Chapter 26 - Carbonyl Compounds Flashcards
What is the simplest aldehyde and what is it used for?
a. The simplest aldehyde is methanal, HCHO.
b. The common name for methanal is formaldehyde, which is used in solution to preserve biological specimens.
What is the simplest ketone and what is it used for?
a. The simplest ketone is propanone, CH3COCH3.
b. The common name for propanone is acetone.
c. Which is used as an important industrial solvent and is also used in nail varnish removers.
What is the oxidation of aldehydes?
a. Aldehydes 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.
b. Ketones do not undergo oxidation reactions.
c. This lack of reactivity is a way of distinguishing between aldehydes and ketones.
What type of reaction occurs in carbonyl compounds and why?
a. Nucleophilic addition reactions occur in carbonyl compounds.
b. The C=O double bond is made up of a sigma and a pi bond.
c. The C=C double bond in alkenes is non-polar.
d. The C=O double bond in carbonyl compounds is polar.
e. Oxygen is more electronegative than carbon.
f. Due to the polarity of the C=O double bond, aldehydes and ketones react with some nucleophiles. A nucleophile is attracted to the and attacks the slightly positive carbon atom resulting in addition across the C=O double bond, resulting in a nucleophilic addition reaction.
What is the reaction of carbonyl compounds with NaBH4?
a. Sodium tetrahydridoborate(III), NaBH4, is used as a reducing agent to reduce aldehydes and ketones to alcohols.
b. This is a nucleophilic addition reaction.
c. The aldehyde or ketone is usually warmed with the NaBH4 reducing agent in aqueous solution.
d. Aldehydes are reduced to primary alcohols by NaBH4, by 2 moles of reducing agent.
e. Ketones are reduced to secondary alcohols by NaBH4, by 2 moles of reducing agent.
f. remember oxygen sandwich.
What is the reaction of carbonyl compounds with HCN?
a. Nucleophilic addition.
b. Hydrogen cyanide is a colourless, extremely poisonous liquid that boils slightly above room temperature, so cannot be used safely in an open laboratory.
c. HCN adds across the C=O bond of aldehydes and ketones.
d. Sodium cyanide and sulfuric acid are used to provide the HCN in the reaction, but the reaction is still potentially very hazardous.
e. This reaction is useful as it provides a means of increasing the length of the carbon chain.
f. For example, propanal reacts with HCN, CH3CH2CHO + HCN => CH3CH2CH(OH)CN. producing a hydroxynitrile.
What is the mechanism for nucleophilic addition reactions to carbonyl compounds?
a. Carbon atom in C=O double bond is electron deficient and attract nucleophiles.
b. In the reaction, NaBH4 provides the nucleophile H- (contains a lone pair, hydride ion)
c. Lone pair of electrons from hydride ions is attracted to and donated to the slightly positive carbon atom in the aldehyde or ketone C=O double bond.
d. A dative covalent bond is formed between the hydride ion and the carbon atom of the C=O double bond.
e. The pi bond in the C=O double bond breaks by heterolytic fission, forming a negatively charged intermediate. (oxygen gains a negative charge).
f. 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 protonated to form an alcohol.
What is the mechanism for the reaction with NaCN/H+?
a. The cyanide ion, CN-, attacks the electron-deficient carbon atom in the aldehyde or ketone.
b. The lone pair of electrons from the cyanide ions, CN-, is attracted to and donated to the slightly positive carbon atom in the aldehyde or ketone C=O double bond. A dative covalent bond forms.
c. The pi bond in the C=O double bond breaks by heterolytic fission forming a negatively charged intermediate. (oxygen gains a negative charge).
d. The intermediate is protonated by donating a lone pair of electrons to a hydrogen ion, to form the product. (you can show protonation to water as in the mechanism with NaBH4.
e. The product is a hydroxynitrile.
f. You have to write the negative charge above the Carbon atom in the cyanide ion.
How can you detect carbonyl compounds?
a. A solution of 2,4-DNP is used to detect the presence of the carbonyl functional group in both aldehydes and ketones.
b. In the presence of a carbonyl group, a yellow or orange precipitate called a 2,4-dinitrophenylhydrazone is produced.
What is the procedure for testing for the carbonyl group with 2,4-DNP?
a. In practical work, 2,4-DNP is normally used dissolved in methanol and sulfuric acid as a pale orange solution called Brady’s reagent. Solid 2,4-DNP can be very hazardous because friction or a sudden blow can cause it to explode.
b. First, add 5cm depth of a solution of 2,4-DNP to a clean test tube. This is in excess.
c. Second, using a dropping pipette, add three drops of the unknown compound. Leave to stand.
d. Third, if no crystals form, add a few drops of sulfuric acid.
e. Finally, a yellow/orange precipitate indicates the presence of an aldehyde or ketone.
How can you distinguish between aldehydes and ketones?
a. You can distinguish between aldehydes and ketones using Tollens’ reagent – a solution of silver nitrate in aqueous ammonia. In the presence of an aldehyde group, a silver mirror is produced. Tollens’ reagent has a short shelf-life and should be made up immediately before carrying out the test.
b. In a clean test tube, add 3cm depth of aqueous silver nitrate.
c. Add aqueous sodium hydroxide to the silver nitrate until a brown precipitate of silver oxide is formed.
d. Add dilute ammonia solution until the brown precipitate just dissolves to form a clear colourless solution. This is Tollens’ reagent.
What is the method for carrying out Tollens’ reagent for an aldehyde?
a. Pour 2cm depth of the unknown solution into a clean test tube.
b. Add an equal volume of freshly prepared Tollens’ reagent.
c. Leave the test tube to stand in a beaker of warm water at about 50 degrees for about 10-15 mins and then observe whether any silver mirror is formed.
d. If the unknown solution contains an aldehyde, then a silver mirror forms. If the unknown solution is a ketone then no reaction is observed.
e. Tollens’ reagent contains silver(I) ions, Ag+, which acts as an oxidising agent in the presence of ammonia. Silver ions are reduced to silver (silver mirror) as the aldehyde is oxidised to a carboxylic acid.
How can you identify an aldehyde or ketone by melting point?
a. The 2,4-dinitrophenylhydrazone precipitate formed in the 2,4-DNP test can be analysed to identify the carbonyl compound.
b. The impure yellow/orange precipitate is filtered to separate the solid precipitate from the solution.
c. The solid is then recrystallised to produce a pure sample of crystals.
d. The melting point of the purified 2,4-dinitrophenylhydrazone is measured and recorded.
e. The melting point is then compared to a database of melting points to identify the original carbonyl compound.
What are carboxylic acids and their derivatives useful for?
a. Carboxylic acids contain both a carboxyl group and a hydroxyl group.
b. They are useful as starting materials or intermediates in the formation of more useful compounds.
c. The painkiller, aspirin, can be synthesis from salicylic acid.
What is the solubility of carboxylic acids?
a. The C=O and O-H bonds in carboxylic acids are polar allowing carboxylic acids to form hydrogen bonds with water molecules.
b. Carboxylic acids up to four carbon atoms are soluble in water.
c. As the number of carbon atoms increases, the solubility decreases as the non-polar carbon chain has a greater effect on the overall polarity of the molecule.
d. Dicarboxylic acids have two polar carboxyl groups to form hydrogen bonds. They are solids at room temperature and dissolve readily in water.
