Topic 5.5 Organic chemistry analysis, synthesis and application


(a) Organic analysis(i) Functional groups

functional group reagent conditions result of positive test
-C=C-  bromine in inert solvent  orange bromine decolourised 
-Cl  warm with NaOH(aq)

add HNO3 then AgNO3 then NH3 (aq) 

white ppt. of AgCl

soluble in dil. NH3 (aq) 

-Br  warm with NaOH(aq)
add HNO3 then AgNO3 then NH3 (aq) 
cream ppt. of AgBr
soluble in conc NH3 (aq) 
-I  warm with NaOH(aq)
add HNO3 then AgNO3 then NH3 (aq) 
yellow ppt. of AgI
Insoluble in conc NH3 (aq) 
-OH  add solid PCl5 acrid steamy fumes of HCl
primary
-CH2-OH
warm with acidified aqueous 
potassium dichromate K2Cr2O7
orange colour changes to 
green product tests +ve for
-CHO
secondary
-CH-OH
  |
warm with acidified aqueous conc.
potassium dichromate K2Cr2O7
orange colour changes to green 
product does not test +ve for
-CHO
tertiary
  |
-C-OH
  |
warm with acidified aqueous conc.
potassium dichromate K2Cr2O7
no change
-C=O 
  |
add 2,4-dinitrophenylhydrazine

warm with Fehling's solution 

yellow ppt. of hydrazone

no change in blue colour 

-CHO  add 2,4-dinitrophenylhydrazine

warm with Fehling's solution 

warm with AgNO3(aq) in NH3(aq)

yellow ppt. of hydrazone

red/brown ppt of Cu2O forms
 
silver mirror forms

-COOH  add NaHCO3 effervescence CO2 formed 
-C=O   -CH-OH
  |           |
 CH3      CH3  
ethanal ethanol
add iodine then aqueous NaOH yellow ppt. and
antiseptic smell of iodoform

Task 5.5a.1 Complete a copy of the table below with names and structural formulae:

functional group compound giving positive test similar compound with giving negative test
 

 

   

Task 5.5a.2 Write an equation for each of the reactions above.
Task 5.5a.3 Predict the result of carrying out an iodoform test on the following compounds: methanol, ethanol, propan-1-ol, methanal, ethanal, propanone.

(ii) Interpretation of physical data

* A pure solid can be identified by its melting temperature, a pure liquid by its boiling temperature. Aldehydes and ketones produce solid derivatives with 2,4-dinitrophenylhydrazine (hydrazones) which have high melting points. The melting points of these derivatives can be used to identify the original ketone or aldehyde.
substance boiling point/oC melting point of hydrazone/oC
propanal 48 156
pentan-2-one 102 141
pentan-3-one 102 156

What are the identities of the following:
Compound A forms a yellow precipitate with  2,4-dinitrophenylhydrazine and an orange precipitate with Fehlings solution. When the yellow precipitate is purified it melts at 156oC.
Compound B forms a yellow precipitate with  2,4-dinitrophenylhydrazine but does not form a silver mirror with Tollens reagent. When the yellow precipitate is purified it melts at 156oC.

The percentage by mass of Carbon and Hydrogen in a hydrocarbon can be found by the following method; 
A known mass of the compound heated in a stream of pure, dry oxygen in the presence of copper (II) oxide when hydrogen and carbon are oxidised, to steam and carbon dioxide respectively.  The steam absorbed in weighed calcium chloride tubes, and carbon dioxide absorbed in weighed bulbs of concentrated potassium hydroxide solution.  The masses of carbon and hydrogen in the sample are found from the increases in mass.

Draw suitable labelled apparatus for the above procedure.
If 7.80g of a hydrocarbon causes calcium chloride to increase in mass by 5.40g and potassium hydroxide to increase in mass by 26.4g what was the formula of the hydrocarbon?

5.5a(iii)(a) interpret simple fragmentation patterns from a mass spectrometer
An organic compound produces ions in a mass spectrometer.  The ions generate pulses of electric current which are sent as signals to a computer (or chart plotter) to be displayed as a mass spectrum.  On the spectrum the large peak on the right is the parent molecular ion and this indicates the relative molecular mass of the compound.  In the spectrometer the molecules are fragmented into positive ions which form a pattern which depends on the structure of the molecule. e.g.  ether

Another compound of relative molecular mass 46 also contains carbon, hydrogen and oxygen has the mass spectrum below.  Identify each fragment and the structural formula.

