6.1.2 Carbonyl Compounds Flashcards
what are the 2 carbonyls
aldehydes and ketones
what are carbonyls
contain the carbonyl functional group C=O
what are aldehydes
- carbonyl functional group found at the end of the carbon chain
- CHO
what are ketones
- carbonyl functional group is joined 2 to other carbon atoms in the carbon chain
- CO
- start with propanone
how do you name aldehydes
- add the suffix -al
- don’t need to name aldehyde group, this is ALWAYS on carbon-1
how do you name ketones
- add the suffix -one
- only needs to be numbered past 5 carbon atoms
what are aldehydes oxidised into
carboxylic acids
what are the reagents required for the oxidation of aldehydes
- acidified potassium dichromate ions
- usually found with K2Cr2O7 and H2SO4
- oxidation agent shown via [O]
- under reflux
what is the equation of oxidation of butanal
butanal + [O] ===> butanoic acid
- (add reagents onto the arrow)
what happens to ketones when oxidised
ketones do not undergo oxidation reactions
what is the reactivity of carbonyls influenced by
the C=O bond
how is the C=O bond structured in carbonyls
- a sigma single bond between the C-O
- and a pi-bond above and below the planes of the carbon and oxygen atoms
what is the difference between the C=C and C=O double bond
- the C=C bond is non-polar (electrophilic addition)
- the C=O bond is polar
explain the polar C=O bond
- O is more electronegative than C
- so the electron-density in the C=O bond lies closer to the O atoms
- so C=δ+ and O=δ-
how does the polar C=O bond affect the reactivity of aldehydes
- can react with NUCLEOPHILES
- via nucleophilic addition
- as can attract them with the Cδ+
what is the reducing agent for carbonyls
NaBH4
- sodium tetrahydridoborate (III)
how are carbonyls reduced
- reduce to alcohols
- usually using a warm carbonyl and NaBH4
- in an aqueous solution
what are aldehydes reduced to
primary alcohols
what is the equation for reduction of aldehydes
aldehyde + 2[H] ===> primary alcohol
- need to add NaBH4 and H2O onto the arrow
- e.g. butanal would go to butan-1-ol
what are ketones reduced to
secondary alcohol
what is the equation for the reduction of ketones
ketone + 2[H] ===> secondary alcohol
- need to add NaBH4 and H2O on the arrow
- e.g. propanone would go to propan-2-ol
what is HCN
hydrogen cyanide
what are the properties of hydrogen cyanide
- colourless, poisonous gas
- boils slightly over room temperature
- cannot be used safely in an open lab
what is used to provide HCN in a lab
- sodium cyanide
- sulfuric acid
- STILL hazardous
why is the addition of HCN useful for carbonyls
- add across the C=O bond
- provide means of increasing the length of the carbon chain
what does HCN and carboxyls turn into
hydroxynitrile
- contains a nitrile C≡N and hydroxyl O-H group
whats the equation of HCN and carboxyls
carbonyl + HCN ===> hydroxynitrile
- need to add H2SO4 and NaCN onto the arrow
- e.g. propanal + HCN ===> hydroxynitrile
- the C≡N and O-H are on each side of the C atom that =O was attached to
what is the basis of the mechanism for the nucleophilic addition of carbonyls
- the carbon atom in the C=O is electron deficient
- so attracts nucleophiles
explain diagrammatically the mechanism for the nucleophilic addition of carbonyls and NaBH4
- the NaBH4 can be considered as donating the hydride ion :H-
1) line goes from the lone pair on H:- to the carbon atom in the C=O
2) need to draw C as δ+ and O as δ-
3) need to draw curly arrow from C=O double bond to the O in the bond
4) next draw the intermediate
5) where the H is attached to the C and there is only a single bond between C-O
6) draw a line from the :O- to a H on a water molecule
7) water molecule should have δ+ H and δ- O
8) draw the alcohol, as the H attached to the O and forms OH
9) and OH- as another product
explain the mechanism of NaBH4 and carbonyls
1) lone pair of e- from the :H- is attracted and donated to the δ+ C in the carbonyl C=O
2) a dative covalent bond forms between the C atom and the H- ion
3) the pi-bond in the C=O breaks down by heterolytic fission, forming a negatively charged intermediate
