6.1.2 - Carbonyls And Carboxylic Acids Flashcards
What is a carbonyl functional group?
Which groups contain a carbonyl group?
- C=O group
- Aldehydes and ketones contain it
What is an aldehyde functional group?
Uses of the simplest aldehyde
- CHO
- Methanal, HCHO, is used to preserve biological specimens
Ketone functional group
Uses of simplest ketone
- CO
- propanone/acetone, CH3COCH, used as an industrial solvent or in nail-varnish removers
Suffix for carbonyl compounds
Aldehyde: -al
Ketone: -one
Which is the first carbon in a carbonyl compound?
The carbon atom of the carbonyl group is always designated as carbon-1
Common uses and applications of octanal
- CH3(CH2)7CHO
- Naturally occurring in citrus oils
- responsible for the smell of oranges
- used as a component of perfumes
- flavour production in food industry
- released in some mating scents for birds (in Alaska) as part of breeding ritual
Oxidation of aldehydes
Oxidised to carboxylic acids Reagents used: - Cr2072-/H+ usually acidified potassium dichromate - dilute H2SO4 Conditions: -Heat under reflux
Do ketones undergo oxidation reactions?
Ketones do not undergo oxidation reactions
Structure of C=O carbonyl group
- the C=O bond is made up of both sigma and pi bonds
- pi bond above and below the plane of the carbon and oxygen atoms, formed by the sideways overlap of p-orbitals
- sigma bond in-between the carbon and oxygen atoms
Difference between carbonyl and alkene group
- C=O is polar, where C=C is not
- this is because oxygen is more electronegative than carbon
- so the electron density lies closer to the oxygen atom than the carbon
- this makes the carbon end of the C=O delta positive, with the Oxygen end being delta negative
What does the polar nature of carbonyl groups allow to happen?
- Aldehydes and ketones can react with some nucleophiles
- A nucleophile is attracted to and attacks the slightly positive carbon atom, resulting in addition across the C=O double bond
- Aldehydes and ketones can therefore undergo nucleophilic addition
(this is different to electrophilic addition in alkene C=C double bonds)
What is sodium tetrahydriborate(III), NaBH4, used for?
- Used as a reducing agent in reactions with aldehydes and ketones to reduce them to alcohols
- The aldehyde is usually warmed with the NaBH4 reducing agent in aq solution
Reducing an aldehyde mechanisms and info.
- Aldehydes are reduced to primary alcohols by NaBH4
Reagents:
- Heated with NaBH4(aq)
E.g. butanal reduced to butan-1-ol
Reducing a ketone mechanism and info.
- Ketones are reduced to secondary alcohols by NaBH4
Reagents:
- heated with NaBH4(aq)
Hydrogen Cyanide, HCN, info.
- Colourless
- Extremely poisonous
- liquid at r.t.
- boils slightly above room temperature
- cannot be used safely in an open laboratory
- in a reaction, sodium cyanide and sulfuric acid are used to provide the hydrogen cyanide
Reaction of HCN with carbonyl compounds
Why is it useful?
- HCN adds across the C=O bond of aldehydes and ketones
- The reaction will form a hydroxynitrile compound
- It is a useful reaction as it provides a means of increasing the length of the carbon chain
What groups does a hydroxynitrile compound consist of?
- hydroxyl group, OH
- Nitrile group, C(triple bond)N
- these can also be called cyanohydrins
Nucleophilic addition of carbonyl compounds
- The carbon atom in the C=O double bond is electron deficient and attracts nucleophiles
- Aldehydes and ketones both react by nucleophilic addition to form alcohols
Mechanism for nucleophilic addition to carbonyl compounds, using NaBH4
- In this reaction NaBH4 contains a hydride ion, ..(lone pair)H-, which acts as a nucleophile:
1. Look in textbook
Mechanism for reaction of carbonyl compounds with NaCN/H+
- In NaCN/H+, A ..(lone pair)CN- acts as the nucleophile in this reaction, which attacks the electron-deficient-carbon atom in the aldehyde of ketone
1. Look in textbook
Aldehyde def
At least one H attached to the carbonyl group
Ketone def
Two carbons attached to the carbonyl
C=O group is in the middle of at least two carbons
Nucleophile def
An atom or group of atoms that is attracted to an electron deficient centre.
They are often negative ions (anions) with a lone pair
They often contain an electronegative atom with a lone pair of electrons and a delta negative charge
Mechanism of nucleophilic addition with carbonyl compounds
- Dipole present on carbonyl - hence it is susceptible to nucleophilic attack
- Nucleophile donates the pair of electrons to the carbon which is electron deficient
- Simultaneously the pi electrons in C=O bond break forming the intermediate
- Extra electron pairs are donated to neighbouring H+ ion to form the alcohol which is stable
Test for aldehydes with Tollen’s reagent mechanism
- positive test shows a “silver mirror”
- this happens because the Tollen’s reacts with OH groups and oxidises them
Test for aldehydes with potassium dichromate
- positive result goes dark green
What is 2,4 NDPH used for?