Sketch the mass spectrum of propanal showing the masses of each fragment.
Task 5.5a(iii)a

5.5a (iii)
(b) interpret simple infra-red spectra
The bonds in organic molecule absorb infra-red radiation.  This happens when the frequency of the radiation matches the natural frequency of vibrations in the bonds.  These might be stretching, or bending vibrations.  A spectrometer shines infra-red light at a sample of an organic material and measures how much of the light is absorbed.  A measure of the frequency (wavenumber) is displayed in the spectrum.  Each bond has its own frequency (wavenumber) and this can be used to identify the bonds present in a compound.

bond

wavenumber/cm-1 seen on spectrum

C-H 2840 - 3095
 C-C 1610 - 1680
C=O 1680 - 1750
C-O 1000 - 1300
C-Cl 700 - 800
O-H 3233 - 3550
2500 - 3300
N-H 3100 - 3500

Infra-red spectrum for propanone

Is substance A in the infra-red spectrum below most likely to be ethyl ethanoate or butane?

Task 5.5aiiib

5.5a(iii)(c) Low resolution nuclear magnetic resonance spectra (NMR)
Hydrogen atoms can be detected using this sort of spectrometry.  The nucleus of a hydrogen atom, the proton, spins and so has a magnetic moment.  This can be aligned or not aligned with a magnetic field .  When electromagnetic radiation of the right frequency is applied resonance occurs and the protons flip from one state to the other and absorb energy.  This absorption of energy is used to detect protons in organic compounds.   The exact resonance frequency for a proton (hydrogen atom) depends on its environment.  For example the frequency is different for hydrogen atoms in CH3, CH2 , C6H5- and in O-H.  Trimethylsilane,  TMS, is used as a standard.  The distance in the spectrum from the TMS peak is called the chemical shift.

Type of proton Chemical Shift (ppm)
R-CH3 0.9
R-CH2 1.3
R-CH2-O- 4.0
C6H5- 7.5
-O-H 5.0
-CHO 9.5



To which functional groups do the protons in the following NMR spectrum belong?  Identify the compound.

Sketch an NMR spectrum for propanal.

An excellent guide to low resolution nmr.

5.5a (iii) (d) The interpretation of simple ultra-violet/visible spectra.
 Some chemical structures absorb electromagnetic radiation in the ultra violet part of the spectrum.  These include conjugated (contain alternate double and single bonds) dienes.  E.g. 1,3-butadiene.  The ultraviolet absorption spectrum for 2,5-dimethyl-2,4-hexadiene is shown below.

Ultra-violet wavelengths are from about 200nm to about 400nm.  Visible light has wavelength between 400nm and 800nm.  b-carotene, which gives carrots their orange colour absorbs at 497nm.  Lycopene, which gives tomatoes their red colour, absorbs at 505nm.  Both of these compounds have 11 conjugated double bonds.

5.5b(i)Pathways for organic synthesis

 Alkanes
Organic reactant Reagent Conditions Organic Products
Alkane 
ethane 
C2H6
halogen 
bromine 
Br2
UV light
inert solvent
halogenoalkane 
bromoethane 
CH3CH2Br
Alkane 
ethane 
C2H6
halogen 
chlorine 
Cl2
UV light
inert solvent
halogenoalkane 
chloroethane 
CH3CH2Cl
Alkenes
Organic reactant Reagent Conditions Organic Products
Alkene 
ethene 
C2H4
halogen 
bromine 
Br2
inert solvent halogenoalkane 
1,2-dibromoethane 
CH2BrCH2Br
unsymmetric alkene 
prop-1-ene 
CH3CHCH2
hydrogen halide 
hydrogen bromide 
HBr
inert solvent halogenoalkane 
2-bromopropane 
CH3CHBrCH3
Alkene 
ethene 
C2H4
potassium manganate(VII) 
KMnO4
alkaline solution ethane-1,2-diol 
CH2OHCH2OH
Arenes
Organic reactant Reagent Conditions Organic Products
arene 
benzene 
C6H6
nitric acid 
sulphuric acid
heat under reflux 
below 60o
concentrated acids
nitrobenzene 
C6H5NO2
arene 
methylbenzene 
C6H5CH3
potassium manganate VII alkaline conditions 
heat under reflux
benzoic acid 
C6H5COOH
arene 
benzene 
C6H6
chloroalkane 
chloromethane 
CH3Cl
anhydrous aluminium chloride as catalyst arene 
methylbenzene 
C6H5CH3
arene 
benzene 
C6H6
acid chloride 
ethanoyl chloride 
CH3COCl
anhydrous aluminium chloride as catalyst ketone 
methylphenylketone 
C6H5COCH3
arene 
benzene 
C6H6
halogen
bromine
Br2
anhydrous aluminium chloride as catalyst halogenoarene
bromobenzene
C6H5Br
diazonium salt 
benzen diazonium chloride C6H5N2+Cl-
phenol 
C6H5OH
below 5oC azo dye 
C6H5N2C6H5OH
diazonium salt 
benzene diazonium chloride C6H5N2+Cl-
2-naphthol below 5oC azo dye 
C10H6(OH)N2C6H5
phenylamine 
C6H5NH2
nitrous acid 
HNO2
below 5o
NaNO2 and dil HCl
diazonium salt 
benzenediazonium chloride C6H5N2+Cl-
nitrobenzene 
C6H6NO2
concentrated hydrochloric acid and tin HCl and Sn heat under reflux phenylamine 
C6H5NH2
phenol
C6H5OH
sodium hydroxide aqueous sodium phenoxide
C6H5ONa
phenol
C6H5OH
bromine aqueous 2,4,6-tribromophenol
C6H2Br3OH
phenol
C6H5OH
ethanoyl chloride   ester
ethylbenzoate
C6H5COOC2H5

