Carbonyl Compounds Flashcards
What is the carbonyl group
C=O
Why is carbonyl polarised
Oxygen is more electronegative than carbon so electrons are more attracted towards the oxygen end of the bond
Why part of the carbonyl reacts with nucleophiles
The carbon
Which part of the carbonyl reacts with electrophiles
The oxygen
Describe aldehydes
Carbonyl group with one hydrogen and one alkyl or aryl (R-CO-H). Names end in -al or -aldehyde.
What are ketones
Carbonyl group with two alkyl or aryl substituents (R-CO-R). Names end in -one
Why are aldehydes usually more reactive than ketones
Inductive effect and steric hindrance
Why does inductive effect make aldehydes more reactive than ketones
The alkyl groups are electron donating (relative to the H), reduce the +ve charge on the carbonyl carbon and make it less attractive to nucleophiles
Why does steric hindrance make aldehydes more reactive than ketones
Alkyl groups get in the way of the approaching reagent. Aldehydes have a small hydrogen atom which does not get in the way of the nucleophile, whereas ketones have two substituents which repel the nucleophile
What happens with addition of water (hydration) of carbonyls
A reversible reaction. Double bond of oxygen to carbon breaks and two OH groups form and bond to carbon. However equilibrium usually favours the starting material
Describe nucleophilic addition of alcohols
Alcohols added to carbonyls is a reversible reaction. The first product is a hemiacetal (formed under an acid or base catalyst). Under an acid catalyst only the hemiacetal reacts to give an acetal.
Describe the nucleophilic addition of alcohols to carbonyls to form hemiacetals and acetals
Ketone + OH group from alcohol (with acid or base catalyst) -> hemiacetal (OH group and O with an open bond) (with an acid catalyst) -> acetal (no more OH group, two O groups with open bonds) and water
What is the significance of the formation of an acetal from the nucleophilic addition of alcohols to carbonyls being reversible
To make an acetal you add excess alcohol and remove water (distill it off). To hydrolyse an acetal you add dilute acid (large excess of water). Le Chatelier’s principle
What is addition of hydrogen (reduction) to carbonyls used for
To make alcohols from aldehydes or ketones.
How is the addition of hydrogen to carbonyls carried out
Done by adding H- (hydride) from reagent like LiAIH4 which gives electrons to oxygen of carbonyl making it -ve. Then adding H+ (from acid) to the reaction flask after which forms OH and so alcohol
What happens during oxidation of carbonyls
Requires an oxidising agent which becomes reduced during the reaction. Aldehyde oxidised to form OH on the H group (carboxylic acid)
What can be used to test for aldehydes
Fehling’s reagent (CU2+ in aqueous sodium tartrate) and Benedict’s reagent (Cu2+ in aqueous sodium citrate) give a reddy born precipitate of Cu2) when in the presence of an aldehyde
What does Tollen’s reagent give when in the presence of an aldehyde
A silver mirror (as is Ag+ in aqueous ammonia)
What does nucleophilic addition of primary amines with aldehydes or ketones produce
An imine
What happens when you add an amine to an aldehyde or ketone
Produces an imine and water. Imine has a C=N bond and the N is bonded to an R group.
