Aldehydes and Ketones Flashcards
Functional group of aldehydes and ketones
C=O, carbonyl group
How are aldehydes produced
By the oxidation of primary alcohols.
An excess of alcohol is warmed with acidified potassium dichromate solution
Conditions for formation of aldehydes from primary alcohols
Heat and distil off the product as it forms
How are ketones formed
By the oxidation of secondary alcohols
Conditions for formation of ketones from secondary alcohols
Heat under reflux
How to produce a carboxylic acid from a primary alcohol
Use excess acidified potassium dichromate instead of excess alcohol
Heat reacting mixture under reflux instead of distilling of the product
Forces between alkane/alkene molecules
Van der Waals forces
Forces between RX molecules
Permanent dipole-dipole interactions
Forces between alcohol molecules
Hydrogen bonding
What forces are found between molecules of an aldehyde/ketone
Permanent dipole-dipole interactions because they are polar molecules
Why do aldehydes and ketones have higher BP’s than alkanes but lower than alcohols
There are Van der Waals forces between molecules of the alkane
There are permanent dipole-dipole interactions between molecules of aldehyde/ketones
There is hydrogen bonding between molecules of the alcohol.
Hydrogen bonds are the strongest intermolecular forces and Van der Waals are the weakest.
More energy is needed to break the stronger intermolecular forces in alcohols
Why are the lower members of aldehydes and ketones soluble in water
They form hydrogen bonds with water molecules
Why does the solubility of aldehydes and ketones decreases as the Mr increases
As the Mr increases, the non-polar hydrocarbon chain which is insoluble becomes larger. Therefore the solubility decreases.
What are aldehydes readily oxidised to
Carboxylic acids
How to distinguish between aldehydes and ketones using Tollen’s reagent
Aldehydes - oxidised to carboxylic acids. Forms a silver mirror
Ketones - No reaction. No colour change, remains colourless
How to distinguish between aldehydes and ketones using Fehling’s solution
Aldehydes - Colour change from blue to red precipitate
Ketones - No reaction. No colour change, remains blue
What happens when HCN is added to a cold solution of a carbonyl compound
It adds across the double bond.
What is the reaction between a carbonyl compound and KCN with dilute H2SO4 (or HCN) called and what are the conditions
Nucleophilic Addition
pH6
Why is the reaction between HCN and carbonyl compounds such as ethanal classified as nucleophillic addition
:CN- ions are nucleophiles as they donate a lone pair of electrons.
It is an addition because HCN adds across the C=O double bond to give a single saturated product
Why is using cyanide ions useful in synthetic organic chem
Increases the length of the carbon chain by 1 carbon atom in the product
What do aldehydes and unsymmetrical ketones form when they react with KCN followed by dilute acid
mixtures of enantiomers
Why is the product of the reaction between aldehydes and ketones always optically inactive
:CN- could attack from above or below/either side of the planar C=O group, giving equal amounts of the two optical isomers. A racemic mixture is formed. A racemic mixture has no effect on plane polarised light because the rotations of the two isomers cancel each other out
Describe the mechanism for the nucleophilic addition of HCN to ethanal
Draw the ethanal molecule with a delta positive charge on the C and delta negative charge on the O
Draw a :CN- molecule below the double bond with an arrow going from the lone pair to the C
Draw an arrow from the double bond to the O
This form a single saturated product with the CN now attached to the C and the O now has a electron pair and a negative charge
Draw another product (same as formed above) and with a H+ion next to it. There is an arrow from the electron pair to the H+ ion
This forms a molecule with both CN and OH. A hydroxynitrile
Why is it necessary to use HCN AND KCN in nucleophilic addition
If no KCN present, [CN-] is very small because HCN is a weak acid and therefore step 1 will be very slow
Step 2 will be too slow if no HCN present
What are aldehydes reduced to and using what
Primary alcohols, using NaBH4
What are ketones reduced to
Secondary alcohols, using NaBH4
What type of reaction is the reduction of aldehydes and ketones using NaBH4 an example of
Nucleophilic Addition
C=O and C=C double bonds have the same electronic structure but their reactions occur by different mechanisms. Explain why
The C=O double bond is polar so is attacked by nucleophiles. The nucleophile attacks the delta positive C atom.
The C=C double bond is not polar. The C=C bond has a high electron density so is attacked by electrophiles. The electrophile attacks the pi bond in C=C. The C=C bond also repels the nucleophile as they are negatively charged ions.
What do nucleophilic additions reactions of carbonyl compounds with KCN, followed by dilute acid produce
hydroxynitriles
Nucleophile
A species which donates a lone pair of electrons to form a dative covalent bond. It is attracted to an electron deficient centre
How to go from a nitrile to a carboxylic acid
Hydrolysis
Reagent : Dilute HCl
Conditions : Heat under reflux
What type of reaction is it when you go from a nitrile to carboxylic acid
Acid hydrolysis
Why is the reaction of propanal with Tollens reagent a redox reaction
Propanal is oxidised to propanoic acid and silver ions are reduced to silver
Why is the reaction of propanal with Fehlings solution a redox reaction
Propanal is oxidised to propanoic acid and Cu 2+ ions are reduced to Cu+ ions in Cu2O
primary alcohol + [O] =
aldehyde + water
secondary alcohol + [O] =
ketone + water
RCHO + 2[H] =
RCH2OH
propanal + 2[H] =
propan-1-ol
How is NaBH4 used in reduction of aldehydes and ketones
In aqeuous solution
Describe the mechanism for the reaction of an aldehyde/ketone with NaBH4 and water
- Draw the displayed formula of the aldehyde/ketone and add the partial charges to the C=O bond.
- There is a :H- ion and there is an arrow coming from the :H- to the C atom of the C=O double bond and an arrow from the middle of the C=O bond to the O.
- This produces a single molecule where the H is now added and the double bond becomes a single bond. There is a negative charge and a lone pair of electrons on the O atom.
- Draw the molecule produced above again and draw the structural formula for H20 besides it.
- There is an arrow from the :O- to the H on the H2O and an arrow from the middle of the H-O bond to the O on the H2O.
- This produces an alcohol + :OH-
Explain why a racemic mixture and not a single optical isomer is obtained when butanone is reduced with NaBH4
:H- could attack from above or below the planar C=O group, giving equal amounts of the two optical isomers. A racemic mixture is formed. A racemic mixture has no effect on plane polarised light because the rotations of the 2 isomers cancel each other out.
dangers of KCN
It is toxic
Solution X reacts with liquid ketones to form a crystalline solid.
This reaction can be used to identify a ketone if the crystalline solid is separated, purified by recrystallisation, and the melting point determined.
Describe how the crystalline solid is separated and purified.
filter / decant in minimum vol of hot solvent
cool / leave (to crystallise) AND filter (under reduced pressure)
Wash with cold solvent/water, and dry (with method)
Explain why NaBH4 reduces 2-methylbutanal but has no reaction with 2-methylbut-1-ene.
H– ion / nucleophile is attracted to δ+ C in 2-methylbutanal
2-methylbut-1-ene has an electron rich C=C
H– ion / nucleophile is repelled by C=C
C=C only attacked by/reacts with electrophiles