Primary alcohols
Organic reactant Reagent Conditions Organic Products
primary alcohol 
ethanol 
C2H5OH
aqueous potassium dichromate VI 
dilute sulphuric acid
distil product mild conditions   aldehyde 
ethanal 
CH3CHO
primary alcohol 
ethanol 
C2H5OH
aqueous potassium dichromate VI 
dilute sulphuric acid
boil under reflux forcing conditions carboxylic acid 
ethanoic acid 
CH3COOH
primary alcohol 
ethanol 
C2H5OH
hydrogen halide 
hydrogen bromide 
HBr
heat under reflux HBr formed in situ from KBr and conc. H2SO4 halogenoalkane 
bromoethane 
C2H5Br
primary alcohol 
ethanol 
C2H5OH
phosphorus and iodine red phosphorus halogenoalkane
iodoethane
C2H5I
primary alcohol 
ethanol 
C2H5OH
carboxylic acid 
ethanoic acid 
CH3COOH
concentrated H2SO4 ester 
ethyl ethanoate 
CH3COOC2H5
primary alcohol 
ethanol 
C2H5OH
acid chloride 
ethanoyl chloride 
CH3COCl
. ester 
ethyl ethanoate 
CH3COOC2H5
primary alcohol 
ethanol 
C2H5OH
phosporus pentachloride .dry  halogenoalkane 
chloroethane 
C2H5Cl
Secondary alcohols
Organic reactant Reagent Conditions Organic Products
secondary alcohol 
propan-2-ol 
CH3CH(OH)CH3
aqueous potassium dichromate VI 
sulphuric acid
conc. acid 
heat under reflux
ketone 
propanone 
CH3COCH3
secondary alcohol 
propan-2-ol 
CH3CH(OH)CH3
hydrogen halide 
hydrogen bromide 
HBr
heat under reflux HBr formed in situ from KBr and conc. H2SO4 halogenoalkane 
2-bromopropane 
CH3CH2(Br)CH3
secondary alcohol 
propan-2-ol 
CH3CH(OH)CH3
carboxylic acid 
ethanoic acid 
CH3COOH
concentrated H2SO4 ester 
2-propylethanoate 
CH3COOCH(CH3)2
secondary alcohol 
propan-2-ol 
CH3CH(OH)CH3
acid chloride 
ethanoyl chloride 
CH3COCl
. ester 
2-propylethanoate 
CH3COOCH(CH3)2
secondary alcohol 
propan-2-ol 
CH3CH(OH)CH3
phosphorus and iodine red phosphorus halogenoalkane
2-iodopropropane
CH3CH(I)CH3
secondary alcohol 
propan-2-ol 
CH3CH(OH)CH3
phosporus pentachloride dry halogenoalkane 
2-chloropropane 
CH3CH2(Cl)CH3
Tertiary alcohols
Organic reactant Reagent Conditions Organic Products
tertiary alcohol 
2-methylpropan-2-ol 
(CH3)3OH
hydrogen halide 
hydrogen bromide 
HBr
heat under reflux HBr formed in situ from KBr and conc. H2SO4 halogenoalkane 
2-bromo-2-methylpropane 
(CH3)3Br
tertiary alcohol 
2-methylpropan-2-ol 
(CH3)3COH
carboxylic acid 
ethanoic acid 
CH3COOH
heat 
concentrated H2SO4
ester 
CH3COOC(CH3)3
tertiary alcohol 
2-methylpropan-2-ol 
(CH3)3OH
acid chloride 
ethanoyl chloride 
CH3COCl
. ester 
CH3COOC(CH3)3
tertiary alcohol 
2-methylpropan-2-ol 
(CH3)3OH
phosphorus and iodine red phosphorus halogenoalkane
2-iodo-2-methylpropropane
(CH3)3I
tertiary alcohol 
2-methylpropan-2-ol 
(CH3)3OH
