Alkyl Halides

Imagine that a pair of crystallizing dishes are placed on an overhead projector as shown in the figure below. An alkene is added to the dish in the upper-left corner of the projector and an alkane is added to the dish in the upper-right corner. A few drops of bromine dissolved in chloroform (CHCl₃) are then added to each of the crystallizing dishes.

The characteristic red-orange color of bromine disappears the instant this reagent is added to the alkene in the upper-left corner as the Br₂ molecules add across the C=C double bond in the alkene.

The other crystallizing dish picks up the characteristic color of a dilute solution of bromine because this reagent doesn't react with alkanes under normal conditions.
If the crystallizing dish in the upper-right corner is moved into the center of the projector, however, the color of the bromine slowly disappears. This can be explained by noting that alkanes react with halogens at high temperatures or in the presence of light to form alkyl halides.

The light source in an overhead projector is intense enough to initiate this reaction, although the reaction is still significantly slower than the addition of Br₂ to an alkene.
The reaction between an alkane and one of the halogens (F₂, Cl₂, Br₂, or I₂) can be understood by turning to a simpler example.

CH₄(g) + Cl₂(g) CH₃Cl(g) + HCl(g)

This reaction has the following characteristic properties.

• It doesn't take place in the dark or at low temperatures.
• It occurs in the presence of ultraviolet light or at temperatures above 250ºC.
• Once the reaction gets started, it continues after the light is turned off.
• The products of the reaction include CH₂Cl₂ (dichloromethane), CHCl₃ (chloroform), and CCl₄ (carbon tetrachloride), as well as CH₃Cl (chloromethane).
• The reaction also produces some C₂H₆.

These facts are consistent with a chain-reaction mechanism that involves three processes: chain initiation, chain propagation, and chain termination.

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Chain Reaction Mechanism

Chain Initiation

A Cl₂ molecule can dissociate into a pair of chlorine atoms by absorbing energy in the form of either ultraviolet light or heat.

Cl2

2 Cl ·

Ho = 243.4 kJ/molrxn

The chlorine atom produced in this reaction is an example of a free radical an atom or molecule that contains one or more unpaired electrons.

Chain Propagation

Free radicals, such as the Cl· atom, are extremely reactive. When a chlorine atom collides with a methane molecule, it can abstract a hydrogen atom to form HCl and a CH₃· radical.

CH4 + Cl·

CH3· + HCl

Ho = -16 kJ/molrxn

If the CH₃· radical then collides with a Cl₂ molecule, it can remove a chlorine atom to form CH₃Cl and a new Cl· radical.

CH3· + Cl2

CH3Cl + Cl·

Ho = -87 kJ/molrxn

Because a Cl· atom is generated in the second reaction for every Cl· atom consumed in the first, this reaction continues in a chain-like fashion until the radicals involved in these chain-propagation steps are destroyed.

Chain Termination

If a pair of the radicals that keep the chain reaction going collide, they combine in a chain-terminating step. Chain termination can occur in three ways.

2 Cl ·		Cl2			Ho = -243.4 kJ/molrxn
CH3· + Cl ·		CH3Cl			Ho = -330 kJ/molrxn
2 CH3·		CH3CH3			Ho = -350 kJ/molrxn

Because the concentration of the radicals is relatively small, these chain-termination reactions are relatively infrequent.
This chain-reaction mechanism for free-radical reactions explains the observations listed for the reaction between CH₄ and Cl₂.

• The reaction doesn't occur in the dark or at low temperatures because energy must be absorbed to generate the free radicals that carry the reaction.

Cl2

2 Cl·

Ho = 243.4 kJ/molrxn

• The reaction occurs in the presence of ultraviolet light because a UV photon has enough energy to dissociate a Cl₂ molecule to a pair of Cl· atoms. The reaction occurs at high temperatures because Cl₂ molecules can dissociate to form Cl· atoms by absorbing thermal energy.
• The reaction continues after the light has been turned off because light is only needed to generate the Cl· atoms that start the reaction. The chain reaction then converts CH₄ into CH₃Cl without consuming these Cl· atoms.

CH4 + Cl·		CH3· + HCl
CH3· + Cl2		CH3Cl + Cl·

• The reaction doesn't stop at CH₃Cl because the Cl· atoms can abstract additional hydrogen atoms to form CH₂Cl₂, CHCl₃, and eventually CCl₄.

CH3Cl + Cl·		CH2Cl· + HCl
CH2Cl· + Cl2		CH2Cl2 + Cl·,	and so on

• The formation of C₂H₆ is a clear indication that the reaction proceeds through a free-radical mechanism. When two CH₃· radicals collide, they combine to form a ethane molecule.

2 CH3·

CH3CH3

Free-radical halogenation of alkanes provides us with another example of the role of atom-transfer reactions in organic chemistry. The net effect of this reaction is to oxidize a carbon atom by removing a hydrogen from this atom.

CH4	+	Cl2		CH3Cl	+	HCl
-4				-2

The reaction, however, doesn't involve the gain or loss of electrons. It occurs by the transfer of a hydrogen atom in one direction and a chlorine atom in the other