Substitution reactions replace a functional group of an organic compound with a second reactant, adding functionality or reactivity. Alkanes can be modified to form new molecules, such as halogenated hydrocarbons or alcohols. SN1 and SN2 reactions compete with each other, with SN2 reactions being more common. Factors such as nucleophile strength and reaction conditions determine the mechanism and result.
A substitution reaction is a chemical reaction in which one constituent of an organic compound, a carbon molecule and other elements, is replaced or replaced by a functional group of a second reactant. Functional groups, reactive subsets of organic compounds, replace hydrogen or other functional groups of lesser activity. A substitution reaction can add functionality or reactivity to alkanes, straight-chain hydrocarbons, and other compounds.
Alkanes, the simplest of the hydrocarbons, consist of straight chains of varying lengths of carbon-carbon covalent bonds surrounded by hydrogen atoms. Covalent bonds between carbon atoms share the outermost electrons to form a stable configuration. Organic chemists replace functional groups at desired points in the carbon structure to build new molecules to be used as end products or precursors to formulations of other useful compounds.
The substitution reaction of an alkane with a halogen, including chlorine, fluorine or bromine, produces halogenated hydrocarbons, also called alkyl halides. Alkyl halides can continue to be modified to form multi-substituted compounds. Common examples include chlorofluorocarbons (CFCs), which were previously used as refrigerant fluids. If the group being added is a hydroxyl group (—OH-) from either reaction in basic solutions or water, alcohols or haloalcohols will be formed.
The carbon-halogen bond is stronger than the covalent bond of the carbon-carbon bond. The halide pulls the electron pair towards itself, leaving the central carbon slightly positive. The substitution in this scenario is called a nucleophilic substitution, as the negatively charged, nucleus-loving, nucleophilic hydroxide group or additional halide atom approaches the alkyl halide from the opposite side of the first halide atom. The negative charge on the approaching group avoids the negative charge on the existing halide group.
One carbon normally bonds with four other atoms in a tetrahedron, shaped like a triangular pyramid. A dexterity of the molecule is possible when replaced by two different groups. Approaching the second nucleophile from a single direction causes the products to have the same three-dimensional configuration. The second nucleophile causes the tetrahedron to tip over as it bonds to the central carbon, much like an umbrella turning inside out in the wind. This is an SN2 substitution reaction: substitution by a nucleophile in a bimolecular reaction.
In an SN1 substitution reaction, the halide takes control of the electron pair for a short time. The now highly positively charged central carbon atom tries to pull its bonds apart as much as possible, forming a planar triangular shape instead of a tetrahedron. The second nucleophile can approach the carbon from either side, forming a mixture of racemic products, equal concentrations of the right and left species of the compound.
SN1 and SN2 reactions compete with each other; SN2 reactions are more common. The strength of the nucleophile, the strength of the displaced group, and the ability of the solvent to support charged species are some of the factors that determine the reaction mechanism. The reaction conditions, especially the temperature, will influence the result.
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