phosporus pentachloride dry halogenoalkane
(CH3)3Cl
Carboxylic acids
Organic reactant Reagent Conditions Organic Products
carboxylic acid 
ethanoic acid 
CH3COOH
alcohol 
ethanol 
C2H5OH
heat 
concentrated H2SO4
ester 
ethyl ethanoate 
CH3COOC2H5
carboxylic acid 
ethanoic acid 
CH3COOH
lithium aluminium hydride dissolved in dry ether alcohol 
ethanol 
C2H5OH
carboxylic acid 
ethanoic acid 
CH3COOH
phosphorus pentachloride dry acid chloride 
ethanoyl chloride 
CH3COCl
carboxylic acid 
ethanoic acid 
CH3COOH
sodium carbonate or 
sodium hydrogencarbonate
  salt
sodium ethanoate
CH3COONa
Primary amines
Organic reactant Reagent Conditions Organic Products
primary amine 
ethylamine 
C2H5NH2
acid 
hydrochloric acid 
HCl
aqueous solution salt 
alkyl ammonium chloride 
C2H5NH3+Cl-
primary amine 
ethylamine 
C2H5NH2
acid chloride
ethanoyl chloride
CH3COCl
  amide
N-ethylethanamide 
CH3CONHC2H5
Halogenoalkanes
Organic reactant Reagent Conditions Organic Products
halogenoalkane 
bromoethane 
C2H5Br
hydroxide ion 
sodium hydroxide 
NaOH
aqueous solution alcohol 
ethanol 
C2H5OH
halogenoalkane 
bromoethane 
C2H5Br
hydroxide ion 
potassium hydroxide 
KOH
heat and distil off product 
ethanolic solution
alkene 
ethene 
C2H4
halogenoalkane 
bromoethane 
C2H5Br
cyanide ion 
potassium cyanide 
KCN
heat under reflux in ethanolic solution nitrile 
propanonitrile 
C2H5CN
halogenoalkane 
bromoethane 
C2H5Br
ammonia heat with concentrated ammonia in a sealed tube amine 
ethylamine etc 
C2H5NH 2
(C2H5)2NH etc
halogenoalkane 
bromoethane 
C2H5Br
magnesium 
Mg
in solution in dry ether grignard reagent 
ethylmagnesiumbromide 
C2H5MgBr
Grignard reagent
Organic reactant Reagent Conditions Organic Products
grignard reagent 
ethylmagnesiumbromide 
C2H5MgBr
methanal 
HCHO
in ether solution followed by dilute acid primary alcohol 
propan-1-ol 
C2H5CH2OH
grignard reagent 
ethylmagnesiumbromide 
C2H5MgBr
aldehyde 
ethanal 
CH3CHO
in ether solution followed by dilute acid secondary alcohol 
butan-2-ol 
CH3CH2CH(OH)CH3
grignard reagent 
ethylmagnesiumbromide 
C2H5MgBr
ketone 
propan-2-one 
(CH3)2CO
in ether solution followed by dilute acid tertiary alcohol 
2-methylpropan-2-ol 
(CH3)3OH
grignard reagent 
ethylmagnesiumbromide 
C2H5MgBr
carbon dioxide 
CO2
in ether solution followed by dilute acid carboxylic acid 
propanoic acid 
C2H5COOH
Ketones
Organic reactant Reagent Conditions Organic Products
ketone 
propanone 
(CH3)2CO
hydrogen cyanide in alkaline conditions 
potassium cyanide used as source of HCN
alcohol, nitrile
2-hydroxy -2-methylpropanonitrile 
CH3C(OH)(CH3)CN
ketone 
propanone 
(CH3)2CO
2,4- 
dinitrophenylhydrazine 
C6H3(NO2)2NHNH2
