ENCYCLOPÆDIA BRITANNICA
chemical compound

Alkyl halides

Structure and physical properties

Alkyl halides (RX, where R is an alkyl group and X = F, Cl, Br, or I) are classified as primary, secondary, or tertiary according to the degree of substitution at the carbon to which the halogen is attached. In a primary alkyl halide, the carbon that bears the halogen is directly bonded to one other carbon, in a secondary alkyl halide to two, and in a tertiary alkyl halide to three.



The methods used to prepare alkyl halides and the reactions that alkyl halides undergo frequently depend on whether the alkyl halide is primary, secondary, or tertiary.

A halogen substituent draws the electrons in the C-X bond toward itself, giving the carbon a partial positive charge (+) and the halogen a partial negative charge (-). The presence of the resulting polar covalent bond makes most alkyl halides polar compounds. Because the bond dipole (the measure of the separation of charge) of a C-X bond is the product of a charge term (largest for fluorine and smallest for iodine) and a distance term (smallest for fluorine and largest for iodine), the molecular dipole moments of alkyl halides do not vary much from one halogen to another (see Table 37).

The most important reactions of organohalogen compounds involve breaking the carbon-halogen bond by processes in which the halogen retains both of the electrons from the original bond and is lost as a negatively charged ion (X-). Consistent with the order of carbon-halogen bond strengths given in Table 37, which shows that the bond to fluorine is the strongest and the bond to iodine the weakest of the carbon-halogen bonds, fluorides are normally observed to be the least reactive of the alkyl halides and iodides the most reactive.

The last column in Table 37 indicates that the boiling points of ethyl halides increase as the atomic number of the halogen increases. With increasing atomic number the halogen becomes more polarizable, meaning that the electric field associated with the atom is more easily distorted by the presence of nearby electric fields. Fluorine is the least polarizable of the halogens and iodine the most polarizable. An increased polarizability is associated with stronger intermolecular attractive forces of the dipole-induced dipole and induced dipole-induced dipole types (see chemical bonding) and therefore with an increased boiling point.

Multiple halogen substitution tends to increase the boiling point: CH3Cl boils at -24º C, CH2Cl2 at 40º C, CHCl3 at 61º C; and CCl4 at 77º C. Multiple fluorine substitution is an exception, however: CH3CH2F boils at -32º C, CH3CHF2 at -25º C, CH3CF3 at -47º C, and CF3CF3 at -78º C. By reducing the molecular polarizability, multiple fluorine substitution weakens the strength of induced dipole-induced dipole attractive forces between molecules. In the liquid state these weakened intermolecular forces are reflected in unusually low boiling points, and in the solid state they are responsible for the novel properties of fluorocarbon polymers.

The densities of alkyl halides are related to intermolecular attractive forces and tend to parallel boiling points, alkyl fluorides being the least dense and alkyl iodides the most dense. In general, alkyl fluorides and chlorides are less dense than water, and bromides and iodides are more dense than water. Alkyl halides are not soluble in water.

Natural occurrence

Estimates place the amount of chloromethane (CH3Cl) that results from natural biological processes at more than five million tons (five billion kilograms) per year. Most of this is produced in the oceans by marine algae and kelp, but terrestrial organisms--especially fungi--also make a contribution. Smaller quantities (less than 250,000 tons per year) enter the atmosphere as a result of volcanic emissions, forest fires, and human activity. Ocean-living organisms are a source of bromomethane (CH3Br) as well as large quantities of iodomethane (CH3I). More than 50 organohalogen compounds, including CHBr3, CHBrClI, BrCH2CH2I, CH2I2, Br2CHCH=O, I2CHCO2H, and (Cl3C)2C=O, have been identified as being present in the Hawaiian red seaweed Asparagopsis taxiformis. Virtually every marine plant that has been assayed has been found to produce organohalogen compounds, many of which have quite complicated structures.

