Halogenoalkanes compounds

Type of halogenoalkane Compound Relative rate of S l reaction... 

There has been much interest recently in the reaction of a, o)-di-halogenoalkanes. 1,2-Dibromoethane reacts with phthalazine to give ethane 1,2-bis-phthalazinium dibromide (1), none of the mono salt being formed directly, but the same dibromo compound and a, a -dipyridyl give the cyclic compound 2. ... 

Halogenoalkanes are compounds in which a halogen atom is bonded to a saturated carbon atom. 

Halogenoalkanes Organic compounds in which one or more hydrogen atoms of an alkane have been substituted by halogen atoms such as chlorine. 

These involve addition of a halogenoalkane to a carbonyl compound. 

The most important means of introducing an alkyl group into an aromatic ring is the Friedel-Crafts reaction. In its simplest form, this is the reaction of an alkyl halide (halogenoalkane) with an aromatic compound, such as benzene, in the presence of a Lewis acid, commonly aluminium chloride (Scheme 3.1). 

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) ... 

Frankland soon prepared more organometallics by reacting other metals with halogenoalkanes. The reaction mixtures were confined in sealed tubes and heated by focusing the sun s rays upon them. Frankland prepared the methyl and ethyl derivatives of mercury, tin and antimony, and also such compounds as oxide of stanethylium (diethyltin oxide, (C2H5)2SnO), which he formulated as C4H5SnO. 

Most simple covalent compounds whose intermolecular forces are London (dispersion) forces, for example iodine (Figure 4-92) and the halogenoalkanes, are poorly soluble in water, but are soluble in less polar or non-polar solvents. Simple covalent compounds whose intermolecular forces are hydrogen bonds are often soluble in water, for example amines, carboxylic acids, amides and sugars, provided they have relatively low molar mass or can form multiple hydrogen bonds. 

A halogenoalkane has a relative molecular mass of 99. Calculations based on elemental analysis of the compound show that it has an empirical formula of CH2CI, What is the molecular formula of this halogenoalkane ... 

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. 

Bromo-2-metbylpropane is a tertiary halogenoalkane and tbe hydrolysis of this compound proceeds by an S l mechanism (Figure 20.7). The hydrolysis is a first-order reaction, which means that the rate doubles if we double the concentration of the halogenoalkane, for instance. However, if we double the concentration of hydroxide ions, OH , the rate does not change at all. The rate depends only on the concentration of the bromoalkane and is independent of the hydroxide ion concentration. Kinetically this means that hydroxide ions cannot be involved in the rate-determining step. The mechanism is shown in Figure 20.9. 

The halogens are found in many organic compounds such as the plastic PVC (poly(choroethene) or polyvinyl chloride) and halogenated hydrocarbons used as solvents, refrigerants and in aerosols. For more details see Uses of halogenoalkanes on pages 222-3. 

Sometimes in industry chemists need to make a new compound with one more carbon atom than the best available organic raw material (the starting compound). Therefore, if we can convert the starting compound to a halogenoalkane, we can then reflux with ethanolic KCN to make a nitrile. We now have an intermediate nitrile with the correct number of carbon atoms. 

If one or more of the hydrogen atoms in an alkane molecule are replaced by halogen atoms, the compound is called a halogenoalkane. 

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 ... 

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