4.8 organic synthesis and analysis Flashcards
what does NMR stand for?
nuclear magnetic resonance
low resolution NMRs = from AS level
high resolution = A level (if talk about splitting patterns)
low resolution Hydrogen NMR:
- each hydrogen environment produces one peak
- the peak area is the number of hydrogens producing that peak
- the scale goes from 10 to 0
how many hydrogen environments does benzene ⏣ have?
1
how many hydrogen environments does chlorobenzene have?
3
carbon-13 NMR:
- identical principles to hydrogen NMR but with carbon atoms
- each peak represents a different carbon environment
- scale runs from 0 to 220
- use data sheet to quote numbers
- when the nuclei absorb the radio frequency radiation, the nuclei move from the lower energy state to a higher energy state
- this absorption is detected and converted into an NMR spectrum
what can be deduced from the analysis of a proton NMR spectrum?
- the no. of peaks = no. of different environments of hydrogen atoms in the molecule
- area under each peak = no. of hydrogen atoms of that type
what is the integration factor of a proton NMR?
- the relative intensity of the peaks (area under the peak)
- (usually written above the peak as hard to read directly)
why are NMR values typically expressed in parts per million e.g 2.0ppm?
- the value of (the chemical shift) δ is generally very small
what is every peak in NMR measured relative to?
- the standard
(TMS / tetramethylsilane)
what is the standard used in NMR spectra?
- tetramethylsilane / TMS
why do we use TMS as a standard?
- as TMS has 12 protons which are all equivalent and 4 carbons, which are also all equivalent
- this means that it gives a single, strong signal in the spectrum which turns out to be outside the range of most other signals, especially from organic compounds
- and TMS is unreactive
- i.e the hydrogens in TMS are all effectively shielded (they dont interfere with H peaks from molecule)
- those hydrogens with a large chemical shift (δ) are said to be ______
- and those with lower values of δ are said to be _____
- deshielded
- shielded
is there always an NMR peak at 0?
- yes because all of them are measured relative to TMS
what is the n+1 rule?
- the peaks in proton NMR are often not single peaks - they are split into a number of peaks very close together
- n = no. of hydrogen atoms on adjacent atoms
- a hydrogen atom with no hydrogen atoms on adjacent carbon atoms will have no splitting and will give a single peak
in n+1 what does n stand for?
- the number of adjacent hydrogen atoms - 3 BONDS AWAY
how many peaks will a hydrogen atom with no hydrogen atoms on adjacent carbon atoms give?
1
- it will have no splitting and will give a single peak known as a singlet
what is 1 peak called in H NMR?
a singlet
what are 2 peaks called in H NMR?
a doublet
what are 3 peaks called in H NMR?
a triplet
what are 4 peaks called in H NMR?
a quartet
what are 5 peaks called in H NMR?
a pentent
what are more than 4 peaks called in H NMR?
multiplet
- for singlet, draw 1 peak
- for doublet, draw 2 peaks of equal size (depending on how many hydrogens in environment)
- for triplet, draw 1, 2, 1
- for quartet, draw 1, 3, 3, 1
how do you work out the number of adjacent hydrogen atoms?
- an adjacent hydrogen atom is defined as the number of hydrogens on the carbon next to the carbon with the hydrogen we are talking about
the splitting of the peaks by adjacent hydrogen atoms in this way is known as what?
coupling
- splitting pattern = singlet, doublet etc
- relative peak area ratio = how many H’s in environment
interpreting proton NMR spectras:
- look at no. of peaks = no. of hydrogen environments
- look at integration no./factor = no. of hydrogens in each environment
- the chemical shift = compare to data booklet
- the splitting/coupling pattern = determined by presence of hydrogen atoms on adjacent carbon atoms
what are the two types of polymerisations we need to know?
- addition polymerisation
- condensation polymerisation
what is condensation polymerisation?
- when large number of molecules join together to form polymer + HCl or H2O
what are the 2 types of condensation polymerisation?
- polyesters
- polyamides
what are polyesters formed from?
EITHER:
- dicarboxic acid + diol —> polyester + water
- diacyl chloride + diol —> polyester + hydrochloric acid
compare polyesters from from dicarboxylic acid + diol and diacyl chloride + diol:
- dicarboxylic acid + diol = lower yield but less toxic products
- diacyl chloride + diol = toxic HCl(g) = white fumes
is esterification reversible?
yes
when using alkaline hydrolysis - per 1 ester bond required how many NaOH?
1
what does addition polymerisation produce?
