26 - Carbonyls And Carboxylic Acids Flashcards
What is a carbonyl function group
C=O
Carbonyl groups:
Aldéhydes
Ketones
Aldéhyde structural formula
CHO
Ketone structural formula
CO
Oxidation of alcohol - aldehyde
Gently heat
Acidified potassium dichromate
Distillation apparatus
Produce aldehyde + water
Oxidation of aldehyde - carboxylic acid
Reflux
What influences the reactivity of ketones + aldehydes
C=O bond
What is a functional group
Area of an organic compound where a chemical reaction can happen
Similarities in structures between alkenes and carbonyl
Both contain a sigma bond between pi bond
Formed from sideways overlapping of p orbitals of adjacent atoms
Differences between alkenes and carbonyl
Chemical properties
Alkenes undergo electrophilic addition reactions
Carbonyl groups undergo nucleophilic addition
C=C - is non polar
C=O - is polar - O is more electronegative
What does the polar C=O bond attract
Reacts with nucleophiles
Attracted to slightly + C
In addition across C=O bond
Nucleophile definition
Electron pair donor
Nucleophilic addition steps
- O atom is more electronegative than C atom
- O atom is partially negative
- C atom is partially positive
- as a result + C attracts a nucleophile
- nucleophile adds itself across C=O
Reducing an aldehyde / ketone
To alcohol
Reagent = sodium tetrahydridoborate NaBH4
Conditions = warmed with NaBH4
Mechanism for nucleophilic addition
With NaBH4
- NaBH4 contains the hydride ion H- = nucleophile
- partial + C attracts H-
- H- donates lone pair to C in the C=O bond
- dative covalent bond is formed between H- and C
- the pi bond in C=O undergoes heterloytic fission
- forming a negatively charged intermediate
- O of intermediate donates lone pair of electrons to H atom in water
INTERMEDIATE PROTONATED
Why can’t HCN be used itself
- uses NaCN and H2SO4
- Because HCN is colourless, poisonous, boils at RT - dangerous
Why is the reaction of carbonyl compounds with HCN useful
Provides a mean of increasing the length of the C chain
Cyanating carbonyl groups with HCN
will produce
hydroxynitriles = nitrile and hydroxyl group
Or cyanohydrins
Reagents and conditions of carbonyl reaction with HCN
Reagents
NaCN / H2SO4
In fume cupboard
Nucleophile in reaction with NaBH4
H-
Hydride ion
Nucleophile in HCN reaction
CN-
HCN mechanism
- partial + charge on C attracts CN-
- CN- donates lone pair to C
- dative covalent bond formed
- pi bond within C=O undergoes heterolytic fission
- negatively charged intermediate uses lone pair to attract H+ from H2SO4
Difference between HCN and NaBH4 reactions
- during reduction H2 is added across C=O
- during cyanation HCN is added
- H2 is not reactive enough to do this - needs NaNH4 reducing agent
- HCN is reactive enough, but extremely poisonous
What do you use to detect carbonyl compounds
Brady’s reagent -
2,4xdinitrophenylhydrazine
What does Brady’s reagent do when carbonyl present
In presence of C=O
Yellow/organic precipitate formed
2,4-dinitrophenylhydrazone
What is 2,4-DNP usually dissolved into make safer
Because hazardous and can explode due to friction
Méthanol and conc.sulfuric acid
How to test for carbonyl groups
1) add excess 2,4DNP solution to test tube
2) add 3 drops of unknown compound - leave to stand
3) if no crystals form add H2SO4
4) IF YELLOW/ ORANGE PRECIPITATE = carbonyl present
Disadvantages of BRADYS REAGENT
can’t distinguish between aldehyde or ketone
Is hazardous dangerous
Using the mp, from Brady’s reagent experiment
To identify carbonyl groups
1) filter out impure solid precipitate from solution
2) solid is recrystallised to produce pure sample of crystals
3) mp of purified 24DNP measured and recorded
COMPARED AGAINST DATABASE OF mp
Why can mp be used to distinguish
Each carbonyl compound produces different crystalline derivatives (orange precipitate) with different mps
How to distinguish between aldehydes and ketones
Using TOLLENS REAGENT
What is tollens reagent
Solution of silver nitrate dissolved in aqueous ammonia
Testing for aldehydes and ketones reagents and conditions
Reagents = tollens reagents
Conditions = heated in 50c water bath for 10 mins
Making tollens reagent
1) add AgNO3 to test tube
2) add aq NaOH until brown precipitate formed (Ag2O)