dil. sulphuric acid 2,4- 
dinitrophenyhydrozone 
C6H3(NO2)2NHNC(CH3)2
ketone 
propanone 
(CH3)2CO
iodine or sodium iodide and chloate(I) alkaline conditions halogenoalkane
tri-chloromethane 
(iodoform)
ketone 
propanone 
(CH3)2CO
sodium borohydride 
NaBH4
aqueous solution secondary alcohol 
propan-2-ol 
CH3CH(OH)CH3
ketone 
propanone 
(CH3)2CO
Lithium aluminium hydride 
LiAlH4
dry
ether solution
secondary alcohol 
propan-2-ol 
CH3CH(OH)CH3
Aldehydes
Organic reactant Reagent Conditions Organic Products
aldehyde 
ethanal 
CH3CHO
hydrogen cyanide in alkaline conditions 
potassium cyanide used as source of HCN
alcohol, nitrile 2-hydroxypropanonitrile 
CH3CH(OH)(CN)
aldehyde 
ethanal 
CH3CHO
2,4- 
dinitrophenylhydrazine 
C6H3(NO2)2NHNH2
dilute sulphuric acid 2,4- 
dinitrophenyhydrozone 
C6H3(NO2)2NHNCHCH3
aldehyde 
ethanal 
CH3CHO
ammoniacal silver nitrate solution 
(Tollen's reagent)
warm in water bath carboxylic acid 
ethanoic acid 
CH3COOH
aldehyde 
ethanal 
CH3CHO
iodine or sodium iodide and chlorate(I) alkaline conditions halogenoalkane
tri-chloromethane 
(iodoform)
aldehyde 
ethanal 
CH3CHO
sodium borohydride 
NaBH4
aqueous solution alcohol 
ethanol 
C2H5OH
aldehyde 
ethanal 
CH3CHO
Lithium aluminium hydride 
LiAlH4
dry
ether solution
alcohol 
ethanol 
C2H5OH
Acid chlorides
Organic reactant Reagent Conditions Organic Products
acid chloride 
ethanoyl chloride 
CH3COCl
water 
H2O
. carboxylic acid 
ethanoic acid 
CH3COOH
acid chloride 
ethanoyl chloride 
CH3COCl
alcohol
ethanol
CH3CH2OH
no water ester 
ethyl ethanoate 
CH3COOC2H5
acid chloride 
ethanoyl chloride 
CH3COCl
ammonia 
NH3
conc. ammonia amide 
ethanamide 
CH3CONH2
acid chloride 
ethanoyl chloride 
CH3COCl
primary amine 
phenylamine 
C6H5NH2
. amide
N-phenylethanamide 
CH3CONHC6H5
Nitriles
Organic reactant Reagent Conditions Organic Products
nitrile 
ethanonitrile 
CH3CN
water acidic heat under reflux carboxylic acid 
ethanoic acid 
CH3COOH
nitrile 
ethanonitrile 
CH3CN
water alkaline heat under reflux salt 
sodium ethanoate 
CH3COONa
nitrile 
ethanonitrile 
CH3CN
lithium aluminium hydride dry ether solution amine 
ethylamine 
CH3CH2NH2
Amides
Organic reactant Reagent Conditions Organic Products
amide 
ethanamide 
CH3CONH2
phosphorus V oxide . nitrile 
ethanonitrile 
CH3CN
amide 
ethanamide 
CH3CONH2
bromine aqueous alkali amine 
methylamine 
CH3NH2
Esters
Organic reactant Reagent Conditions Organic Products
ester 
ethyl ethanoate 
CH3COOC2H5
water conc. sulphuric acid alcohol, acid
ethanol, ethanoic acid 
C2H5OH, CH3COOH
ester 
ethyl ethanoate 
CH3COOC2H5
water aqueous sodium hydroxide alcohol
ethanol, sodium ethanoate
C2H5OH, CH3COONa

