Module 6 - [ch26] (carbonyls and carboxylic acids) Flashcards
What is the functional group of aldehydes and ketones and what distinguishes one from another?
carbonyl functional group
C=O
the position of the C=O differs
Where are the functional group of carbonyls, aldehydes and ketones found?
aldehydes - the carbonyl functional group is found at the end of a carbon chain
ketone - the functional group is joined to two carbon atoms in the carbon chain
what are aldehydes oxidised to?
carboxylic acids
conditions for oxidation of an aldehyde
refluxed with acidified potassium dichromate
dichromate solution turns from orange to green
What is the difference in the way carbonyls react and alkenes react
alkenes - C=C non-polar bond
carbonyls - C=O polar bond
carbonyls react with some nucleophiles by nucleophilic addition
alkenes react with electrophiles by electrophilic addition
Why is the C=O bond polar
Oxygen is more electronegative than carbon
the electron density lies closer to the oxygen atom making the oxygen end slightly negative and the carbon end delta positive
What is NaBH4 used as reacting with carbonyl compounds
reducing agent
what are aldehydes and ketones reduced to
aldehydes - primary alcohols
ketones - secondary alcohols
conditions for the reduction of carbonyl compounds
NaBH4
gently warmed with reducing agent in aqueous solution
+ 2[H] (reducing agent e.g)
what type of reaction is that of carbonyl compounds with HCN
addition reaction
describe hydrogen cyanide
colourless, extremely poisonous liquid
advantages of reaction of carbonyls with HCN
increases the chain length
aldehyde + HCN –H2SO4/H2O–>
hydroxynitrile
what type of reaction is the one by which aldehydes and ketones react to form alcohols
nucleophilic addition
describe mechanism for reaction of a carbonyl with NaBH4
lone pair on hydride ion (from NaBH4) attracted to delta plus charge on carbon atom
dative covalent bond formed between hydride ion and carbon atom
Pi bond breaks apart by heterolytic fission making oxygen negative (intermediate)
Lone pair on oxygen donated to a delta positive hydrogen in a water molecule
= alcohol + OH-
describe mechanism for reaction of a carbonyl with HCN
lone pair of electrons on cyanide ion attacks electron deficient carbon atom
dative covalent bond formed
Pi bond breaks by heterolytic fission
Oxygen donates lone pair to hydrogen ion
hydroxynitrile formed
where is the negative charge on a CN
on the carbon atom
-:CN
What is used to detect the presence the presence of the carbonyl functional group?
brady’s reagent
2,4 DNP
2,4 dinitrophenylhydrazine
what is the positive test for the carbonyl functional group
2,4 DNP
yellow/orange precipitate forms
What is the issue with using 2,4 DNP to identify organic compounds
how is this overcome
gives a positive result for both aldehydes and ketones
further tests need to be carried out to distinguish between the two
taking melting point of precipitate, compare to database, distinguishes aldehydes from ketones
What is Tollen’s reagent? What is one of it’s properties?
a solution of silver nitrate in aqueous ammonia
silver complex ions in colourless solution
has a short shelf-life so must be made up immediately before carrying out a test
How do you test for an aldehyde
tollen’s reagent
gently warm (water bath 50 degrees)
produces a silver mirror
How do you prepare tollen’s reagent
Add silver nitrate in a test tube AgNO3(Aq)
Add aqueous sodium hydroxide NaOH until a brown precipitate of silver oxide Ag2O formed
Add dilute ammonia solution until precipitate dissolves to form clear and colourless solution
what is an carboxylic acid
an organic acid with the carboxyl functional group
Which carboxylic acids are soluble in water
carboxylic acids with up to 4 carbon atoms
Why are carboxylic acids soluble in water
C=O
O-H
polar bonds which can form hydrogen bonds with water molecules
How does solubility of carboxylic acids change as chain length increases
solubility decreases
as chain length gets longer, the non-polar carbon chain has a greater effect on the overall polarity of the molecule than the carboxyl group
what are dicarboxylic acids
have two polar carboxyl groups
solids at room temp and dissolve readily in water
what type of acid are carboxylic acids
weak acids
partially dissociates
HCOOH <-> H+ HCOO-
carboxylic acid + metal
PRODUCTS
carboxylate salt + hydrogen gas
give a general formula for
carboxylic acid + metal
acid(COOH) + metal -> (negative acid ion)(metal positive ion) + H2(g)
what type of reaction is that of a carboxylic acid and a metal
redox
carboxylic acid + metal oxide
carboxylate salt + water
both Hs from H2O from the acid
carboxylic acid + alkalis
carboxylate salt + water
one H from H2O from the acid, the other from the alkali
carboxylic acid + carbonate
carboxylate salt + water + carbon dioxide
how do you test for the carboxyl group
reaction with carbonate
What is a derivative of a carboxylic acid
contain the acyl group
O
// R - C
\
what are four carboxylic acid derivatives
esters
acyl chlorides
acid anhydrides
amide
naming acyl chlorides
e.g propanoic acid
-oyl chloride
propyl chloride
how is an acid anhydride formed
removal of water from two carboxylic acid molecules
what is esterification
reaction of alcohol with a carboxylic acid to form an ester
alcohol warmed with a carboxylic acid with a small amount of concentrated sulfuric acid (catalyst)
alcohol + carboxylic acid
ester + water
what is hydrolysis
chemical breakdown of a compound in the presence of water (or in aqueous solution)
what is acid hydrolysis
ester heated under reflux with dilute aqueous acid
ester broken down by water with acid acting as a catalyst
reversible
ester + water ->
carboxylic acid + alcohol
alkaline hydrolysis
ester is heated under reflux with aqueous hydroxide ions
irreversible
ester + hydroxide ions ->
carboxylate ion + alcohol
how do you prepare an acyl chloride?
carboxylic acid + thionyl chloride
thionyl chloride
SOCl2
carboxylic acid + thionyl chloride ->
What are the safety precautions regarding this reaction?
acyl chloride + sulphur dioxide(g) + hydrochloric acid(g)
SO2(g) an HCl(g) harmful gases
reaction should be carried out in a fume cupboard
why are acyl chlorides useful?
they are very reactive
can be converted into carboxylic acids derivates with good yields
acyl chloride + alcohol ->
ester + HCl
acyl chloride + phenol ->
ester + HCl
phenol
____
___/ O \
HO \ ____/
acyl chloride + water ->
carboxylic acids + HCl fumes
acyl chloride + ammonia ->
primary amide + ammonium chloride
COCl + 2NH3 -> CONH2 + NH4Cl
acyl chloride + primary amine ->
secondary amide + HCl
acid anhydrides
less reactive than acyl chlorides
but react in the same way
what type of intermolecular forces do molecules with the carbonyl group have
permanent dipole-dipole due to the polar C=O
which bond in carbonyl compounds is usually responsible for its reactions and why
C=O
the polarity of the bond
(difference in electronegativity between C and O)
what is fehling’s solution and how is it used
copper complex ions
blue
test for an aldehyde, blue -> brick red
reduction to Cu+2 ions
why does a silver mirror form when an aldehyde is added to tollen’s reagent
Ag+ ions are reduced to Ag(s)
what is a reducing agent for aldehydes and ketone and what ions does it release in solution
NaBH4
H- ions
how do you create a carboxylic acid from an aldehyde
oxidation
acidified potassium dichromate (K2Cr2O7/H2SO4)
