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Chapter 6: Nucleophilic Substitution and Elimination Reactions of Alkyl Halides Back To Chapter

6.1: Alkyl Halides

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Alkyl Halides

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6.2: Nucleophilic Substitution Reactions

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Alkyl halides, or haloalkanes, are organic compounds comprised of an alkyl group and a halogen atom bonded to an sp³ hybridized carbon.

Each carbon atom is defined by its proximity to the halogen atom. The carbon bonded directly to the halogen is denoted “alpha-carbon”; the carbon next to it is called “beta-carbon” and so forth.

Depending on the number of alkyl groups on the alpha carbon, alkyl halides are further classified as primary, secondary, and tertiary.

Nomenclature of alkyl halides follows similar IUPAC rules as for alkanes. Halogens are regarded as substituents and named fluoro-, chloro-, bromo-, and iodo-.

First, count the carbons on the longest chain. Here, five corresponds to pentane. Secondly, find and name the substituents. Here it is bromo-, chloro-, and methyl.

Next, number the parent chain and assign a locant to the substituents giving them the lowest possible number. Lastly, assemble the substituents alphabetically. This compound is called 4-bromo-1-chloro-4-methyl pentane.

If the chain contains a double bond, the double bond has precedence and obtains the lowest number giving the compound 3-fluoro-1-butene.

If a molecule is cyclic, chiral, and with multiple substituents, the substituents are likewise assigned the lowest locant possible, and the groups are alphabetically ordered. The stereocenters are indicated at the beginning of the name, giving (1R,2R)-3-butyl-1,2-dichloro-5-ethylcyclohexane.

Alkyl halides are used as building blocks in syntheses because of their reactivity. Going down group 17, the halogen’s atom size and bond length increase, while electronegativity and basicity decrease.

The higher electronegativity of halogens leads to the withdrawal of electron density from the adjacent carbon making the alpha carbon electrophilic and, hence, more reactive.

The alkyl halide’s reactivity is further influenced by the stability of the halogen’s conjugate base. Iodide, bromide, and chloride are stable yet weak conjugate bases of their strong hydrohalic acids. Because of their stability, they can be more easily substituted by a stronger base, making them good leaving groups.

6.1: Alkyl Halides

Structural Properties

Alkyl halides are halogen-substituted alkanes wherein one or more hydrogen atoms of an alkane is replaced by a halogen atom such as fluorine, chlorine, bromine, or iodine. The carbon atom in an alkyl halide is bonded to the halogen atom, which is sp³-hybridized and exhibits a tetrahedral shape.

Unlike alkyl halides, compounds in which a halogen atom is bonded to an sp²-hybridized carbon atom of a carbon-carbon double bond (C=C) are called vinyl halides. Whereas aryl halides are compounds in which a halogen atom is bonded to an sp²-hybridized carbon atom of an aromatic ring.

Furthermore, depending on the degree of substitution at the carbon atom containing the halogen, alkyl halides are classified into primary, secondary, and tertiary alkyl halides. The carbon bonded to a halogen atom is called the α- carbon. The carbon atom linked to the α-carbon is called β-carbon. The adjacent carbons are subsequently denoted by greek alphabets (γ, δ, and so forth).

Nomenclature

Naming alkyl halides follows the general IUPAC rules of organic chemistry:

Identify the longest carbon chain and name it.
Find and name the substituent(s).
Number the longest carbon chain and designate a number, or locant, for each substituent.
Assemble the substituents in alphabetical order.

Reactivity and Applications

Alkyl halides are versatile compounds that serve as solvents, pesticides, and intermediates in the preparation of dyes, drugs, and synthetic polymers. The chemical reactivity of alkyl halides differs depending upon the structural classification. Alkyl halides are classified into primary, secondary, and tertiary alkyl halides depending on the degree of substitution on a carbon-bearing halogen.

Furthermore, the halogen atom’s electronegativity plays a significant role in the alkyl halide’s reactivity. Except for iodine, other halogens have a significantly greater electronegativity than carbon. Due to electronegativity differences between carbon and halogen atoms, the covalent bond between the atoms is polarized; thus, the carbon carries a partial positive charge and the halogen atom a partial negative charge. Consequently, the carbon attached to the halogen is an electrophile.

In a periodic table, going down the halogen family, the electronegativity decreases; inversely, the halogen atom size increases. Thus, the carbon-halogen bond length increases while bond strength and polarity decrease from fluorine to iodine, making it easier to break the bond.

Apart from these factors, the relative stability of the corresponding conjugate bases, or halide anions, also influences the alkyl halide's chemical nature. The stability of halide anions can be measured with respect to the hydrohalic acids' relative acidities. The strongest hydrohalic acid, hydrogen iodide, has a pK_a of -11. Meaning it fully dissociates to the most stable conjugate base, the iodide ion, and a proton. Thus, iodine is an excellent leaving group. Except for hydrofluoric acid with a pK_a of 3.2, other hydrohalic acids have pK_a values of less than 0, making them strong acids with weak and stabilized conjugate bases, which are excellent leaving groups.

Tags

Alkyl Halides Structural Properties Halogen-substituted Alkanes Fluorine Chlorine Bromine Iodine Sp3-hybridized Tetrahedral Shape Vinyl Halides Aryl Halides Primary Alkyl Halides Secondary Alkyl Halides Tertiary Alkyl Halides Î±-carbon Î²-carbon Nomenclature IUPAC Rules Reactivity Applications

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