Reactions of Alkyl Halides

Summary

• Alkyl halides are organic compounds containing carbon – halogen bond.
• The polarizability of carbon – halogen bond greatly influences the chemical reactivity of alkyl halides.
• Alkyl halides undergo nucleophilic substitution reaction.
• Substitution reaction can follow S_N1 or S_N2 mechanisms. 
• Both mechanisms follow different conditions and offer different products.
• Alkyl halides also form an important Grignard reagent which is commonly utilized to make carbon–carbon bonds. Thus, alkyl halides are important precursors to increase the carbon chain length.
• Keep reading for more facts about reactions of alkyl halides.

Haloalkanes or Alkyl Halides

Haloalkanes or commonly known as alkyl halides are organic compounds containing halogen atoms directly bonded to a carbon atom. They are derived from alkanes or alkenes. Halides are an important class of organic compounds being precursors in many organic syntheses. They find many industrial applications such as flame retardants, organic solvents, and propellants. The general structure of alkyl halides are represented as:

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. 

Also, moving from top to bottom in the halogen group of the periodic table, the size of the p orbital increases. This means electronic clouds in halogen atoms are more diffused. This eventually influences the overlapping with carbon sp3 orbitals and hence the strength of carbon – halogen bond decreases from fluorine to iodine. It also changes the bond length of the C – X bond from fluorine to iodine, top to bottom. 

Read more about The Reaction of Alkanes

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.

Nucleophilic substitution reactions

Nucleophile stands for “nucleus loving”. It is an atom or group of atoms that is rich in electron density and reacts (attacks) with electron-deficient sites. In the same way, electrophile (electron loving) is an electron-deficient atom or group of atoms seeking electrons. The nucleophilicity of a nucleophile depends upon its structure. Common nucleophile as listed here

Class	Nucleophile	Relative activity
Very powerful nucleophile	I-, HS-, RS-	>105
Good nucleophile	Br -, HO-, RO-, CN-, N3-	104
Fair nucleophile	NH3, Cl-, F-, RCO2-	103
Weak nucleophile	H2O, ROH	1
Very weak nucleophile	RCOOH	10-2

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. 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

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.

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: 

As mentioned earlier, primary, secondary & tertiary follow slightly different mechanisms in nucleophilic substitution reactions. For example, the rates of reaction for bromides of methyl, ethyl, isopropyl, and tert-butyl compounds are significantly different. Methyl bromide undergoes SN2 reaction quickly while tert-butyl bromide does not react to the same bromide anion. The reason for this behavior can be explained on the basis of steric hindrance. 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. 

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. 

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. Adding even stronger nucleophiles does not influence the rate of reaction. The reaction thus follows first-order kinetics where the rate of reaction is independent of concentration and nature of nucleophile. Moreover, nucleophile does not participate in the rate-determining step. This reaction mechanism is called an unimolecular nucleophilic substitution reaction and is denoted as S_N1.

The rate equation can be

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.

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

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 

S_N1 reactivity: methyl < primary < secondary < tertiary

S_N2 reactivity: tertiary > secondary > primary > methyl 

S_N2 is a bimolecular nucleophilic substitution reaction taking place in a single step while SN1 is a unimolecular substitution reaction taking place in more than one step. The order of reaction for both mechanisms is also different. Both pathways are important for the synthesis of various organic compounds.

Examples of nucleophilic reaction of alkyl halides

Preparation of ether via williamson synthesis

This is a traditional method of preparing ether using alkyl halides and alkoxide. The reaction is more successful when modeled towards S_N2 mechanism conditions. That’s why ence methyl halides and other primary alkyl halides are the best starting point. 

Preparation of amines

Alkylamines can be prepared by the reaction of alkyl halides and ammonia. The reaction if not controlled may proceed in further steps to further alkylation of amines making, monomethyl, dimethyl, and trimethyl derivatives. 

Preparations of thiols

Sulfur is the element most like oxygen. The sulfur analogue of alcohols (R-OH) is called thiol (R-SH). These thiols are prepared by S_N2 reaction where sulfur source e.g. thiourea can react with an alkyl halide to form urea and thiol. The reaction goes through an intermediate; isothiouronium salt that on hydrolysis forms thiol.

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.

Dehydrohalogenation: Elimination reaction of alkyl halides

Dehydrohalogenation is the removal of elements of hydrogen halide (HX) from an alkyl halide. It is one the most easily utilized methods of preparation of alkenes by β elimination. The reaction is carried out in the presence of a strong base such as sodium hydroxide or potassium hydroxide in ethyl alcohol as a solvent.

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

That’s why alkyl iodides have more tendency to follow this elimination mechanism and form alkenes. 

Frequently Asked Questions

What are the most common reactions of alkyl halides?

Alkyl halides mostly undergo nucleophilic substitution reactions due to their polar nature.

How are alkenes produced from alkyl halides?

Alkenes are produced from alkyl halides by dehydrohalogenation reactions. HX component of alkyl halide is removed from alkyl halide in the presence of a strong base to produce alkene.

CH₃CH₂CL + KOH → KCL +CH₂CH₂ +H₂O

What is the order of reactivity of alkyl halides?

The decreasing order of reactivity of alkyl halides (RX) is:

RI > RBr > RCl > RF

How are alkyl halides converted to alcohol?

Alkyl halides are converted to alcohol by nucleophilic substitution reactions. In these reactions, OH^- ions of water act as a nucleophile, attacking partial positive carbon of RX and producing alcohol.

RX + H₂O→ ROH + HX

Books for further study

1. Morrison, R. T., and R. N. Boyd. "Organic chemistry 5th edition." (1987).
2. Cary, A, F. Organic Chemistry, 3^rd edition, (1996). 
3. Volhardt K, P, C. Organic Chemistry, (1987).
4. Smith M, B and March, J. March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 5^th edition (2001).

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