- from an alkene —> polyalkene
does addition polymerisation have a high yield?
yes - as it has no by products
what are some common addition polymers?
- PTFE
- PVC
- polystyrene
what is PTFE commonly used in?
- non-stick frying pans
what is PVC commonly used in?
- plastic, doors and window frames
what is polystyrene commonly used for?
packaging materials
when is addition polymerisation used (instead of condensation)?
if:
- the main polymer chain only consists of carbon
- if the monomer is an alkene
what is a disadvantage of having a condensation reaction with 1 monomer?
it could self polymerise
what are the 2 things that polyamides are formed from?
EITHER:
- dicarboxylic acid + diamine —> polyamide + water
- di acyl chloride + diamine —> polyamide + HCl(g)
what does di acyl chloride + diamine form?
polyamide + HCl (g)
what does dicarboxylic acid + diol form?
polyester + water
what does di acyl chloride + diol form?
polyester + hydrogen chloride (HCl(g))
what does dicarboxylic acid + diamine form?
- polyamide + water
what observation does the formation of HCl (g) (hydrogen chloride) give?
white fumes
what are 4 condensariob polymers you need to know?
- PET
- nylon 6
- nylon 6,6
- kevlar
what is PET used in?
found in plastic bottles
what is nylon 6 used in?
- used in clothes
what is nylon 6,6 found in?
clothes
what is kevlar used for?
army uniform
what is special about the condensation polymer nylon 6?
- it is self polymerising
(only 1 monomer)
what is reflux?
the process of heating a reaction mixture but not evaporate the products
why is there not a stopper at the top of the condenser of a reflux apparatus
as otherwise pressure would build up and it could explode
why should you heat a reflux apparatus with a hot water bath and not a bunsen burner?
as organic materials are flammable
what else could you use instead of a hot water bath to heat in reflux?
- an oil bath
- if need very high temperatures (above 100°C)
also could use anti-bumping granules in reflux
for organic compounds, can you assume that 1cm^3 is equal to 1g?
no
what is the density equation?
density = mass / volume
(can be expressed in a variety of units so be careful when converting between units)
what are some separation methods?
- chromatography:
• TLC/paper chromatography
• gas chromatography
• high performance liquid chromatography (HPLC) - distillation
- fractional distillation
- separating funnel
what are the different chromatography methods?
• TLC/paper chromatography
• gas chromatography
• high performance liquid
chromatography:
- can be used to help determine the purity of a compound
- is routinely used to resolve different enantiomers that may have similar affinities for the stationary/mobile phase
- also used in conjunction with mass spectrometry techniques which provide information about fragment ions which can be matched to a database to match compounds to spectra
- if a compound is completely pure, there will be little trace of additional peaks or spots in any chromatogram
what does TLC (thin layer chromatography) separate material out depending on?
- the solubility in the mobile phase (solvent)
- and the affinity to the stationary phase (paper/TLC plate)
what is the mobile phase in TLC chromatography?
the solvent
what is the stationary phase in TLC chromatography?
the paper / TLC plate
why is a sealed system used in TLC chromatography? (e.g with a beaker and a lid)
- to increase the speed of the procedure
- and to reduce loss of solvent
what happens if a component is insoluble in TLC chromatography?
- it will remain on the pencil line
- if this happens, repeat the experiment with a different solvent
if a molecule is more attracted to the mobile phase, it moves (further/less far) up the paper/TLC plate and so will have a (higher/lower) Rf value?
- further
- higher
should the Rf value ever exceed 1?
no
how to calculate the Rf value?
= distance moved by compound / distance moved by solvent
the more a substance rises up the plate the (greater/lower) Rf value it has?
greater
sometimes the spots in TLC chromatography are not visible. how can they be seen?
- can be viewed under a UV lamp or the TLC plate by spraying with other chemicals such as ninhydrin which shows the spots up
the stationary phase TLC chromatography:
- for TLC, normally a silica based stationary phase is used
- this is polar
- therefore, depending on the affinity the analyte has for the stationary phase will dictate distance it travels up the TLC plate
- we say that if an analyte interacts with the stationary phase, it adsorbs onto it
for TLC, what kind of stationary phase is normally used?
- a silica based stationary phase
- (this is polar)
if an analyte interacts with the stationary phase, it ____ onto it
adsorbs
what can be done if the different compounds can have a similar affinity for the mobile phase and move similar amounts?
- two way separation
- using a different solvent (mobile phase) could mean that the analytes could be separated by a second solvent that they may have a different affinity for
how does two-way separation work?
- put in 1 solvent
- remove + dry
- turn 90°
- put in 2nd solvent
- remove + dry
how could you stain TLC chromatography / amino acids to see the spots?
- spray ninhydrin
- or UV lamp
what is gas chromatography?
- a column/tube
- filled with a solid or liquid adsorbed onto a solid, which acts as the stationary phase
- an inert carrier gas is the mobile phase used to separate the volatile components of a mixture
- usually carried out under pressure at high temperatures
- at the end of the column, a range of detectors (usually mass spectrometer) can be used to identify each component and the retention time for each component recorded
- the amount of time spent in the column will depend on the substance
- different substances will therefore exit the column at different times
- therefore can tell how many different substances make up a mixture by how many peaks on a spectrum
in gas chromatography (GC) the column/tube has a special coating on the inside
in gas chromatography, what is the stationary phase?
the coating on the tube
in gas chromatography, what is the mobile phase?
the carrier gas
what is the retention time in gas chromatography?
- the time it takes for a compound to elute (exit) the column
in what state are analytes?
gaseous
- and are kept gaseous, sometimes by heating the column
the larger the area underneath the peak after elution and detection, the (more/less) of that substance there is in a certain sample?
more
is peak area proportional to the amount of substance present in gas chromatography?
yes
- in gas chromatography, if polar coating and inject A + B (A=polar, B=non-polar), so A sticks, B carried by gas
- if B goes through faster than A, is the first peak on graph, because B elutes faster than A
in gas chromatography, if it has a quicker elution time, it has a (higher/lower) affinity to the mobile phase
higher
what’s the difference between gas chromatography and high performance liquid chromatography?
(the principles are the same)
- but in HPLC the column is filled with solid particles and the sample being tested is dissolved in a suitable solvent
- this mixture is then forced through the column at high pressure
- the main difference = the nature of the mobile phase
- GC = gas, HPLC = liquid
what state is the mobile phase in gas chromatography? in HPLC?
- GC = gas
- HPLC = liquid
when do you use GC compared to HPLC?
- depending on boiling points due to GC being in gas form and HPLC in liquid form
why do different substances have different retention times?
- the pressure used (bc that affects the flow rate of the solvent)
- the nature of the stationary phase (not only what material made of but also particle size)
- the exact composition of the solvent
- the temperature of the column
when is distilliation used to separate mixtures?
when you have two liquids in a mixture that have different boiling points
when is fractional distilliation used to separate mixtures?
- when the substances you are trying to separate have boiling points that are very similar
what is the difference in apparatus between distillation and fractional distillation?
- in fractional distillation, glass beads are placed in the column
why does fractional distillation have glass beads in the column?
- as have large surface area
- which encourages the vapour to dissipate (lose) heat and condense
what flask is used in fractional distillation?
a round-bottomed flask
when is a separating funnel used to separate a mixture?
- when you have a mixture of two immiscible liquids (dont dissolve in each other) of different densities
how does a separating funnel work?
- the two immiscible liquids need to be different densities
- the more dense liquid is the bottom layer
in separating funnels, normally, organic compounds are less dense BUT NOT ALWAYS
why can’t you have a cork in a separating funnel?
it won’t separate if it’s got a cork
when is recrystallisation done?
- to improve the purity of a crude (not pure) substance
if a question says ‘how to improve purity of a substance’ what is it talking about?
recrystallisation
how is recrystallisation performed?
- dissolve the crude product in the minimum volume of hot solvent (hot solvent = dependent on question)
- filter the mixture hot - this removed any insoluble impurities
- allow mixture to cool and crystallise
- wash crystals with ice cold solvent
- dry in warm oven below melting point of the substance
what factors might be considered when deciding on a synthesis route?
- type of reaction
- reagents
- atom economy
- byproducts
- conditions
what condition is required for alkenes to undergo free radical substation with halogens?
- uv light
name the process in which alkenes can react to form polymers?
addition polymerisation
what reagents and conditions are required for a halogenoalkane to form a nitrile?
- KCN in ethanol / ethanolic KCN
- warm/reflux set up
name the mechanism and products formed when halogenoalkanes react with ammonia?
- mechanism: nucleophilic substitution
- products: amines
name the two types of reactions between halogenoalkanes and hydroxides. how can you influence which reaction will occur?
- nucleophilic substitution - produces alcohols
- elimination - produces alkenes
- aqueous conditions are required for the nucleophilic substitution reaction and anhydrous ethanolic conditions required for the elimination reaction
how can alcohols react to form alkenes?
