Alkyl Halides

DEEPSHIKHA

DEPARTMRNT OF CHEMISTRY

Alkyl halide or haloalkanes are formed by the replacement of hydrogen atoms in an aliphatic hydrocarbon by halogen atoms (Fluorine, chlorine, bromine or iodine). They can also be manufactured from any organic precursors such as alkanes, alkenes, or alcohols and carboxylic acids.

Classification Of Alkyl Halide

Alkyl Halide can be classified on the basis of various aspects. They are as follows.

Number of Halogen Atoms

Here, the classification mainly depends on whether they contain one, two, or more halogen atoms in their structure. Under this category we have;

1. Mono Haloalkane

Example: CH₃-CH₂-X [Where X can be Cl, F, Br or I]

2. Dihaloalkane

Example: X-CH₂-CH₂-X [Where X can be Cl, F, Br or I]

3. Trihaloalkane

Example: X-CH₂-CHX-CH₂-X [Where X can be Cl, F, Br or I]

The Position of Halogen atom Along the Chain of Carbon Atom

The classification depends on how the halogen atom is positioned on the chain of carbon atoms.

Primary alkyl halide

Secondary alkyl halide

Tertiary alkyl halide

Primary Alkyl Halide

In this type of haloalkanes, the carbon which is bonded to the halogen family will be only attached to one other alkyl group. It doesn’t matter how much bulky group is attached to it.

Some examples of primary haloalkanes are,

Secondary Alkyl Halide

In this type of haloalkanes, the carbon atom which is bonded with the halogen atom is joined directly to the other two alkyl groups which can be the same or different. Some examples are:

Tertiary Alkyl Halide

In this type of haloalkanes, the carbon atom which carries the halogen atom is directly bonded to three alkyl group. This alkyl group maybe with a combination of the same or different. Some examples are,

Alkyl halides are colourless when they exist in pure form. But, bromides and iodides develop colour when exposed to light. Many volatile halogen compounds have a sweet smell.

Physical Properties

Boiling and Melting Points

Methyl chloride, methyl bromide, ethyl chloride and some chlorofluoromethanes are in the form of gas at room temperature

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Higher members are liquids or solids.

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As we know, molecules of organic halogen compounds are polar in nature.

Due to greater polarity and greater molar mass as compared to parent hydrocarbon, the intermolecular force of attraction is stronger in halogen derivatives.

Density

Bromo-derivatives, iodo-derivatives and polychloro derivatives of hydrocarbons are heavier than water.

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The density increases with an increase in the number of carbon atoms, halogen atoms and atomic mass of halogen atoms.

Solubility

The haloalkanes are less soluble in water.

The nature of Carbon – Halogen bond

In an alkyl halide molecule, the carbon – halogen bond is made up of a sp3 hybrid overlapping with the p orbital of the halogen atom. Here the interesting point is the difference in electronegativity of carbon and halogen atoms. Halogens are known for their high electronegativity e.g. F & Cl. This induces dipolar character in carbon – halogen bond of alkyl halide. Thus, electron density along the C – X bond is more inclined towards halogen giving halogen partially negative charge and carbon partially positive charge. 

The polar character of the C – X bond

Reactions of alkyl halides

This polar character of the C – X bond is responsible for some of the characteristic chemical reactions of alkyl halides. The partial positive charge at the carbon center guides the attack of nucleophiles and drives nucleophilic substitution reactions.

A substitution reaction where a nucleophile replaces another nucleophile is known as a nucleophilic substitution reaction. These substitution reactions are a characteristic feature of alkyl halides. As described earlier, the induced dipolar nature of the C – X bond creates partial positive & negative charges on carbon and halogen centers. Here in the presence of a strong nucleophile (stronger than already attached to carbon), incoming nucleophile replaces the halogen.