Several naturally occurring halogen-containing substances have pharmaceutical applications. An example is the antibiotic chloramphenicol produced by Streptomyces venezuelae.



Fluorine-containing natural products are relatively rare, the most prominent examples being -fluoro fatty acids. (The prefix indicates that the substitution occurs at the end of a chain.) Fluoroacetic acid, FCH2CO2H, occurs in the South African plant Dichapetalum cymosum and is quite toxic. A related Dichapetalum species contains 16-fluorohexadecanoic acid, FCH2(CH2)14CO2H, which is also poisonous when ingested because of its subsequent metabolic conversion to fluoroacetic acid.

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Contents of this article:

Introduction
    General considerations
       The periodic table
       Trends in the chemical properties of the elements
    Classification of compounds
    Inorganic compounds
       Nomenclature of inorganic compounds
          Binary compounds
             Binary ionic compounds
             Binary molecular (covalent) compounds
          Nonbinary compounds
             Ionic compounds containing polyatomic ions
             Acids
             Compounds with complex ions
       Water
          Significance of water
          Structure of water
             Structures of ice
             Significance of the structure of liquid water
          Behaviour and properties
             Water at high temperatures and pressures
             Physical properties
             Chemical properties
                Acid-base reactions
                Oxidation-reduction reactions
       Oxides
          Metal oxides
          Nonmetal oxides
             Oxides of nitrogen
             Oxides of phosphorus
             Oxides of carbon
                Carbon monoxide
                Carbon dioxide
                Carbon suboxide
             Oxides of sulfur
          Peroxides
             Hydrogen peroxide
             Superoxides
       Oxyacids and their salts
          Oxyacids of nitrogen and their salts
             Nitric acid and its salts
             Nitrous acid and its salts
          Oxyacids of phosphorus and their salts
             Orthophosphoric acid and its salts
             Phosphorous acid and its salts
             Hypophosphorous acid and its salts
          Oxyacids of sulfur and their salts
             Sulfuric acid
                Preparation
                Formation of salts
                Reactions and uses
             Sulfurous acid
          Carbonic acid and its salts
             Carbonate and hydrogen carbonate salts
             Other carbonic acids
       Carbides, nitrides, phosphides, and sulfides
          Carbides
             Preparation
             Classification
                Saltlike carbides
                Interstitial carbides
                Covalent carbides
          Nitrides
             Preparation
             Ionic nitrides
             Interstitial nitrides
                Covalent nitrides
                Boron nitride
                Cyanogen
                Sulfur nitrides
          Phosphides
          Sulfides
       Hydrides
          Saline hydrides
          Metallic hydrides
          Covalent hydrides
          Ammonia
             Preparation
             Physical properties
             Chemical reactivity
             Industrial uses
             Derivatives of ammonia
       Inorganic polymers
          General characteristics
          Major classes of inorganic polymers
             Borates
             Silicates
             Silicones
             Silanes
       Boranes and carboranes
          Boranes
             Structure and bonding
             Reactions and synthesis
          Carboranes
             Structure and bonding
             Reactions and synthesis
       Coordination compounds
          General considerations
             Coordination number
             Ligands and chelates
                Mononuclear, monodentate
                Polydentate
                Polynuclear
             Nomenclature
          Historical review
          Structure and bonding
             Valence bond theory
             Crystal field, ligand field, and molecular orbital theories
             Isomerism
                Ionization isomerism
                Coordination isomerism
                Ligand isomerism
                Linkage isomerism
                Geometric isomerism
                Optical isomerism
             Geometry of compounds of various coordination numbers
          Principal types
             Aqua complexes
             Halide complexes
             Metal carbonyls
             Metal nitrosyls
             Metal cyanides and isocyanides
             Metal clusters
             Isopoly and heteropoly anions
             Organometallic complexes
          Reactions
             Lability and inertness
             Principal types
                Acid-base
                Substitution
