Halogenoalkanes are otherwise simple alkanes that contain one or more members of the halogen family. In practice, the halogens found in organic molecules are chlorine (Cl), bromine (Br), fluorine (F), and iodine (I). Some texts refer to this class of compounds as haloalkanes or alkyl halides. The chemical literature will frequently use the terms interchangeably.

Note: The X in R-X represents a generic halogen atom.

Naming Halogenoalkanes

Haloalkanes are named by adding a prefix to the name of the alkane from which they are derived. The prefix denotes the particular halogen used.

F = Fluoro-
Cl = Chloro-
Br = Bromo-
I = Iodo-

If other substituents need to be named, all prefixes are still put in alphabetical order. When necessary, numbers identify substituent locations.

Example names of halogenoalkanes

	IUPAC name	Common name
CH₃F	FluoroMethane	Methyl fluoride
CH₃Cl	ChloroMethane	Methyl chloride
CH₃Br	BromoMethane	Methyl bromide
CH₃I	IodoMethane	Methyliodide
CH₂F₂	DiFluoroMethane	Methylene fluoride
CH₂ClCH₂	DiChloroMethane	Methylene chloride
CH₂FCl	ChloroFluoroMethane
CHBrClF	BromoChloroFluoroMethane
HCCl₃	TriChloroMethane	Chloroform
CHX₃	Haloforms (X=halogen)
CCl₄	TetraChloroMethane	Carbon tetrachloride
CH₃CHCl₂	1,1-DiChloroEthane
	1,6-DiChloro-2,5-DiMethylHexane

Physical properties

R-X bond polarity: most polar C—F > C—Cl > C—Br > C—I least polar

atom \|	electronegativity \|	difference from C (= 2.5) \|
F	4.0	1.5
Cl	3.0	0.5
Br	2.8	0.3
I	2.5	0.0

The difference in electronegativity of the carbon-halogen bonds range from 1.5 in C-F to almost 0 in C-I. This means that the C-F bond is extremely polar, though not ionic, and the C-I bond is almost nonpolar.

Physical appearance: Halogenoalkanes are colourless when pure. However bromo and iodo alkanes develop colour when exposed to light. Many volatile halogen compounds have sweet smell.

Boiling point: Halogenoalkanes are generally liquids at room temperature. Halogenoalkanes generally have a boiling point that is higher than the alkane they are derived from. This is due to the increased molecular weight due to the large halogen atoms and the increased intermolecular forces due to the polar bonds, and the increasing polarisabilty of the halogen.

For the same alkyl group, the boiling point of halogenoalkanes decreases in the order RI > RBr > RCl > RF. This is due to the increase in van der Waals forces when the size and mass of the halogen atom increases.

For isomeric halogenoalkanes, the boiling point decreases with increase in branching.

Density: Halogenoalkanes are generally more dense than the alkane they are derived from and usually more dense than water. Density increases with the number of carbon and halogen atom. It also increases with the increase in mass of halogen atom.

Solubility: The halogenoalkanes are only very slightly soluble in water, but dissolve in organic solvents. This is because for dissolving halogenoalkanes in water the strong hydrogen bonds present in water have to be broken. When dissolved in organic (non polar) solvents, the intermolecular attractions are almost same as that being broken.

Bond Length: shortest C—F < C—Cl < C—Br < C—I longest

bond	length (pm)
C-F	138
C-Cl	177
C-Br	193
C-I	214

Larger atoms means larger bond lengths, as the orbitals on the halogen is larger the heavier the halogen is. In F, the orbitals used to make the bonds is 2s and 2p, in Cl, it's 3s and 3p, in Br, 4s and 4p, and in I, 5s and 5p. The larger the principal quantum number, the bigger the orbital. This is somewhat offset by the larger effective nuclear charge, but not enough to reverse the order.

Chemical properties

Bond strength: strongest C—F > C—Cl > C—Br > C—I weakest

bond	strength (kJ mol^-1)
C-F	484
C-Cl	338
C-Br	276
C-I	238

Bond reactivity: least reactive C—F < C—Cl < C—Br < C—I most reactive

Stronger bonds are more difficult to break, making them less reactive.

Reactions

Substitution reactions of halogenoalkanes

R-X bonds are very commonly used throughout organic chemistry because their polar bonds make them reasonably reactive. In a substitution reaction, the halogen (X) is replaced by another substituent (Y). The alkyl group (R) is not changed.

Reminder: The ":" in a chemical formula represents a pair of unbound electrons (a "lone pair").

A general substitution reaction

Y: + R—X → R—Y + X:

Substitutions involving halogenoalkanes involve a type of substitution called Nucleophilic substitution, in which the substituent Y is a nucleophile. A nucleophile is an electron pair donor. The nucleophile replaces the halogen, which becomes a leaving group. Nucleophilic substitution reactions are abbreviated as S_N reactions.

For general discussion of these reaction, we can use "Nu:" or "Nu:^-" to represent a generic nucleophile.

