Halogenoalkane structure

Abstract Room temperature ionic liquids (ILs) have attracted considerable attention as novel reaction media over the last decade. By virtue of their unique properties, ILs have been proposed as alternative solvents. Structurally, most of the ILs that have been investigated to date are based on imidazolium, pyridinium and ammonium cations, associated with polyatomic anions such as chloroaluminates, tetrafluoroborate, hexafluorophoshate and bis-triflimide. Although these salts have positive properties, imidazole, pyridine and halogenoalkanes come from petroleum feedstocks that are neither green nor sustainable. Renewable resources may represent a valid alternative to synthesized new ILs an alternative able to take into accounts both the ecological and economic requirements. 

Halogenoalkanes undergo competitive substitution and elimination reactions. The ratio of products derived from substitution and elimination depends on the structure of the halogenoalkane, the choice of base or nucleophile, the reaction solvent and the temperature. Sn2 reactions are normally in competition with E2 reactions, while S jl reactions are normally in competition with El reactions. 

The halogenoalkanes are an important and useful group of compounds. Their structure is straightforward in that the halogen atom simply replaces a hydrogen atom in the given hydrocarbon structure, whether that structure is a straight or branched chain. 

There is in fact another mechanism by which the nucleophilic substitution of halogenoalkanes can be achieved. The major factor that determines which mechanism takes place in a particular case is thought to depend primarily on the structure of the halogenoalkane. 

Hgure 20.8 The structure of the intermediate (transition structure) in an S 2 reaction involving a a primary halogenoalkane, and b a tertiary halogenoalkane (the bulky R groups make this mechanism difficult)... 

When ammonia is reacted with an excess of a halogenoalkane, a mixture of amines can be formed. If we start with an excess of 1-bromopropane, give the structural formula and name of the tertiary amine formed. 

The mechanism is determined by the structure of the halogenoalkane involved in the reaction. 

In a tertiary halogenoalkane, the carbon atom bonded to the halogen atom is also bonded to three other carbon atoms (alkyl groups). For example, 2-bromo-2-methylpropane is a tertiary halogenoalkane. Its structure is shown in Figure 16.5. 

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