Nucleophilic substitution reactions

A simple illustration is shown below:

This substitution reaction is greatly influenced by nature of attacking nucleophile, substrate, and leaving group. In primary or secondary alkyl halides, attack of incoming nucleophile and displacement of leaving group takes place in a single step, however, in bulky tertiary alkyl halides, attack of incoming nucleophile and displacement of leaving group takes place in two steps. Hence, these substitution reactions follow two different mechanisms known as S_N1 and S_N2 reactions. 

S_N2 reaction mechanism

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A simple example of the SN2 mechanism is the reaction of methyl bromide with hydroxide ion (base). The attack of incoming hydroxide at a partially positively charged carbon center displaces the bromide. The formation of a new bond of hydroxide ion and cleavage of bromide bond to carbon center takes place in a single step. Actually, the nucleophile pushes off the leaving group from carbon. The kinetic data suggest that the rate of reaction is directly proportional to the concentration of both methyl bromide and hydroxide ion. Thus it is first order in each reactant and second-order overall reaction.

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Rate of reaction = k [ CH₃Br ] [ ^-OH ]

Therefore, is it a bimolecular reaction where both attacking nucleophile and substrate are involved in the formation of a transition state. In this transition state, carbon is bonded to both the incoming nucleophile and the departing leaving group. Such reaction is known as bimolecular nucleophilic substitution reaction denoted as S_N2.

An interesting aspect of the SN2 mechanism is the 100% inversion of configuration at the reaction site. It means the incoming nucleophile attacks carbon from the side opposite to the leaving group. In the end, the incoming nucleophile gets attached to the position opposite to the leaving group.

This can be shown as: 

Methyl is a smaller group compared to tert-butyl. For methyl, the attack of incoming nucleophiles from the backside is relatively easy because the three hydrogens attached do not pose any hindrance to the attacking group. While in tert-butyl group, three bulky methyl groups are attached to the electrophilic carbon center offering steric hindrance (opposition) to the incoming nucleophiles. 

 nucleophiles. 

S_N1 reaction mechanism

As mentioned in the previous section, tert-butyl bromide does not undergo substitution via the SN2 reaction mechanism. These bulky halides actually undergo nucleophilic substitution reaction following a slightly different mechanism, called the S_N1 mechanism. 

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S_N1 mechanism.

The kinetic data shows that the rate of reaction depends only upon tert-butyl bromide. The nucleophile (water in the above equation) does not contribute to the rate of reaction. 

Rate of reaction = k [ (CH₃)₃CBr ]

Thus reaction mechanism can be written as 

In the first step, alkyl halide dissociates to form a carbocation and a halide ion. The formation of the carbocation is a fundamental feature of the S_N1 mechanism.

The carbocation formed in the first step reacts rapidly with a water molecule (nucleophile). The step completes nucleophilic substitution and forms an oxonium ion.

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The last step is a fast acid-base reaction. Here water acts as a base and removes the proton from oxonium ion to give the final product i.e. tert-butyl alcohol. It is clear that S_N1 is an ionization mechanism. 

Comparing S_N2 vs S_N1 mechanism

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In summary, both mechanisms involve the formation of slightly different intermediates. The relative rate of reaction for S_N1 and S_N2 reactions changes by changing the alkyl group. We may characterize it as 

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S_N1 reactivity: methyl < primary < secondary < tertiary

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S_N2 reactivity: tertiary > secondary > primary > methyl 

Examples of nucleophilic reaction of alkyl halides

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Preparation of ether via williamson synthesis

Preparation of amines

Other chemical reactions of alkyl halides

Preparation of Grignard reagent

The grignard reagent is an organomagnesium compound. It is the most important organometallic compound among the family members. It can be prepared directly by alkyl halide and magnesium in anhydrous diethyl ether solvent. This organomagnesium reagent act as bronsted base and is used in various organic syntheses. This is used to synthesize carbon–carbon bonds.

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This elimination reaction also depends upon the nature of the alkyl group and halide attached. Its rate of reaction increases with decreasing strength of carbon – halogen bond. Hence dehydrogenation increases for halides as 

F < Cl < Br < I

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