Organic Chemistry Flashcards
What is a chiral molecule?
A chiral molecule is a molecule containing a chiral carbon atom, which has four different groups bonded to it.
What is the name of a chiral isomer?
Enantiomer.
What is the key property to enantiomers?
Enantiomers rotate the plane of plane-polarised light by equal amounts but in opposite directions.
What happens to a 50:50 mixture of both optical isomers?
A raecemic mixture occurs where they solution is not optically active as both the rotations cancel out.
Explain why Sn1 nucleophilic substitution always results in a raecemic mixture.
The carbocation intermediate is trigonal planar therefore the OH- can attack from two sides that result in different isomers which produces a 50:50 mixture of both isomers.
Explain why in Sn2 nucleophilic substitution the product is always the opposite enantiomer to the reactant.
The OH- always attacks from the same side therefore the molecule undergoes chiral inversion.
What happens in nucleophilic addition in terms of chirality?
The CN- can attack from either side which results in a raecemic mixture.
Explain why carbonyl compounds have slightly higher boiling points that equivalently sized alkanes.
Carbonyl compounds are polar therefore they form permanent dpdp forces which are stronger than london forces.
Describe and explain the trend in solubility for carbonyl compounds.
Small compounds (C<5) tend to be miscible with water as they can form hydrogen bonds by as they get larger, more hydrogen bonds are disrupted with water than there are formed with the carbonyl compound which decreases the solubility of that compound dramatically.
What can 2,4-DNPH be used to test for? What is the colour change in a positive test?
It can be used to test for a C=O bond in a compound. The colour change in a positive test is yellow/orange solution -> yellow/orange precipitate
What is the product when a primary alcohol is fully oxidised in acidic conditions?
Carboxylic acid (-COOH group is protonated)
What is the product when a primary alcohol is fully oxidised in alkaline conditions?
Carboxylate salt (-CO2 is deprotonated)
Describe the conditions required to make a carboxylic reaction from a primary alcohol.
Solution must be heated under reflux.
Potassium dichromate(VI): K2Cr2O7(aq).
Dilute sulfuric acid: H2SO4(aq).
Acid must be distilled out at the end
What is the colour change when you oxidise a primary alcohol into a carboxylic acid?
Orange to Green
Name the product when a primary alcohol is oxidised by Tollens’ reagent and the colour change that occurs.
It forms a carboxylate salt and forms a silver mirror on the inner surface of the test tube.
Describe the preparation of Tollens’ reagent.
Add NaOH to AgNO3 to form Ag2O ppt.
Re-dissolve ppt in NH3 dropwise.
Name the product when a primary alcohol is oxidised by Fehling’s/Benedict’s solution and the colour change that occurs.
It forms a carboxylate salt and the colour change is from a blue solution to a red precipitate.
What does lithium aluminium hydride (LiAlH4) do?
It reduces carbonyl compounds into their respective alcohols as it acts as a source of H- ions.
What is the necessary conditions when using lithal?
Lithal must be dissolved in a dry ether as this prevents it from hydrolysing: LiAlH4 + 4H2O -> LiOH + Al(OH)3 + 4H2
Describe roughly what occurs in the “iodoform reaction”.
A carbonyl compound is oxidised to a carboxylate salt.
What are the products in an iodoform reaction.
A carboxylate salt and iodoform (CHI3)
What organic molecules can be oxidised by iodine? (Iodoform reaction)
Ethanol
Ethanal
Methyl secondary alcohol
Methyl ketone
Describe the products of nucleophilic addition reaction of a ketone with HCN/KCN.
It forms a nitrile compound with an alcohol group.
Describe the nucleophilic addition mechanism of a ketone with HCN/KCN?
First step:
CN from KCN acts as a nucleophile and C=O bond breaks.
Second step:
O- takes H+ from HCN.
Overall:
CN- from KCN is regenerated and HCN is used up.
Describe the conditions that are required for nucleophilic addition.
HCN and KCN (which is what edexcel want you to write)
You could also use KCN and H2SO4 (this generates HCN in the situation which is safer) or you could use HCN with a pH of 9.21 (which maintains a equilibrium of HCN dissociation that keeps HCN and CN- concentrations large at all times)
Explain why carboxylic acids have higher boiling points that equivalently sized alkanes and alcohols.
Carboxylic acids form strong hydrogen bonds.
They are also polar therefore form dpdp forces.
The molecules can also pair up to form dimers which doubles the strength of their london forces.
Overall this means that their intermolecular forces are quite strong.
Describe the trend in solubility for carboxylic acids.
Small compounds (C<5) tend to be miscible with water as they can form hydrogen bonds but as they get larger, more hydrogen bonds are disrupted with water than there are formed with the carboxylic acid which decreases the solubility of that molecule dramatically.
What is made when a nitrile is hydrolised with HCl or NaOH (give answer for both)?
A carboxylic acid when heated under reflux with dilute HCl and a carboxylate salt when heated under reflux with NaOH.
What is the product when a carboxylic acid reacts with PCl5 and what are the observations?
An acyl chloride.
Steamy white fumes which turn damp blue litmus paper red.
What can PCl5 be used to test for?
An OH group.
What forms when a carboxylic acid reacts with an alcohol? What other condition is required for this reaction to work?
An ester.
Concentrated sulfuric acid.
How can an acyl chloride form a carboxylic acid and describe the solution it forms.
It can react with water to create a carboxylic acid. This creates a very acidic mixture as a weak acid is formed (carboxylic acid) and a strong acid is formed (HCl).
What forms when an alcohol reacts with an acyl chloride and why is this the preferred method of creating this type of compound?
It makes an ester and its preferred because it is the fast and complete method. The other way to make an ester with an alcohol is by reacting it with PC5 and then another alcohol.
What is formed when an acyl chloride reacts with ammonia?
Amides
What is the name of an ester derived from and which order do they go in? (give an example)
The alcohol derived part and then the acid derived part. e.g. Methyl ethanoate or Ethyl butanoate
Explain the low boiling points in esters.
Moderate london forces.
Permanent dpdp forces.
No hydrogen bonding.
Explain the difference in aqueous solubility of esters and alkanes
Ester group can form H bonds whereas the carbon chains cannot.
Describe how esters can be made from carboxylic acids and why this method is not efficient. Also Explain.
Carboxylic acids can react with alcohols in the presence of concentrated sulfuric acid to form esters. Carboxylic acids aren’t very susceptible to nucleophiles and alcohols are weak nucleophiles. The reaction is also reversible as water can hydrolise the product therefore the reaction is slow, incomplete and has a low % yield.
What is the good and bad way of forming esters? (e.g. methyl ethanoate)
Bad way:
Alcohol + carboxylic acid (w/ H2SO4) -> ester + water
Slow, incomplete, low % yield
Good way:
Acyl chloride + alcohol -> ester + HCl
Fast, complete, high % yield
How can an ester be converted back into carboxylic acids and alcohols?
Esters can be hydrolysed in acidic conditions but this is a bad method as it has a low % yield due to the reaction being reversible.
What is formed when esters are hydrolysed in alkaline conditions? What is good about this reaction?
A carboxylate salt and an alcohol. The reaction is irreversible and OH is a good nucleophile therefore it is fast and has a high % yield.
What is the better method of hydrolysing esters? Heating under reflux with HCl or NaOH.
Heating under reflux is with NaOH is better as this is fast, complete and has a high % yield.
Doing this with HCl is slow, incomplete and therefore has a low % yield.