C26 - Carbonyl Compounds Flashcards
What is the carbonyl functional group?
C=O
What are the two main carbonyl compounds?
Aldehyde and ketone
Where is the carbonyl group found in a aldehyde?
- At the end of the chain
- Carbon atom is attached to one or two hydrogen atoms
- Written as CHO in structural formula
Where is the carbonyl group found in a ketone?
- Joined to two carbon atoms in the carbon chain
= Written as CO in structural formula
What are the suffixes for aldehydes and ketones?
Aldehyde - al
Ketone - one
What can aldehydes be oxidised into?
Carboxylic acids
What are the reagents and conditions for the oxidation of an aldehyde?
- Reflux [O]
- K2Cr2O7
- dilute H2SO4
What is the difference between the double bonds found in alkenes and carbonyl compounds?
- C=C bond in Alkenes is non-polar
- C=O bonds in carbonyl compounds is polar
What does the polarity of the bond in the carbonyl group allow?
Aldehydes and ketones can react with some nucleophiles
- Nucleophile is attracted to and attacked delta +ve carbon atom resulting in addition across the C=O double bond
- Nucleophilic addition reaction
What are aldehydes and ketones reduced to?
Aldehydes - primary alcohol
Ketones - secondary alcohol
What are the reagents and conditions for the reduction of aldehydes and ketones?
NaBH4
H20
Heat
What are the properties of hydrogen cyanide?
- colourless
- extremely poisonous
- liquid
- boiled slightly above room temp.
What is I used to provide the hydrogen cyanide in its reaction with carbonyl compounds?
Sodium Cyanide (NaCN) H2SO4
Whatcopmpund is formed form the reaction of HCN with carbonyl compound?
Hydroxynitrile (or cyanohydrins)
What fucntiondsla groups do hydroxynitriles contain?
- OH
- CN
What is the mechanism for the reaction of carbonyl compounds with NaBH4?
- NaBH4 contains the hydride ion H- which acts as the nucleophile
1. Lone pair of electrons from hydride ion is attracted ti and donated to delta +ve carbon atom in the aldehyde or ketone C=O bond
2. A dative covalent bond is formed between he hydride ion and the C atom of the C=O bond
3. The Pi-bond in the C=O bond breaks by heterolytic fission forming a negatively charged intermediate (H2O)
4. The oxygen atom of the intermediate donated a lone pair of electrons to a hydrogen atom in the water molecule
5. Intermediate has now been protonated to form an alcohol.
What is the mechanism for the reaction of a carbonyl compound with NaCN/H+
- NaCN contains the ion CN- which acts as a nucleophile
1. Lone pair of electrons from the CN- ion is attracted to and donated to the delta +ve C atom in the aldehyde or ketone C=O bond forming a dative covalent bond.
2. The pi-bond in the C=O bond breaks by heterolytic fission forming a negatively charged intermediate
3. Intermediate is protonated by donating a lone pair of electrons to a H+ ion to form the hydroxynitrile
What is 2,4-DNP?
2,4-dinitrophenylhydrazine
Used to detect presence of carbonyl group
What is Brady’s reagent?
2,4-DNP dissolved in methanol and sulphuric acid as a pale orange solution
Why is 2,4-DNP normally dissolved for practical work?
Solid 2,4-DNP can be very hazardous because friction or a sudden blow can cause it it explode
How do you test for the carbonyl group ion aldehydes and ketones?
- Add excess of 2,4-DNP to clean test tube
- Add a few drops of unknown compound using dropping pipettes and leave to stand
- If no crystals form add a few drops of H2SO4
- Yellow/orange precipitate indicates presence of an aldehyde or ketone
What is tollen’s reagent?
Solution of silver nitrate in ammonia
Distinguishes between aldehydes and ketones
Why does tollen’s reagent need to be made up immediately before carrying out a test?
It has a short shelf life
How is tollen’s reagent made?
- Add 3cm depth of aqueous silver nitrate, AgNO3 (aq) to test tube
- Add NaOH (aq) to silver nitrate until brown precipitate of silver oxide, Ag2O is formed
- Add dilute NH3 solution until brown precipitate just dissolves to form colourless solution
How is the test for aldehydes carried out?
- Add equal volumes of unknown solution and tollen’s reagent to a test tube
- Leave test tube to stand in beaker of warm water approx. 50 degrees for 10-15 mins
- Silver mirror will form in the presence of an aldehyde group
Why does a silver mirror form in the presence of an aldehyde group?
- Tollen’s reagent contains Ag+ (aq) ions which act as an oxidising agent in the presence of NH3
- In the reaction silver ions are reduced to silver as the aldehyde is oxidised to a carboxylic acid
How can an aldehyde/ketone be identified by melting point?
- The impure 2,4-DNP yellow orange precipitate is filtered to separate the solid from the solution
- Solid is then recrystallised to produce a pure sample of crystals
- Melting point of purified 2,4-DNP is measured and recorded
- M.P is then compared to database of melting points to identify original carbonyl compound
What is the carboxyl functional group
-COOH
What is the suffix for carboxylic acids?
-oic acid
What is carbon-1 in a carboxylic acid?
Always the carbon in the carboxyl group
Why are carboxylic acids able to form hydrogen bonds with water molecules?
Polarity of C=O and O-H bonds in carboxyl groups
Which carboxylic acids are soluble in water?
Carboxylic acids with up to 4 carbon atoms
How does the number of carbon atoms affect the solubility of carboxylic acids?
As no. Of carbon atoms increases, solubility of decreases as non-polar carbon chain has greater affect on the overall polarity of the molecule
What are the properties of dicarboxylic acids?
- Have 2 polar carboxyl groups on each end to from hydrogen bonds
- Solids at room temp.
- Dissolve readily in water
What kind of acids are carboxylic acids?
Weak - partially dissociate
What reactions to carboxylic acids take place in?
Redox - with metals
Neutralisation - with bases (alkalis, metal oxides and carbonates)
What is the product of redox and neutralisation reactions with carboxylic acids?
Carboxylate salts
What takes place in redox reactions of carboxylic acids?
Aqueous solutions of carboxylic acids react with metals in a redox reaction to form hydrogen gas and the carboxylate salt
What would be observed in a redox reaction of carboxylic acids?
Metal disappears and effervescence as hydrogen gas is evolved
What takes place in neutralisation reaction of carboxylic acids with metal oxides or alkalis ?
Carboxylic acid react with metal oxides or alkalis to form carboxylate salt and water
What takes place in neutralisation reaction of carboxylic acids with carbonates?
Carboxylic acid reacts with carbonates to form carboxylate salt, carbon dioxide and water
What would be observed in a neutralisation reaction of carboxylic acid with carbonates if the carboxylic acid was in excess?
Solid carbonate would disappear
How would you test for the carboxyl group in carboxylic acids?
Carboxylic acids are only common organic compounds that are sufficiently acidic to react with carbonates
Useful for distinguishing them from phenols as they are not acidic enough to reach with carbonates