What does the fact that the nucleophilic addition of primary amines and carbonyls is reversible mean
Imines are easily hydrolysed. Have to distil off water to shift equilibrium
What reactions do hydrazines undergo
Nucleophilic addition to form hydrozones and water. Hydrozones are reasonably stable towards hydrolysis
When is glucose a hemiacetal
When it is in its cyclic form
How does glucose form a hemiacetal
Undergoes intramolecular hemiacetal formation to form a ring. Glucose exists predominantly in its cyclic form
What functional groups does glucose have
Both an aldehyde and alcohol functional groups
What is an example of an aldehyde in equilibrium with a hemiacetal
Glucose
What is an example of a ketone in equilibrium with a hemiacetal
Fructose
What is fructose in its open chain form
A ketone
What is fructose in its cyclic form
A hemiacetal
What is sucrose
A disaccharide consisting of glucose and fructose. Two hemiacetals
What is the functional groups of carboxylic acids and how do you name them
-COOH (CO2H) functional group. Names end in -oic acid
What is the formula of propanoic acid
CH3CH2CO2H
What are the bonding and physical properties of carboxylic acids
Have higher boiling points than corresponding alkanes and alcohols. High boiling points due to strong hydrogen bonds between acid molecules
What do carboxylic acid molecules form in liquid and gas
Hydrogen bonded dimers. H of OH on one molecule forms hydrogen bond with O on other molecule and vice-versa
How are carboxylic acids synthesised
Alcohol oxidised to aldehyde which is oxidised to carboxylic acid. Use H2SO4 or KMnO4 as reagents
Describe the acidity of carboxylic acids
The OH proton can be lost to bases. So in water can lose H to form caboxylate ion (-ve) and H3O+
Describe the structure of a carboxylate ion
Charge is shared between both oxygen atoms (resonance). Both C-O bonds are the same length. Bond length is between that of a single and double bond
Describe acid strengths
In terms of electron withdrawing groups they stabilise the negatively charged carboxylate and increase in acid strength. Large Ka indicated a stronger acid
Give examples of where carboxylic acids are important intermediated in metabolism of carbohydrates
Glycolysis, citric acid cycle
Order anydride, ester and acid chloride in terms of most to least reactive
Acid chloride > anhydride > ester
Describe reactions of acid derivates
RCOY (+ nucleophile) -> RC)Nu + Y-. Y is the leaving group which departs during the reaction taking a pair of electrons with it
What are acid halides useful for
Making acid derivatives. They are prepared from acids. RCOOH + COCl2 -> RCOCl. Could also used PCl3
How do you convert acid halides into acid derivatives
By reacting them with nucleophiles.
How do you make a carboxylic acid from acid chloride
Add H2O
How do you make an ester from acid chloride
Add R’OH
How do you make a primary amide from acid chlride
Add NH3
How do you make a secondary amide from acid chloride
Add R’NH2
What de acid halides lose when reacting with nucleophiles to be converted into other acid derivatives
The halide (e.g. Cl)
What are the carboxylic acid derivateves
Acid halide (X= F, Cl, Br, I), ester, acid anhydride, amide
What are the molecules that can be formed when a carbonyl group is bonded to an electronegative element
Propanoyl chloride, ethyl acetate, ethanoic anhydride, N-methyl-propioamide
What test can you use to distinguish and amine from an amide
Amines react with ninhydrin to give a blue colour, amides do not.
What is an amide
C=O and NH2
What is an amine
Just NH2
How can amides can have 2 forms
The C and N can form a partial double bond giving the O a negative charge
Which 2 ways can esters be synthesised
From acid chlorides (add R’OH) or using an alcohol, carboxylic acid and strong acid catalyst (HCl or conc. H2SO4)
How do you make esters from alcohols, carboxylic acid and strong acid catalyst
Add excess alcohol to push the equilibrium towards the products
How do you hydrolyse an ester to a carboxylic acid and alcohol
Use dilute aqueous acid
Describe basic ester hydrolysis (saponification)
Ester (+NaOH and water) -> carboxylate salt and alcohol
Why is basic ester hydrolysis not reversible
Once formed the carboxylic acid is deprotonated by the base to form a salt. The negatively charged carboxylate is not attacked by the alcohol nucleophile. The reaction is irreversible.
How much base do you need to add to basic ester hydrolysis
One equivalent of base is used up, therefore need to add at least one equivalent
What is saponification (basic ester hydrolysis) used for
To make soaps from fats
How do you remove the carboxylic acid after saponification
By adding a stronger acid (e.g. HCl) to protonate it. Convert carboxylic salt to carboxylic acid
How do you produce biodiesel and what catalyst is used
By reacting methanol with a fat. Reaction is catalysed by NaOCH3. Biodiesel is fatty acid methyl ester
What are uses amides
Polymers- wool and silk are natural polyamides (proteins)
How can you test for proteins/peptides
Use Biuret test. GIves a purple/ violet colour with proteins/ peptides, stays blue otherwise
What is nylon
An artificial polyamide
How are polyesters made
Reaction of a diester (or acid) with a diol.
What is an example of a polyester
PET (polyethylene terephalate)
