TOPIC 17 : ORGANIC CHEMISTRY II Flashcards
Explain what is meant by a chiral molecule and say how you would recognise an asymmetric carbon atom.
A chiral molecule is one that has no plane of symmetry. An asymmetric carbon atom is one that is attached to 4 different groups.
Why doesn’t a racemic mixture have any effect on the plane of polarisation of plane polarised light?
It is a 50/50 mixture of the two enantiomers. One of these is rotating the plane
of polarisation anti-clockwise, and the other is rotating it by exactly the same amount clockwise. They cancel each other out, so there is no overall effect on the light.
Define structural isomer
Same molecular formula different structural formulae
Define Chain isomers
Compounds with the same molecular formula but different structures of the carbon skeleton
Define Position isomers
Compounds with the same molecular formula but different structures due to different positions on the same functional group on the same carbon skeleton
Define functional group isomers
Compounds with the same molecular formula but with atom arranged to give different functional groups
What causes positional isomerism
Functional groups found at different positions on a chain
What are stereoisomers
Stereoisomers have the same structural formulae but have a different spatial arrangement of atoms
What causes chain isomerism
Different chain lengths due branching
What causes functional group isomerism
The position of atoms causes different functional groups eg = ketones and aldehydes
Explain how geometric isomerism arises
When there is a double bond between Carbons causing restricted rotation around the double bond and two different groups/atoms attached to both ends of the restricted double bond
What are the two types of stereoisomers
Geometrical isomerism ( E/Z) and optical isomerism
What is a Z isomer how do we determine if an isomer is a Z isomer
A Z isomer is when the priority groups on either side are on the same side
What is an E isomer how do we determine if an isomer is an E isomer
If the priority groups are on opposite sides then it is an E isomer
What is a priority group
The atom with the larger atomic proton number
When does optical isomerism occur
Occurs in carbon compounds with 4 different groups of atoms attached to a carbon
What shape are optical isomers
Tetrahedral
Define a racemic mixture
A racemic mixture is a mixture containing equal amounts of two enantiomers (optical isomers) that rotate plane polarised light in opposite directions by the same amount
What are enantiomers
Optical isomers
What is special about optical isomers
They have similar chemical and physical properties but rotate plane polarised light by the same amount in different directions
How is a racemate mixture formed use an example
Nucleophilic addition of HCN to aldehydes and unsymmetrical ketones from both sides forms a racemate
What is optical activity
When an optical isomer rotates plane polarised light
What is a chiral molecule
A carbon atom that has 4 different groups attached
Give an example of how optical activity provides evidence for SN1 mechanisms. DESCRIBE the mechanism for 2 bromo-butane reacting with a OH- ion
The Br breaks away from the haloalkane to form a carbocation intermediate
The OH- ion can attach from other side resulting in different enantiomers and a racemate forms
Because a racemate forms they will be no optical activity in the products proving SN1 mechanisms
Give an example of how optical activity provides evidence for SN2 mechanisms
Describe the mechanism for 2 bromo- butane with a OH - ion
Give an example of how optical activity provides evidence for SN2 mechanisms
Draw the mechanism for 2 bromo- butane with a OH - ion
In an SN2 mechanism there is no intermediate species formed
The OH- attacks the carbon and the bromine breaks off
If the reactant was chiral than during the reaction the opposite enantiomer would form
How else can we get a racemic mixture? Draw the mechanism
We can form a racemate by the electrophilic addition of HBr to an unsymmetrical alkene
What test would you use to distinguish between hexane and hex-1-ene ? Give the results of the test for each substance
Bromine water
Bromine water is an orange solution of bromine and it becomes colourless when an alkene is present as alkenes decolourise bromine water
What is meant by non-superimposable
Cannot be placed above one another and look the same
Define plane polarized monochromatic light
Light that only oscillates in one plane and is one colour
State how separate samples of 2 - enantiomers could be distinguished in a laboratory
Observe the directions they rotate plane polarised light
What is formed during a reaction undergoing SN1
Carbocation intermediate
What is involved during an SN2 mechanism
Two molecules are involved, the nucleophile attacks the carbon cation when the halogen breaks off
How can we separate samples of optical isomers be distinguished
Pass plane polarized light through the isomers and the isomers will rotate the plane polarized light in opposite directions.
