Halogenation, of alkanes

When a mixture of an alkane and chlorine gas is stored at low temperatures in the dark, no reaction occurs. In sunlight or at high temperatures, however, an exothermic reaction occurs. One or more hydrogen atoms of the alkane are replaced by chlorine atoms. This reaction can be represented by the general equation 

The reaction is called chlorination. This process is a substitution reaction, as a chlorine is substituted for a hydrogen. 

An analogous reaction, called bromination, occurs when the halogen source is bromine. 

Chlorination of hydrocarbons is a substitution reaction in which a chlorine atom is substituted for a hydrogen atom. Likewise in bromination reactions, a bromine atom is substituted for a hydrogen atom. 

If excess halogen is present, the reaction can continue further to give polyhalogenated products. Thus, methane and excess chlorine can give products with two, three, or four chlorines.  

The rest of this chapter describes a second method for preparing alkyl halides, one that uses alkanes as reactants. It involves substitution of a halogen atom for one of the alkane s hydrogens. 

Volume II of Organic Reactions, an annual series that reviews reactions of interest to organic chemists, contains the statement Most organic compouncJs burn or explode when brought in contact with fluorine.  

The alkane is said to undergo fluorination, chlorination, bromination, or iodination according to whether X2 is F2, CI2, Br2, or I2, respectively. The general term is halogenation. Chlorination and bromination are the most widely used. 

The reactivity of the halogens decreases in the order F2 CI2 Br2 I2. Fluorine is an extremely aggressive oxidizing agent, and its reaction with alkanes is strongly exothermic and difhcult to control. Direct fluorination of alkanes requires special equipment and techniques, is not a reaction of general applicability, and will not be discussed further. 

Chlorination of alkanes is less exothermic than fluorination, and bromination less exothermic than chlorination. Iodine is unique among the halogens in that its reaction with alkanes is endothermic and alkyl iodides are never prepared by iodination of alkanes. 

Sample Problem 15.1 Draw all the constitutional isomers formed by monohalogenation of (CHglpCHCHpCHs with CI2 and hv. 

Substitute Cl for H on every carbon, and then check to see if any products are identical. The starting material has five C s, but replacement of one H atom on two C s gives the same product. Thus, (CHsJpCHCHaCHs affords four monochloro substitution products. 

Alkyl chlorides or bromides can be obtained from alkanes by reaction with chlorine or bromine gas, respectively, in the presence of UV light. The reaction involves a radical chain mechanism. 

The ease of halogenation depends on whether the hydrogen atom is bonded to a primary, secondary or tertiary carbon atom. A tertiary hydrogen atom is more reactive because reaction with a halogen atom (X ) produces an intermediate tertiary radical, which is more stable (and therefore more readily formed) than a secondary or primary radical (see Section 4.3). 

Chloroalkanes, such as CHjCb (dichloromethane), are common solvents in organic synthesis 

This is a substitution reaction as a hydrogen atom on the carbon is substituted for a Cl or Br atom. A mixture of halogenated products is usually obtained if further substitution reactions can take place. 

Primary, secondary and tertiary halogenoalkanes are defined in Section 2.1 

Whenever a radical reacts with a stable single or double bond, a new radical is formed in the products. 

This radical reaction is typically seen with the nonpolar C-H bonds of alkanes, which cannot react with polar or ionic electrophiles and nucleophiles. 

A radical X also adds to the n bond of a carbon-carbon double bond. One electron from the double bond is used to form a new C - X bond, and the other electron remains on the other carbon originally part of the double bond. 

Although the electron-rich double bond of an alkene reacts with electrophiles by ionic addition mechanisms, it also reacts with radicals because these reactive intermediates are also electron deficient. 

A radical, once formed, rapidly reacts with whatever is available. Usually that means a stable o or n bond. Occasionally, however, two radicals come into contact with each other, and they react to form a a bond. 

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Chlorination of methane and halogenation of alkanes generally proceed by way of free radical intermediates Alkyl radicals are neutral and have an unpaired electron on carbon... 

Cation (Section 1 2) Positively charged ion Cellobiose (Section 25 14) A disacchande in which two glu cose units are joined by a 3(1 4) linkage Cellobiose is oh tamed by the hydrolysis of cellulose Cellulose (Section 25 15) A polysaccharide in which thou sands of glucose units are joined by 3(1 4) linkages Center of symmetry (Section 7 3) A point in the center of a structure located so that a line drawn from it to any element of the structure when extended an equal distance in the op posite direction encounters an identical element Benzene for example has a center of symmetry Cham reaction (Section 4 17) Reaction mechanism m which a sequence of individual steps repeats itself many times usu ally because a reactive intermediate consumed m one step is regenerated m a subsequent step The halogenation of alkanes is a chain reaction proceeding via free radical intermediates... 

Halogenation (Sections 4 14 and 12 5) Replacement of a hy drogen by a halogen The most frequently encountered ex amples are the free radical halogenation of alkanes and the halogenation of arenes by electrophilic aromatic substitution... 

Chain reaction (Section 4.17) Reaction mechanism in which a sequence of individual steps repeats itself many times, usually because a reactive intermediate consumed in one step is regenerated in a subsequent step. The halogenation of alkanes is a chain reaction proceeding via free-radical intermediates. 

Simple alkyl halides can be prepared by radical halogenation of alkanes, but mixtures of products usually result. The reactivity order of alkanes toward halogenation is identical to the stability order of radicals R3C- > R2CH- > RCH2-. Alkyl halides can also be prepared from alkenes by reaction with /V-bromo-succinimide (NBS) to give the product of allylic bromination. The NBS bromi-nation of alkenes takes place through an intermediate allylic radical, which is stabilized by resonance. 

Another characteristic of many radical reactions is that, once initiated, they often proceed with great rapidity owing to the establishment of fast chain reactions of low energy requirement, e.g. in the halogenation of alkanes (3, cf. p. 323) ... 

Multiple substitutions almost always occur in the halogenation of alkanes. 

Table 2-1 Selectivity of Chlorine and Bromine in Free-Radical Halogenation of Alkanes ...

Halogenation of alkanes had long been known, and in 1930 the kinetics of the chlorination of chloroform to carbon tetrachloride were reported by Schwab and Heyde (equation 40), while the kinetics of the chlorination of methane were described by Pease and Walz in 1931. Both of these studies showed the currently accepted mechanism, which was extended to reactions in solution by Hass et al. in 1936. The free radical halogenation mechanism of other alkanes was described by Kharasch and co-workers, ° including side chain halogenation of toluene. 

Other methods for the preparation of alkyl halides are electrophilic addition of hydrogen halides (HX) to alkenes (see Section 5.3.1) and free radical halogenation of alkanes (see Section 5.2). 

Radical reactions are often called chain reactions. All chain reactions have three steps chain initiation, chain propagation and chain termination. For example, the halogenation of alkane is a free radical chain reaction. 

The halogenation of alkanes in the presence of sulphur dioxide yields alkanesulphonyl chlorides (5.79), and these are made in large quantities for conversion to metal alkanesulphonates (used as emulsifiers in polymerizations) or to nitrogen-containing derivatives. The sulphur dioxide acts by trapping the alkyl radical it does not terminate the chain mechanism, and so quantum yields can be very high (—2000). 

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