Chapter 17.1, 17.2 & 17.3 - Chirality, Carbonyls & Carboxylic acids (L) Flashcards
What are carbonyl compounds?
Compounds with a C=O bond
What are the two types of carbonyl compounds?
Aldehydes and ketones
What is the structural difference between aldehydes and ketones?
In aldehydes, there is a hydrogen bonded to the carbonyl group, in ketones there is not
Where are aldehydes always found and what does this mean locant-wise?
They are always found at the start or end of the chain. This means they do not require a locant
Where can’t ketones be found?
At the end or start of the chain
What’s the difference between C=C and C=O bonds?
Carbon and oxygen have different electronegativities, meaning the C=O bond is a polar bond
What does the fact that C=O is a polar bond mean for reactions?
It means the delta positive carbon can be attacked by nucleophiles
How does the smell of aldehydes vary?
Short aldehydes = unpleasant smells
Long aldehydes = pleasant smells
What is the smell of short ketones?
Solvents
Out of alcohols, carbonyl compounds and alkanes, what is the order of boiling point?
Highest Alcohols Carbonyl compounds Alkanes Lowest
Why do carbonyl compounds have a higher boiling point than alkanes?
Carbonyl compounds have both London and dipole-dipole intermolecular forces, whereas alkanes only have London forces
Why do carbonyl compounds have dipole-dipole forces and alkanes do not?
Because carbonyl compounds are polar, and alkanes are non-polar
Why do alcohols have the highest boiling point?
Because they can form hydrogen bonds between each other, as well as having London and dipole-dipole
Why can’t carbonyl compounds form hydrogen bonds despite being polar?
Because there are no hydrogen atoms attached directly to the oxygen
Why are small carbonyl compounds soluble?
Because they can form hydrogen bonds with water
What is the trend in solubility as carbonyl chain length increases and why?
As the chain gets longer, solubility decreases. This is because the greater number of London forces interferes with the formation of hydrogen bonds
Which is easier to oxidise aldehydes or ketones and why?
Aldehydes are very easy to oxidise because of the hydrogen atom attached to the carbonyl group. On the contrary, ketones are hard to oxidise as they have no hydrogen attached
How many polar bonds are present in carboxylic acids?
3
C=O
C-O
O-H
When are carboxylate ions formed?
When a carboxylic acid loses the H from the O-H bond
What is meant by chirality?
A molecule is called chiral if it cannot be superimposed on its mirror image by any combination of rotations and translations
How can you identify a chiral centre?
A carbon with 4 different atoms or groups attached
What is polarimetry?
The use of a polarimeter to measure the amount of optical activity
What does a polariser do?
Converts unpolarised light into plane polarised light
What is the structure of a carboxylate ion?
O
- C
O
There are two identical carbon-oxygen bonds
What is the smell of butanoic acid?
Smell of stale sweat
What is the boiling point of carboxylic acids in comparison to other organic compounds and why?
The presence of three polar bonds results in strong hydrogen bonding, which means they have a relatively high boiling point in comparison to other organic compound
What happens to boiling point in carboxylic acids as chain length increases and why?
As chain length increases, boiling point increases because the number of London forces between the non-polar hydrocarbon chains increase
How does solubility change in carboxylic acids as chain length increases?
Solubility decreases as the non-polar hydrocarbon chain increases in length, as it forms more London forces which interfer with the formation of hydrogen bonds with water molecules
What two organic molecules oxidise to carboxylic acids?
Primary alcohols and aldehydes
What is the method to oxidise to a carboxylic acid?
Primary alcohol + oxidising agent + heat under reflux
When the oxidation is complete the mixture is fractionally distilled to obtain a pure sample
What is the oxidising agent in oxidising to carboxylic acids?
Acidified potassium dichromate
Other than oxidation, how else can carboxylic acids be formed?
Nitrile + dilute acidic or alkaline conditions + heat under reflux
What is the formula for acidic hydrolysis to a carboxylic acid?
Nitrile + 2H2O + H+ -> Carboxylic acid + NH4+
What is initially produced in alkaline hydrolysis to a carboxylic acid, and how is this overcome?
A carboxylate ion, however this can easily be converted into a carboxylic acid with the addition of a dilute acid
What is the formula for alkaline hydrolysis to a carboxylic acid?
Nitrile + H2O + OH- -> Carboxylate ion + NH3
Can carboxylic acids be reduced to aldehydes and why?
No carboxylic acids cannot be reduced to aldehydes, because all aldehydes are more easily reduced that carboxylic acids, meaning any aldehyde produced will also immediately be reduced to a primary alcohol
What is produced from the neutralisation of carboxylic acids?
Carboxylate salts
What is the most common carboxylate salt?
Sodium ethanoate
What is formula for the production of sodium ethanoate from ethanoic acid?
CH3COOH + NaOH -> CH3COONa + H2O
Ethanoic acid + sodium hydroxide -> sodium ethanoate + water
What type of compound are carboxylate salts?
Ionic
What is the functional group of acyl chlorides?
COCl
What occurs in the halogenation of carboxylic acids and what are formed?