5.5b (ii) Apparatus and safety in organic synthesis
Organic compounds may be hazardous because of:
    Flammability.
Use in small amounts avoids the undue risk of fire.  Avoid naked flames.  Use electrical heaters
    Toxicity
The use of small amounts, fume cupboards, gloves and normal laboratory safety procedures  reduces the risk of harmful amounts of a chemical entering the body by inhalation, ingestion or by skin absorption.
    Non-biodegradability
Some substances do not decay naturally in the environment.  The hazard is reduced by using small quantities, and pouring waste solvents in a suitable container rather than pouring it down the sink.
When carrying out organic reactions safety goggles should always be worn.
Some common apparatus for organic preparation: Quickfit apparatus such as a

pear shaped flask


Liebig condenser   T connector                                  thermometer holder
      
Separating funnel   delivery tubes                            
         

5.5b (iii) Practical techniques
Mixing can involve adding a liquid to a solid or another liquid using a separating funnel.
Mixing can involve adding a gas to a liquid through a delivery tube into the liquid.

boiling under reflux, 
Practice setting up reflux apparatus.  (Needs Microsoft Word.)
         
fractional distillation, 


Filtration under reduced pressure (filter pump and Buchner funnel an flask), 
 
Recrystallisation
This is used to purify an impure organic solid.
1. Choose a suitable solvent.  The solvent is suitable if the product is insoluble in the cold solvent but soluble in the hot solvent.
2. Dissolve the impure sample in the minimum volume of hot solvent.
3. Filter the hot solution through hot apparatus and collect the filtrate.  This removes solid impurities which were insoluble in the solvent.
4. Allow the filtrate to cool so that crystals of the product form.
5. Again filter the mixture under reduced pressure.  Soluble impurities are now removed.
6. Wash the residue with a little cold solvent.
7. Dry the residue which should then be the pure product.

Describe with a series of labelled diagrams how compound X can be purified using the information below.

Solvent Solubility of X in cold solvent Solubility of X in hot solvent
water 0.01 gcm-3 0.02 gcm-3
50% water 50% ethanol 0.03 gcm-3 0.06 gcm-3
ethanol 0.06 gcm-3 3.65 gcm-3


Determination of melting temperature
A pure solid has a sharp melting point which can be found in a data book.  If a solid product has been purified it can be identified from its melting point or if we know what it is we can tell if it is pure.  Impurities lower the melting point.

 
1. A liquid is chosen for the boiling tube so that its boiling point is well above the melting point of the solid.
2. The apparatus is set up as shown.
3. The apparatus is heated gently with stirring until the first solid is seen to melt when the temperature is recorded.
4. After the sample is melted it is allowed to cool.
5. When the first crystals of solid appear in the sample the temperature is recorded again.

Comment on the purification of X by student A and B given the data below.

MP of X in data book/oC MP of X by student A/oC Mp of X by student B/oC
128 129 126

Determination of boiling temperature
   In both apparatus the bulb of the thermometer is below the surface of the liquid but not touching the side of the glass.
Heating with a variety of sources
Heating with a naked Bunsen flame is a fire hazard with flammable organic compounds.  Alternatives include using an electrically heated hot plate, using an electrical heating mantle which can surround a round bottom flask or using a water bath.
Other apparatus

5.5b(iv) The principles of fractional distillation

Raoult's Law

Raoult's law states that the saturated vapour pressure of a component in a mixture is equal to the product of the mole fraction of that component and the saturated vapour pressure of that component when pure.

For a mixture of A and B obeying Raoult's law pA = poA * XA

where pA = partial vapour pressure of A in the solution

poA = vapour pressure of pure A

XA = mole fraction of A in the solution

mole fraction of A in mixture A+B, XA = nA/nA+nB

nA= no. moles A

nB= no. moles B.

pT = poA * XA + poB * XB

where pT = total vapour pressure

This law is obeyed by mixtures of similar compounds. They form IDEAL SOLUTIONS. The substances A and B form an ideal solution if the intermolecular forces A----A, A----B, B----B are all equal. As these forces are alike the vapour pressure of an ideal mixture is not increased or decreased because the escape of molecules is neither helped nor hindered. Also for an ideal solution there is:

(1) no enthalpy change on mixing;

(2) no volume change on mixing.

Diagrams for Idea Solutions


Task 5.5biv.1 Pure hexane has a vapour pressure of about 340 mmHg and pure pentane has a vapour pressure of about 310 mmHg.  Sketch a vapour pressure composition graph and a boiling point composition graph for a mixture of these two compounds.

Fractional distillation

This technique has a number of important applications:

(1) used to separate the components of liquid air; The air is compressed and cooled to liquefy it. Fractions are oxygen -183oC, argon -186oC, and nitrogen -196oC.

(2) used to separate fractions from petroleum; The fractions are bitumen >350oC, fuel oil 300 oC, diesel 240 oC, kerosene 200 oC, naptha 120 oC, petrol 40 oC, LPG <25 oC.

(3) used to produce whisky and other alcoholic drinks. Fractions are ethanol 78 oC and water 100 oC.

Principles of fractional distillation

Fractional distillation relies on the fact that the composition of a two liquid mixture in equilibrium with its vapour is different in the liquid and vapour phases. Real mixtures are not ideal and show deviations from Raoult's law.

The particles of one liquid surround those of the other and change the forces on them so affecting their tendency to escape from the liquid.

(1) Negative from deviations Raoult's law:

If the attractive forces between the different particles from the two liquids are stronger than the attractive forces in pure liquids, then the particles will be held in the liquid more strongly. Therefore fewer particles will escape, thus the vapour pressure will be lower than predicted by Raoult's law. Boiling points for any composition of this mixture will be higher than predicted.