Nu:^- + R—X → R—Nu + X:^-
Nu: + R—X → R—Nu⁺ + X:^-

**Common Nucleophiles**
Reagent	Nucleophile	Name	Product	Product name
NaOH/KOH	:O^-H	Hydroxide	R—OH	Alcohol
NaOR'	:O^-R'	Alkoxide	R—O—R'	Ether
	:S^-H	Hydrosulfide	R—SH	Thiol
NH₃	:NH₃	Ammonia	R—NH₃⁺	Alkylammonium ion
KCN	:C^-N	Cyanide	R—CN	Nitrile
AgCN	Ag-CN:	Silver cyanide	R-NC	isonitrile
	:C^-≡C—H	Acetylide	R-C≡C—H	Alkyne
NaI	:I^-	Iodide	R—I	Alkyl Iodide
R'^-M⁺	:R'^-	Carbanion	R-R'	Alkane
KNO₂	O=N—O	Nitrite	R–O—N=O	Alkyl nitrite
AgNO₂	Ag—Ö—N=O	Silver nitrite	R—NO₂	Nitroalkane
LiAlH₄	H	Hydrogen	RH	alkane
R'COOAg	R'COO^-	Alkanoate	R'COOR	Ester

Example: Suggest a reaction to produce the following molecule.

Answer:

+

OR

+

Any halogen could be used instead of Br

Reaction mechanisms

Nucleophilic substitution can occur in two different ways, but we only consider the S_N2 mechanism.

Nucleophilic Substitution Reaction Mechanism

In nucleophilic substitution, a nucleophile attacks a molecule and takes the place of another nucleophile, which then leaves. The nucleophile that leaves is called the leaving group. A nucleophile has a lone pair of electrons and is often negatively charged. The leaving group is the part of the organic molecule which breaks free and forms a new nucleophile.

S_N2

The S_N2 reaction is a 1-step reaction where the attacking nucleophile, because of its higher affinity for and stronger bonding with the carbon, forces the leaving group to leave. These two things happen in a single step.

In S_N2 reactions, the single step of the nucleophile coming together with the reactant from the opposite side of the leaving group, is the key to its rate. Because of this, the rate is dependent on both the concentration of the nucleophile as well as the concentration of the reactant. The higher these two concentrations, the more frequent the collisions. Thus the reaction rate is a second-order reaction:

Rate = v = k[Nu:][RX] (where Nu: is the attacking nucleophile)

There are primarily three things that affect whether an S_N2 reaction will take place or not. The most important is structure. That is whether the halogenoalkane is on a methyl, primary, secondary, or tertiary carbon. The other two components that determine whether an S_N2 reaction will take place or not, are the nucleophilicity of the nucleophile and the solvent used in the reaction.

Reactivity Due to Structure of S_N2

CH₃X > RCH₂X > R₂CHX >> R₃CX

The structure of the halogenoalkane has a great effect on mechanism. CH₃X & RCH₂X are the preferred structures for S_N2. R₂CHX can undergo the S_N2 under the proper conditions (see below), and R₃CX rarely, if ever, is involved in S_N2 reactions.

S_N2 nucleophilic substitution of bromine with a generic nucleophile

The reaction takes place by the nucleophile attacking from the opposite side of the bromine atom. Notice that the other 3 bonds are all pointed away from the bromine and towards the attacking nucleophile. When these three bonds are C-H bonds, there's very little steric hindrance of the approaching nucleophile. However, as the number of R groups increases, so does the steric hindrance, making it more difficult for the nucleophile to get close enough to the carbon, to expel the bromine atom. In fact, tertiary carbons (R₃CX) are so sterically hindered as to prevent the S_N2 mechanism from taking place at all.

In the case of this example, a secondary carbon, there is still a great deal of steric hindrance and whether the S_N2 mechanism will happen will depend entirely on what the nucleophile and solvent are. S_N2 reactions are preferred for methyl halides and primary halogenoalkanes.

Another important point to keep in mind, and this can be seen clearly in the example above, during an S_N2 reaction, the molecule undergoes an inversion. The bonds attached to the carbon are pushed away as the nucleophile approaches. During the transition state, these bonds become planar with the carbon and, as the bromine leaves and the nucleophile bonds to the carbon, the other bonds fold back away from the nucleophile. This is particularly important in chiral or pro-chiral molecules, where an R configuration will be converted into an S configuration and vice versa.

Examples:

OH^- + CH₃—Cl → HO—CH₃ + Cl^-

OH^- is the nucleophile, CH₃—Cl is the electrophile, HO—CH₃ is the product, and Cl^- is the leaving group.

or,

Na⁺I^- + CH₃-Br → I-CH₃ + Na⁺Br^-

The above reaction, taking place in acetone as the solvent, sodium and iodide disassociate almost completely in the acetone, leaving the iodide ions free to attack the CH-Br molecules. The negatively charged iodide ion, a nucleophile, attacks the methyl bromide molecule, forcing off the negatively charged bromide ion and taking its place. The bromide ion is the leaving group.

Leaving Group

The leaving group is the group on the substrate that leaves. In the case of a halogenoalkane, this is the halide ion that leaves the carbon atom when the nucleophile attacks. The tendency of the nucleophile to leave is

Relative Reactivity of Leaving Groups

I^– > Br^– > Cl^– >> F^–

Fluoride ions are very poor leaving groups because they bond very strongly and are very rarely used in halogenoalkane substitution reactions.

Elimination Reactions

With alcoholic potassium hydroxide, halogenoalkanes lose H-X and form the corresponding alkene.

Nucleophilic substitution reactions and elimination reactions share a lot of common characteristics, on top of which, the reactions can sometimes compete and, since their products are different, it's important to understand them both. Without understanding both kinds of mechanisms, it would be difficult to get the product you desire from a reaction.

In addition, the S_N2 reaction will be referenced quite a bit by way of comparison and contrast, so it's probably best to read that section first and then continue here.

Elimination reactions create alkene products from halogenoalkane reactants. Elimination, unlike substitution, does not occur with methyl halides because the reaction creates a double bond between two carbon atoms and methylhalides have only one carbon.