- alcohols can undergo dehydration with concentrated sulfuric acid (catalyst) to form alkenes
- this is an elimination reaction
what are the various products formed when alcohols react with acidified potassium dichromate (VI)?
- primary alcohols are oxidised to aldehydes and then carboxylic acids
- secondary alcohols are oxidised to ketones
- tertiary alcohols do not undergo oxidation with acidified potassium dichromate (VI)
what are produced when alcohols react with carboxylic acids?
esters
give the reducing agent required for aldehydes and ketones to form alcohols?
NaBH4 in water with methanol
give the reactants and mechanism name for the formation of hydroxynitriles from aldehydes
- reactants = HCN
- mechanism = nucleophilic addition
give the reducing agent required for carboxylic acids to form alcohols?
LiAlH4
what are produces when nitriles undergo reduction with LiAlH4?
amines
give the reactants and mechanism name for the formation of nitrobenzene from benzene
- reactants = concentrated H2SO4 and concentrated HNO3
- mechanism = electrophilic substitution
how can arenes form benzoic acid?
- benzoic acid is produced when side chains on arenes undergo oxidation.
- alkaline manganate (VII) followed by acidification is the most effective reagent
- OH-/KMnO4 and then dilute H2SO4
what are produced when phenol reacts with dizonium salts?
azo compounds
what is the product obtained when phenol reacts with nitric acid?
nitrophenol
give the reactants and mechanism name for the production of a phenylketone from benzene
- reactants = acyl chloride and AlCl3 catalyst
- mechanism = electrophilic substitution
how can ethanoic acid be formed from chloroethane?
- nucleophilic substitution with aqueous NaOH
- followed by oxidation with the oxidising agent acidified potassium dichromate (VI)
- chloroethane —> ethanol —> ethanoic acid
how can 2-propylamine be formed from propene?
- electrophilic addition with HBr
- followed by nucleophilic substitution with NH3
- propene —> 2-bromopropane —> 2-propylamine
a synthetic route is chosen to produce an organic compound. in terms of reagents, why might this route be favoured?
- the reagents might be renewable
- the reagents may have minimal safety concerns
how are by-products linked to the selection of a particular synthetic route?
- a synthetic route with less harmful by-products is preferred as there are fewer safety and environmental concerns
- the process is more sustainable if the by-products can be used in another industry
what condition is required to ensure a primary alcohol forms a carboxylic acid upon oxidation?
- reflux
how can you separate an insoluble product from a solution?
filtration:
- filter paper placed in a funnel over a conical flask
- pour the mixture through the funnel
- the insoluble product is left on the filter paper
describe how to carry out simple distillation
- heat a solution in a round bottomed flask using a bunsen burner
- the solvent evaporates then cools in the condenser
- the pure liquid is collected in a beaker
how can water soluble impurities be separated from an insoluble organic product?
- once the reaction is complete, pour the mixture into a separating funnel and add water
- shake the funnel and then leave the mixture to settle
- two distinct layers will form since the organic layer and aqueous layer will not mix
- the tap on the separating funnel can then be opened and each layer can be run off separately
what is one way that polyesters are formed?
- when dicarboxylic acids react with diols in a condensation reaction
what is one way that polyamides are formed?
- when dicarboxylic acids react with diamines in a condensation reaction
draw the repeat unit of nylon-6:
-CH2CH2CH2CH2CH2C=ONH-
what is the displayed formula of the monomer used to make nylon-6?
- cyclic
- 6 carbons 1 nitrogen
- H bonded to the nitrogen at position 2
- =O bonded to carbon at position 1
which two monomers are used to make nylon-6,6?
- 1,6-diaminohexane
- hexanedioic acid
how is PET produced?
- formed from benzene-1,4-dicarboxylic acid and ethane-1,2-diol
- the two monomers react together and ester links form between them
how can you determine the melting point of an organic solid?
- pack a small sample of the solid into a capillary tube. place the tube inside a heating element
- increase the temperature of the heating element until the sample turns to liquid
- compare the melting point to data book values to identify the solid
how can the melting point of a substance be used to evaluate its purity?
- a pure substance will have an exact sharp melting point
- an impure substance will melt over a range of temperatures
how can NMR be used to identify a molecule?
- NMR allows you to identify the different fragments in the molecule which can then be used to predict the structure of the molecule
on a proton NMR spectrum, what does the ratio if the areas under the peaks indicate?
- the relative number of protons in each environment
what are equivalent protons?
protons in the same environment