3) add dilute NH3 until brown precipitate dissolves to clear solution
Testing for aldehyde
Using tollens reagent
Practical
1) pour unknown solution into test tube
2) add equal volumes of fresh Tollens reagent (short shelf life)
3) leave for 10 mins in water bath at 50c
IF SILVER MIRROR FORMED = ALDEHYDE PRESENT
What is oxidised and reduced in Tollens reagent experiment
Ag+ acts as an oxidising agent in presence of ammonia (NH3)
Ag+ is reduced to Ag = silver mirror
Aldehyde is oxidised to carboxylic acid
Adv and disadv
Of tollens reagent
Ad = can identify and aldehyde from a carbonyl group
Disad = can’t confirm if a ketone is present
- short shelf life
Why can’t tollens reagent identify is a ketone is present
Ketones are not easily oxidised
Carboxyl functional groups
Carbonyl C=O
Hydroxyl O-H
Explanation or the water solubility of carboxylic acids in terms of hydrogen bonding
C=O and O-H are polar bonds
Allowing carboxylic acids to form hydrogen bonds with H20 molecules
Hydrogen bonding between Carboxylic acids and H2O
The O in the OH group has a lone pair of electrons that can hydrogen bond with a H on H2O
when do carboxylic acids stop being soluble
More than 4 carbons in the carbon chain
Why are carboxylic acids only soluble with up to 4 carbons
As the number of Cs in the car in chain increases
The polarity or the Carboxylic acid decreases
And the solubility of the Carboxylic acid decreases
As the non polar chain has a greater effect on the overall polarity
Strength of carboxylic acids
Weak
Partially dissociate
HCOOH —> H+ + HCOO- carboxylate ion
Naming carboxylates
= salts of CA
End names is
- OATE
Réactions CAs can react with
Metals
Metal oxides
Alkalis
Carbonates
Identifying CAs
Only common organic compound
Sufficiently acidic to react with carbonates
What is a derivative of carboxylic acid
A compound that can be hydrolysed to form the parent carboxylic acid
Name carboxylic acids derivatives
Esters
Acrylic chlorides
Acid anhydrides
Amuses
Naming esters
from carboxylic acids
Remove -oic acid
Replace with -oate
First word = the alkyl chain attached to O from COO
Propanoic acid —> ethyl propanoate
Name acyl chlorides
From carboxylic acids
Remove -oic acid
Replace with -oyl chloride
Propanoic acid —> propanoyl chloride
Naming acid anhydrides
Removal of water from 2 carboxylic acids
Remove acid
And anhydride
Ethanoic acid —> ethanoic anhydride
What is estérification
Reaction of alcohols with carboxylic acid
to form an Ester
Reagents and conditions for estérification
Reagents:
Conc. H2SO4 catalyst
Conditions:
- the reaction is reversible - need to separate products as formed
Small esters - warm and distil out = because more volatile
Large Ester - reflux and use fractional distillation to separate
Making ethyl propanoate
Practical
1) pour 2cm3 of ethanol and propanoic acid into polling tube
2) add a few drops H2SO4
3) place boiling tube in 80c water bath leave for 5 mins
4) pour contents into Na2CO3 beaker - removes un reacted carboxylic acids
How to identify if an Ester has formed
Smell the fruity scent
See oily floating Ester drops on surface of water
What is hydrolysis of esters
To form alcohols
Chemical breakdown of a compound of presence of water / aqueous solution
Acid hydrolysis of an Ester produces
Carboxylic acid and alcohol
The Ester is broken down by water with acid catalyst
Reagents and conditions of acid hydrolysis
Reagents
- dilute aqueous acid catalyst
Conditions
- Ester heated under reflux with catalyst
Alkaline hydrolysis of an Ester
Produces
Carboxylate ion and alcohol
Reagents and conditions of alkaline hydrolysis
Reagents
- OH-
Conditions
- heat under reflux with an alkaline
Preparation of acyl chlorides
Carboxylic acid + SOCl2
Directly from their parent carboxylic acid
Acyl chloride + SO2 + HCl
Reaction with thionyl chloride
- SO2 and HCl evolved as gases - leaving acyl chloride
Reagents and conditions of making an acyl chloride
Add SOCl2 to carboxylic acid
Conditions
- fume cupboard
Acyl chloride + alcohol
Ester + HCl
Acyl chloride + phenols
Esters + HCl
Acyl chloride + H2O
Carboxylic acid + HCl
Acyl chloride + ammonia
primary Amides + NH4Cl
Acyl chloride + primary amine
Éthanoyl chloride + methyl amine
N- methylethanamide + HCl
Acid anhydride + alcohol
Warmed together
Carboxylic acid + Ester
