Chapter 26 - Carbonyl Compounds Flashcards
what is the carbonyl functional group and what is it not
C = O
where the carbon is connected to other carbons or hydrogens NOT an oxygen or anything else
what is the difference between aldehydes and ketones and what is their general formula
- aldehydes have the C=O bond at the end of a carbon chain
- Ketones do not have it at the end
CnH2nO
what occurs in the oxidation of a primary alcohol and what are the reactants and conditions
primary alcohol + [O] —> aldehyde + water
- acidified potassium dichromate
- distillation
OR
primary alcohol + 2[O] —> carboxylic acid + water
- acidified potassium dichromate
- reflux
what occurs in the oxidation of an aldehyde and what are the conditions
aldehyde + [O] —> carboxylic acid
- reflux
- potassium dichromate (acidified)
what occurs in the oxidation of a secondary alcohol and what are the conditions
secondary alcohol + [O] —> Ketone + water
- reflux
- acidified potassium dichromate
do ketones further oxidise
no
what is the colour change for all of these oxidations
orange —> green
what are the differences between the C=C double bond and the C=O bond
- they have the same structure, a sigma bond and a pi bond
- however C=O is polar because the oxygen is more electronegative, C=C is not
- C=O attracts nucleophiles as the carbon is delta +ve
- C=C has an electron dense region so attracts electrophiles
what do aldehydes and ketones reduce to and what is the reducing agent and conditions
- aldehydes reduce to primary alcohols
- Ketones reduce to secondary alcohols
- the reducing agent is NaBH4(aq)
- represented by [H]
- solution is warmed
what is the mechanism for the reduction of aldehydes and ketones
1) NaBH4 splits to form a Hydride ion
2) the lone pair on the hydride ion attacks the delta+ve carbon forming a daitive covalent bond
3) the Pi bond of the C=O breaks by heterolytic fission forming a negatively charged intermediate
4) the intermediate is protonated (from H2O) to form an alcohol
what is the reaction of carbonyl compounds with HCN
- HCN adds across the double bond =
- H2SO4 and NaCN is used to provide the HCN as the reaction occurs
- it increases the length of the carbon chain
- forms effectively an alcohol with a cyanide group on
what is the mechanism for the reaction of carbonyls with HCN
1) HCN splits into H+ and CN-
2) the lone pair of electrons on the carbon attacks the delta+ve carbon
3) the rest happens as before but with H+ not H2O
what are the three ways to detect or distinguish between carbonyls
- Brady’s
- Tollens’
- Acidified Dichromate
what is the brady’s reaction
1) add 5cm depth of 2,4 DNPH
2) add a few drops of the unknown substance
3) if no crystals form, add H2SO4
4) an orange precipitate indicates an aldehyde or ketone
- if a carbonyl compound is present, an orange precipitate forms
what is the Tollens’ reagent reaction and how can we make Tollens
1) add approx. 3cm depth of AgNO3 (aq) to a test tube
2) add NaOH to form a brown precipitate
3) add dilute ammonia until the brown precipitate dissolves - this is tollens
4) add equal vols of tollens and unknown to a test tube
5) leave for 15mins at 50 degrees
if a silver mirror forms then its an aldehyde, otherwise it’s a ketone
what is the acidified dichromate experiment
1) add a few drops of H2SO4 and K2Cr2O7 to a test tube
2) if there is a colour change of orange —> green then its an aldehyde, otherwise its a ketone
what is the extension to the brady’s (2,4DNPH) reaction that can be used to identify the exact carbonyl compound
1) filter the precipitate
2) recrystallise the precipitate and leave to dry
3) find its melting point and compare to standard values
what is a carboxylic acid and what is the carboxyl group
- a carboxylic acid is an organic acid which contains the carboxyl functional group
- it contains a carbonyl group and a hydroxyl group COOH
what is the solubility of carboxylic acids
- they dissolve well in water as the polar C=O and O-H groups can disrupt and form hydrogen bonds with the water molecules
- solubility decreases with chain length
- up to butanoic acid are soluble
how do the melting and boiling points of carboxylic acids compare
- higher boiling points than similar length or molecular sized alcohols
- because they’re able to form more hydrogen bonds
