This lesson describes haloalkane substitution reactions. The structure and function of nucleophiles, electrophiles, and haloalkanes are discussed. The mechanisms of SN1 and SN2 reactions are discussed.
Imagine sleeping and suddenly you hear a noise in the house. Your heart begins to beat faster and all of a sudden there is a flood of adrenaline racing through your body. Have you ever wondered how adrenaline is activated so quickly?
The precursor to epinephrine (adrenaline) is a nucleophile that grabs a methyl group from another molecule in the body called SAM (S-Adenosyl methionine). This allows epinephrine to be activated quickly in 'fight or flight' situations like when things go bump in the night.
Organic chemists use a similar reaction, called nucleophilic substitution, to put new functional groups on molecules.
What is a Nucleophile?
A nucleophile is a molecule that is electron rich. Many times you can identify these because they have lone pair electrons on an atom or a negative charge. For example, a hydroxide ion has a negative charge and is a nucleophile.
Hydroxide is a nucleophile - note the lone pairs of electrons and the negative charge.
Nucleophiles attack electrophiles, which are electron deficient. Many times, these atoms have a positive charge. Remember, positive and negative charges are attracted to each other. This is what we observe in nucleophilic substitution reactions.
What is a Haloalkane?
Haloalkanes or alkyl halides are carbon molecules that are attached to a halogen (F, Br, Cl, or I). The carbon is less electronegative than the the halogen, so the carbon will have a partial positive charge. The halogen will have a partial negative charge. This means that the carbon of the haloalkane can be attacked by a nucleophile.
The carbon of the alkyl halide has a partial positive charge while the chlorine has a partial negative charge
Alkyl halides are classified based on the adjacent carbon.
primary alkyl halide - halogen is attached to a carbon with two hydrogens
secondary alkyl halide - halogen is attached to a carbon with one hydrogen
tertiary alkyl halide - halogen is attached to a carbon with no hydrogens
Types of Substitution Reactions
Haloalkanes can react in two types of substitution reactions: SN1 and SN2.
An SN1 reaction is a substitution (S) reaction that involves a nucleophile (N) and is unimolecular (1). Unimolecular means that the speed of the reaction depends on only one molecule: the nucleophile.
In an SN1 reaction, the halogen-carbon bond breaks, resulting in a positively charged carbon. The nucleophile is attracted to the positive charge, and a new bond is formed. This is a two-step mechanism and most often occurs with tertiary alkyl halides.
An SN2 reaction is a substitution (S) reaction that involves a nucleophile (N) and is bimolecular (2). Bimolecular means that the speed of the reaction depends on both the nucleophile and the haloalkane.
In an SN2 reaction, the nucleophile attacks the partially positive carbon next to the halogen. This is called a backside attack because of the way the nucleophile approaches the carbon. Then, the halogen leaves the molecule. This occurs in one step, so it is a concerted reaction.
This reaction occurs most often with primary and secondary haloalkanes. Tertiary haloalkanes have too large an alkyl group for a backside attack to occur.
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In SN2 reactions, the identity of the halogen affects how fast the reaction occurs. The haloalkane that reacts the fastest is alkyl iodide. Alkyl bromides are the next fastest, followed by alkyl chlorides, with alkyl fluorides being the slowest. Therefore, you can move from I on the periodic table, up through the halogens to get slower and slower reactions.
In SN2 reactions, the halogen affects the speed of the reaction. I is fastest, then Br, Cl, and finally F as the slowest
Nucleophiles are electron rich molecules while electrophiles are electron deficient molecules. Haloalkanes are carbon molecules attached to a halogen - F, Cl, Br, or I. They are electrophiles.
Haloalkanes are classified based on the adjacent carbon.
primary alkyl halide - carbon with two hydrogens
secondary alkyl halide - carbon with one hydrogen
tertiary alkyl halide - carbon with no hydrogens
Haloalkanes can undergo two types of substitution reactions:
SN1 - The halogen leaves the haloalkane and the nucleophile attacks the positive charge on the carbon.
SN2 - The nucleophile attacks the carbon attached to the halogen, replacing the halogen.
Tertiary haloalkanes undergo SN1 reactions while secondary and primary haloalkanes undergo SN2 reactions. Organic chemists use these reactions to change the functional group on molecules. The haloalkane that reacts the fastest is alkyl iodide, followed by alkyl bromides, then alkyl chlorides, then alkyl fluorides.
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