chapter 29 Flashcards
what is a TLC?
quick and inexpensive analytical technique used to indicate how many components there are in a mixture
how does TLC work?
capillary tube spots small sample of solution
baseline drawn in pencil
substances mixture forms weak bonds with silica surface (adsorption)
different components in different mixtures have different affinities for adsorbent so bind with different strengths
placed in beaker with solvent and cover lid
separation occurs as solvent carries components to different extents
may need UV lamp to see
how do you calculate Rf value?
distance dot travelled / distance solvent travelled
what is gas chromatography useful for?
separating and identifying volatile organic compounds present in mixture
what is the mobile phase in gas chromatography?
inert carrier gas
what is the stationary phase in gas chromatography?
high boiling point liquid absorbed onto inert solid support
what does separation depend on in gas chromatography?
depends on differing solubility of components in liquid stationary phase
how does gas chromatography work?
small amount of volatile mixture injected into apparatus
mobile carrier gas carries components through column which contains liquid stationary phase
more soluble a liquid is in stationary phase the slower it passes through
different components reach detector at different times
what is retention time?
time taken for each component to travel through column
what can retention times be used for?
to identify compounds present by comparing known values
what can peak integrations be used for (area under each peak)?
to determine concentrations of components in each sample
what can relative peak areas be used for?
give relative proportions of different compounds
what are calibration curves needed for?
to turn peak area into concentration
test for alkene?
add bromine water drop wise
orange colour disappears
test for haloalkane?
aqueous silver nitrate solution and ethanol
water bath 50 degrees
C - white
B - cream
I - yellow
test for carbonyl group?
add 2,4-DNP (Brady’s reagent)
orange precipItate will form
what is the test for aldehydes?
add Tollen’s reagent
warm in water bath
produces silver mirror
how do you test for primary and secondary alcohols and aldehydes?
add acidified dichromate
warm in water bath
colour change orange to green
how do you test for phenols?
test with pH paper
pH shows phenol is weak acid
add sodium carbonate solution
no reaction shows its a phenol
or
add bromine water drop wise
if it becomes decolourised and white precipitate forms then it is a phenol
how does a nuclei have non-zero nuclear spin?
if it has an odd number of nucleons
gives them magnetic moment
what is NMR?
extremely powerful analytical tool
gives information on number and type of different carbon or hydrogen environment in a molecule
how does NMR work?
sample dissolved in solvent
placed in NMR tube
placed inside strong external magnetic field and spun
C13 or H2 align with or against external magnetic field
energy difference between two possibilities
radio frequency applied to sample nuclei
can absorb radiation and flip between two spin states (resonance)
why do H1 and C13 in different environments need different frequencies?
electrons surrounding nucleus shift energy and radio frenquency
why is TMS used for standard reference chemical in chemical shift scale?
inert
non-toxic
produces single peak in NMR spectrum
