TESTING OF ORGANIC COMPOUNDS Flashcards
1
Q
distillation
A
- purifies and separates desired products from undesired products
- uses boiling points to separate two or more molecules
- two types - simple distillation and fractional distillation
2
Q
simple distillation
A
- technique relies on a difference of at least 50 degrees celsius in boiling point between the components to obtain an effective seapration
- equipment used:
- round bottom flask: contains the mixture to be separated and* boiling chips (distribute heat and avoid explosion)*
-
condenser: glassware used to cool hot vapours
- vapours condense back to liquid
- distillate: the cooled vapour which has condensed back to liquid and collected in the beaker
- liquids w higher boiling points are left in the round-bottom flask
3
Q
fractional distillation
A
- enables separation of liquids that have boiling points that are close together
- commonly used to separate volatile liquids from a reaction mixture
- a volatile liquid is one that vapourises easily at low temperatures (low boiling point)
- the vapours from the distillation flask contain a higher concentration of more volatile or lower boiling point components than the liquid in the flask
- vapours rise up the fractionating column until they reach a height where the temperature is low enough for condensation to occur
- as the condensed liquid trickled back down the it is reheated by vapours rising from the distillation flask - causes some condensed liquid to evaporate
- vapour now has a higher concentration of the low boiling component
- process of evaporation and condensation repeats multiple times throughout the length of the fractionating column - increasing the concentration of the most volatile component in the vapour
- temp at the top of the fractionating column remains relatively stable
- vapour eventually reaches condenser - it is cooled and the distillated drips into the collection vessel
- the distillate consists of components that condense over a small temp range near the boiling point of the desired organic compound
4
Q
melting point determination
A
- temp at which melting occurs depends on structure of organic molecule
- pure organic molecule has a sharp melting point
- melting point range: DIFFERENCE between the temp at which the sample begins to melt and the temp at which sample completely melts
- melting point range of 0.5 - 2 degrees celsius is considered small//narrow → indicating pure compound
- in pure organic compounds - all the molecules are the same
- intermolecular attractions are maximised - molecules can pack together in an orderly arrangemet
- this is why pure organic substances has a sharp melting point
- in a mixture of 2 or more organic compounds (impure) the molecules cannot pack in an orderly array
- intermolecular forces are disrupted - less heat is needed to melt this mixture - melting point is lower
- impure solid has a broader melting point range because regions of the crystal structure contain different amount of impurity
- low melting point and a wide melting point range of more than 2 degrees celsius usually indicated an impure substance
- mixed melting point determination - can be performed to confirm identity of unknown solid
5
Q
testing for carbon-carbon double bonds
A
- addition reaction between alkene and bromine can be used to test for the presence of a carbon carbon double bond
- red-orange colour of bromine rapidly decolourises - become colourless → colourless dibromo product forms
- bromine doesn’t react w alkanes
- can be used to distinguish between alkanes and alkenes
6
Q
testing for hydroxyl functional group
A
- can be determined by esterification reaction
- mixture of organic compound and ethanoic acid is gently heated with a few drops of concentrated sulfuric acid
- if there is a hydroxyl gorup - characteristic smell of an ester will be detected
- to determine if the organic compound is primary, secondary or tertiary, acidified potassium dichromate or acidified potassium permanganate can be used
-
acidified potassium dichromate:
- primary or secondary - orange colour of the dichromate ion will be reduced to Cr 3+ → green in colour
- no colour change if tertiary
-
acdified potassium permanganate
- primary or secondary - purple coplour of permanganate ions will be reduced to Mn2+ - colourless
- no colour change if tertiary
7
Q
testing for carboxyl functional group
A
- sodium hydrogen carbonate can be used to test for the presence of carboxylic acid
- when acids react w metal hydrogen carbonate → carbin diocide, salt and water are formed
- effervescence (fizzing) is observed when the carboxyl group is present
- presence of CO2 gas can be confirmed using lime water test
- when calcium hydroxide reacts with CO2 - calcium carbonate precipitate forms which turns the solution milky or cloudy
Ca(OH)2 (aq) + CO2 (g) → CaCO3 (s) + H2O (l)
8
Q
iodine number
A
- used to measure degree of unsaturation of fats and oils
- iodine number - mass of iodine in grams that reacts with 100 grams of fat or oil
- the more unsaturated a fat or oil, the higher the iodine number
- oils with higher iodine numbers are grenerally more reactive, less stable, softer and more susceptible to oxidation that fats or oils with lower iodine numbers
- vegetable oils tend to have more unsaturated fats than animal fats - higher iodine values
iodine decolourises when it reacts with unsaturated hydrocarbons
9
Q
volumetric analysis procedure
A
- preparation of standard solution
- preparation of sample aliquots
- preparation of burette
- titration (produce 3 concordant titres)
- calculation of the unknown concentration
10
Q
prep standard solution
A
-
standard solution: solution of accurately known conc.
