Aliphatic compounds are the open chain compounds. The open chain hydrocarbons are named as the aliphatic hydrocarbons. In these aliphatic hydrocarbons, if one or more hydrogens are replaced or substituted with halogens, then those are called as halogen compounds. Halogen compounds can be defined as the compounds that contain halogens as their functional group.
Classification and nomenclature:
- Depending upon the number of other carbon atoms attached directly to the carbon atom to which the halogen is attached, the compound is called as primary, secondary or tertiary compound.
- The halogen compounds are also called as halides. As these aliphatic halogen compounds are the alkyl derivatives, these can also be called as alkyl halides.
- These alkyl halides like the alkyl compounds, can be given two kinds of names.
- Common names for simpler halides
Note: Similar name doesn’t always mean the same classification.
Eg: isopropyl chloride – secondary chloride
Isobutyl chloride – primary chloride
- IUPAC names in which the compound is simply named as an alkane with a halogen as a side chain.
General methods of preparation:
- Halogenation of certain hydrocarbons:
Halogenation is nothing but the addition of halogens. Here in this process, to synthesize alkyl halides, the aliphatic hydrocarbons are halogenated. i.e. halogens are added to the aliphatic hydrocarbons.
- Addition of hydrogen halides to alkenes and alkynes:
In this method of preparation, hydrogen halides (H-X) are added to alkenes and alkynes. Here in this process, the unsaturated bond will move away from the atom on which the halogen attacks and the π-electrons will shift to the adjacent atom on which the halogen is attached. The hydrogen will attack the atom on which the π-electrons are present.
- Halide exchange:
This method of preparation is used to prepare the required halogen compounds. If we have any halogen compound, we need some other halogen derivative, then the available halogen compound is subjected to react with the halogen that is needed. In this case the halogens will get exchanged and the required halogen derivative will be obtained. Here the halogen presents in the compound we select should be less reactive than the halogen we need.
- From alcohols:
In this, the alcoholic hydroxyl (‘-OH’) will get substituted with halogen. The reason for this reaction is that the halogen is more reactive than the hydroxyl group and is capable of breaking the existing bond between carbon and hydroxyl group. As the halogen is more electronegative than the hydroxyl group, this carbon atom will get attracted towards halogen more than the hydroxyl group.
- From silver salts of carboxylic acids:
The silver salts of carboxylic acids react with halogen in presence of carbon tetrachloride, forming an unstable intermediate.
That intermediate will eventually undergo decarboxylation thermally to give alkyl halide.
This reaction of formation of alkyl halides from silver salts of carboxylic acids inn presence of carbon tetrachloride is called as Hundiecker reaction.
- Preparation of poly halides:
Poly halides means more than one halide. This occurs when halogen has been added to unsaturated hydrocarbons like alkenes and alkynes.
When halogens are added to alkenes, vicinal dihalides will be formed.
When halides are added to alkynes, tetrahalides will be formed.
- Preparation of vinyl halides:
Rearrangement of Carbocations:
Rearrangement means arrangement in another manner. That means, the first arrangement is destroyed and again will be arranged in such a way that the molecule will be more stable. Any molecule or ion will rearrange itself or reposition itself to attain more stable state. Likewise, carbocations also will move from an unstable form/state to a more stable form/state. This movement occurs through various structural recognitional shifts within the molecule. Carbocation will undergo mainly two such kind of shifts to attain more stability.
Hydride shift:
Hydride shift means movement of hydrogen from one carbon to its neighbor carbon. This is seen oftenly with the reaction of alcohol with hydrogen halides.
Step 1: Protonation:
Step 2: Dehydration:
Step 3: Rearrangement or hydride shift:
Alkyl shift:
This means movement of alkyl group from one carbon atom to its neighbor carbon atom.
Step 1: Protonation:
Step 2: Formation of carbocation or Dehydration:
Step 3: Rearrangement or alkyl shift:
