mod 8 Flashcards
how does IR work
- solution of the unknown sample to be tested is placed in cuvette and a sample of solvent by itself is placed in another cuvette (reference)
- the IR source is split into 2 beams using mirrors and passed through both cuvettes
- the beams pass through a monochromator to isolate wavelengths to analyse
- the difference in the absorbance between both cuvettes is due to IR absorbed by the substances
- the beam hits the detector which converts the information into a digital signal
choice of reactants
retrosynthetic analysis is the method by which scientists begin with the desired product and work backwards to determine required reactants
- e.g in organic compounds –> break long chains and consider which functional groups react
UV visible spectrophotometry
is based on the same principles as colorimetry, but it can also measure UV spectrum (190nm-380nm) absorbance
- no complementary absorbance but we take the peak absorbance
- select wavelength to test using UV spectrophotometer (uses monochromator not colour filter)
compound purity
spectrometric techniques (IR, NMR, MS) can be used to determine purity
- additional peaks
- missing peaks
- overlapping peaks when -CH2- are buried in long alkyl chains
hydrogen chemical environment
look at:
- what the hydrogen atom is bonded to
- what the atom the H is bonded to is bonded to
if they are the same = same carbon environment
- H atoms attached to the same C atom are identical
- OH has its own H environment
pH test
if it has acidic properties –> blue litmus will turn red
chemical shifts
to record C and H shifts, they are compared to the reference compound tetramethylsilane (TMS)
- TMS is chemically inert with 1 C and H environment hence strong baseline
fragmentation in MS
high energy electrons strike molecular ions and fragment them into smaller parts:
molecular ion –> cation + radical
- cation will be deflected and hit detector to form another peak
- fragments (m/z) < parent molecular ion
important complexes
memorise table
emission and absorption
emission = black background with coloured lines
absorption = coloured background with black lines
reaction conditions for exo equilibrium reactions
- too high temp = equation shifts left decreasing yield + safety concerns
- too low temp = reduces ROR
- need modernate-high temp
how AAS works steps
- hollow cathode lamp has a filament composed of element to be analysed
- the lamp is heated to emit the specific wavelength of the elements specific to the wavelengths absorbed by the element
- the solution is sprayed into the flame to atomise the substance (converts ions to free neutral gas atoms)
- light leaving the flame passes through a monochromator, separating and selecting a single wavelength
- the selected wavelength strikes a detector which determines the intensity and calculates he absorbance (by comparing to lamp’s intensity)
testing for sulfate and phosphate ions
- test for SO4(2-) by first adding acid then Ba2+ ions
- test for PO4(2-) by first adding base then Ba2+ ions
principles of infrared spectroscopy
molecules absorb infrared radiation and transition from lower to higher vibration levels
- vibrational levels are called modes (i.e stretching, bending)
- only modes that change the dipole moment (polarity through magnitude or direction) of molecular with be infrared active
reaction conditions for gas equilibrium reactions
- too high pressure = safety
- too low pressure = reduces ROR
- need to decide based on pressure conditions
isotopes
additional very small peaks due to the presence of isotopes
- usually have very small abundance
performing colorimetry in a school lab
- filtered light beam passes through analyte
- light beam strikes the detector which converts light into electricity
- the final result is an absorbance value
- absorbance must be compared o those of standard solutions
- machine must be calibrated (using blank with only solvent to set baseline at 0) by testing standard solutions = graph should be linear
bromine water test
test for C=C (carbon carbon double bonds)
- unsaturated carbons undergo addition reactions with hydrogen or hydrogen halides
- bromine water contains:
Br2(aq) + H2O(l) –><— HOBr(aq) + H+(aq) + Br-(aq)
- bromine is brown and will decolourise when reacted with unsaturated hydrocarbons
advantages of MS
- used for quantitative and qualitative analysis
