Organic synthesis and analysis

What happens to the signal on the H NMR when the hydrogen atoms are closer to electronegative elements and why?

What happens to the signal on the H NMR when the hydrogen atoms are closer to electronegative elements and why?

• signal produced is further downfield

  • electronegative elements deshield nuclei of H atoms

What does the peak area represent?

number of hydrogens producing that peak

What is the scale of an H NMR?

0 to 10 but backwards

What does the splitting of a peak on an H NMR spectrum depend on?

how many hydrogens are on adjacent carbon atoms

What are miscible liquids?

liquids that will mix with each other

What are immiscible liquids?

liquids that don’t mix with each other → some form definite layers but some form droplets within the other liquid

What are insoluble solids?

solids that don’t dissolve in liquid

What are soluble solids

solids that dissolve in liquid

What is simple distillation and which substances does it work for?

• method of separating miscible liquids

• used if boiling temperature of 2 products differs by a significant amount (20°C or more)

• used to separate volatile liquids from a non-volatile mixture

What is fractional distillation and which substances does it work for?

• use of a fractional column → enables more efficient separation of product to occur

• used to separate miscible liquids that have close boiling points (<20°C)

What is vacuum distillation and which substances does it work for?

• separation under reduced pressure

  • reduced pressure reduces the boiling temperature → avoids decomposition at high temperatures

• used for miscible liquids that decompose at boiling temperature or have a high boiling temperature

What is steam distillation and in which industry is it used? Give an example of its use

• passing of steam through substance to remove volatile compounds with the steam which then condense in the receiving flask

• used in perfume industry to get essential oils out of flowers

What is solvent extraction? Use the extraction of caffeine from tea as an example

• solvent extraction uses the differing solubilities of a compound in 2 immiscible solvents to extract the compound required

• compounds can have different solubilities in different solvents

• For example:

  • caffeine has a solubility of 22mg cm⁻³ in water and 140mg cm⁻³ in dichloromethane

  • to extract caffeine from tea, tea steeped in hot water and then shaken with dichloromethane

  • maximum amount of caffeine extracted in dichloromethane layer whilst tannins left behind in water

  • dichloromethane and water don’t mix and form 2 layers which can be separated by a separating funnel

How are insoluble solids separated from liquid?

• simple filtration → filter paper and funnel

  • quicker when using fluted filter paper → only 1 layer of paper and not as much paper touching funnel

• vacuum filtration → Buchner funnel

  • once solid in funnel, needs to be washed with appropriate solvent and dried at room temperature or in drying oven below melting point

How are soluble solids obtained from a solution?

crystallisation

What needs to be done before crystallisation can occur?

• coloured impurities need to be removed by boiling solution with decolourising charcoal

• charcoal then filtered off while solution still hot → removes coloured impurities absorbed by charcoal

How is crystallisation carried out?

• after removing coloured compounds, concentrate solution by boiling

• if solution concentrated enough crystals will form when solution cools

  • if not, then boil again to concentrate more → careful as several solvents are highly flammable (use water bath or electrical heater)

Crystals formed in crystallisation need to be purified. How is this carried out?

• dissolve crystals in minimum amount of hot solvent

• filter to remove impurities

• allow to cool

• filter

• wash with small amount of solvent

• dry between 2 pieces of filter paper below melting point temperature

How are melting points used to indicate the purity of a solid?

• pure compounds

  • sharp (within 1°) melting point

• impure compounds

  • lower melting temperature than expected

  • melts over a range of temperatures (not a fixed temperature)

Why is it difficult to obtain a melting point for aldehydes and ketones?

often exist as liquids or low melting temperature solids

How are the melting points of ketones and aldehydes measured?

• ketone/aldehyde reacted with 2,4-dinitrophenylhydrazone to give a derivative

  • derivatives are usually red-orange solids and have an easier to measure melting temperature

• melting temperature can be compared with a table of melting temperatures to identify the starting aldehyde/ketone

Which 2 methods can be used to find the melting temperature of a compound and which one is better for flammable substances?

• electrical heating → use for flammable substances

• heating bath method

What is condensation polymerisation?

the joining of many monomer molecules to form a long chain polymer, with the loss of a small molecule (usually water)

What functional groups do monomers involved in condensation polymerisation usually contain?

• -OH

• NH₂

• COOH

What are the differences between addition polymerisation and condensation polymerisation?

addition polymerisation:

• contain double bond (alkenes)

• double bond breaks and polymer only contains single bonds → chain is exclusively carbon

• no other products formed

condensation polymerisation

• contain functional groups → -OH, NH₂, COOH

• 2 molecules join with the loss of a small molecule → removed from functional groups on end of carbons of adjoining molecule

  • chain isn’t exclusively carbon

• small product formed as well as polymer

How are polyesters made?

condensation polymerisation

What is the most common polyester and which monomers is it made from? What are its properties and structure?

