Organic Synthesis and Analysis

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Organic Analysis and Synthesis ­ Answers
1. Calculating percentage composition is normally carried out by combustion analysis. A known mass of the
substance is burned and masses of water and carbon dioxide are produced. To calculate percentage
composition:
Mass of carbon dioxide mass of carbon % carbon
Mass of water mass of hydrogen % hydrogen
% Oxygen = 100 ­ (% carbon + % hydrogen)
2. Calculating percentage composition:
1) Calculate % carbon: 6.60 × 12
44 = 1.80g of carbon
% Carbon = 1.80
2.90 × 100 = 62.1%
2 = 0.300g of hydrogen
2) Calculate % hydrogen: 2.70 × 18
% Hydrogen = 0.300
2.90 × 100 = 10.3%
3) Calculate % oxygen: 100 - (62.1 + 10.3) = 27.6%
3. The empirical formula of a compound is obtained by dividing the percentage composition of each element by its
atomic mass and then dividing through by the smallest.
% Divide by Ar Divide by smallest
Carbon 62.1 62.1/12.0 = 5.175 3.0
Hydrogen 10.3 10.3/1.0 = 10.3 5.97 = 6.0
Oxygen 27.6 27.6/16.0 = 17.25 2.0
4. Finding the empirical formula:
5. The molar mass can be found from the m/e value of the molecular ion in the mass spectrum. If the unknown can
be titrated, the number of moles of a given mass can be estimated.
6. The test for the C=C group is to add bromine water. Compounds that contain a C=C group quickly decolourise the
brown bromine water and do not give a precipitate.
7. Halogenoalkanes contain a halogen atom, which on warming with aqueous NaOH, is removed by hydrolysis.
Excess dilute nitric acid is added to this solution to neutralise the NaOH. On addition of a solution of silver nitrate,
a precipitate is obtained:
A white precipitate soluble in dilute ammonia solution proves the presence of chlorine in the organic
compound.
A cream precipitate, insoluble in dilute ammonia but soluble in concentrated ammonia, shows the
presence of bromine in the organic compound.
A pale yellow precipitate, insoluble in both dilute and concentrated ammonia, proves the presence of
iodine in the original organic compound.
8. Testing for alcohols:
Add PCl5: Compounds with an ­OH group give steamy fumes of HCl, so all alcohols and carboxylic acids give
a positive result. However, alcohols do not give bubbles of CO2 on addition of Na2CO3
Warm with ethanoic acid: In the presence of concentrated sulphuric acid, all alcohols form esters. If the
product is poured in a beaker of dilute NaHCO3, the characteristic odour of an ester will be detected.
Acidified potassium dichromate: Primary and secondary alcohols turn orange potassium dichromate
solution to a green solution of Cr (III). Tertiary alcohols are not oxidised, so the solution remains orange.
To distinguish between primary and secondary alcohols, distil of the organic product and add Tollen's
reagent. Only the oxidised product of the primary alcohol forms a silver mirror.
IR Spectrum: Alcohols, as well as carboxylic acids, have a broad band in the IR spectrum at approximately
3000 cm-1. Carboxylic acids, but not alcohols, also have a peak at approximately 1700 cm-1.
9. Both aldehydes and ketones give a yellow or orange precipitate when a few drops of
2,4-dinitrophenylhydrazine are added. The infrared spectra of aldehydes and ketones have peaks about 1700
cm-1, as do the IR spectra of all other compounds with a C=O group.

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There are several chemical tests to differentiate between aldehydes and ketones:
Fehling's solution: The carbonyl compound is warmed with Fehling's. Aldehydes give a red precipitate but
ketones do not react, so the solution remains blue.
Tollen's reagent: Tollen's reagent is made by adding a few drops of NaOH to silver nitrate solution and
dissolving the precipitate in dilute ammonia solution. If the carbonyl compound is gently warmed with
Tollen's reagent, aldehydes give a silver mirror, but not ketones.…read more

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Allow the beaker of water to cool, stirring continuously. Note the temperature when bubbles stop coming
out of the capillary tube and the liquid begins to suck back into the capillary tube.
The average of these two temperatures is the boiling temperature of the liquid.
16. Method for melting point determination:
Inset some of the pure solid into a capillary tube and then attach the tube open end upwards to a
thermometer with a rubber band.
Place the thermometer into a bath of liquid.…read more

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Identification BPt MPT
23. There are several ways in which the length of the carbon chain can be increase:
Nucleophilic substitution with KCN: Potassium cyanide reacts with primary, secondary and tertiary
halogenoalkanes. The conditions are heat under reflux in a solution of ethanol and water. This reaction
increases the carbon chain length by one carbon atom. The product is a nitrile, which can be reduced to a
primary amine or hydrolysed to a carboxylic acid.…read more

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Conditions: Mix dry reagents
34. Acid chloride to amide:
Equation: RCOCl + 2NH3 RCONH2 + NH4Cl
Reagent: Aqueous ammonia
Conditions: Mix at room temperature
35. Nitration of benzene:
Equation: C6H6 + HNO3 C6H5NO2 + H2O
Organic Product: Nitrobenzene
Reagent: Concentrated nitric acid
Conditions: Mix with concentrated sulfuric acid at 60 degrees
36. Friedel-Crafts alkylation:
Equation: C6H6 + C2H5Br C6H5C2H5 + HBr
Organic Product: Ethylbenzene
Reagent: bromoethane
Conditions: Dry, in the presence of an anhydrous aluminium chloride catalyst
37.…read more

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Carcinogenic compounds: Should not be used and a suitable alternative should be used instead.
45. Combinatorial chemistry involves rapid synthesis or the computer simulation of a large number of different but
often structurally related molecules or materials. It is often used in the development of new pharmaceutical
compounds as compounds can be quickly tested for activity as drugs. The number of products produced = Nx,
where N is the no. of reactants and x is the no. of steps taken.
46.…read more

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