IB Chemistry home > Syllabus 2016 > Organic chemistry > Synthetic pathways

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.

synthesis

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:

full screen version

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

CH3CH=CH2 + HBr CH3CHBrCH3

Step 2: The 2-bromopropane is warmed with aqueous sodium hydroxide forming propan-2-ol.

CH3CHBrCH3 + NaOH(aq) CH3CH(OH)CH3 + NaOBr

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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
Reaction type
Reagents
Conditions
Free radical substitution
Halogen
UV light
Example:
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
Reaction type
Reagents
Conditions
Electrophilic addition
Hydrogen halide
inert solvent/RT
Example:
CH2=CH2 + HBr CH3CH2Br
3. Alkene dihalogenoalkane
Reaction type
Reagents
Conditions
Electrophilic addition
halogen
inert solvent/RT
Example:
CH2=CH2 + Br2 CH2BrCH2Br
4. Alkene alkane
Reaction type
Reagents
Conditions
hydrogenation
hydrogen
150ºC/nickel catalyst
Example:
CH2=CH2 + H2 CH3CH3
5. Alkene alcohol
Reaction type
Reagents
Conditions
hydration/addition
water
170ºC/sulfuric acid catalyst
Example:
or steam
or phosphoric acid/300ºC
CH2=CH2 + H2O CH3CH2OH
6. Alkene poly(alkenes)
Reaction type
Reagents
Conditions
polymerisation
-
TiCl3 (Zeigler-Natta) catalyst
Example:
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
Reaction type
Reagents
Conditions
nucleophilic substitution
NaOH(aq)
aqueous/heat
Example:
CH3CH2Br + NaOH CH3CH2OH + NaBr
8. Halogenoalkane dihalogenoalkane
Reaction type
Reagents
Conditions
free radical substitution
halogen
UV light/RT
Example:
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
Reaction type
Reagents
Conditions
oxidation [O]
potassium dichromate(VI)/H+(aq)
aqueous/distillation
Example:
CH3CH2OH + [O] CH3CHO + H2O

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)
Reaction type
Reagents
Conditions
oxidation [O]
potassium dichromate(VI)/H+(aq)
aqueous/reflux
Example:
CH3CH(OH)CH3 + [O] CH3COCH3 + H2O

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
Reaction type
Reagents
Conditions
dehydration
concentrated phosphoric acid
170ºC
Example:
CH3CH(OH)CH3 + H3PO4 CH2=CHCH3 + H3PO4.H2O

However, at 140ºC an ether is formed:

12. 1º Alcohol Ether
Reaction type
Reagents
Conditions
dehydration
concentrated phosphoric acid
140ºC
Example:
2CH3CH2OH + H3PO4 CH3CH2OCH2CH3 + H3PO4.H2O

These last two dehydration reactions are NOT required in the 2016 syllabus


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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
Reaction type
Reagents
Conditions
oxidation [O]
potassium dichromate(VI)/H+(aq)
reflux
Example:
CH3CHO + [O] CH3COOH

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From Halogenoalkanes

14. Halogenoalkane nitrile
Reaction type
Reagents
Conditions
nucleophilic substitution
KCN/ ethanolic solution
heat
Example:
CH3CH2Br + KCN CH3CH2CN + KBr
15. Halogenoalkane amine
Reaction type
Reagents
Conditions
nucleophilic substitution
NH3
heat
Example:
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
Reaction type
Reagents
Conditions
reduction
(i)LiAlH4/ether
(ii) H+(aq)
heat
or
H2
Ni catalyst/150ºC
Example:
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
Reaction type
Reagents
Conditions
condensation
carboxylic acid/amine
heat
Example:
CH3COOH + CH3CH2CH2NH2 CH3CONHCH2CH2CH3 + H2O

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
Reaction type
Reagents
Conditions
condensation/esterification
carboxylic acid/alcohol/H2SO4
heat
Example:
CH3COOH + CH3CH2CH2OH CH3COOCH2CH2CH3 + H2O

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