Why do alcohols have higher boiling points than alkanes
Alcohols have higher boiling points than an alkane with the same number of carbon atoms due to hydrogen bonding.
What is formed from the oxidation of a primary alcohol
You get an aldehyde if you distill or a carboxylic acid if you reflux or oxidise the aldehyde
What is formed from the oxidation of a secondary alcohol
ketone
What is formed from the oxidation of a tertiary alcohol
They do not oxidise
What reagent is used to oxidise the alcohols or aldehydes
Potassium dichromate (K2 Cr2 O7)
What does the chromium species get reduced to and state the colour change
The orange dichromate ion (Cr2O72-) is reduced to the green Cr3+ ion
How would you prepare to form an aldehyde from a primary alcohol
Add potassium dichromate solution K2Cr2O7 to excess alcohol and distill off the aldehyde as soon as it forms
Describe what happens when a reaction mixture is refluxed and why it is necessary in this case for complete oxidation to ethanoic acid
A mixture of liquids are heated to boiling point for a long time
Vapour is formed which escapes from the liquid mixture and condenses and returns to the liquid mixture
Any ethanol and ethanal that evaporated can be oxidised
What are the circumstances for us including numbers to show the position of the double bond in ketones
When ketones have 5 CARBONS OR MORE you need to add a number to show the position of the double bond eg pentan-2-one
When is the prefix oxo used
It is used for compounds that contain a ketone group in addition to a carboxylic acid or aldehyde attached
What reagents are used in oxidation and how can they easily be expressed
Potassium dichromate it can easily be expressed as [O]
What reagents are used in reduction and how can they easily be expressed
LiAlH4 and it can easily be expressed as [H]
Why must LIALH4 be dissolved in dry ether
LiALH4 reacts violently with water and alcohols and the dry ether prevents this.
Describe the solubility of small carbonyls
Small carbonyls are soluble in water because they can form hydrogen bonds in water
Describe the solubility of pure carbonyls
They cannot hydrogen bond but bonds instead by permanent dipole bonding
What are the two other reagents that can oxidise an aldehyde into a carboxylic acid and what are the conditions,reaction and observations of each
Tollens Reagent=
Conditions= heat gently
Reaction and observation = aldehydes only are oxidised by tollens reagent into carboxylic acids and the silver ions are reduced into silver atoms and a silver mirror is produced,
Ketones result in no change
Fehling’s solution=
Conditions= heat gently
Reactions= aldehydes only are oxidised by fehling’s solution into a carboxylic acid and the copper ions are reduced to copper oxide
Observation= BLUE CU 2+ ions in solution change to a red precipitate of Cu2O. KETONES DO NOT REACT
What test do we use to identify carbonyl compounds and describe the process
Brady’s reagent (2,4, DNP) - product is an orange precipitate
Rinse the carbonyl with brady’s reagent
Filter precipitate
Recrystallise the precipitate using maximum amount of hot ethanol
Measure the melting and boiling temperature and dry purified product
Refer to data to determine carbonyl
A mechanism of the reaction between an aldehyde and hydrogen cyanide name what type of mechanism this is and condiitons
Nuclophilic addition - Conditions= Room temperature and pressure, HCN in presence of KCN
Why is Hydrogen cyanide in the presence of KCN
The KCN increases the concentration of the CN- ion
What is a nucleophile
Donates an electron pair, negatively charged and is attracted to areas of low electron density
What carbonyls can react with iodine in the presence of sodium hydroxide
Only carbonyls with a methyl group next to the C=O bond can do this reaction. Ethanal is the only aldehyde that reacts. Methyl ketones mostly react
Write the equation for ethanal + iodine sodium hydroxide and state the product formed
CH3COCH3 +3I2 +4NaOH > CHI3 + 3NaI + 3H2O
The product formed is CHI3 which is a yellow precipitate with an antiseptic smell
What is the name of the salt formed from carboxylic acid and the name the salt from ethanoic acid
A carboxylate, ethanoic acid= ethanoate
Describe the solubility of carboxylic acids in water
Small carboxylic acids can dissolve in water (up to butanoic acid) after this solubility decreases.
