carbonyls and carboxylic acids chp 26 Flashcards
what is a carbonyl group
C=O
what structural similarities do aldehydes and ketones share
They both have a carbonyl group (C=O)
Where is the carbonyl group found in aldehydes
- Is found at the end of the carbon chain
- the carbon (C=O) is attached to 1 or 2 hydrogens
Where is the carbonyl functional group found in ketones
- The carbonyl group is joined to 2 carbon atoms in the carbon chain
- The ketone group is written as CO
How would you name aldehydes and ketones
You add the suffix -al for aldehydes and -one for ketones after the name of the longest carbon chain.
what is the product of oxidising aldehydes
draw reaction
- They can be oxidised into carboxylic acids when refluxed with acidified dichromate (Na2Cr2O7 / K2Cr2O7 with dilute H2SO4)
What oxidation reactions do ketones undergo
They undergo none
What reaction mechanism can carbonyl comnpounds undergo
- Nucleophilic addition reactions
- This is due to the polarity of the C=O group, which attracts nucleophiles
what influences the reactivity of aldehydes and ketones
The C=O bond, made up of both a sigma-bond and a pi-bond
Why do carbonyl groups and alkenes react differently if they both contain a double bond
- The C=C bond in alkenes is non-polar
- The C=O bond in a carbonyl compound is polar
What makes a C=O bond polar
- Oxygen is more electronegative than carbon
- The electron density in the double bond lies closer to the oxygen atom that to the carbon.
- ^This makes the carbon end slightly more positive and the oxygen end slightly more negative
What reaction mechanism does the C=C bonds in an alkene undergo
Electrophilic addition (NOT NUCLEOPHILIC ADDITION)
What is a nucleophile
A compound/molecule that donates electron pairs to areas of low electron density
What reducing agent is used when reducing ketones and aldehydes
- Sodium tetrahydridoborate (III)
- NaBH4
What conditions are used for the reduction of aldehydes and ketones
- warm
- reducing agent (NaBH4)
What is the product of reducing a aldehyde
- They are reduced into primary alcohols
What is the product of the reduction of ketones
- They are reduced into secondary alcohols
How can you write a reducing group in a reaction equation
2[H]
How does HCN (cyanide) react with carbonyl compounds
It adds across the C=O bond of aldehydes and ketones
What can be used to provide the hydrogen cyanide in a reaction due to hydrogen cyanide being dangerous
Sodium cyanide and sulphuric acid are used to provide the hydrogen cyanide in the reaction
why is the reaction of hydrogen cyanide so important for organic chemistry
- The reaction is very useful as it provides a way to increase the length of the carbon chain
Why is hydrogen cyanide dangerous to use in the lab
- It is a colourless, extremely poisonous liquid that boils slightly above room temperature
What type of reaction is the reaction of carbonyl groups with hydrogen cyanide
- its an addition reaction
What functional groups does the organic product formed from the reaction of an aldehyde with hydrogen cyanide contain
- It contains a hydroxyl group -OH, and a nitrile C≡N
- These compound are classed as hydroxynitriles (or cyanohydrins)
Draw the reaction mechanism for the nucleophilic addition of NaBH4 with a carbonyl compound
Explain the mechanism for the nucleophilic addition of NaBH4 with a carbonyl compound
- Hydride ion has lone pair
- Dative covalent bond forms between hydride ion and carbon in C=O
- C=O (pi-bond) breaks by heterolytic fission
- negative intermediate formed
- oxygen atom from carbonyl group donates lone pair to hydrogen in water molecule
- Intermediate has been protonated to form an alcohol
Draw the reaction mechanism for the reaction of a carbonyl group and cyanide
Explain the reaction mechanism for the reaction of carbonyl compunds with cyanide
- electron pair from :CN- attracts and donated to δ+ carbon in carbonyl group
^dative covalent bond forms - pi-bond in C=O bond breaks by heterolytic fission,
^forms negatively charged intermediate - intermediate protonated by donating lone pair to a hydrogen ion,
- The product is a hydroxynitrile
what is used to detected the presence of the carbonyl functional group in aldehydes and ketones, what does a positive and negative result look like
- Bradys reagents (2,4-dinitrophenylhydrazine)
- In the presence of a carbonyl group a yellow/orange precipitate is produced
- the reaction between bradys reagent and aldehydes and ketones is a condensation reaction
How is Bradys reagent used in the lab to test for carbonyl groups
- 2,4-DNP dissolved in methanol and sulfuric acid as a pale orange solution (this is then called Bradys reagent)
what is the method for testing for carbonyl groups in aldehydes and ketones
1)Add 5cm depth of a solution of 2,4 - DNP to a clean test tube. This is in excess
2)Using a dropping pipette, add 3 drops of the unknown compound. leave to stand
3)If no crystals form, add a few drops of sulfuric acid
4)A yellow/orange precipitate indicates the presence of an aldehyde or ketone
what can be used to distinguish between aldehydes and ketones
Draw what is oxidised and reduced during a positive result
- tollens’ reagent (a solution of silver nitrate in aqueous ammonia)
- in the presence of an aldehyde group, a silver mirror is produced
what is the method for producing tollens reagent
- add 3cm depth of (aq) silver nitrate, AgNO3 to a test tube
- Add (aq) sodium hydroxide, NaOH, to silver nitrate until brown precipitate of silver oxide forms
- Add dilute ammonia solution until brown precipitate just dissolves to form a clear colourless solution
What is the method for carrying out the test for aldehydes
1)Pour 2cm depth of the unknown solution into a test tube
2)Add an equal volume of the freshly prepared pollens reagent