what are the 3 main reactions of carboxylic acids
- with SOCl2 to form Acyl Chlorides
- with itself to form acid anhydrides
- with alcohols to form esters
what sort of acids are carboxylic acids, give an equation
- they are weak acids
CH3COOH —> CH3COO- + H+
REVERSIBLE REACTION - partially dissociate in water
what is a derivative of a carboxylic acid
a derivative of a carboxylic acid is a compound that can be hydrolysed to form the parent carboxylic acid
what is hydrolysis
“hydrolysis is the chemical breakdown of a compound in the presence of water or in aqueous solution”
what are esters
they are compounds which contain the ester functional group of
RCOOR
what is the main physical property of esters
they are more volatile than alcohols as they have no OH group so can’t form as many hydrogen bonds
what is the equation for making esters from carboxylic acids (esterification)
what is the sort of reaction and what is the catalyst
carboxylic acid + alcohol —> ester + water
REVERSIBLE REACTION
Conc. H2SO4
how can we make esters: esterification
1) add approx. 2cm^3 of both a carboxylic acid and an alcohol to a test tube
2) add a few drops of concentrated sulphuric acid (catalyst)
3) heat at 80 degrees for 5 mins
4) add sodium carbonate to react with any excess acid
what are the two types of ester hydrolysis
acidic and alkaline
what occurs in acidic ester hydrolysis, what is the use of the acid
ester + water —> carboxylic acid + alcohol
REVERSIBLE REACTION
- effectively the reverse of esterification
- heated under reflux, acid acts as a catalyst
- reversible reaction
- any dilute aqueous acid
what occurs in alkaline ester hydrolysis
- known as saponification
- irreversible
Ester + OH- —> carboxylate ion + alcohol
how can we form acyl chlorides from carboxylic acids
carboxylic acid + Thionyl chloride (SOCl2) —-> acyl chloride + sulphur dioxide(g) + Hydrogen Chloride(g)
what are the 4 main reactions of acyl chlorides
- with alcohols/phenols to form esters
- with water to form carboxylic acids - hydrolysis
- with 2NH3 to form primary amides
- with CH3NH2 and other primary amines to form secondary amides
how do acyl chlorides react with phenols and alcohols
Acyl Chloride + alcohol —> ester + HCl(g)
how do acyl Chlorides react with water (hydrolysis) to form carboxylic acids
- violent reaction
- steamy HCl(g) fumes
Acyl Chloride + water —> Carboxylic acid + HCl(g)
how do Acyl Chlorides react with ammonia
Acyl Chloride + 2NH3 —-> Primary amide + ammonium chloride
how do acyl chlorides react with primary amines
acyl chloride + primary amine —-> secondary amide + acyl ammonium chloride
how is an acid anhydride formed
carboxylic acid + carboxylic acid —> acid anhydride + water
how does an acid anhydride react with alcohols (esterification)
acid anhydride + alcohol —-> ester + carboxylic acid
which of the reactions of carboxylic acids and its derivatives are reversible and why might they have low yields
esterification/hydrolysis of esters
dissociation of a carboxylic acid
they are in equilibrium and therefore cannot have a 100% yield
what is the organic product formed in the Tollens’ reagent reaction/redox systems
Aldehyde oxidises to a carboxylic acid, silver ions reduce to silver solid
carboxylic acid is organic product
what is the name of the product of the hydrogen cyanide and carbonyl compound reaction
Hydroxynitrile
how can hydroxynitriles react with water
they can be hydrolysed with only water or under acidic and alkaline conditions
what occurs in acidic hydrolysis of hydroxynitriles, what are the reactants
- heat under reflux with a dilute acid
CH3CN + 2H2O + HCl —> CH3COOH + NH4+Cl- - instead of forming an ammonium salt as you do with just water, it (the ethanoate ions) reacts further with the H+ ions to form a carboxylic acid
why are acyl chlorides more acidic than carboxylic acids in water
- they will react with water to form HCl and a carboxylic acid, HCl is a strong acid
- whereas carboxylic acids only slightly dissociate
what makes a molecule able to form a silver mirror with Tollens’ reagent and therefore what is the only carboxylic acid which does
it must have a C-H bond as these are the only bonds with a low enough bond enthalpy for the molecule to be oxidised by Tollens’/ reduce Tollens’ to silver
Hence methanoic acid is the only carbox acid which can
