Standard Questions Flashcards
Outline a simple chemical test to detect the presence of a carbonyl and determine its identity
Add 2,4-Dinitrophenylhydrazine (2,4-DNP)
Orange Precipitate hydrazone will form if a carbonyl is present (Aldehyde or Ketone)
To determine the identity purify the hydrazone precipitate by recrystallization
Measure the melting point of the purified precipitate
Compare the melting point with data table values to identify the aldehyde or ketone
Outline a simple chemical test to distinguish between a Ketone and an aldehyde
Add Tollens Reagent (Ammoniacal Silver Nitrate) Warm in water bath to 60°C
Aldehyde – A silver mirror will form as Silver reduced Ag+(aq) + e- Ag(s)
Ketone – No visible observation
Why does Sodium Borohydride have to be aqueous to act as a reducing agent?
It dissolves in water so the ions dissociate NaBH4 -> Na+ + BH4
Dative bond breaks releasing Hydride ions (H-)in solution
What type of reaction takes place in reduction of a carbonyl and what type of bond fission occurs?
Nucleophilic addition; Heterolytic fission as both electrons go to one atom
Describe in words the mechanism of reduction of a carbonyl
Nucleophile is attracted to δ+ C
The lone pair of electrons is donated to form a dative bond
The π electron pair goes to the oxygen atom
Causing the π bond to break
Why are short chain (
Highly polar C=O bond and O-H bonds in carboxylic acid molecules for H bonds with polar water molecules
As the number of carbons increases a greater proportion of the molecule is non polar which is hydrophobic. This does not attract H2O and cannot form H bonds with H2O molecules.
The more O’s with lone pairs on a compound the more soluble
Describe a simple chemical test to differentiate a carboxylic acid from phenol and an alcohol
Add a carbonate, the carboxylic acid will react the others wont
Why are saturated fats solid at room temperature?
Higher melting point
More intermolecular forces between molecules More energy required to overcome intermolecular forces
Why do unsaturated fats have lower melting points than saturated fats?
Lower Boiling point
Less intermolecular forces between molecules #
Less energy to overcome
Explain why triglycerides are soluble in non-polar solvents and not in water
There are Van Der Waals forces between triglycerides
There is Van Der Waals forces between triglycerides and non-polar solvent
Triglycerides cannot hydrogen bond to water (enough)
Because there are not enough suitable oxygen atoms/Long Hydrocarbons chains
Why do bromine atoms substitute the H atoms on positions 2, 4 and 6 in a reaction with phenol?
Most reactive hydrogens due to the position of the highly electronegative O atom
Uses of Phenols
Plastics, Antiseptics, disinfectants & Resins for Plants
What can be deduced from an a 13C NMR spectrum
The different types of carbon present, from chemical shift values
The possible structures of the molecule
What can be deduced from a High resolution Proton NMR spectrum
TMS is a standard for chemical shift measurements and produces a peak at δ=0 (ppm)
What information can be deduced from an integration trace
Relative number of protons in each environment
Why are deuterated solvents used? State an example
So they do not produce a chemical shift/peak an example is CDCl3
What is the use of MRI and what is its relation to NMR?
MRI uses the same technology as NMR to obtain diagnostic information about internal structures in body scanners
Describe a proton exchange using D2O
A proton NMR spectrum is run on the sample of the compound under investigation
A small amount of D2O is added to another sample; and another scan run
You can now compare the two spectra produced, to identify the –NH, -OH by identifying which peak/s are not present in the second spectra as these will have been removed
Explain how D2O can be used to confirm which protons are responsible for peak ‘x’
D replaces OH (or NH) proton/Protons are labile
The Peak for OH (or NH) protons disappears
Describe and explain the different ways that a high resolution NMR spectrum can give information of a molecule
Chemical Shift values give the type of proton environment
The number of peaks gives the number of different proton environments
Relative peak areas give the number of protons in each environment
Splitting gives the number of adjacent protons o Using n+1 rule so e.g. Doublet means one proton on adjacent carbon
D2O can be used to identify –OH and –NH protons
In TLC how can you use the chromatogram to identify the compounds
Measure how far each spot travels relative to the solvent front
Calculate the Rf value Using Equation 𝑅𝑓 = 𝐷𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑚𝑜𝑣𝑒𝑑 𝑓𝑟𝑜𝑚 𝑏𝑎𝑠𝑒𝑙𝑖𝑛𝑒 𝑏𝑦 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡/ 𝐷𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑚𝑜𝑣𝑒𝑑 𝑓𝑟𝑜𝑚 𝑏𝑎𝑠𝑒𝑙𝑖𝑛𝑒 𝑏𝑦 𝑆𝑜𝑙𝑣𝑒𝑛𝑡 𝑓𝑟𝑜𝑛𝑡
Compare the Rf value found with data table values for the type of compound
State disadvantages of chromatography
Similar compounds have similar retention times/Rf values due to similar structutes
Unknown compounds have no reference retention times/Rf values
It can be difficult to find a solvent that separates all the components in a mixture (too soluble will not retain on the stationary phase and if not soluble enough then components will hardly move)
Discuss the advantages of GC-MS (Gas Chromatography-Mass Spectroscopy)
Provides a far more analytical tool than chromatography alone
Gas chromatography separates but cannot conclusively identify components
Gas Chromatography can give the relative proportions from the relative peak areas
Mass Spectroscopy identifies structures by a computer comparing fragment ions with a spectral database
Describe (Solid) Thin Layer Chromatography (TLC)
Stationary Phase: Solid, Separation by adsorption
Mobile Phase: Liquid e.g. Solvent