3)Leave the test tube to stand in a beaker of warm water at 50°C for about 10-15 mins and then observe weather any silver mirror is formed
what is reduced and oxidised in the reaction between an aldehyde and tollens reagent
- silver ions are reduced to silver
- aldehyde is oxidised to a carboxylic acid
what is the method for using melting point to identify the carbonyl compound
1) The impure solid is filtered to separate the solid precipitate from the solution
2)The solid then recrystallised to produce a pure sample of crystals
3)The melting point of the purified 2,4-DNP is measured and compared to a database of known melting points
What pre-existing groups does a carboxyl group contain
- carbonyl group
- hydroxyl group
what are the main organic reactions that carboxylic acid is involved in
- Ester Formation: Carboxylic acids + alcohols → esters
- Acyl Chloride Formation: Carboxylic acids + SOCl2 → acyl chlorides
- Amide Formation: Carboxylic acids + amines → amides
- Decarboxylation: Carboxylic acids → alkanes (conc H2SO4 or nickel and high temps)
- acid anhydride formation: 2 carboxylic acid -condensation-> acid anhydride
what suffix is used to name a carboxylic acid
- oic acid is added to the stem of the longest carbon chain
are carboxylic acids soluble
- The C=O and O-H in carboxylic acids are both polar and so can form hydrogen bonds
- carboxylic acids with up to 4 carbon atoms are soluble
- As carbon chain increases the solubility decreases as more and more of the molecule is non-polar
What are dicarboxylic acids and are they soluble
- these are molecules with 2 carboxyl groups
- they solid at room temp but dissolve readily in water
are carboxylic acids weak or strong acids
- they are classified as weak acids
- when dissolved they only partially dissociate
what reagents are used in neutralisation and redox reactions involving carboxylic acids, what products are always formed
- redox reactions with metals
- Neutralisation reactions with bases (alkalis, metal oxides, carbonates)
- carboxylate salts are formed
what is the suffix used in the nomenclature of the carboxylate ion in a carboxylate salt
changes from -oic acid to -ate
what is produced in the reaction between carboxylic acids and metal
- its a redox reaction
- Hydrogen gas and carboxylate salt is formed
What is produced from a carboxylic acid + metal oxide reaction
salt + water
carboxylic acid + alkalis = ?
salt + water
what can be used to distunguish between phenols and carboxylic acids
- A carbonate can be used
- carboxylic acid + carbonate would show CO2 is given off as effervesence
- phenols are not acidic enough to react with carbonates
carboxylic acids + carbonates =?
salt + carbon dioxide + water
what is meant by a derivative of a carboxylic acid
- a compound than can be hydrolysed to form the parent carboxylic acid
draw an acyl group
what are examples of carboxylic acid derivatives
esters, acyl chlorides, acid anhydrides and amides
How do you name esters
- remove the -oic acid suffix from the parent carboxylic acid and replace with -oate.
- The alkyl chain attached to the oxygen atom of the COO group is then added as the first word in the name
How do you name acyl halides
- remove the oic acid and replace with -noyl halides
How are acid anhydrides formed
- formed by the removal of water from 2 carboxylic acids molecules
- what reaction brings about esterification
- what conditions are needed
- The reaction of an alcohol with a carboxylic acid to form an ester.
- alcohol is warmed
- small amount of concentrated H2SO4 needed
what can esters be hydrolysed by
- aqueous acid or alkali
what is acid hydrolysis in reference to esterification
- it is the opposite of esterification
- what are the products of an acid hydrolysis of an ester
- what are the conditions to carry out the reaction
- ester is heated under reflux with dilute aqueous acid
- ester broken down by water, with the acid acting as a catalyst
- products are a carboxylic acid and an alcohol
^alcohol is the group from bonded to the oxygen of the ester
what is alkaline hydrolysis also known as
saponification
characteristics of alkaline hydrolysis of esters
- it is irreversible
- the ester is heated under reflux with aqueous hydroxide ions
- products are an ester ion and an alcohol (that was alkyl group attached to oxygen)
How can acyl halides be prepared
- reacting the parent carboxylic acid with thionyl halide, SOX2
- the other products are SO2 and HX (evolved as gases)
- reaction should be done in fume cupboard
How are acyl chlorides useful in organic synthesis
- they are very reactive
- can be easily converted into carboxylic acid derivatives with good yield
- they react with nucleophiles by losing the chloride ion but retaining the C=O
acyl halide + alcohol = ?
- esters + H-halide
acyl chlorides/acid anhydrides + phenols = ?
- phenyl esters + HCl
neither reaction needs an acid catalyst
acyl chloride + water = ?
- carboxylic acid + HCl
- violent reaction, evolutions of dense steamy HCl fumes
During a reaction between acyl chloride and ammonia/amine what does the ammonia/amine act as
Acts as nucleophiles by donating a lone pair of electrons on the nitrogen atom to an electron-deficient species
acyl chloride + ammonia =
- primary amides + ammonium chloride
what is meant by a primary amide
- the nitrogen in the amide group is only attached to one carbon atom
primary amine + acyl chloride = ?
- secondary amine
what is meant by a secondary amine
- The nitrogen atom is attached to 2 carbon atoms
How do acid anhydrides react with phenols, water, ammonia and amines
- they react similarly to acyl chlorides
- they are less reactive than acyl chlorides
draw the reaction mechanism of the reaction between acyl chlorides and nucleophiles
