Analytical Techniques, Alcohols, Haloalkanes Flashcards
infrared radiation corresponds to…
the energy required to make covalent bonds, causing the bonds to vibrate more and absorb energy
energy of infrared radiation depends on
its wavelength or frequency
energy required to make a covalent bond vibrate depends on
- the mass of the atoms (heavier ones vibrate more slowly)
2. strength of the bonds (stronger ones vibrate faster)
bonds in different functional groups
absorb different frequencies of infrared light
an infrared spectrum shows
the frequencies absorbed by a sample at each frequency
pollution monitoring
remote sensors analyse IR spectra of vehicle emissions. They detect and measure CO, CO2 and hydrocarbon concentrations
greenhouse effect
- light from sun heats up the ground and much of the heat radiates away from the ground as infrared radiation
- gases in the atmosphere containing C=O O-H and/or C-H bonds absorb certain frequencies of IR as they correspond to the natural frequency of the bond
- they re-emit the energy as radiation into the atmosphere close to the earths surface and so some of the heat is trapped in the atmosphere
where does H2O come from
evaporation of sea lake and river water, O-H bonds
where does CO2 come from
volcanoes respiration and burning and decay of organic matter, C=O bonds
where does CH4 come from
released in the production of oil and natural gas, from rotting organic matter eg in landfill sites and released by animals such as cows during digestion, C-H bonds
effects of global warming
- unpredictable
- rainfall patterns are changing leading to flooding in places and droughts in others
- higher temperatures and longer growing seasons may improve crop yields in some places and decrease in others due to drought
- storms and hurricanes are becoming more frequent and violent
ways of attempting to reduce global warming
- developing fuel alternatives eg wind solar tidal nuclear power
- developing bio fuels for transport
- developing new cars with more efficient engines to reduce CO2 produced per mile
- incentives available to householders for converting to renewable sources of energy such as solar panels which don’t release co/2
why is spectra of molecules more complicated
due to the possible combinations of isotopes and due to fragmentation of the molecules
what happens to molecules when analysed in a mass spectrometer
they become ions - molecular ions M+
haloalkanes general formula
CnH(2n+1)H
physical properties of haloalkanes
- have a permanent dipole
- not sufficiently polar to be soluble in water
- melting and boiling points vary according to strength of ID:ID due to increasing number of e- rather than decreasing strength of permanent dipole dipole interactions
CH3I> Ch3Br> CH3Cl> Ch3F
formation of haloalkanes
- from alkanes - free radical substitution (X2 UV light)
- from alkenes - electrophilic addition reaction (HX or X2)
- from alcohols - substitution reaction (H2SO4 NaX)
nucleophile
an electron pair donor
substitution reaction
one atom or group is replaced by another atom or group
reflux
process of continuous evaporation and condensation. a condenser is used to prevent volatile components from escaping
nucleophilic substitution reactions
- nucleophiles have a lone pair of electrons and often a negative charge
- haloalkanes have a polar C-X bond ( X= halogen atom)
- the lone pair of electrons on the nucleophile is attracted to the delta positive carbon of the haloalkane
with an aqueous alkali (hydrolysis)
reagents: NaOH (aq)
conditions: water as the solvent, heat under reflux
water could be used as the reagent instead of OH- ions but the reaction would be incredibly slow so OH- is a much better nucleophile
investigating the rate of hydrolysis method
- haloalkane is mixed with water, silver nitrate and ethanol at 60 degrees
- the reaction with h2o is slow so the production of x- is low
- as x- is produced a ppt forms with Ag+
- rate of production of the ppt is monitored to find the rate of hydrolysis
equations for investigating rate of hydrolysis
- CH3X + H2O -> CH3OH + HX
then
X- (aq) + Ag+ (aq) -> AgX (s)
observations for investigating rate of hydrolysis
CH3I -> Yellow ppt almost immediately
CH3Br -> cream ppt after a few minutes
CH3Cl -> white ppt formed slowly
CH3F -> no change observed as AgF is soluble
conclusions for investigating rate of hydrolysis
- rate is dependent of strength of C-X bond
- C-F > C-Cl > C-Br > C-I
- C-F bond is strongest as it is the shortest bond and most polar - additional ionic character strengthens the bond, reflected in the bond enthalpy
- C-F is strongest so hardest to break and therefore slowest to react with the nucleophiles
- C-I is the weakest so easiest to break and quickest to react with the nucleophile
reaction with CN- (to extend carbon chain)
reagents: NaCN
conditions: ethanol solvent
3 BONDS BETWEEN C AND N
reaction with NH3
reagents: excess NH3 and then NaOH
conditions: ethanol solvent
first salt formation: Ch3Br + NH3 -> CH3NH3+ + Br-
Amine generation: CH3NH3+ + OH- -> CH3nh2 + OH-
combustion haloalkanes
hx is produced in addition to the usual combustion products
- as a result, emissions from combustion of haloalkanes must go through basic scrubbers to neutralise the HX and prevent pollution (acid rain)
CH3Cl2Cl + 3O2 -> 2Co2 + 2H2O + HCl
uses of haloalkanes
anaesthetics eg halothane