Describe the acidity of carboxylic acids
They are weak acids in water and only partially dissociate
What is a dimer
Two carboxylic acids bonded together by a covalent bond
Explain how the ability of ethanoic acid to dimerise affects its boiling point
The boiling point will be higher as the molecule will be larger and contain twice the number of electrons meaning greater london forces and more energy is required to break them
Suggest an explanation for why sodium ethanoate is a solid at room temperature while ethanoic acid is a liquid
Sodium ethanoate is an ionic lattice so will have strong electrostatic forces of attraction between oppositely charged ions. Lots of energy is required to overcome this attraction. Ethanoic acid has intermolecular forces between molecules which require less energy to overcome
What are the three ways you can prepare a carboxylic acid
Full oxidation of primary alcohols, Oxidation of aldehydes, hydrolysis of nitriles
Name the reagent, conditions and the reaction of the hydrolysis of nitriles
Reagent= Dilute HCL/sulfuric acid
Conditions= Heat under reflux
Reaction = Nitrile > Carboxylic acid
What is formed when you react an acid + metal
Salt + hydrogen
What is formed when you react an acid + alkali
Salt + water
What is formed when you react an acid + carbonate
Carbon dioxide + salt + water
What is the only carboxylic acid that can be oxidised by using oxidizing agents and what does it form
Methanoic acid is the only one as its structure is effectively an aldehyde group it forms carbonic acid which decomposes to carbon dioxide
Describe the reaction between carboxylic acids and phosphorus chloride and describe the conditions and observations and write an equation for the reaction of ethanoic acid reacting with phosphorus chloride
Reaction = Carboxylic acid > acyl chloride
Conditions= Room temperature
Observations= Misty fumes of HCL
Equation= CH3COOH + PCL5 > POCL3 + HCL+ CH3COCL
What is formed when you react a carboxylic acid with alcohol and what is special about this reaction and the conditions
Carboxylic acid + alcohol > salt + water
It requires a strong acid catalyst (H2SO4), the reaction is slow and needs to be heated under reflux it is also reversible
How do you name an ester
The part ending in yl comes from the alcohol that formed it
The part ending in anoate comes from the carboxylic acid
How can esters be hydrolysed
They can be hydrolysed either by heating under reflux with acid or NaOH
Name one way an ester can be hydrolysed and state the conditions, reagents and a reaction equation using the ester ethyl propanoate
Heating under reflux with dilute NaOH
Ethyl propanoate + NaOH > sodium propanoate + ethanol
This reaction is not reversible and goes to completion
Name another way an ester can be hydrolysed and state the conditions, reagents and a reaction equation using the ester ethyl propanoate
Heating under reflux with dilute HCL acid and heat under reflux
Ethyl propanoate + water»_space; Propanoic acid + ethanol
What are the conditions and reagents and products for reacting an acyl chloride with water and state the observation
Products= Carboxylic acid Reagent = water Conditions= room temperature Observation= Steamy white fumes of HCL
What are the reagents,conditions and products for reacting an acyl chloride with alcohol and state the observation
Products= Ester Reagent = alcohol Conditions= room temperature Observation= Steamy white fumes of HCL
What are the reagents,conditions and products for reacting an acyl chloride with ammonia and state the observation
Products= primary amide Reagent = ammonia Conditions= room temperature Observation= white smoke of NH4Cl is give off
What are the reagents,conditions and products for reacting an acyl chloride with primary amines
Products= Secondary amide Reagent = Primary amine Conditions= room temperature Observation= Steamy white fumes of HCL
What are the two types of polymerisation
Addition polymerisation and condensation polymerisation
How to produce polyesters
- diol + diacarboxylic acid > Water + polyester
2. diol + diacyl dichloride > HCL + polyester
Why Is chloroethanoic acid more acidic than ethanoic acid
Chlorine is more electronegative and therefore withdraws the electrons from COOH decreasing COOHs electron density and less negative so it becomes more stable and therefore chloroethanoic acid is more acidic
Why does increasing chain length of a carboxylic acid mean it is less acidic
Increasing the chain length increases electron releasing alkyl groups attached which make the COOH more negative and less stable so it is weaker
What can polyesters be hydrolysed by
- Acid or alkali
- With HCL a polyetser will be hydrolysed and spilt up into the original dicarboxylic acid and diol
- With NaOH a polyester will be hydrolysed and split up into the diol and dicarboxylic acid salt
