Carbonyls And Carboxylic Acids
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What group defines a carbonyl compound?
The C=O bond.
What are some examples of carbonyl compounds?
• Aldehydes • Amides • Carboxylic acids • Ketones
What is the typical structure of an amide?
C=O bond adjacent to an N-H group
What are some examples of carbonyl compounds?
• Aldehydes • Amides • Carboxylic acids • Ketones
What is the typical structure of an amine?
N-H group bonded to an alkyl group.
Why do different aldehyde molecules have different boiling points?
Though both exhibit similar PD-PD interactions and neither form hydrogen bonds, those with longer carbon chains will exhibit stronger London forces and thus boiling point will be greater.
Why can’t ketones form hydrogen bonds with other molecules?
They do not contain the O-H group, thus there is not a hydrogen atom in a solution of ketones ONLY that can be attracted to the lone pair on the oxygen. However this i present in water thus ketones can form bonds with H2O instead.
How can we oxidise aldehydes to Carboxylic acids?
By heating under reflux in the presence of acidified potassium dichromate, an oxidising agent.
What type of reaction mechanism are carbonyl compounds typically involved in?
Nucleophilic addition.
Why do carbonyl compounds partake in nucleophilic addition mechanisms?
The C=O bond comprised a sigma and pi bond. This is polar due to differing electronegativity, so each atom has a partial charge. This means the carbon atom is susceptible to nucleophilic attack.
What reducing agent is typically used for nucleophilic addition, to reduce a ketone?
NaBH4 provides a H- hydride ion with a lone pair of electrons.
Describe the typical process for a nucleophilic addition.
• Nucleophile is attracted to delta-positive carbon, forcing electrons from pi-bond to oxygen • This forms O-, which attracts the delta-positive H+ from a neighbouring water molecule to form an OH alcohol group. • An OH- ion is also released.
How can we practically produce hydroxynitriles through nucleophilic addition?
• Reaction of NACN and H2SO4 to produce a highly reaction CN- nucleophile - note this isn’t stable on its own so must be produced in situ • This then undergoes nucleophilic substitution to form hydroxy-nitrile
Why is the nucleophilic addition of CN- a useful synthetic process?
This process extends the length of the carbon chain. We can then perform other synthetic processes on the nitro group to form other useful substituent groups.
What is an example of a useful compound we can form from a hydroxynitrile - and how do we form it?
By hydrolysis of the hydroxynitrile we replace the N group with OH to form a COOH functional group.
How can we distinguish between aldehydes and ketones?
Additional of Tollens’ reagent - only aldehydes will form a silver mirror on the surface of the test tube.
How can we distinguish between ketones and aldehydes/alcohols?
Ketones DO NOT oxidise in the presence of acidified potassium dichromate - the solution remains orange rather than changing colour to green.
How can we confirm the presence of aldehydes OR ketones exclusively?
Addition of 2,4-DNPH (Brady’s reagent) where an orange-yellow ppt is formed for a positive result.
How can we use 2,4-DNPH to identify a carbonyl compound?
• React carbonyl compound • Filter out ppt • Recrystallise using hot ethanol • Dry then measure and compare melting point against dataset
Why are Carboxylic acids highly soluble in water?
The C=O and O-H groups are highly polar and can both form hydrogen bonds.
Why does solubility of Carboxylic acids decrease with chain length?
The influence of the hydrogen-bonding groups decreases with increasing chain length, as the non-polar hydrocarbon chain takes influence.
How do we name Carboxylic acids with >1 COOH groups?
-dioic acid will be the suffix.
What is a Carboxylic acid derivative?
A compound that hydrolyses into a Carboxylic acid.
Give some examples of Carboxylic acid derivatives.
• Acyl chloride • Acid anhydride • Hydroxynitrile