Ch 26 Carbonyl Compounds Flashcards
Functional group of aldehydes and ketones
Carbonyl functional group C=O
Aldehydes carbonyl functional group at…… and structural formula…..
End of carbon chain CHO
Simplest aldehyde
HCHO Methanal Common name formaldehyde Used in solution to preserve biological specimens
Ketone carbonyl functional group is positioned…. and structural formula is…..
Joined to two carbon atoms CO
Simplest ketone
CH3COCH3 Propanone Common name is acetone Important industrial solvent and is also used in nail varnish removers
Oxidation on aldehydes vs ketones
Ketones dont oxidise, lack of reactivity to distinguish between ket and ald Aldehydes can be oxidised to carb acids when REFLUXED with ACIDIFIED DICHROMATE IONS Cr2O72- / H+ usually as mixture of K2CR2O7 and H2SO4 Dichromate solution changes from orange to green
Oxidation of aldehydes
Reactivity of ald and key influenced by
Nature of C=O double bond made up of pi and sigma bonds.
Significant difference in bonds of carbonyl compounds and alkenes
The C=C double bond in alkenes is non polar The C=O double bond in carbonyl compounds is polar Because o more electroneg than C so e dens in double bond lies closer to O than C Carbon end slightly + O slightly -
Aldehydes and ketones react with some nucleophiles because of
Due to polarity of the C=O double bond Nucleophile is attracted to and attacks the slightly + C atom resulting in addition across the C=O double bond Nucleophilic addition
To reduce aldehydes and ketones to alcohols what is used
Ald/ ketone warmed with sodium tetrahydridoborate (III) NaBH4, reducing agent in aqueous solution
reducing an aldehyde equation
Reducing a ketone equation
Reaction of carbonyl compounds with HCN
Adds acroos C=O bond of ald and ket Colourless and extremely poisonous liquid that boils just above room temp Sodium cyanide and sulfuric acid are used to provide HCN in reaction Very useful reaction, provides means of increasing the length of the carbon chain
Organic product from reaction of ald with HCN contains which two functional groups?
A hydroxyl group -OH and a nitrile group C_=N (3 bonds between C AND N) Compounds are classed as hydroxynitriles (or cyanohydrins)
Mechanism for reaction with NaBH4
C group in C=O double bond is electron deficient and attracts nucleophiles so react by nucleophilic addition Insert diagram see book for annotations
Mechanism for reaction with NaCN/H+
Insert diagram, see book for annotations
Structure of 2,4-dinitrophenylhydrazine
To detect presence of carbonyl functional group in both ald and ket….
A solution of 2,4- dinitrophenylhydrazine (2,4-DNP) (Brady’s reagent) Normally uses dissolved in methanol and sulfurique acid as pale orange solution Brady’s Solid DNP v. Hazardous because friction can cause explosion In presence if carb a yellow or orange precipitate called a 2,4- dinitrophenylhydrazone is produced
+ Reaction of ald and ketones with 2,4 DNP
see book
How do u distinguish between ald and ket
Once compound ideantified as carb add Tollens reagent (solution of silver nitrate in aqueous ammonia) to fresh sample When ald a silver mirror is produced
Testing for ald group with tollens reagent
Reaction of tollens and ald
Tollens contains silver (I) ions Ag+ which act as oxidising agent in presence of ammonia Silver ions are reduced to silver as ald is oxidised to carboxlic acid
Identifying ald or ketone by melting point
What two groups does the carboxyl group contain?
Carbonyl grp and hydroxyl group Carboxyl functional group is found in carboxylic acids, funct group is COOH not CO and OH
Where are carboxylic acids found
Found in medicines fruit juices vinegar and rhubarb leaves Used as intermediates for formation of things like aspirin which can be synthesised from salicylic acid
How can carboxlic acids form hydrogen bonds with water
C=O AND O-H bonds are polar Carb acids up to 4 carb atoms are soluble in water E.g dicarboxylic acids have 2 polar carboxyl groups to form H bonds, solid and took temp and readily dissolve As number of carb atoms increases solubility decreases as non polar carbon chain has greater effect on polarity of molecule
Strength of carb acids
Weak Methanoic acid HCOOH is simplest When dissolved in water carb acids partially dissociate HCOOH(aq) double arrow H+ (aq) + HCOO-(aq)
Which reactions do carb acids take place in
Redox with metals Neutralisation with bases In these reactions carb acids form carboxylate salts
Redox reactions or carb acids
Aq carb acids react with metals to form H gas and carboxylate salt Metal dissolves (disappears) and effervescence
Neutralisation reaction of carb acid with metal oxides
To form salt and water Eg Ethanoic acid + calcium oxide = calcium ethanoate and water
Neutralisation reaction of carb acids with alkalis
Form salt and water Eg Ethanoic acid +sodium hydroxide = sodium ethanoate and water
Neutralisation reaction of carb acids with carbonates
Carbon dioxide formed If carb acid in excess a solid carbonate would disappear
Test for carboxyl group
Neutralisation reaction of carb acids with carbonates means can distinguish carb acids from eg phenols (not acidic enough) Carb acids are only common organic compound sufficiently acidic to react with carbonates
What is derivative of carb acid
Compound that can be hydrolysed to form parent carboxylic acid They have common sequence of atoms in struc called acyl group Esters, acyl chlorides, acid anhydrides and amides are all derivatives of carb acids
How to name Ester
Remove oic and replace with oate Alkyl chain attached to oxygen of COO group is first word of name (alcohol part)
How to name acyl chlorides
Remove oic acid and replace with oyl chloride
How are acid anhydrides formed
Removal of H20 from two carb acid molecules
Describe estérification
Alcohol warmed with carb acid with small amount of conc sulfuric acid (catalyst)
How to make ethyl propanoate
Describe acid hydrolysis
Easter can be hydrolysed by aqueous acid (reverse of estérification) Ester is heated under reflux with dilute aq acid Ester broken down by water with acid acting as catalyst
Describe alkaline hydrolysis
Also know as saponification and is irreversible Ester heated under reflux with aq hydroxide ions
How to prepare acyl chlorides
Reaction of parent carboxylic acid with thionyl chloride SOCl2 Other products are gases SO2 and HCL leaving only SOCl2 Reaction in fume cupboard as products are harmful
How do acyl chlorides react
Very reactive Useful as easily converted into carb acid derivatives eg esters with good yields React with nucleophiles by losing chloride ion while retaining C=O bond
Reaction of acyl chlorides with alcohols to form esters
Why can carb acids not react with phenols but …… can to form esters
Carb acids not reactive enough Acid chlorides and acid anhydrides both much more reactive than carb acids and react to produce phenyl esters Neither reaction needs acid catalyst
What happens when water is added to acyl chloride
Violent reaction Dense steamy hydrogen chloride fumes form Carb acid formed
How does ammonia and amines react with acyl chlorides
Act as nucleophiles by donating lone pair of electrons on N atom to electron deficient species Reaction forms amides
How does ammonia react with acyl chlorides
Forms primary amide where nitrogen atom connected to one carbon atom
How does PRIMARY amine react with acyl chloride
Same as ammonia to form SECONDARY amide where nitrogen attached to two carbon atoms
Acid anhydrides vs acyl chlorides
React in similar way Acid an. Less reactive so useful in lab preps
2(CH3CO)2O + C6H5OH ARROW CH3COOC6H5 + CH3COOH
ethanoic anhydride + phenol to phenyl ethanoate + ethanoic acid
Mechanism or reaction between acyl chlorides and nucleophiles
+ see book for annotations