- identify isotopes
- very sensitive, requires small sample
- accurate and fast
precipitation titration: equations
- Ag reacts with unknown anion to precipitate
Ag(aq) + X(-) –> AgX(s) - Ag reactions with dichromate to precipitate
2Ag(aq) + CrO4(2-)(aq) –> Ag2CrO4(s)
chemical shift formula
sample frequency - reference (TMS) frequency / spectrometer frequency
fertilisers
have nitrates and phosphates, causing:
- algae growth (eutrophication)
- decreases O2 in water
- algae blocks sunlight - limiting photosynthesis
stretching mode
2 atoms oscillate back and forth, stretching the bond between them
- at ground level a molecule stretches to a certain degree
- with infrared energy, the molecule stretches to a greater extent –> higher energy stretching mode
IR graph
- xaxis plots wave numbers –> higher wave no. = higher frequency = shorter wavelength = higher energy
- yaxis plots transmittance
= 10^-absorbance x 10%
= light leaving cuvette/light entering cuvette x100% - if no light absorbed, transmittance = 100% (baseline)
availability of reactants
chemical production facilities are best located near the production site of the raw materials it uses:
- reduce cost of transportation (e.g ethanol produced near farms supplying sugar cane)
- unless material is abundant (e.g haber process uses NaCl from ocean)
reading IR
- remove fingerprint region (right of 1500)
- look at wave number on reference sheet
- memorise table about depth and breadth
nucleus spins for NMR
- applied external magnetic field splits nucleus into 2 levels (parallel/low = poles align with magnetic field) (antiparallel/high = poles don’t align with magnetic field)
- when radio-wave pulse of correct frequency is applied –> aligned will absorb energy and transition to higher unaligned state
- nucleus oscillates between the levels called resonance
reaction conditions for endo equilibrium reactions
- need high temp but safe
bending mode
a bending mode is a molecular vibration where the angle between 2 bonds oscillates from small to large
- at ground level a molecule bends to a certain degree
- with infrared energy, the molecule bends to extreme angles –> higher energy stretching mode
what is yield
the amount of product made from the reactants.
- equilibrium reactions with yield<theoretical yield never goes to completion
- for a multistep process (A–>B–>C–>D) %yield = %yield(A–>B) x %yield(B–>C) x %yield(C–>D)
the percentage yield actually made is:
= experimental yield/theoretical yield x 100%
advantages of IR
- identify functional groups
- match substances with computer databases
- very sensitive –> small sample
- accurate and fast
conducting complexation reactions
- form precipitate by adding solution to metal cation
- dissolve the precipitate using NH3(aq)
Beer-lambert law states that:
A = epsilon x l x c = positively sloped line
- epsilon = molar absorption coeff in cm per mol per L (specific to each substance)
- c = concentration in mol per L
- l = cell/cuvette length in cm
- A = absorption (unitless value btw 0-1)
carbonate test
add sodium carbonate or sodium bicarbonate to produce the reaction:
acid + carbonate –> salt + water + CO2(g)
- confirm CO2 through limewater test (turns milky)
colourimetry is
used to find the concentration of a solution based on he intensity of is colour (absorption of the complimentary colour)
- more light absorbed = more intense colour = higher conc
- transition from lower to higher energy level
flame tests
atomise metal ions from ground to excited state –> transition back down and release photons
- flame colour is the frequency of photons released (unique to elements)
- only used for cations
performing colorimetry in a school lab steps:
- choose an appropriate coloured filter or set a wavelength to be measured for the machine
- set the baseline of he colorimeter at 0 with blank
- find the absorbance of a set of standard solutions containing the analyte
- plot this on a graph and draw a calibration curve –> normally straight line (conc vs absorbance on x&y axis)
- measure the absorbance of the unknown analyte solution
- interpolate the concentration of the unknown analyte solution from the calibration curve
testing for acetate
use confirmatory test for acetate with iron(III) chloride
- acetate doesn’t precipitate with metal ions
cation testing and anion testing and halide testing
look at flowcharts