4) the O- atom of the intermediate donates a lone pair of electrons to a H atom in a water molecule, and the intermediate becomes protonated to form an alcohol
explain diagrammatically the mechanism for the electrophilic addition between carbonyls and HCN (NaCN and H+)
- the :C-N ion acts as a nucleophile (negative charge on the C not N)
1) draw a line from the lone pair of electrons on the :C-N ion to the C in the carbonyl
2) draw δ+ and δ- onto the C and O
3) draw a line from the C=O double bond to the δ- O
4) draw the intermediate
5) consists of CN attached FROM THE C onto the C, and the C=O turned into C-O, where the O is :O-
6) draw an arrow from the lone pair on the :O- to a H+ ion
7) draw the final product
explain the mechanism for the nucleophilic addition of carbonyls with HCN
- the cyanide ion :CN- attacks the electron-deficient carbon atom on the carbonyl
1) the lone pair of electrons on the :CN- is attracted and donated to δ+C in the C=O, forming a dative double bond
2) the pi-bond in the C=O breaks via heterolytic fission, forming a negatively charged intermediate
3) the intermediate is protonated by donating a lone pair of electrons from the :O- to a hydrogen ion, to form the product
- (could also be done via water, as in NaBH4)
how can the carbonyl functional group C=O be detected
- use a solution of 2,4-dinitrophenylhydrazine
- also known as 2,4-DNP/ 2,4-DNPH
- also referred to as Brady’s solution
- detects presence of C=O by forming a yellow precipitate called 2,4-dinitrophenylhydrazone
how is Brady’s solution form
- 2,4-DNP is usually dissolved in methanol and sulfuric acid
- forms the pale orange Brady’s solution
why is pure 2,4-DNP not really used when testing for C=O
- in solid form can be hazardous
- and friction or sudden blows can cause explosion
what is the process of testing for the C=O functional group
1) add 5cm depth of 2,4-DNP to a clean test tube (this is in excess)
2) use a dropping pipette to add 3 drops of the unknown compound and leave to stand
3) if no crystals form, add a few drops of H2SO4
4) if you get a yellow/orange precipitate, shows presence of aldehyde/ketone
how do you test for a ketone OR an aldehyde
use Tollen’s agent
what is Tollen’s reagent
- a solution of silver nitrate in aqueous ammonia
what do you need to remember when using Tollen’s reagent
- it has a short shelf-life
- so should be made immediately before carrying out the test
how does Tollen’s reagent show the presence of an aldehyde
- if ALDEHYDE is present, should form a silver mirror
how do you make Tollen’s reagent
1) in a clean test tube, add 3cm depth of AgNO3(aq)
2) add aqueous sodium hydroxide NaOH until a brown precipitate of silver oxide forms Ag2O
3) add dilute ammonia to the solution until brown dissolves to colourless = Tollen’s reagent
how do you test for carbonyls using Tollen’s reagent
1) pour 2cm depth of unknown solution into clean test tube
2) add equal volumes of freshly prepared Tollen’s
3) leave the test tube in beaker of warm (50°C) water for 10-15 minutes
4) if a silver mirror forms, aldehyde is present, and if no reaction, then ketone is present
how does Tollens reagent distinguish between ketones and aldehydes
- Tollens’ contains silver (I) ions = Ag+ (aq)
- can react as an oxidising agent in presence of ammonia
- Ag+ gets reduced to Ag as the aldehyde is oxidised to a carboxylic acid
- Ag+(aq) + e- ===> Ag(s)
- causes silver mirror
- whereas carbonyl + [O] goes to carboxylic acid
what is a method for specifically identifying a ketone or aldehyde present
- through melting points
- where the orange precipitate formed from 2,4-DNP can be analysed further
how would you compare the melting points of carbonyls to identify
1) impure yellow/orange solid is filtered to separate the solid precipitate from the solution
2) the solid is then recrystallised to form a pure sample of crystals
3) the melting point of the pure crystals (which at this point would be 2,4-dinitrophenylhydrazONE) is measured and recorded
4) the MP can be compared to a database or data table of melting points, to identify the compound