- prepared by dissolving primary standards or diltuin stock solutions
- ideal primary standard:
- readily obtained in pure form so that the amount in moles can be accurately calculated from their mass
- known chemical formula
- easy to store w/o absorbing water vapour or reacting w gases
- high molar mass (minimise effect of errors in weighing)
- is inexpensive
- the standard solutions needs to be titrated against a standard solution to determine its actual concentration - becomes standardised solution
- to maximise precision and accuracy, volumetric flask should only be rinsed with water to remove any trace chemicals from the glassware
11
Q
prep aliquot
A
- known volume of sample transferred to conical flask with pipette
- fixed volume known as aliquot
- indicator may need to be added
- to maximise precision and accuracy
- conical flask should only be rinsed w deionised water to remove any trace chemicals
- rinsing w other solutions can cause unintended reactions and affect the result
- pipette should be rinsed with the solution it is to be filled with
- rinsing with water will dilute the aliquot (pipette is hard to dry as it is small)
- conical flask should only be rinsed w deionised water to remove any trace chemicals
12
Q
prep burette
A
- burette contains the standard solution → delivers accurate volume of solution to the aliquot
- to maximise precision and accuracy
- burette should only be rinsed with the solution to be filled with (rinsing w water dilutes the solution → burette is narrow)
13
Q
titration to produce 3 concordant titres
A
- solutions are mixed until equivalence point is reached
- equivalence point: the point at which the number of moles of both reactants are in the mole ratios of the balanced chemical equation
- end point: permanent colour change of solution in conical flask
- indicator is chosen so that the colur change at the end point occurs near the equicvalence point
- amount dispensed from burette → titre
- calculated by subtracting initial reading from final
- all readings to 2 decimal places
- to minimise random errors, titration is repeated several times to obtain concordant titres
- reading within 0.10mL of each other
- average fo 3 concordant titres used in calculation
14
Q
errors in volumetric analysis
A
- accuracy depends on calibration of equipment used and uncertainties associated w analytical equipment
- parallex error - not reading from bottom of meniscus
15
Q
dilution factor
A
- dilution factor = volume of the diluted solution/volume of the undiluted solution
- It is often necessary to dilute a solution by adding water to it to reduce its concentration, before carrying out a titration. This is done to obtain concentrations that will result in titres that are within the range of the burette.
16
Q
mass spectrometry
A
- quantitative technique to detect concentrations in parts per billion to parts per trillion
- mass specturm of an unknown sample can be compared to mass spectra in database to find its identity
17
Q
principles of mass spectrometry
A
ionisation
- sample is injected to an ionisation chamber
- sample is bombarded by a stream of electrons
- knocks one or more electrons from the particle - resulting in positively charged ions
- molecule is said to have been ionised
deflection
- positively charged ions are deflected by a magnetic field
- ions are separated in a magnetic field based on there mass (m)/charge (z) ratio (m/z)
- ions with greater m/z will be deflected the least (heavier)
detection
- the ions pass through a detector based on their m/z values
- data is recorded as a mass spectrum
18
Q
fragmentation
A
- inside the ionisation chamber, high energy electrons from the electron beam ionise the sample by knocking off electrons in the molecule to produce a positively charge moelcular ion
- renmoving electrons weakens the bonds and can cause covalent bonds to break - resulting in the molecule to fragment - break apart
- Any bond in the molecular ion can break, so fragments can be single atoms, small groups of atoms, or large sections of a molecule
- However, as electrons have been lost, these fragments are always positively charged.
- write all fragments with a positive charge