The OH group is replaced by a halogen (e.g. chlorine) forming an acyl (chloride)
Why must the halogenation of carboxylic acids occur in anhydrous conditions?
Because both the reagents and the acyl chloride product react with water
What is the reagent in the halogenation of carboxylic acids to an acyl chloride?
Phosphorus pentachloride
How do you name acyl chlorides?
You use the suffix -oyl chloride
For example propanoyl chloride
What are enantiomers?
Optical isomers that are mirror images of one another, but are non-superimposable.
How do enantiomers affect plane-polarised light?
It rotates it
In polarimetry, if the plane of polarised light is rotated clockwise, what is the substance described as?
Dextrorotatory
In polarimetry, if the plane of polarised light is rotated anti-clockwise, what is the substance described as?
Laevorotary
In polarimetry, if the angle of rotation is a negative value what is direction of the rotation?
Anti-clockwise
In polarimetry, if the angle of rotation is a positive value what is direction of the rotation?
Clockwise
What is a racemic mixture?
A mixture containing equal amounts of two enantiomers
Why do racemic mixtures display no optical activity in polarimetry?
Because each enantiomer rotates the plane of light to the same degree in opposite directions
What is unpolarised light?
(Normal) light that has oscillations in all planes
What is the optical activity of Sn2 products?
The optical activity of the product will be an inversion of the reactant. E.g. if the reactant was laevorotatory, the product would be dextrorotatory
What is the optical activity of Sn1 products?
The product will display no optical activity
Why will the product of Sn1 mechanism display no optical activity?
As it will produce a racemic mixture, in which each enantiomer will rotate the plane of light to the same degree in opposite directions, resulting in no net rotation/optical activity
Which mechanism (Sn1 or Sn2) is most common for primary halogenoalkanes?
Sn2
Which mechanism (Sn1 or Sn2) is most common for secondary halogenoalkanes?
Neither, both mechanisms can occur, with the conditions and actual halogenoalkane dictating which is more common
Which mechanism (Sn1 or Sn2) is most common for tertiary halogenoalkanes?
Sn1
What is the transition state of the Sn2 mechanism?
The original halogenoalkane with an extra OH attached. The carbon will have 5 bonds (the bond to the halide and the OH are partial bonds)
What is the product of the first step in the Sn1 mechanism?
A carbocation with a planar shape, which can equally be attacked from either the left or the right by the OH- ion, hence forming a racemic mixture
What is the common reducing agent for both ketones and aldehydes?
Lithium tetrahydridoaluminate (also known as lithium aluminium hydride)
What must the lithium tetrahydridoaluminate be dissolved in for the reduction to take place?
Dry ether
What 4 reagents can be used to oxidise aldehydes but not ketones?
Acidified potassium dichromate
Fehling’s solution
Benedict’s solution
Tollen’s reagent
What is the positive colour change for oxidisation with acidified potassium dichromate?
If an aldehyde is present, it will change from an orange solution to a green solution
What is the positive colour change for oxidisation with Fehling’s solution and Benedict’s solution?
If an aldehyde is present, it will change from a deep blue solution to a red precipitate
What is the positive colour change for oxidisation with Tollen’s reagent?
If an aldehyde is present, it will change from a colourless solution to a silver mirror
In aldehyde oxidation with acidified potassium dichromate, what causes the colour change?
The reduction of chromium from +6 to +3
In aldehyde oxidation with Fehling’s solution and Benedict’s solution, what causes the colour change?
The conversion of a copper (II) complex to copper (I) oxide
In aldehyde oxidation with Tollen’s reagent, what causes the colour change?
The conversion of a silver (I) complex to metallic silver
What is Tollen’s reagent also known as?
Ammoniacal silver nitrate
How do you perform the iodoform test/reaction?
If a carbonyl compound is added to an alkaline solution of iodine and the mixture is warmed and then cooled, a pale yellow precipitate sometimes forms
What is the iodoform reaction a positive test for?
Carbonyls with the CH3CO group
What aldehydes contain the CH3CO group?
Only ethanal
What ketones contain the CH3CO group?
Methyl ketones (ketones with the carbonyl group as the second carbon in the chain)
Why will some alcohols also form a positive test in iodoform reactions?
Because oxidising conditions are used, so the alcohols will be oxidised to carbonyls. Therefore to work out whether the alcohol will show a positive test in iodoform, first imagine the alcohol in its carbonyl state
What are the reaction conditions for the reaction between carbonyls and hydrogen cyanide?
The reagent is hydrogen cyanide, and the reaction is carried out in an aqueous alkaline solution containing potassium cyanide (KCN)
What is the type of mechanism for the reaction between carbonyls and hydrogen cyanide?
Nucleophilic addition
What is the name of the product of the reaction between carbonyls and hydrogen cyanide?
Hydroxynitriles
What would the name of the organic product from the reaction between propanal and hydrogen cyanide?
2-hydroxybutanenitrile
In hydroxynitriles, which carbon is often the first carbon in nomenclature?