• PET (polyethylene terephthalate)

• made from ethane-1,2-diol and benzene-1,4- dicarboxylic acid

• contains ester link → can be broken down by hydrolysis

• behaves like an ester

• not easily biodegradable

How are polyamides formed?

condensation polymerisation of a diamine and a carboxylic acid

What kind of molecule is Nylon-6,6 and what do the numbers indicate?

• polyamide

• numbers denote the amount of carbons in each starting molecule of the starting monomers

How is Nylon-6,6 made?

• made from hexane-1,6-dioic acid and hexane-1,6-diamine

  • hexane-1,6-dioic acid made by:

    • reducing benzene → cyclohexane

    • oxidising cyclohexane → cyclohexanol and cyclohexanone

    • cyclohexanol and cyclohexanone oxidised → hexanedioic acid

How is Nylon 6 made and how is this different from Nylon-6,6?

• the polymerisation of 6-aminohexanoic acid → contains both amino and carboxyl group

  • made from caprolactam which is made from benzene

• different to Nylon-6,6 as Nylon 6 only uses one starting monomer

What type of molecule is Kevlar and how is it produced? What are its properties?

• polyamide

• produced from benzene-1,4-dioic acid and benzene-1,4-diamine

• 5x stronger than steel

• can be produced as fibre → bullet proof vests

What is chromatography?

a technique used to separate substances from a mixture by their slow movement, at different rates, through or over a stationary phase

What are the 2 different phases in chromatography?

• mobile phase → solvent that moves through paper, carrying different substances with it

• stationary phase → contained on paper and doesn’t move through it

What is the difference between thin layer chromatography and paper chromatography?

• thin layer chromatography

  • uses a layer of silica or aluminium oxide coated onto a glass plate as the stationary phase

• paper chromatography

  • uses water trapped onto cellulose fibres of paper as stationary phase

How is chromatography carried out in both thin layer and paper chromatography?

• spots of starting substances dissolved into a suitable solvent are placed at bottom of paper or plate

• this is placed onto mobile phase solvent → must be below line of spots

• solvent rises up paper or plate → separates mixture into different spots

• when solvent front has reached suitable place → paper or plate taken out and dried

• measurements made

  • distance of spot from starting line

  • distance of solvent front from starting line

How is an R𝒻 value calculated?

R𝒻 = distance travelled by spot ÷ distance travelled by solvent front

What are the problems with thin layer and paper chromatography and how can these be overcome?

• dots might not completely separate when using a particular solvent

  • rotate dried chromatogram 90° and use a different solvent → 2-way separation

• the dots may be colourless and therefore unable to be identified

  • spray with ninhydrin and gently warm paper

    • blue spots appear → common for amino acids

  • shine UV light onto plate

    • if compounds are fluorescent then will show colour

  • coat plate with fluorescent material and expose to UV light

    • dots show as dark spots on fluorescent background

What are the advantages of using thin layer chromatography over paper chromatography?

• faster

• thin layer on plates can be made from variety of materials

• uses in forensic science

What are the mobile and stationary phases of gas chromatography?

• mobile phase: gas

• stationary phase: high boiling point liquid that is absorbed onto a solid

How does gas chromatography work?

• sample gases carried through column by inert gas

• time taken for compound to exit column = retention time

  • compounds that are less polar/lower boiling points will come off column first

• relative area of each peak gives percentage of each compound present in mixture

Which factors determine the efficiency of separation of compounds in gas chromatography?

• volatility of compound itself

• column temperature

• length of column

• flow rate of carrier gas

When would compounds be able to be identified by retention time and why?

• when all conditions are kept exactly the same

• retention times vary enormously

What can happen after gas chromatography to identify the separated components produced? What is this method called?

• components of mixture are led into a mass spectrometer

• known as GC-MS

What is high performance liquid chromatography (HPLC) and when is it used?

• HPLC is when a column is packed with uniform solid particles and sample is dissolved in solvent

• solution then forced at high pressure through the column

• used when a compound has a high boiling point as might start to decompose

Give an example of when high performance liquid chromatography might be used (hint: sport)

drug testing athlete’s urine

What is another term for the peak area?

integration number/factor

What determines chemical shift?

• environment around H atoms → proximity to electronegative atoms

  • nearer H is to electronegative atom, more deshielded it is = greater chemical shift

What is coupling?

splitting of the peaks by adjacent hydrogen atoms

What is the (n+1) rule?

• n+1 rule determines the way in which each peak splits depending on the total number of hydrogen atoms of adjacent atoms

• n = number of hydrogen atoms on adjacent carbon atoms

What are the names of the split peaks according to the value of n?

• n = 0 → singlet

• n = 1 → doublet

• n = 2 → triplet

• n = 3 → quartet

• n = 4 → quintet

• n = 5 → sextet

• n = 6 → septet

What is the standard used in NMR spectra? Why is this molecule used?

• tetramethylsilane (TMS)

• used as it gives a single, strong signal outside the range of most other signals

Why do the same hydrogen environments contribute to the same peak?

identical hydrogen atoms absorb radiation at the same frequency

What is C-13 NMR?

• identical to H NMR but with carbon atoms

• each peak represents different C environment

• scale runs from 0 to 220