(2) Positive deviations from Raoult's law:

If the attractive forces between the different particles from the two liquids are weaker than the attractive forces in the pure liquids, then the particles will be held in the liquid less well. Therefore more particles will escape, thus the vapour pressure will be higher than predicted by Raoult's law. Boiling points for any composition of this mixture will be lower than predicted.

A plot of temperature against composition is needed to explain how fractional distillation works.

For a liquid mixture of composition C1 it will boil at temperature T. It will be in equilibrium with vapour at composition C2. Vapour C2 will condense and then boil to give a mixture richer again in B. Eventually the distillate will be pure B. The residue in the flask will be A.
Task 5.5b(iv).2 Methanol has a boiling point of 338K and ethanol has a boiling point of 351K.  Sketch a temperature composition curve for this mixture and use it to explain how they can be separated.  State the names of the distillate and the residue.

5.5c(i) The solubility of pharmaceuticals

Drugs can be made to target fatty tissue like lipids in the body.  These chemicals will have a large number of CH3 or CH2 groups.  The following pharmaceutical inhibits cholesterol synthesis and acts within fatty tissue.

Other pharmaceuticals like penicillin are targeted at non-fatty tissue and must be more water soluble.  For this the molecules must contain hydroxyl groups to form hydrogen bonds to water or an ionic group.  Penicillin-G shown below prevents the growth of cell walls in bacteria and so is an antibiotic.  One draw back is that it is very soluble (due to its ionic group) and is eliminated very rapidly by the kidneys so has to be given in large doses.

Task  5.5c(i).1 Describe the inplications for solubility of ziagen, Aczone Gel
 and other drugs found at
http://www.rxlist.com/drugs/alpha_a.htm


5.5c(ii) The use of organic fertilisers
Inorganic fertilizers like ammonium nitrate and ammonium sulphate are very soluble, can be leached easily which may lead to the eutrophication of rivers and lakes.  Other problems are that they decrease soil pH, and for osmotic reasons, cause burning and foliage decay in some plants.
Urea is an organic fertilizer which only slowly releases nitrogen to the soil, has a high proportion of nitrogen and does not immediately change soil pH.  It undergoes hydrolysis in the soil to form ammonia.
H2N-CO-NH2 + H2O -----> CO2 +2NH3
The process is catalysed by urease made by soil bacteria.  The ammonia is converted to nitrates in the soil which can then be absorbed by plants.  Alternatives include manure, hoof and horn, or dried blood.
Task 5.5c(ii).1 State steps in the process of eutrophication.
Task 5.5c(ii).2 Explain equilibrium giving low pH.
Task 5.5c(ii).3 Explain water loss to soil through osmosis.

5.5c(iii) Uses of esters, oils and fats
Esters find a use as food flavourings because of their characteristic fruity smells.  Ethyl methanoate is used in raspberry essence.  3-methylbutylethanoate is used in pear essence.
Margarine is made from an ester of a long chain carboxylic acid containing carbon to carbon double bonds.  This unsaturation makes the original ester a liquid oil.  A typical source is peanuts or sunflower seeds.  The amount of unsaturation is reduced in an addition reaction with hydrogen using a nickel catalyst.  The resulting edible solid fat is margarine.  The nature of fats and oils and saturation.  (requires Powerpoint)
Essential oils are so called because they are extracted from the essence of plants.  The bark of Birch trees can be used to make oil of wintergreen, of use in relief of muscle pain.  This is methyl-2-hydroxylbenzoate.  Many of these essential oils are hydrocarbons or simple derivatives.  Limonene can be separated from citrus peel.  It is a methyl cyclohexene with a CH3CCH2 group attached.
The chemistry involved in the production of soaps and soapless detergents
Detergent- A substance that acts as a cleaning agent, improving the ability of water to wash things.

Why is a detergent necessary?
Though very effective at removing water-soluble dirt (hydrophilic), water is poor at removing material that does not dissolve in water (hydrophobic). The main problem is caused by greasy fats and carboxylic acids, which tend to bind other “dirt” particles to the skin or fabric. This prevents them being washed away by water. A detergent works by enabling water to mix with and remove greasy materials.

Types of detergents
(i) Soapy detergents (ii) Soapless detergents
All detergents form a lather easily when shaken with PURE water. But the soapy detergent forms a lather with greater difficulty with tap water (tap water 300 mg/litre Ca2+ ); that is, a lather is only possible if a lot of soap is used, so in effect the soap is wasted in hard water. On the other hand, the soapless detergent forms a lather readily in hard water. These differences can be explained if the chemical nature of these materials is considered.