The carbon which is attached to the nitrogen (the carbon in the nitrile group)
How do you revise the mechanism for the reaction between carbonyls and hydrogen cyanide?
Draw it out each time this flashcard comes up until you get it 100% right by memory
Include all curly arrows, relevant lone pairs and dipoles!
Why is a racemic mixture produced from the reaction between carbonyls and hydrogen cyanide?
Because the carbonyl molecule is planar, which means there is an equal chance of the cyanide ion attacking from either side of the plane, meaning a racemic mixture is formed
Will the produce of the reaction between HCN and carbonyls display optical activity and why?
No it will not display optical activity, as it produces a racemic mixture
What is Brady’s reagent also known as?
2,4-dinitrophenylhydrazine
What does Brady’s reagent provide a positive test for?
The presence of a carbonyl compound
What is the positive colour for the presence of a carbonyl compound using Brady’s reagent?
Bright orange coloured precipitate
What else can Brady’s reagent be used for?
To identify exact individual carbonyls
How can Brady’s reagent be used to identify exact individual carbonyls?
The compound formed from the reaction with Brady’s (the orange precipitate) can be filtered, purified and dried, and their melting points measured. These measured melting points can then be compared to data book melting points to identify the carbonyl present
What are the compounds formed from the reaction with Brady’s reagent known as?
Derivatives
What is the general formula of acyl chlorides?
RCOCl
What is formed from the reaction between water and acyl chlorides?
A carboxylic acid and hydrogen chloride gas is formed
What happens in the reaction between acyl chlorides and water?
The OH and chlorine swap places
What is formed from the reaction between acyl chlorides and alcohols?
An ester and hydrogen chloride gas is formed
What happens in the reaction between acyl chlorides and alcohols?
The hydrogen from the OH joins with the Cl to form HCl, and the rest of the alcohol joins the molecule where the Cl was
What is formed from the reaction between acyl chlorides and ammonia?
An amide and hydrogen chloride gas are formed.
However the hydrogen chloride goes on to react with unreacted NH3 to form NH4Cl
What does the amide group look like?
O=C-NH2
What is the organic product of the reaction between amines and acyl chlorides?
N-substituted amides
What does the N indicate in N-substituted amines?
That there is an alkyl group also attached to the N. It is essentially used as a locant, for example N-methyl-ethanamide would indicate that there is a methyl group attached to the N
Do secondary amines react with acyl chlorides and what is the organic product?
Yes, and the N on the organic product will have two attached alkyl groups. This can be named using N again, e.g. N,N-dimethyl-ethanamide would indicate that there was two methyl groups attached to the N
Do tertiary amines react with acyl chlorides and why?
No they do not, as they have no H atom to react with the Cl to form hydrogen chloride
What is the functional group of esters?
O=C-O-R
R
or
-COO-
What reacts to form esters?
Carboxylic acids and alcohols
What is the equation for the hydrolysis of esters in acid solution?
Ester + Water ⇌ Alcohol + Carboxylic acid
This reaction is reversible
What is the catalyst used in the hydrolysis of esters/the formation of esters from alcohols and carboxylic acids?
A sulfuric acid catalyst
What is the equation for the hydrolysis of esters in alkaline solution?
Ester + NaOH -> Carboxylate salt + Na + Alcohol
What is the disadvantage of using hydrolysis of esters in alkaline solution?
A carboxylate ion is formed, however this can easily be overcome by adding a dilute acid (H+) to form a carboxylic acid
What is the advantage of using hydrolysis of esters in alkaline solution?
The reaction is not reversible so goes to completion
How do you name esters?
The first part of it (before the -OO-) is the second word, and the second part of it (after the -OO-) is the first work
For example:
CH3COOCH3
Would be methyl ethanoate
Which part of the name of the ester indicates the carboxylic acid and which part the alcohol that came together to form it?
The first part is the alcohol, the second part the carboxylic acid.
For example:
Methyl ethanoate would be formed from methanol and ethanoic acid
2 uses of esters
Pleasant smells given off by esters means they are often used in perfumes
Triesters can be hydrolysed to form a common ingredient of soaps, sodium stearate
What are the two main differences between addition and condensation polymerisation?
In condensation polymerisation-
- each time two monomers join together, another small molecule is formed as a by product (often water)
- usually the monomers reacting together are different
To form a polyester, what must the two monomers have?
A reactive group at each end of the monomers, often that reactive group is an OH group
2 uses of polyesters
Food and drink packaging
Many types of clothing
What can sodium carbonate and hydrogencarbonate be used to test for and why?
Carboxylic acids, as they react with carboxylic acids to form a salt, carbon dioxide and water
How can you convert a carbonyl into a hydroxynitrile?
Carbonyl + HCN (in the presence of KCN) -> Hydroxynitrile
How can you convert a nitrile into a carboxylic acid?
Nitrile + heat under reflux with dilute HCl -> Carboxylic acid
To form an ester from an alcohol and a carboxylic acid, what must also be present?
H2SO4 (sulfuric acid)
What observation can you make if an ester has been formed?
Fruity smell
