Syllabus ref: 10.2 Syllabus ref: 20.1 Syllabus ref: 20.2 A synthetic pathway is the route taken to prepare a specific product. Chemists must be able to think in reverse occasionally to work backwards from a desired product and consider from which particular substances it could be prepared. |
Schematic diagram
A synthetic pathway is the route taken to prepare a specific product. For IB examinations syntheses of up to four steps are required. A schematic diagram of all of the required reactions is shown below:
A question may ask for a pathway to go from a given alkene to a specific alcohol, in which case the reagents and conditions required for the first step (alkene to haloalkane) must be given, before the second stage, reaction of the haloalkane to form the alcohol.
Step 1: alkene haloalkane Step 2: haloalkane alcohol |
Example: Give the reagents and conditions required for the synthesis of propan-2-ol starting from propene. Step 1: Propene is reacted with hydrogen bromide giving 2-bromopropane
Step 2: The 2-bromopropane is warmed with aqueous sodium hydroxide forming propan-2-ol.
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From alkanes
Alkanes are not particularly useful for synthesis as they have no functional groups and all the bonds are strong. In general, they are made into more useful alkenes by thermal or catalytic cracking.
They do, however, undergo free radical substitution with halogens in UV light at RT.
1. Alkane
halogenoalkane
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---|---|---|
Reaction type
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Reagents
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Conditions
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Free radical substitution
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Halogen
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UV light
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Example:
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CH3CH3 + Br2
CH3CH2Br + HBr
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From Alkenes
Alkenes are very useful synthetic tools as they undergo many reactions (far more than required for the IB).
2. Alkene
halogenoalkane
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---|---|---|
Reaction type
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Reagents
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Conditions
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Electrophilic addition
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Hydrogen halide
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inert solvent/RT
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Example:
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CH2=CH2 + HBr
CH3CH2Br
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3. Alkene
dihalogenoalkane
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---|---|---|
Reaction type
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Reagents
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Conditions
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Electrophilic addition
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halogen
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inert solvent/RT
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Example:
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CH2=CH2 + Br2
CH2BrCH2Br
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4. Alkene
alkane
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---|---|---|
Reaction type
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Reagents
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Conditions
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hydrogenation
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hydrogen
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150ºC/nickel catalyst
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Example:
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CH2=CH2 + H2
CH3CH3
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5. Alkene
alcohol
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---|---|---|
Reaction type
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Reagents
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Conditions
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hydration/addition
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water
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170ºC/sulfuric acid catalyst
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Example:
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or steam
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or phosphoric acid/300ºC
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CH2=CH2 + H2O
CH3CH2OH
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6. Alkene
poly(alkenes)
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---|---|---|
Reaction type
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Reagents
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Conditions
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polymerisation
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-
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TiCl3 (Zeigler-Natta) catalyst
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Example:
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nCH2=CH2
-[CH2CH2]n-
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From halogenoalkanes
Halogenoalkanes are very useful synthetic tools, as reactions usually proceed easily due to the dipole that exists between the carbon and halogen atoms. The halide ion formed in nucleophilic substitution is a good leaving group (i.e. it is stable) driving the reaction to the side of the products.
7.
Halogenoalkane
alcohol
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---|---|---|
Reaction type
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Reagents
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Conditions
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nucleophilic substitution
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NaOH(aq)
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aqueous/heat
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Example:
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CH3CH2Br + NaOH
CH3CH2OH + NaBr
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8. Halogenoalkane
dihalogenoalkane
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---|---|---|
Reaction type
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Reagents
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Conditions
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free radical substitution
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halogen
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UV light/RT
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Example:
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CH3CH2Br + Br2
CH3CHBr2 + HBr
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From alcohols
Alcohols are not generally thought to be very useful for synthesis, but they do undergo some reactions. Primary alcohols form aldehydes (alkanals) which must be distilled off before they react further.
9. 1º Alcohol
aldehyde
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Reaction type
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Reagents
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Conditions
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oxidation [O]
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potassium dichromate(VI)/H+(aq)
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aqueous/distillation
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Example:
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CH3CH2OH + [O]
CH3CHO + H2O
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Secondary alcohols form ketones under the same conditions. This time there is no danger of further reaction and the procedure may be carried out under reflux before distilling off the product.