Soapy detergents
Animal fats are in fact esters made from long chain carboxylic acids and the compound glycerol. The alkaline hydrolysis or saponification (soap making) of animal fats gives glycerol and the sodium or potassium salt of the carboxylic acid. It is the salt which is the soap.
            O
               //
      R-C
               \
                O-Na+ (R is the hydrophobic hydrocarbon chain)
(e.g. toilet soap, gels in shampoos, shaving creams especially K+ salts)

The charged -COO- group (carboxylate ion) at the end of a soap enables it to dissolve in water. Water molecules attach themselves to this end of the soap molecule via hydrogen bonds and the oil/ grease molecules attach around the alkyl with Van der Waals forces. The forces between water and grease are thus much increased, so that the grease is lifted off the surface in the form of small globules. These can be rinsed away.

Advantages:
Reaction of soap is more favourable with skin- more agreeable than with other detergents.

Disadvantages:
They do not remove some types of dirt well
They do not clean very efficiently in hard water, as they react with calcium and magnesium ions to form insoluble calcium/ magnesium salts - scum.  The scum is insoluble calcium or magnesium salt.
2R-COONa (aq) + Ca2+(aq) -----> Ca(R-COO)2(s) + 2Na+(aq)
Made from vegetable/ animal fats - often expensive

Soapless detergents
These are very much synthetic materials, they work in a similar way to soaps but a sulphonate group (-SO2-O-) or sulphate group (-O-SO2-O) replace the carboxylate group as the hydrophilic component.  The reaction of oil and concentrated sulphuric acid will produce a soapless detergent.

              O                                       O
               ||                                        ||
       R-O-S-O-Na+      also  R-C6H4-S-O-Na+
               ||                                        ||
              O                                       O

(e.g. dishwasher powder, washing powder)
The process of sulphonation gives rise to these detergents.

Advs:
The Ca2+/ Mg2+ compounds formed with soapless detergents are more soluble in water. Thus there is no wastage in hard water.
Made from the by-products of oil refining, therefore quite cheap to make.

Disadvs:
First soapless detergents were not biodegradable- the side chains cannot be degraded. Thus when discharged in rivers, the microorganisms present in the water can not destroy them. Thus causing foaming in rivers and streams. This has been overcome by modifying the structure of the detergents.  Chains without branches were introduced which are biodegradable.

Task 5.5d(iii) In the following molecules on the web page below explain if they are soapy or soapless, which part of the molecule dissolves in water and which part in oil and state what if anything would happen if calcium ions were present.
http://www.elmhurst.edu/~chm/vchembook/558detergent.html


5.5d(iv) Uses and properties of polymers

Monomer
Polymer
Properties
Uses
ethene 
CH2=CH2
polyethene
-(CH2-CH2)n-
low mp. soft, flexible plastic bags, squeezy bottles, washing up bowls, buckets
propene
CH3CH=CH2
polypropene -(-CH(CH3)
-CH2-)n-

high tensile strength
water repellent
low melting point
low density, 
tougher than polyethene

ropes, sacks
carpets and curtains

ropes

chloroethene (vinylchloride)
CHCl=CH2
polychloroethene
(polyvinylchloride PVC)
-(-CHCl-CH2-)n-
harder, less flexible than polyethene, electrical insulator raincoats, guttering, floor tiles, packaging, covering electrical wire
tetrafluroethene 
CF2=CF2
polytetrafluroethene 
-(-CF2-CF2-)n-
low friction on surface non-stick coatings on frying pans
phenylethene (styrene)
C6H5CH=CH2
poly(phenylethene)
polystyrene 
-(-CH(C6H5)-CH2-)n-
softens at low temperature
thermal insulator
moulded shapes
coffee cups
foam packaging
ethane-1,2-diol 
H-O-CH2-CH2-O-H
benzene- 
1,4-dicarboxylic acid 
H-O-COC6H5-CO-O-H
polyester
terylene 
H-O-(COC6H5-CO-
O-CH2-CH2-O)n-H
stability to light
resistance to abrasion
high tensile strength
low melting point
net curtains
conveyor and drive belts
ropes, safety belts
permanent pleating
hexanedioic acid 
HOOC-(CH2)4-COOH
1,6-diaminohexane 
H2N-(CH2)6-NH2
polyamide (nylon-6.6) 
HO-(OC-(CH2)4-CO-
HN-(CH2)6-NH)n-H
high tensile strength
low melting point
low affinity for water
resistance to abrasion
ropes, parachutes,  stockings
permanent pleating
easy drying garments
conveyor belts, brushes