- what is a positive result?
Used to test for ketones or aldehydes - test for carbonyl compounds with the C=O double bond.
- orange/brown precipitate will form
Testing for different carbonyl compounds using melting point
D,,Dodd,d,d,e
How to make Tollen’s reagent
- React AgNO3 and NaOH (aq)
Why does Tollen’s cause aldehydes to form a solver mirror/layer?
Silver ions in Tollen’s reagent are reduced
The silver ions then form solid silver (no oxidation number)
The aldehyde is then oxidised
Hence the silver layer forms
Why does 2,4 NDPH not react with other carbony-containing functional groups such as carboxylic acids, ester, or amides?
- With a -COOH or a die the compound acts as a base
- this leaves the carboxylate ion negatively charged
- so the ion is unable to be attacked by a nucleophile
- In aldehydes and ketones the carbocation is positively charged
- so the carbocation can be attacked by a nucleophile
Uses of esters
- Flavourings - e.g. fruit flavourings, etc.
- Scents - e.g. soap
- nail varnish remover
Functional group of Carboxylic acids
-COOH
pH strength of carboxylic acids
- Classified as weak acids
- this is because when dissolved in water, they partially dissociate
Reactions of carboxylic acids
- Redox with metals
- neutralisation with bases - alkalis, metal oxides, carbonates etc.
Esterification of carboxylic acid mechanism
Reagents:
- alcohol + carboxylic acids
Conditions
- reflux
- conc. H2S04
H2SO4 is a dehydrating agent - removes water - causes equilibrium to move to the right
- this increases yield of the ester
Esterification of acid anhydrides mechanism
Reagents:
- alcohol + acid anhydride
Conditions:
- reflux
- dry conditions
Equation:
Ethanoic anhydride + methanol —> methyl ethanoate + ethanoic acid
- Reaction is not reversible
- Acid anhydrides are less toxic than acts chlorides
What is hydrolysis
- what is it in context of esterification?
- A reaction in which water or OH ions break a compound in two
- hydrolysis is the opposite of esterification and occurs under reflux
What conditions does hydrolysis occur in?
- Reflux conditions
Example of acidic hydrolysis reaction of ester
Methyl ethanoate + water —> Ethanoic acid + methanol
Alkaline hydrolysis reaction of esters example
Methyl ethanoate + Sodium hydroxide —> sodium ethanoate - methanol
Use of sulphur dichloride oxide and info
- SOCl2 replaces the -OH group in carboxylic acids to produce an acyl chloride, sulphur dioxide and thionyl chloride
Symbol equation for reaction between ethanoic acid and thionyl chloride
CH3COOH(l) + SOCl2(l) —> CH3COCl(l) + SO2(g) + HCL(g)
Different reagents and product that acrylic chlorides can be used in
Amides:
- Reagents: concentrated ammonia, NH3
Alcohols:
- Reagents: H2O
Conditions:
To make ester: reagents: - alcohols with a methyl group Conditions: - heat - catalyst
To make methylamines(?):
- Reagents: primary amines
Equations and products of reactions with acyl chlorides
Ddijej
Test for carboxylic acids
- React with a metal carbonate - Sodium Carbonate
- Effervescence should occur
Butanal and butanone both react with 2,4-DNPH (Brady’s reagent) to produce orange precipitates.
Outline how the mixtures containing these orange precipitates can be used to distinguish between butanal and butanone.
(3 Marks)
- Recrystallise/purify the precipitate
- Measure melting points of each precipitate
- Compare with known values
How to prepare acyl chlorides and why it should be carries out in a fume hood
- React carboxylic acid with Thionyl chloride (SOCl2) to produce the acyl chloride
- Replace the OH on the carb acid with a Cl atom
- Produces SO2 and HCl gas
- both of these products are toxic
- needs to be done in a fume hood to prevent inhalation of toxic gases
Acid Anhydride formation
- React two carboxylic acids together
- produce acid anhydride and a molecule of water - condensation reaction
- e.g. two ethanoic acid molecules react together to form ethanoic anhydride
Butanal and butanone both react with 2,4-dinitrophenylhydrazine to produce mixtures containing orange precipitates.
Outline how the mixtures containing these orange precipitates can be used to distinguish between butanal and butanone.
(3 Marks)
Purify/recrystallise the precipitate
Measure the melting points of both
Compare the melting points with known values (online, etc.)