10.
2º Alcohol
ketone (alkanone)
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Reaction type
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Reagents
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Conditions
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oxidation [O]
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potassium dichromate(VI)/H+(aq)
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aqueous/reflux
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Example:
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CH3CH(OH)CH3 + [O]
CH3COCH3 + H2O
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Alcohols can also undergo different reactions with dehydrating acids such as concentrated phosphoric acid or concentated sulfuric acid. The nature of the products depends on the conditions used.
At 170ºC the alcohol dehydrates directly to an alkene.
11. 1º Alcohol
Alkene
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Reaction type
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Reagents
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Conditions
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dehydration
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concentrated phosphoric acid
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170ºC
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Example:
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CH3CH(OH)CH3 + H3PO4
CH2=CHCH3
+ H3PO4.H2O
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However, at 140ºC an ether is formed:
12. 1º Alcohol
Ether
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Reaction type
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Reagents
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Conditions
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dehydration
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concentrated phosphoric acid
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140ºC
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Example:
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2CH3CH2OH + H3PO4
CH3CH2OCH2CH3
+ H3PO4.H2O
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These last two dehydration reactions are NOT required in the 2016 syllabus
From aldehydes (alkanals)
Aldehyde reactions are not included in the IB pathways, except for oxidation to carboxylic acids. The conditions required are the same as for oxidation of alcohols.
13. Aldehyde
carboxylic acid
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Reaction type
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Reagents
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Conditions
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oxidation [O]
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potassium dichromate(VI)/H+(aq)
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reflux
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Example:
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CH3CHO + [O]
CH3COOH
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From Halogenoalkanes
14.
Halogenoalkane
nitrile
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Reaction type
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Reagents
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Conditions
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nucleophilic substitution
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KCN/ ethanolic solution
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heat
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Example:
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CH3CH2Br + KCN
CH3CH2CN + KBr
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15. Halogenoalkane
amine
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---|---|---|
Reaction type
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Reagents
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Conditions
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nucleophilic substitution
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NH3
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heat
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Example:
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CH3CH2Br + NH3
CH3CH2NH2 + HBr
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From Nitriles
Nitriles are useful as an intermediate when extending the length of the carbon chain by one carbon atom. The nitrile is reduced to the amine by a variety of reducing agents, such as hydrogen gas in the presence of a nickel catalyst, or lithium aluminium hydride in ethoxyethane.
16.
Nitrile amine
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Reaction type
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Reagents
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Conditions
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reduction
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(i)LiAlH4/ether
(ii) H+(aq) |
heat
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or
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H2
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Ni catalyst/150ºC
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Example:
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CH3CH2CN + 4[H]
CH3CH2CH2NH2
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From Carboxylic acids and derivatives
Amines react with carboxylic acids and their derivatives to form amides, a class of compounds containing the functional group -CONH1. This reaction is important as amides are an integral part of natural and synthetic polymers.
Polyamides are synthetic polymers consisting of hydrocarbon chains all linked together by amide linkages, -CO-NH-. In synthetic polyamides there are usually only two types of hydrocarbon section per molecule.
Proteins (also called polypeptides) are natural large molecules consisting of hydrocarbon sections (with branched attachments in some cases) linked together by amide (in biology called peptide) linkages. In natural polypeptides there may be many different types of hydrocarbon section per molecule.
17.
Carboxylic acid + amine
amide
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Reaction type
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Reagents
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Conditions
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condensation
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carboxylic acid/amine
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heat
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Example:
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CH3COOH + CH3CH2CH2NH2
CH3CONHCH2CH2CH3
+ H2O
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Alcohols react with carboxylic acids forming esters. Once again the importance of this reaction lies in the fact that if there are two functional groups per molecule a polymer, in this case polyester is formed. Polyesters are important synthetic fabrics.
18.
Carboxylic acid + alcohol
ester
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---|---|---|
Reaction type
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Reagents
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Conditions
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condensation/esterification
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carboxylic acid/alcohol/H2SO4
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heat
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Example:
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CH3COOH + CH3CH2CH2OH
CH3COOCH2CH2CH3
+ H2O
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