applications of UV-Vis specrophotometry
used in two settings:
1. scan over the whole UV Vis range (190nm-200nm)
- determine which peak is best to analyse then measure at a specific wavelength
- determine which wavelengths the material absorbs the most intensely
2. measure at one specific wavelength
heavy metal ions
high density metals harmful to living organisms (bioaccumulate and have neurotoxins) found in:
- natural deep aquifers
- industrial waste
- mining
- combustion of fuels with metals
principles of nuclear magnetic resonance
NMR used to map the atomic structure of organic molecules
- nuclear spin is the angular momentum for its nucleus
- nuclei produce magnetic field with distinct north and south poles
- mainly C and H
disadvantages of IR
- doesn’t provide location of groups
- no info on size or MM
- not as sensitive as mass spec or UV vis spec
- only detects changes in dipole moment
HNMR spec
- signal splitting due to neighbouring H atoms in different environments showing close adjacent peaks
- no. of peaks = n+1 (n is the total no. of hydrogen atoms attached to neighbouring C atoms)
- don’t add H bonded to non-carbons to ‘n’ number
- no. of signals = no. of H environments
- peak at ppm 0 is of TMS reference
- downfield (increasing PPM) = H env is closer to electronegative atoms and vise versa for upfield
reaction conditions to optimise yield and ROR in industrial synthesis
- use a catalyst
- use a higher temperature
- use high pressure
- use correct molar ratio (or slight excess of reactants if equilibrium reaction to shift right)
precipitation titration
a volumetric analysis for the concentration of an ion in solution
- use Ag(NO3) as standard solution as Ag precipitates with most anions
- once precipitate is formed –> excess Ag will accumulate in the solution and be detected using detector
how does mass spec work
- sample is vapourised and injected into ionisation chamber
- in the chamber, an electron gun produces a stream of electrons to bombard the sample
- some electrons form a molecular ion (M + e- –> M+. + 2e-) - positive molecular ions are accelerated out of the chamber by an electric field that is orientates so only positive charges are accelerated
- molecular ion travels through a strong electromagnet causing ions to curve
- degree of deflection into arc is determined by mass-to-charge (m/z) ratio of particular ion
- at a given electromagnetic strength only ions of one (m/z) will reach detector –> rest hit wall and ejected - detector records the intensity of the stream of ions striking it
- then, the electromagnetic strength shifts ions of all (m/z) can be detected
- the mass spectrum plots the abundance (no. of ions reaching detector) at each m/z
oxidation test
add acidified KMnO4(aq)/Kr2Cr2O7(aq) to known alcohol
- if the pink/purple disappears its a primary or secondary alcohol
- if the major product is carboxylic acid (test acidity using litmus) = primary alcohol
- if the major product is a ketone (neutral test using litmus) = secondary
- if the colour doesn’t change use sodium test (bubbling) to confirm tertiary alcohol
electromagnetic spectrum
the electromagnetic spectrum includes (from 10^6nm to 10^-13nm) radio-waves, micro-waves, IR, colours (750-400nm), UV, X-ray, Gamma rays.
- only a small portion are visible to human eye
- energy increases and wavelength decreases as nm gets smaller
commercial synthesis factors and questions
ROR = how long does it take one cycle of the reaction to complete?
Yield = how much of the product is made per cycle?
Environmental = how much byproduct wastage? cost to dispose it?
Purity = cost to purify?
Safety = is this procedure safe?
colorimetry wheel
yellow –><– purple
red –><– green
orange –><– blue
CNMR spec
- only C13 is detected
- one signal per carbon environment
- use reference sheet to identify bond via PPM
absorbance calculation with light
A = log10(Io/I) = epsilon x l x c
- Io = light entering
- I = light leaving
- transmission is the ratio of intensity if light leaving to the intensity of light entering = Io/I x 100%
minimising precipitation titration error
since excess Ag(NO3) might be reacted to detect endpoint:
- perform blank titration with Ag(NO3) and the same volume aliquot of water with CrO4(2-) indicator
- wait for solution to turn red
- subtract this volume from regular titration volume to get true titre
what is industrial synthesis?
