Organic Synthesis Test Questions with Correct Answers Graded A+
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). - Answers True.
This technique is called combustion analysis.
Test for alkene and observations - Answers Add bromine water, orange bromine is decolourised.
Test for halogenoalkane and observations - Answers 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? - Answers 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) - Answers Add PCl5 (s). Steamy fumes of HCl
Primary alcohol - Answers Add K2Cr2O7 (aq) and dilute H2SO4 (aq) and warm. Orange to green.
Secondary alcohol - Answers Add K2Cr2O7 (aq) and dilute H2SO4 (aq) and warm.
Orange to green.
Tertiary alcohol - Answers Add K2Cr2O7 (aq) and dilute H2SO4 (aq) and warm.
No reaction (stays orange).
Spectroscopic / Chromatographic Methods - Answers 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 - Answers 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 - Answers Rf values can be used to identify the amino acids that result from
the hydrolysis of proteins
, NMR spectra - Answers Number and relative intensity of peaks and the splitting pattern can be used to
distinguish between structural isomers.
What are the Physical Methods? - Answers - Boiling point determination
- Melting point determination
Boiling point determination - Answers 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 - Answers 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 - Answers 1. Separating funnel
2. Solvent extraction of water soluble organic compounds
3. Distillation
4. Steam Distillation
5. Recrystallisation
Separating funnel - Answers 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 - Answers 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? - Answers Rate, yield, cost of
reagents and energy use.
What is yield greatly affected by? - Answers 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...? - Answers The aim is to use the minimum number of steps.
What are the most common hazards? - Answers - Toxicity of gases
- Toxicity or corrosive nature of liquids and solids
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). - Answers True.
This technique is called combustion analysis.
Test for alkene and observations - Answers Add bromine water, orange bromine is decolourised.
Test for halogenoalkane and observations - Answers 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? - Answers 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) - Answers Add PCl5 (s). Steamy fumes of HCl
Primary alcohol - Answers Add K2Cr2O7 (aq) and dilute H2SO4 (aq) and warm. Orange to green.
Secondary alcohol - Answers Add K2Cr2O7 (aq) and dilute H2SO4 (aq) and warm.
Orange to green.
Tertiary alcohol - Answers Add K2Cr2O7 (aq) and dilute H2SO4 (aq) and warm.
No reaction (stays orange).
Spectroscopic / Chromatographic Methods - Answers 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 - Answers 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 - Answers Rf values can be used to identify the amino acids that result from
the hydrolysis of proteins
, NMR spectra - Answers Number and relative intensity of peaks and the splitting pattern can be used to
distinguish between structural isomers.
What are the Physical Methods? - Answers - Boiling point determination
- Melting point determination
Boiling point determination - Answers 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 - Answers 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 - Answers 1. Separating funnel
2. Solvent extraction of water soluble organic compounds
3. Distillation
4. Steam Distillation
5. Recrystallisation
Separating funnel - Answers 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 - Answers 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? - Answers Rate, yield, cost of
reagents and energy use.
What is yield greatly affected by? - Answers 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...? - Answers The aim is to use the minimum number of steps.
What are the most common hazards? - Answers - Toxicity of gases
- Toxicity or corrosive nature of liquids and solids
