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:

Organic Synthesis: Aliphatic Compounds
Organic Synthesis: Aliphatic Compounds
Organic Synthesis: Aliphatic Compounds
Organic Synthesis: Aliphatic Compounds

Alkanes

Alkanes are made from:

  • Crude oil – the fractional distillation of crude oil.
  • Alkeneselectrophilic addition of hydrogen to alkenes using a nickel catalyst.

Alkenes

Alkenes are made from:

  • Alkanes – steam cracking of alkane
  • Halogenoalkaneselimination reaction by refluxing ethanolic potassium or sodium hydroxide ions with a halogenoalkane.
Organic Synthesis: Aliphatic Compounds
Organic Synthesis: Aliphatic Compounds
  • Alcoholselimination reaction (dehydration) of alcohols with concentrated sulfuric or phosphoric acid as a catalyst.
Organic Synthesis: Aliphatic Compounds
Organic Synthesis: Aliphatic Compounds

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

  • Alkeneselectrophilic addition of hydrogen halides to alkenes.
Organic Synthesis: Aliphatic Compounds
Organic Synthesis: Aliphatic Compounds

Alcohols

Alcohols are made from:

  • Alkeneselectrophilic addition of alkenes with water using a concentrated phosphoric acid (catalyst). The reaction takes place at 350 °C and 60 atm.
Organic Synthesis: Aliphatic Compounds
Organic Synthesis: Aliphatic Compounds
  • Halogenoalkanesnucleophilic substitution reaction by reflux with aqueous potassium hydroxide solution
Organic Synthesis: Aliphatic Compounds
Organic Synthesis: Aliphatic Compounds
  • Aldehydes and Ketonesnucleophilic addition / reduction with sodium borohydride (NaBH₄). Aldehydes make primary alcohols, ketones make secondary alcohols.
Organic Synthesis: Aliphatic Compounds
Organic Synthesis: Aliphatic Compounds

Aldehydes

Aldehydes are made from:

  • Primary Alcoholspartial oxidation of primary alcohols by heating and distillation using acidified potassium dichromate (K₂Cr₂O₇)

Ketones

Ketones are made from:

  • Secondary Alcoholsoxidation of a secondary alcohol with an oxidizing agent such as acidified potassium dichromate (K₂Cr₂O₇)

Hydroxynitriles

Hydroxynitriles are made from:

  • Aldehydes and Ketonesnucleophilic addition reactions of sodium cyanide followed by dilute acid like sulfuric acid.
Organic Synthesis: Aliphatic Compounds
Organic Synthesis: Aliphatic Compounds

Carboxylic Acids

Carboxylic acids are made from:

  • Aldehydesoxidation of aldehydes by heat and reflux with excess oxidising agent, for example acidified potassium dichromate (K₂Cr₂O₇)
  • Acyl Chlorides / Acid Anhydridesnucleophilic addition elimination reaction between water and acyl chloride/ acid anhydride.
Organic Synthesis: Aliphatic Compounds
Organic Synthesis: Aliphatic Compounds

Esters

Esters are made from:

  • Carboxylic acids and Alcoholsesterification reaction by heating carboxylic acid and alcohol with sulfuric acid catalyst
  • Acyl Chloride/ Acid Anhydride and Alcoholsnucleophilic addition elimination reaction between alcohols and acyl chlorides or acid anhydrides at room temperature.
Organic Synthesis: Aliphatic Compounds
Organic Synthesis: Aliphatic Compounds

Amides

Primary amides are made by:

  • Acyl Chlorides / Acid Anhydridesnucleophilic addition elimination reaction between acyl chlorides/ acid anhydrides with ammonia at room temperature

Secondary amides are made by:

  • Acyl Chlorides / Acid Anhydridesnucleophilic addition elimination reaction between acyl chlorides/ acid anhydrides with primary amines at room temperature

Amines

Amines are made by:

  • Halogenoalkanesnucleophilic substitution reaction of alcoholic ammonia with halogenoalkanes. Further substitution occurs to make secondary, tertiary amines and quaternary ammonium salts
Organic Synthesis: Aliphatic Compounds
Organic Synthesis: Aliphatic Compounds
  • Nitrile – Reduction in dry ether with lithium aluminium hydride

Nitriles

Nitriles are made from:

  • Halogenoalkanesnucleophilic substitution reaction with KCN in ethanol/ water mixture heated under reflux.
Organic Synthesis: Aliphatic Compounds
Organic Synthesis: Aliphatic Compounds
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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

Organic Synthesis: Aliphatic Compounds
Organic Synthesis: Aliphatic Compounds

Answer:

  1. 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.

  2. Reaction 3: Nucleophilic addition reactions – add KCN followed by dilute acid
→What is organic synthesis?

Organic synthesis is the process of creating new organic compounds or modifying existing compounds to create a desired product.

→What are aliphatic compounds?

Aliphatic compounds are a class of organic compounds that contain only carbon and hydrogen atoms, and do not contain any aromatic rings.

→What is the difference between aliphatic and aromatic compounds?

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.

→What is the process of organic synthesis in aliphatic compounds?

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.

→What is the importance of organic synthesis in aliphatic compounds?

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.

→What are the common methods used in organic synthesis of aliphatic compounds?

The common methods used in organic synthesis of aliphatic compounds include substitution reactions, addition reactions, and elimination reactions.

→What is a substitution reaction in organic synthesis?

A substitution reaction in organic synthesis is a reaction in which a functional group is replaced by another functional group.

→What is an addition reaction in organic synthesis?

An addition reaction in organic synthesis is a reaction in which two or more molecules combine to form a new compound.

→What is an elimination reaction in organic synthesis?

An elimination reaction in organic synthesis is a reaction in which a bond is broken, resulting in the formation of two new compounds.

→How are reagents and catalysts used in organic synthesis of aliphatic 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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