Advanced Organic Synthesis –
Strategies for Efficient Molecule
Construction
The percentage composition or empirical formula of an organic compound can be
found by burning it in dry excess oxygen and measuring the amounts of the products
formed (H2O and CO2). - True. This technique is called combustion analysis.
Test for alkene and observations - Add bromine water, orange bromine is
decolourised.
Test for halogenoalkane and observations - Hydrolyse with NaOH (aq); Acidify with
dilute nitric acid and add AgNO3 (aq)
Chloroalkane: White ppt
Bromoalkane: Cream ppt
Iodoalkane: Yellow ppt
Confirm by adding dilute then conc. ammonia
Why do we acidify it with dilute nitric acid? - To get rid of NaOH so it doesn't react
with AgNO3 to produce a ppt. Otherwise you'll get a ppt. that confuses you.
Alcohols (all OH groups) - Add PCl5 (s). Steamy fumes of HCl
Primary alcohol - Add K2Cr2O7 (aq) and dilute H2SO4 (aq) and warm. Orange to
green.
Secondary alcohol - Add K2Cr2O7 (aq) and dilute H2SO4 (aq) and warm.
Orange to green.
Tertiary alcohol - Add K2Cr2O7 (aq) and dilute H2SO4 (aq) and warm.
No reaction (stays orange).
Spectroscopic / Chromatographic Methods - Once the functional group(s) in the
molecule have been identified by chemical tests, chemists use spectroscopic
methods in order to identify the specific compound.
- IR fingerprint
- Rf values in chromatography
- NMR spectra
IR fingerprint - The fingerprint region of the unknown compound is checked against a
database of known compounds. The unknown sample must be extremely pure.
Rf values in chromatography - Rf values can be used to identify the amino acids that
result from the hydrolysis of proteins
NMR spectra - Number and relative intensity of peaks and the splitting pattern can
be used to distinguish between structural isomers.
, What are the Physical Methods? - - Boiling point determination
- Melting point determination
Boiling point determination - The boiling point of an organic liquid can be determined
and the value compared with literature values. The two main drawbacks with this
method are:
- The boiling points of similar substances are often quite loose together.
- Impurities, and variation in atmospheric pressure, alter the boiling point. Impurities
usually raise the boiling point.
Melting point determination - Due to the problems associated with boiling point
determination it is commonplace in organic synthesis for a liquid to be converted to a
solid derivative. The derivative is then purified (by recrystallisation) and its melting
point determined. Impurities make the melting point range broader and lower.
Methods of separation and purification - 1. Separating funnel
2. Solvent extraction of water soluble organic compounds
3. Distillation
4. Steam Distillation
5. Recrystallisation
Separating funnel - This is used to separate immiscible liquids. This is often used
when separating an organic liquid which has been used to extract a chemical from
water.
Distillation - A volatile liquid may be separated from involatile materials by simple
distillation. Cooling the receiver flask minimises loss of product caused by
evaporation. The boiling point range of the product (determined during distillation)
indicates its degree of purity.
What are the most important economic factors in designing a synthesis? - Rate,
yield, cost of reagents and energy use.
What is yield greatly affected by? - Yield is greatly affected by the number of steps in
a synthetic pathway - the more steps there are the lower the yield will be.
So in general...? - The aim is to use the minimum number of steps.
What are the most common hazards? - - Toxicity of gases
- Toxicity or corrosive nature of liquids and solids
- Flammability of reactants or products
- Exothermic reactions
Toxicity of gases - Use a fume cupboard
Toxicity or corrosive nature of liquids and solids - use gloves
Flammability of reactants or products - Use a water (or oil) bath for heating; not a
naked flame
Strategies for Efficient Molecule
Construction
The percentage composition or empirical formula of an organic compound can be
found by burning it in dry excess oxygen and measuring the amounts of the products
formed (H2O and CO2). - True. This technique is called combustion analysis.
Test for alkene and observations - Add bromine water, orange bromine is
decolourised.
Test for halogenoalkane and observations - Hydrolyse with NaOH (aq); Acidify with
dilute nitric acid and add AgNO3 (aq)
Chloroalkane: White ppt
Bromoalkane: Cream ppt
Iodoalkane: Yellow ppt
Confirm by adding dilute then conc. ammonia
Why do we acidify it with dilute nitric acid? - To get rid of NaOH so it doesn't react
with AgNO3 to produce a ppt. Otherwise you'll get a ppt. that confuses you.
Alcohols (all OH groups) - Add PCl5 (s). Steamy fumes of HCl
Primary alcohol - Add K2Cr2O7 (aq) and dilute H2SO4 (aq) and warm. Orange to
green.
Secondary alcohol - Add K2Cr2O7 (aq) and dilute H2SO4 (aq) and warm.
Orange to green.
Tertiary alcohol - Add K2Cr2O7 (aq) and dilute H2SO4 (aq) and warm.
No reaction (stays orange).
Spectroscopic / Chromatographic Methods - Once the functional group(s) in the
molecule have been identified by chemical tests, chemists use spectroscopic
methods in order to identify the specific compound.
- IR fingerprint
- Rf values in chromatography
- NMR spectra
IR fingerprint - The fingerprint region of the unknown compound is checked against a
database of known compounds. The unknown sample must be extremely pure.
Rf values in chromatography - Rf values can be used to identify the amino acids that
result from the hydrolysis of proteins
NMR spectra - Number and relative intensity of peaks and the splitting pattern can
be used to distinguish between structural isomers.
, What are the Physical Methods? - - Boiling point determination
- Melting point determination
Boiling point determination - The boiling point of an organic liquid can be determined
and the value compared with literature values. The two main drawbacks with this
method are:
- The boiling points of similar substances are often quite loose together.
- Impurities, and variation in atmospheric pressure, alter the boiling point. Impurities
usually raise the boiling point.
Melting point determination - Due to the problems associated with boiling point
determination it is commonplace in organic synthesis for a liquid to be converted to a
solid derivative. The derivative is then purified (by recrystallisation) and its melting
point determined. Impurities make the melting point range broader and lower.
Methods of separation and purification - 1. Separating funnel
2. Solvent extraction of water soluble organic compounds
3. Distillation
4. Steam Distillation
5. Recrystallisation
Separating funnel - This is used to separate immiscible liquids. This is often used
when separating an organic liquid which has been used to extract a chemical from
water.
Distillation - A volatile liquid may be separated from involatile materials by simple
distillation. Cooling the receiver flask minimises loss of product caused by
evaporation. The boiling point range of the product (determined during distillation)
indicates its degree of purity.
What are the most important economic factors in designing a synthesis? - Rate,
yield, cost of reagents and energy use.
What is yield greatly affected by? - Yield is greatly affected by the number of steps in
a synthetic pathway - the more steps there are the lower the yield will be.
So in general...? - The aim is to use the minimum number of steps.
What are the most common hazards? - - Toxicity of gases
- Toxicity or corrosive nature of liquids and solids
- Flammability of reactants or products
- Exothermic reactions
Toxicity of gases - Use a fume cupboard
Toxicity or corrosive nature of liquids and solids - use gloves
Flammability of reactants or products - Use a water (or oil) bath for heating; not a
naked flame