2 or more reactants combine to form a product and to be successful they need:
- identified reactions
- most efficient pathway for conversion
goals of commercial synthesis
maximise the quantity of product generated per unit of time
- quantity of production = reaction rate x yield
Atomic absorption spectroscopy
a quantitate technique used to determine the concentration of metal cations
- based on absorption spectrum
- different metal atoms absorb different wavelengths of light to transition between energy levels
- follows beer lamberts law
principles of mass spectrometry
mass spectrometry is a quantitative (structural determination of pure substances) and qualitative (determines substances in mixture) technique used to analyse solids, liquids and gases
flame test colours
Ba2+ = apple green
Ca2+ = brick red
Mg2+ = none
Pb2+ = gray
Ag2+ = none
Cu2+ = blue-green
Fe2+ = green/gold
Fe3+ = orange/brown
K+ = purple
Na+ = yellow
carbon chemical environment
look at:
- what the C atom is bonded to
- what the atom the C is bonded to is bonded to
if they are the same = same C environment/identical
- if there is axis of symmetry, the corresponding C on the other side is identical
NMR radio frequency needed to transition the nucleus depends on
- nucleus identity
- chemical environment of nucleus
what is atom economy
a concept used to explore the proportion of reactants converted to products through:
atom economy = molecular weight of desired product/molecular weight of all reactants x 100%
- detects wasteful products so we aim to minimise them to optimise atom economy
sodium test
test for hydroxyl (OH) group by reacting substance with metallic sodium:
alcohol + sodium –> sodium alkoxide + H2
- vigorous bubbling –> use pop test to confirm H2
- must anhydrous alcohol (remove water using dehydrating agent) as sodium+water=explosive reaction
- can’t determine whether primary, secondary or tertiary
metal complexes from complexation reaction
forms when ligands donate electron pairs to a central cation to form coordinate covalent bonds
- coordination no. = no. of ligands bound to single metal cation
- overall charge = metal cation charge + ligand charges
- final metal complex is a lewis product
precipitation titration: Mohr’s method
Mohr’s method used to analyse Cl-, Br-, CN- solutions using Ag(NO3) titrant and CrO4(2-) indicator.
1. place a standard solution of Ag(NO3) in burette
2. 25mL aliquots of unknown are placed in conical flask with K2CrO4(2-) indicator –> initially yellow colour
3. at equivalence point all unknown ions are consumed
4. when more Ag(NO3) is added, there is an excess that will precipitate out into a red colour
5. colour change from yellow to red is the endpoint
common fragments in MS
(mass = fragment)
15 = CH3+
17 = OH+
29 = CH2CH3+, CHO+
31 = CH2OH+
43 = CH3CH2CH2+, CH3CHO+
45 = COOH+, CH3CHOH+
how to read mass spec
- base peak is the most abundant fragment at 100% –> the other peaks are in proportion to the base peak
- the last peak is the molecular ion peak = molar mass
- find other fragmented peaks by memorising m/z ratios
disadvantages of MS
- doesn’t distinguish hydrocarbons with same molecular mass
- no conclusive evidence for functional groups
- not appropriate for on field testing
esterification test
react alcohol or carboxylic acid with pure acetic acid under reflux conditions with a concentrated sulfuric acid catalyst
- fruity smell is produced –> could be alcohol or carboxylic acid
- test may fail for tertiary alcohols
lucas’ reagant test
react known alcohol with ZnCl2 dissolved in concentrated HCl(aq) to form an alkyl chloride (white organic layer on top of test tube)
- no reaction = primary alcohol
- reacts over 1 hr = secondary alcohol
- reacts immediately = tertiary alcohol
