Halogenoalkanes production

The halogenoalkane products of these substitution reactions have uses as solvents (dry cleaning for example), anaesthetics and fire retardants. Dichloromethane, for instance, can be used as a solvent glue for polystyrene in model making (Figure 10.67). 

Many mechanisms in organic chemistry start with an acid/base reaction. This may be just a simple Bronsted-Lowry protonation of a hydroxyl group, which results in the activation of a C-OH bond or it may be a Lewis acid/base reaction as, for example, when aluminium trichloride complexes with a halogenoalkane in the first step of the Friedel-Crafts reaction. In each case, the initial intermediate usually reacts further and leads to the desired product. In inorganic chemistry, the acid/base reaction may be all that is of interest, e.g. the treatment of a carbonate with an acid to liberate carbon dioxide. However, it is unusual in organic chemistry for the acid/base reaction to be an end in itself. It is for this reason that acid/base characteristics are normally considered as a property of the molecule, similar to the nucleophilic and electrophilic properties to which they are closely related, rather than as a fundamental reaction type as is the case in inorganic chemistry. 

For an elementary bimolecular reaction, the two molecules involved may be the same or different. The number of product molecules is almost always one or two. We have already come across reactions of this type the Sn2 hydrolysis of halogenoalkanes involves reaction between two different molecules reaction (2.5) involves two molecules of the same compound, buta-1,3-diene (1) ... 

Helwig and Hanack (1985) were, however, successful with a synthetic method that was useful for the formation of alkenediazonium salts, namely the elimination of hydrogen halide from 1-halogenoalkanal (4-toluene)sulfonylhydrazones and dissociation of the product, the alkene(4-toluene)sulfonyldiazenes, into alkenediazonium salts with the help of Lewis acids (see Sect. 2.10, Scheme 2-109). By analogy, 1,2-dihalogenoalkanal (4-toluene)sulfonylhydrazones should form alkynediazonium salts. This pathway was indeed successful (2-111). 

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 regioselective addition of HX to alkenes produces the more substituted halogenoalkane, which is called the Markovnikov (Markovnikoff) product. Markovnikov s rule states that on addition of HX to an alkene, H attaches to the carbon with fewest alkyl groups and X attaches to the carbon with most alkyl groups. This can be explained by the formation of the most stable intermediate carbocation. 

Halogenoalkanes are saturated molecules but the halogen atom can be replaced by other atoms or groups in substitution reactions. This means that halogenoalkanes are very useful in reaction pathways that enable us to synthesize a range of important organic products. 

This is particularly true where tertiary halogenoalkanes are involved, as the El mechanism favoured in these reactions involves the same first step - the production of the intermediate carbocation. As a result of this competition between the two processes going on simultaneously, it is very difficult to obtain clean products in these reactions. A single product will always have to be separated from a number of by-products. 

In the presence of strong bases, NH-imidazoles are alkylated via the imidazolyl anion, for example, Na-imidazolide (prepared from imidazole and NaOH) undergoes 1-alkylation (->8, R = alkyl) with halogenoalkanes and dialkyl sulfates [378]. Because of the ambident character of unsymmetrical imidazolyl anions (e.g., 9), base-induced alkylation of 4- or 5-substituted imidazoles affords mixtures of 1,4- and 1,5-disubstituted products, for example ... 

This reaction is not really suitable for preparing specific halogenoalkanes, because we get a mixture of substitution products. In the reaction between methane and chlorine, the products can include dichloromethane, trichloromethane and tetrachloromethane as well as chloromethane. These other products result from propagation steps in which a chlorine free radical attacks a halogenoalkane already formed. For example ... 

When an alkene is bubbled through a concentrated solution of a hydrogen halide (HF, HCl, HBr, HI) at room temperature, the product is a halogenoalkane. For example ... 

When an aqueous solution of sodium hydroxide is added to a halogenoalkane, a nucleophilic substitution reaction takes place. The halogen atom in the halogenoalkane is replaced by an —OH, hydroxyl group, so the organic product formed is an alcohol ... 

Here the nucleophile is the ammonia molecule. Ethylamine is a primary amine, as the nitrogen atom is attached to only one alkyl group. If the ammonia is not in excess, we get a mixture of amine products. This is because the primary amine product will act as a nucleophile itself and will attack halogenoalkane molecules, forming secondary amines, and so on. The secondary amine formed above would be (CH3CH2)2NH, called diethylamine. You can read more about amines in Chapter 27, page 401. 

Sometimes the starting compound from a raw material does not have enough carbon atoms in its molecules to make the desired product. An extra carbon atom can be added by adding the nitrile functional group, —C=N. Remember that these can be made from halogenoalkanes ... 

Addition, substitution, and telomerization can occur when solutions or suspensions of chlorinated aromatic substrates in halogenoalkane solvents are subjected to direct low-temperature (<25 C) fluorination, and fluoro-aromatics may be obtained by dehydrohalogenation of appropriate products ie.g., see Elcheme 1). Fluorination of hexafluorobenzene in 1,1,2-trichlorotri-... 

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