Chapter 11: Alkyl Halides – Properties, Radical Reactions, and Stereochemistry

Alkyl Halides

Classification and Structure

Alkyl halides are organic compounds in which a halogen atom (F, Cl, Br, I) is bonded to an sp3 hybridized carbon atom. They are fundamental intermediates in organic synthesis and fall into three major categories:

• Haloalkane (alkyl halide): Halogen attached to an alkane carbon.

• Alkenyl halide (vinyl halide): Halogen attached to an alkene carbon.

• Aryl halide (haloarene): Halogen attached to an aromatic ring.

Students should be familiar with the nomenclature of these molecules, following IUPAC rules.

Physical Properties

Alkyl halides are generally polar due to the electronegativity of the halogen atom, which withdraws electron density from the carbon atom. This polarity influences their physical properties:

• Boiling and melting points: These are determined by the size and polarizability of the halogen. Larger, more polarizable halogens lead to higher boiling points.

C–X	μ (Debye)	C–X bond length (Å)
CH3F	1.85	1.39
CH3Cl	1.87	1.78
CH3Br	1.83	1.94
CH3I	1.63	2.14

C–X	b.p. (°C)
CH3F	-38
CH3Cl	-12
CH3Br	4
CH3I	42

Alkanes and Radicals

Reactivity of Alkanes

Alkanes possess only strong σ (sigma) bonds and lack the π (pi) bonds found in alkenes and alkynes, making them relatively unreactive and nonpolar. Radical chemistry is essential to activate alkanes for further reactions.

• Radical: An atom or molecule with an unpaired electron.

• In reaction mechanisms, the movement of an unpaired electron is shown by a half-headed arrow (fishhook).

Bond Cleavage: Homolytic vs. Heterolytic

Bond breaking can occur in two ways:

• Homolytic cleavage: Each atom retains one electron, forming two radicals. $A-B \xrightarrow{homolytic} A^{\cdot} + B^{\cdot}$

• Heterolytic cleavage: One atom retains both electrons, forming ions. $A-B \xrightarrow{heterolytic} A^{+} + B^{-}$

Radical Stability

Radical stability parallels carbocation stability: more alkyl groups attached to the radical carbon increase stability due to hyperconjugation and inductive effects.

• Tertiary radical > Secondary radical > Primary radical > Methyl radical

Halogenation of Alkanes

General Reaction and Conditions

Alkanes react with halogens (Cl2, Br2) under high temperature or light (hv) to form alkyl halides via a radical mechanism.

• Example reactions:

$CH_4 + Cl_2 \xrightarrow{\Delta \text{ or } hv} CH_3Cl + HCl$ $CH_3CH_3 + Br_2 \xrightarrow{\Delta \text{ or } hv} CH_3CH_2Br + HBr$

Mechanism Steps

The halogenation of alkanes proceeds in three steps:

1. Initiation: Homolytic cleavage of the halogen molecule forms two halogen radicals. $Br_2 \xrightarrow{\Delta \text{ or } hv} 2Br^{\cdot}$

2. Propagation: Halogen radical abstracts a hydrogen from the alkane, forming an alkyl radical, which then reacts with another halogen molecule. $Br^{\cdot} + H-CH_3 \rightarrow HBr + CH_3^{\cdot}$ $CH_3^{\cdot} + Br_2 \rightarrow CH_3Br + Br^{\cdot}$

3. Termination: Radicals combine to form stable molecules, ending the chain reaction. $Br^{\cdot} + Br^{\cdot} \rightarrow Br_2$ $Br^{\cdot} + CH_3^{\cdot} \rightarrow CH_3Br$ $CH_3^{\cdot} + CH_3^{\cdot} \rightarrow CH_3CH_3$

Regioselectivity: Bromination vs. Chlorination

Bromination is more selective than chlorination, favoring the formation of more stable radicals and thus more substituted alkyl halides.

Product	Chlorination (%)	Bromination (%)
2-chloropropane	60	—
1-chloropropane	40	—
2-bromopropane	—	97
1-bromopropane	—	3

Radicals and Alkenes

Radical Addition to Alkenes

Alkenes react with HBr in the presence of peroxides to yield anti-Markovnikov products via a radical mechanism. Without peroxides, Markovnikov addition occurs.

• Markovnikov addition: HBr adds so that Br attaches to the more substituted carbon. $CH_3CH_2CH=CH_2 + HBr \rightarrow CH_3CH_2CHBrCH_3$ (2-bromobutane)

• Anti-Markovnikov addition: In the presence of peroxides, Br attaches to the less substituted carbon. $CH_3CH_2CH=CH_2 + HBr \xrightarrow{peroxide} CH_3CH_2CH_2CH_2Br$ (1-bromobutane)

No carbocation intermediate is formed in the radical mechanism, so rearrangement does not occur. This reaction is specific to HBr due to thermodynamic factors.

Allylic Bromination with NBS

Allylic bromination can be achieved using N-bromosuccinimide (NBS), which selectively brominates the allylic position of alkenes via a radical mechanism.

• NBS: Provides a low, steady concentration of Br2 for selective allylic bromination.

• Mechanism involves formation of an allylic radical and subsequent reaction with Br2.

Radical Substitution Products

Product Distribution

The number of different alkyl halides formed depends on the number of unique hydrogens in the alkane. More substituted positions yield major products due to radical stability.

• Example: When reacting branched alkanes with Br2 and light, the major product is typically the one formed via the most stable (tertiary) radical intermediate.

Stereochemistry

Chirality and Racemic Mixtures

Substitution at a chiral center can lead to the formation of racemic mixtures of stereoisomers. Radical intermediates are planar, allowing attack from either side and resulting in both enantiomers.

• Example: Bromination of a chiral alkane can yield both R and S enantiomers in equal amounts.

Additional info: Radical reactions are crucial for the functionalization of otherwise inert alkanes and alkenes, expanding the synthetic utility of these molecules in organic chemistry.