Carbonyls and Carboxylic Acids (Chapter 26) Flashcards
What is the carbonyl functional group?
C=O
Describe the aldehyde functional group
- CHO
- Carbonyl functional group found at the end of the carbon chain (the carbon is attached to 1 or 2 hydrogen atoms)
- Ending: -al
Describe the ketone functional group
- CO
- Carbonyl functional group joined to 2 carbon atoms in a carbon chain (∴ simplest is propanone)
- Ending: -one
What is the equation for the oxidation of aldehydes?
RCHO + [O] => RCOOH
What reactions do ketone not undergo?
Oxidation
What is the difference between the C=C bond and the C=O bond?
The C=C bond is non-polar and the C=O bond is polar (due to the electronegativity difference between carbon and oxygen)
What reaction can carbonyls undergo and why?
Nucleophilic addition
- Due to the polarity of the C=O bond, aldehydes and ketones react with some nucleophiles
- The nucleophile is attracted to and attacks the delta position carbon atom, resulting in addition across the C=O bond
How are carbonyls reduced?
- They are reduced to alcohols in the presence of NaBH4 (reducing agent) and water
- The carbonyl is warmed with NaBH4 in an aqueous solution
- 2 [H] needed per 2 hydrogens wanted to be added
What is the full name of NaBH4?
Sodium tetrahydridoborate (III)
What is the equation for the reduction of aldehydes to primary alcohols?
RCHO + 2[H] => RCH2OH + OH-
What is the equation for the reduction of ketones to secondary alcohols?
RCOR + 2[H] => RCH(OH)R + OH-
What is the by product of reduction of carbonyls?
OH-
What are aldehydes reduced to?
Primary alcohols
What are ketones reduced to?
Secondary alcohols
Describe the reaction of carbonyl compounds with HCN (hydrogen cyanide)?
- HCN adds across the C=O bond
- NaCN and H2SO4 are used to provide HCN for the reaction
- The reaction produces hydroxynitriles (2 functional groups: OH- and C=-N nitrile)
What are the characteristics of HCN that mean that NaCN and H2SO4 need to be used?
HCN is a colourless gas which is very poisonous
Why is the reaction of carbonyl compounds and HCN useful?
1) it provides a means of increasing the length of a carbon chain as you are making a C-C bond
2) it is the only way to synthesise a tertiary alcohol
3) can go from CN to COOH with H+ or H2O to make amino acids
What is the equation of an aldehyde and HCN?
RCHO + HCN => RCH(OH)(CN)
Describe the mechanism of nucleophilic addition in the context of reduction of carbonyls with NaBH4
1) the lone pair of electrons from the hydride ion (H-), the nucleophile, is attracted and donated to the delta positive carbon atom in the C=O bond
2) a dative covalent bond is formed between the hydride ion and the carbon atom of the C=O bond
3) the pi-bond in the C=O bond breaks by heterolytic fission, forming a negative charged intermediate
4) the oxygen atom of the intermediate donates a lone pair of electrons to hydrogen atom in a water molecule - the intermediate has been protonated to form an alcohol (quench step)
Describe the mechanism of nucleophilic addition in the context of reaction of carbonyls with NaBH4?
1) the lone pair of electrons from the cyanide ion (CN-), a nucleophile, is attracted and donated to the delta carbon atom in the C=O bond
2) a dative covalent bond is formed between carbon and the cyanide ion
3) the pi bond in the C=O bond breaks by heterolytic fission, forming negatively charged intermediate
4) the intermediate is protonated by donating a lone pair of electrons to the H+ ion, forming a hydroxynitrile
How do you test for the presence of a carbonyl group?
1) add a solution of 2,4-DNP/Brady’s reagent to the test solution
2) in the presence of C=O, a yellow/orange ppt is formed
3) purify the 2,4-DNP derivatives (precipitates) using recrystallisation and heat them to find their melting points and compare to the known values of the melting points in a data table to identify the original carbonyl compound
How does the Tollen’s reagent test work?
1) Tollen’s reagent (a solution of AgNO3 in aqueous ammonia) contains Ag+ ions, which act as a reducing agent in the presence of ammonia
2) Ag+ ions are reduced to Ag as the aldehyde is oxidised to a carboxylic acid - ketones do not react as they cannot be oxidised
Equations: 1) Ag+ + e- => Ag (silver mirror)
2) RCHO + [O] => RCOOH
How do you test for the presence of aldehydes (distinguish between aldehydes and ketones)?
1) add Tollen’s reagent
2) in the presence of an aldehyde group, a silver mirror is produced
3) no reaction takes place in the presence of ketones
Describe the characteristics of carboxylic acids
- Carboxylic acids are organic acids which contain the carboxyl functional group (carbonyl and hydroxyl groups)
- To name: add the suffix -oic acid to the stem of the longest carbon chain
- The carboxyl carbon atom is always designated as carbon 1
What is the carboxyl group?
A carbonyl and hydroxyl group
Describe the solubility of carboxylic acids
1) the C=O and OH bonds in carboxylic acids are polar, allowing them to form hydrogen bonds with water molecules
2) carboxylic acids up to 4 carbon atoms are soluble in water - as the number of carbon atoms increases, the solubility decreases as non-polar carbon chain has a greater effect on the overall polarity of the molecules
3) dicarboxylic acids have 2 polar carboxyl groups to form hydrogen bonds ∴ they dissolve readily in water and are solids at room temperature