pesticides eg DDT
CFCs
what is an organohalogen
contain atleast one halogen atom joined to a carbon chain. they are not naturally occuring and so are not broken down naturally in the environment
use of organohalogens
pesticides - DDT solvents - CHCl3 trichloromethane dry cleaning solvents - C2H2Cl2, C2HCl3 making polymers - c2h3cl, c2f4 flame retardant - cf3br refrigerants - f2ccl2, hcclf2
ozone
o3
what is ozone and where is it found
found in the stratosphere where it absorbs most of the dangerous UV radiation from the sun. further depletion of the ozone layer would result in more uv radiation reaching the surface of the earth increasing chances of sunburn and skin cancers
ozone being formed and broken down
o2 -> 2o (radicals)
o2 + o -> o3 (radicals react with oxygen)
equlibriumm of o2 + o -> o3
cause of ozone depletion by cfcs
chlorofluorocarbons are extremely stable compounds due to their c-x bonds which compose slowly over thousands of years. they used to be used as refrigerants, in air conditioning units and as aerosol propellants. they remain stable in the atmosphere until they reach the stratosphere which may take many years. uv radiation breaks them down, leading to chlorine radicals
chlorine radicals and o3
ccl2f2 -> cclf2 radical + cl radical
nitrous oxide radicals
form from aircraft pollution in the stratosphere and naturally in thunderstorms
destruction of ozone by radicals
propagation step 1: R + o3 -> RO radical + o2
propagation step 2 -> RO radical + O -> o2 + radical
O3 + O -> 2O2
because CFCs are so stable ones already released will continue to destroy the ozone for years to come
how to solve the problem of CFCs
biodegradable alternatives such as hydrocarbons and hcfc’s now used wherever possible. co2 is now used to expand polystyrene for packaging. however, some cfcs are still used and nitrogen oxides are still produced by aircraft. International agreements are required due to air pollution affecting all countries
primary alcohol
has 1 r group
secondary alcohol
has 2 r groups diol
tertiary alcohol
has 3 r groups triol
physical properties alcohols
- polar due to difference in electronegativity between the O and H in the OH groups
- higher mp than alkanes of similar molecular weight
- all common alcohols are liquid at rtp due to hydrogen bonding between the molecules
- smaller alcohols are water soluble due to hydrogen bonding being established between OH groups in the alcohol and water
- methanol ethanol and propanol all soluble
- larger alcohols - less soluble due to longer non polar carbon chains
- all toxic to varying degrees - methanol highly toxic and small quantities cause blindness and death
combustion of alcohols
c2h5oh + 3o2 -> 2co2 + 3h2o
- flammable and used as fuels
- ethanol made by fermentation is a biofuel
- ethanol/methanol an be used as petrol additives
- methylated spirits = ethanol with methanol added so unfit to drink
elimination of water alcohol
reagent: alcohol
conditions: h3po4 catalyst and heat
removed H-O-H
oxidation reactions
reagents: acidified potassium dichromate VI K2Cr2O7 and h2so4
conditions: dependent on the alcohol and target product
observations: orange cr2o72- turns green cr3+
primary alcohol oxidation
RCh2OH -> RCHO (aldehyde) -> RCOOH (carboxylic acid)
ethanol can be oxidised to ethanal and then oxidised again to ethanoic acid
overall: C2H5OH + 2[O] -> Ch3COOH + H2O
distillation of primary alcohol
aldehyde produced (lower BP so forms a gas at lowest temp and is collected as the distillate)
reflux of primary alcohol
carboxylic acid produced ( continuous evaporation and condensation - the aldehyde produced reacts further)
secondary alcohols
go to ketones
R2CHOH -> R2CO
further oxidation not possible due to C-C bonds being too strong to break
tertiary alcohols
no reaction as bonds too strong to break. dichromate solution stays orange
distinguishing alcohols
- to identify a 3 from a 1 and 2 add k2cr2o7 and 1 and 2 will turn from orange to green
- to identify 1 and 2 oxidise the alcohol fully (reflux) and isolate the product (distill)
- 1 alcohol turns to carboxylic acid which turns litmus pink
- 2 turns to ketone only
substitution reactions forming haloalkanes
alcohols react with hydrogen halides to form haloalkanes. the hydrogen halide is formed insitu from the reaction of the sodium halide with sulphuric acid.
reagents: h2so4 and nabr(s)
conditions: Heat and reflux
formation of HBr: h2so4 + nabr -> nahso4 + hbr
then: ch3ch2oh + hbr -> ch3ch2br + h2o
overall: ch3ch2oh + h2so4 + nabr -> ch3ch2br + nahso4 + h2o
heating under reflux
used for prolonged heating at a well defined temperature without loss of reactants or products
distillation
used to remove the desired product from a mixture of organic compounds either during or after a reaction
separating immiscible liquids
using a separating funnel. the organic layer is usually the upper layer. the tap is opened and then closed when the entire aqueous layer has been removed
shaking with a base
eg nahco3(aq) to neutralise an acid catalyst -> carried out in a separating funnel so the aqueous and organic layers can be separated using a funnel
using anhydrous salt as a drying agent
removes water from an organic liquid and then is filtered
redistillation
used to purify an organic liquid by only collecting the fraction at the correct boiling point of the desired compound
