carbonyl compounds and identifying aldehydes and ketones Flashcards
What compounds contain the carbonyl functional group
- aldehydes
- ketones
What is the carbonyl functional group
C=O
Describe the structure of an aldehyde
- the carbonyl functional group is found at the end of a carbon chain
- the carbon atom of the carbonyl group is attatched to one or two hydrogen atoms
What is the structural formula of the aldehyde group
CHO
What is the formula of the simplestt aldehyde
- HCHO
Describe the structure of a ketone
- the carbonyl functional group is joined to two carbon atoms in a carbon chain
What is the structural formula of a ketone group
CO
what is the simplest ketone and give the formula
CH3COCH3
propanone
What is another word for methanala and its use
- formaldehyde
- used in solution to preserve biological specimens
What is another name for propanone and give its use
- acetone
- important in industrial solvent and used in nail varnish remover
What is the suffix of an aldehyde group when naming them
al
What is the suffix of a ketone when namine them
one
Describe the oxidation of aldehydes
- adehydes can be oxidised to carboxylic acids when refluxed with acidified dichromate (VI) ions Cr2O72-/H+ usually as a mixture of sodium or potassium dichromate (VI) K2Cr2O7 and dilute sulfuric acid H2SO4
What is a way of distinguishing ketones from aldehydes
- using potassium dichromate
- aldehyde will turn green
- ketones will show no change as they do not undeergo oxidation reactions
Describe the difference of reactivity between carbonyl and alkene functional groups
- C=C double bond in alkenes is non polar
- C=O double bond in carbonyl is polar
Describe the reactivity of aldehydes and ketones
- it is influenced by the nature of the carbon oxygen double bond
- oxygen is more electronegative than carbon
- the electon density in the double bond lies closer to the oxygen atom than the carbon
- makes the carbon end of the bond slightly positive and the oxygen ened slightly negative
- due to the polarity of the C=O double bonds aldhydes and ketons react with ome nucleophiles
- nucleophile is attracted to and attacks the slightly positive carbon atoom reslting in addition across C=O double bond
- reaction is nucleophillic addition
What is NaBH4 and what reaction is it used for
- used as a reducing afent to reduce aldehydes and ketones to alcohols
Describe how you turn an aldehyde or a ketone into a into an alcohol
- warming the aldehyde or ketone with NaHB4 reducing agent in an aqueous solution
What are aldehydes reduced to
primary alcohols
W
What are ketones reduced to
secondary alcohols
Describe the reaction of carbonyl compounds with hydrogen cyanide
- hydrogen cyanide HCN adds across the C=O double bond of aldeydes and ketones
*
Describe why NACN used when reacting HCN with carbonyl compounds
- HCN is colourless and extremely poisonous liquid that boils slightly above room temperature so cannot be used safely in an open larbatory
- sodium ccyanide and sulfuric acid are used to provide the hydrogen cyanide in the reaction
What is the product of an aldehyde reacting with HCN and what are the functioncal groups
- hydroxynitrile
- -OH
- Ctriple bond N
Why is the reaction of carbonyl compounds and HCL useful
- increasing the length of the carbon chain
What does the carbon atom in the C=O bond do
- it is electron defifcient attracting nucleophiles
What is the reaction mechanism for aldehydes and ketones
nucleophillic addition
Describe the mechanism reaction with NaBH4 with a carbonyl group
- the lone pair of electrons from the hydride ion H- is attracted and donated to the positive dipole on the carbon atom in the aldehyde or ketone C=O double bond
- a dative covalent bond is formed between the hydride ion and the carbon atom of the C=O double bond
- the pi bond in the C=O double bond breaks by heterolytic fission forming a negativley charged intermediate
- the oxygen atom of the intermediate donates a lone pair of electrons to a hydrogen atom in a water molecule the intermediate has then been protatnated to form an alcohol
Describe the mechanism for the reaction with NaCn/H+
- the cyanide attacks the electron deficient carbon atom in the aldehyde or ketone
- the lone pair of electrons is attracted and donated to the partial positive chargew on the carbon atom in the aldehyde or ketone C=O double bond a dative covalent bond forms
- the pi bond in the C=O double bond breaks via heterolytic fission forming a negativley charged intermediate
- the intermediate is protonated by donating a lone pair of electrons to a hydrogen ion to form the product
- the product is a hydroxynitrile
What is used to detect a carbonyl group
- a solution of 2,4-dinitrophenylhydrazine 2,4-DNP also known as bradys reagent is used to detect the carbonyl functional group
- a yellow or orange precipitate called 2,4-diitrophenylhydrazone is produced
Why is the solid 2,4-DNP dangerous and how do we use it in practicals
- normally used dissolved in methanol and sulfuric acid
- can be hazardous because of friction or a sudden blow can cause it to explode
outline the method for testing for a carbonyl group
- add 5cm depth of a solution of 2,4-dinitrophenylhydrazine to a clean test tube. This is in excess
- using a dropping pipette add three drops of unknown compounds leave to stand
- if no crytals form add a few drops of sulfuric acid
- a yellow/orange preipitate indicates the prescence of an aldehyde or ketone
Describe the test to distinguish between aldehydes and ketones
- tollens reagent
- solution of silver nitrate in aqueous ammonia
- in prescence of an aldehyde group a silver mirror is produced
Describe the method for testing for an aldehyde group
- in a clean test tube add 3cm depth of aqueous silver nitrate AgNO3(aq)
- add aqueous sodium hydroxide to the silver nitrate until a brwon precipitate of silver oxide Ag2Ois formed
- add dilute ammonia solution until the brown precipitate just dissolves to form a clear colourless solution - this is tollens reagent
- pour 2cm depth of the unknown solution into a clean test tube
- add equal volume of freshly prepared tollen reagent
- leave the test tube to stand in a beaker of warm water for about 10 to 15 minutes and observed wether any silver mirror is formed
Describe how tollens reagent identifies ann aldehyde
it contains silver ions which act as an oxidising agent in the precence of ammonia
solver ions are reduced to silver as the aldehyde is oxidised to a carboxylic acid
Describe the method for identify an aldehyde or a ketone by melting point
- impure yellow/orange solid is filtered to seperate the solide precipitate from the solution
- the solid is then recrystallied to produce a pure sample of crystals
- the metling point of purified 2,4-dinitrophenylhydrazone is measured and recorded
- the melting pointd id then compared to a database or data table of melting point to identify the original carbonyl compoud
