Answer 8

carbonyls tend to be more reactive than alkenes, as the C=O bond is polar, due to the electronegativity difference between C and O, causing electron density to lie closer to the O, creating a dipole, so nucleophiles are attracted to the slight positive charge on the C atom
whereas C=C bonds are non polar

Question 9

give the reagents and conditions to go from an aldehyde/ketone to an alcohol

Answer 9

reagent - NaBH4
conditions - aqueous
- just write it as [H] in equations, so you don’t have to worry about side products, but don’t forget to balance!

Question 10

give the reagents and conditions to go from an aldehyde/ketone to a hydroxynitrile

Answer 10

reagents - NaCN + H2SO4 to form HCN in situ - HCN can be used in equations
conditions - aqueous

Question 11

why is HCN alone not used as a reagent

Answer 11

HCN is a colourless + poisonous liquid, and is also very volatile as it boils slightly above room temp, so it cannot be safely used in a lab

Question 12

give the reagents and conditions to go from an aldehyde to a carboxylic acid

Answer 12

reagents - K2Cr2O7
conditions - H2SO4 + heat under reflux
colour change from orange to green will occur

• this reaction can also be used to identify an aldehyde

Question 13

carboxylic acid definition

Answer 13

a compound with C=O carbonyl and OH hydroxyl functional groups bonded to the same carbon atom

Question 14

why do carboxylic acids have high water solubility

Answer 14

C=O and O-H bonds are both polar, and these groups allow carboxylic acids to form hydrogen bonds with H2O

Question 15

what structural factors affect the solubility of carboxylic acids

Answer 15

carbon chain length - as the length of the alkyl chain increases, solubility decreases as this is non polar

number of carboxyl groups - as number of carboxyl groups increases, solubility increases - dicarboxylic acids are very soluble

Question 16

how do carboxylic acids act as acids

Answer 16

carboxylic acids are weak acids, as they only partially dissociate when dissolved in water, forming H+ and a carboxylate ion

carboxylic acids can undergo the same redox (with metals) and neutralisation (with bases) reactions as other acids - in both they will form carboxylate salts

Question 17

outline a test for carboxylic acids

Answer 17

• react sample with Na2CO3
  POSITIVE= effervescence as gas is released, if this gas is bubbled through limewater it will turn limewater cloudy - this is because a neutralisation reaction occurs, carboxylic acids are the only common organic group acidic enough to react with carbonates

Question 18

what would be observed in a carboxylic acid was reacted with Na

Answer 18

effervescence and bubbling

Question 19

give the equation to produce an ester

Answer 19

alcohol + carboxylic acid <> ester + water
- this is a condensation reaction
- it is also a reversible reaction

Question 20

give the reagents and conditions to produce esters

Answer 20

reagents - alcohol + carboxylic acid
conditions - conc H2SO4 + reflux
- acid must be conc as too much water will cause the position of the equilibrium to shift and move to the reactants side, decreases yield of ester

Question 21

what are the 2 methods of hydrolysing esters

Answer 21

acid hydrolysis and alkali hydrolysis

Question 22

outline the process of acid hydrolysis of esters

Answer 22

• ester is heated under reflux with dilute H2SO4 - it should be dilute as more water will move position of equilibrium to the reactant side, increasing break down of ester
• products are alcohol and carboxylic acid

Question 23

outline the process of alkali hydrolysis

Answer 23

• ester is heated under reflux with aqueous NaOH
• products are carboxylate ion and alcohol
• this is preferable, as it is not a reversible reaction so no ester will be reformed and therefore there will be a higher yield of products

Question 24

give the equation for producing acid anhydrides

Answer 24

2 carboxylic acid&raquo_space; acid anhydride

Question 25

outline how to name acid anhydride

Answer 25

replace acid with anhydride and if 2 acids are different, arrange names alphabetically
e.g. ethanoic anhydride
ethanoic propanoic anhydride

Question 26

give other methods of producing esters

Answer 26

reacting acid anhydrides/acyl chlorides/carboxylic acids with alcohols/phenols

Question 27

give the reagents and equation and conditions for the formation of acyl chlorides

Answer 27

reagents - thionyl chlorice (SOCl2)
carboxylic acid + SOCl2&raquo_space; acyl chloride + SO2 + HCl
- this reaction must take place in a fume hood, due to release of SO2 and HCl

Question 28

what reactions can acyl chlorides undergo

Answer 28

acyl chlorides are very reactive, and can easily be converted into carboxylic acid derivatives
e.g. esters

Question 29

give the reagents and conditions to go from an acyl chloride to a carboxylic acid

Answer 29

reagents - water
conditions - fume cupboard, this is a very vigorous reaction, and produces dense steam and HCl fumes
byproducts - HCl

Question 30

name the mechanism that acyl chlorides normally undergo

Answer 30

nucleophilic addition elimination

Question 31

give the reagents and by products to go from an acyl chloride to a primary amide

Answer 31

reagent - 2 ammonia + acyl chloride
byproducts - NH4Cl

Question 32

give the reagents and byproducts to go from a primary amide to a secondary amide

Answer 32

reagent - 2 primary amide + acyl chloride
byproducts - RNH3+Cl-
- the RNH3+ group comes from one of the primary amides

Question 33

compare the reactions of acyl chlorides and acid anhydrides

Answer 33

acid anhydrides react similarly to acyl chlorides with alcohols, phenols, water, ammonia - although they produce a carboxylic acid rather than HCl

however acid anhydrides are much less reactive than acyl chlorides

Question 34

what is another way than the production of esters can be more efficient/with higher yields

Answer 34

using acyl chlorides instead of carboxylic acids as the reaction is not reversible