Organic Synthesis - Organic Synthesis: Aliphatic Compounds (A-Level Chemistry)
Organic Synthesis: Aliphatic Compounds
Synthesis of Aliphatic Compounds
A-Level Chemistry exam boards requires you to know the synthesis of different organic compounds. The flow diagram below summarises the synthesis of each functional group in aliphatic compounds.
We will also go through each organic compound, so you can revise the synthesis of each, step-by-step:
Alkanes
Alkanes are made from:
- Crude oil – the fractional distillation of crude oil.
- Alkenes – electrophilic addition of hydrogen to alkenes using a nickel catalyst.
Alkenes
Alkenes are made from:
- Alkanes – steam cracking of alkane
- Halogenoalkanes – elimination reaction by refluxing ethanolic potassium or sodium hydroxide ions with a halogenoalkane.
- Alcohols – elimination reaction (dehydration) of alcohols with concentrated sulfuric or phosphoric acid as a catalyst.
Halogenoalkanes
Halogenalkanes are made from:
- Alkanes – free radical substitution reaction by adding a halogen to an alkane in the presence of UV light
For example, to make bromoethane (CH₃CH₂Br) from ethane:
1. Initiation: Br₂ → 2Br∙
2. Propagation: CH₃CH₃ + Br∙ → HBr + CH₃CH₂∙
CH₃CH₂∙ + Br₂ → CH₃CH₂Br + Br∙
3. Termination: CH₃CH₂∙ + Br∙ → CH₃CH₂Br
- Alkenes – electrophilic addition of hydrogen halides to alkenes.
Alcohols
Alcohols are made from:
- Alkenes – electrophilic addition of alkenes with water using a concentrated phosphoric acid (catalyst). The reaction takes place at 350 °C and 60 atm.
- Halogenoalkanes – nucleophilic substitution reaction by reflux with aqueous potassium hydroxide solution
- Aldehydes and Ketones – nucleophilic addition / reduction with sodium borohydride (NaBH₄). Aldehydes make primary alcohols, ketones make secondary alcohols.
Aldehydes
Aldehydes are made from:
- Primary Alcohols – partial oxidation of primary alcohols by heating and distillation using acidified potassium dichromate (K₂Cr₂O₇)
Ketones
Ketones are made from:
- Secondary Alcohols – oxidation of a secondary alcohol with an oxidizing agent such as acidified potassium dichromate (K₂Cr₂O₇)
Hydroxynitriles
Hydroxynitriles are made from:
- Aldehydes and Ketones – nucleophilic addition reactions of sodium cyanide followed by dilute acid like sulfuric acid.
Carboxylic Acids
Carboxylic acids are made from:
- Aldehydes – oxidation of aldehydes by heat and reflux with excess oxidising agent, for example acidified potassium dichromate (K₂Cr₂O₇)
- Acyl Chlorides / Acid Anhydrides – nucleophilic addition elimination reaction between water and acyl chloride/ acid anhydride.
Esters
Esters are made from:
- Carboxylic acids and Alcohols – esterification reaction by heating carboxylic acid and alcohol with sulfuric acid catalyst
- Acyl Chloride/ Acid Anhydride and Alcohols – nucleophilic addition elimination reaction between alcohols and acyl chlorides or acid anhydrides at room temperature.
Amides
Primary amides are made by:
- Acyl Chlorides / Acid Anhydrides – nucleophilic addition elimination reaction between acyl chlorides/ acid anhydrides with ammonia at room temperature
Secondary amides are made by:
- Acyl Chlorides / Acid Anhydrides – nucleophilic addition elimination reaction between acyl chlorides/ acid anhydrides with primary amines at room temperature
Amines
Amines are made by:
- Halogenoalkanes – nucleophilic substitution reaction of alcoholic ammonia with halogenoalkanes. Further substitution occurs to make secondary, tertiary amines and quaternary ammonium salts
- Nitrile – Reduction in dry ether with lithium aluminium hydride
Nitriles
Nitriles are made from:
- Halogenoalkanes – nucleophilic substitution reaction with KCN in ethanol/ water mixture heated under reflux.
Worked Example:
1. Describe the types and reactions condition given in reaction pathway 1 and 2
2. Compound Z contains hydroxynitrile groups. Describe the type and reaction conditions for reaction 3
Answer:
- Reaction 1: Nucleophilic substitution reaction heating and reflux with aqueous sodium hydroxide
Reaction 2: The alcohol groups have been oxidized to form aldehyde groups. Heat and distil with acidified potassium dichromate solution.
- Reaction 3: Nucleophilic addition reactions – add KCN followed by dilute acid
Organic synthesis is the process of creating new organic compounds or modifying existing compounds to create a desired product.
Aliphatic compounds are a class of organic compounds that contain only carbon and hydrogen atoms, and do not contain any aromatic rings.
Aliphatic compounds are made up of only carbon and hydrogen atoms, while aromatic compounds contain a ring of carbon atoms with alternating double bonds and have a distinctive odor.
Organic synthesis in aliphatic compounds involves the use of reagents and catalysts to create new bonds between carbon atoms, or to modify existing bonds to form the desired product.
Organic synthesis in aliphatic compounds is important because it allows for the creation of new compounds with specific properties, such as improved stability, solubility, and reactivity.
The common methods used in organic synthesis of aliphatic compounds include substitution reactions, addition reactions, and elimination reactions.
A substitution reaction in organic synthesis is a reaction in which a functional group is replaced by another functional group.
An addition reaction in organic synthesis is a reaction in which two or more molecules combine to form a new compound.
An elimination reaction in organic synthesis is a reaction in which a bond is broken, resulting in the formation of two new compounds.
Reagents and catalysts are used in organic synthesis of aliphatic compounds to facilitate and control the reaction process, increasing the speed and yield of the desired product.
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