                Isomerization
                Oxidation-reduction
          Synthesis
          Uses
             Dyes and pigments
             Extraction and separation of metals
             Chemical analysis
             Sequestering
             Catalysis
             Biology
    Organometallic compounds
       General considerations
          Defining characteristics
          Historical developments
       s- and p-block organometallic compounds
          The stability and reactivity of organometallic compounds
          The synthesis of s- and p-block organometallic compounds
             Formation of alkyllithium and Grignard reagents
             Double displacement
             Redistribution
             Hydrometallation
             Reduction
             Carbanion character
             -hydrogen elimination
       d- and f-block organometallic compounds
          Metal carbonyls
             The structure of metal carbonyls
             Zero-oxidation-state metal carbonyls
             Metal carbonyl anions
          Compounds with metal-carbon bonds
             Simple alkyl ligands
             Alkylidene ligands
             Alkylidyne ligands
             Alkene and alkyne ligands
             Polyene ligands
             Cyclic polyene ligands
       Metal clusters
       Organometallic compounds in catalysis
          Hydrogenation
          Hydroformylation
          Alkene polymerization
Organic compounds
    General considerations
       Historical developments
       Carbon bonding
       Functional groups
          Alkanes
          Alkenes
          Alkynes
          Aromatics
          Alcohols, phenols, and ethers
          Aldehydes and ketones
          Carboxylic acids and their derivatives
          Amines and thiols
          Halides
          Polyfunctional compounds
       Chemical synthesis
          General considerations
          General approach to synthesis
          Evaluation of a synthetic method
          Isolation and purification of products
       Spectroscopy of organic compounds
          Ultraviolet and visible (UV-visible) spectroscopy
          Infrared (IR) spectroscopy
          Nuclear magnetic resonance (NMR) spectroscopy
          Proton magnetic resonance spectroscopy
          Carbon-13 magnetic resonance spectroscopy
          Mass spectrometry
       Reaction types
          Substitution reactions
          Elimination reactions
          Addition reactions
          Hydrolysis
          Condensation
          Acid-base reactions
    Hydrocarbons
       Aliphatic hydrocarbons
          Alkanes
             Nomenclature
             Three-dimensional structures
             Cycloalkanes
             Stereoisomerism
             Physical properties
             Sources and occurrence
             Chemical reactions
          Alkenes and alkynes
             Bonding in alkenes and alkynes
             Natural occurrence
             Synthesis
             Physical properties
             Chemical properties
             Polymerization
       Aromatic hydrocarbons
          Arenes
             Structure and bonding
             Nomenclature
             Physical properties
             Source and synthesis
             Reactions
          Nonbenzenoid aromatic compounds
             Annulenes and the Hückel rule
             Polycyclic nonaromatic compounds
    Alcohols, phenols, and ethers
       Alcohols
          Structure and classification
          Nomenclature
             IUPAC nomenclature
             Common names
          Physical properties
             The effect of hydrogen bonding on boiling points
             Solubility properties
          Commercially important alcohols
             Methanol
             Ethanol
             2-Propanol
          Sources
             Natural products
             Reduction of carbonyl compounds
             Hydration of alkenes
             Displacement of halides
             Using Grignard and organolithium reagents
          Reactions
             Oxidation
             Biological oxidation
             Dehydration to alkenes
             Substitution to form alkyl halides
             Esterification
             Acidity of alcohols: formation of alkoxides
       Phenols
          Nomenclature
          Physical properties
          Natural sources
          Synthesis
             Hydrolysis of chlorobenzene (the Dow process)
             Oxidation of isopropylbenzene
             General synthesis of phenols
          Reactions
             Acidity of phenols
             Oxidation
             Electrophilic aromatic substitution
             Formation of phenol-formaldehyde resins
       Ethers
          Nomenclature
          Physical properties
          Complexes of ethers with reagents
          Synthesis
             Williamson ether synthesis
             Bimolecular dehydration
          Reactions
             Cleavage
             Autoxidation
          Epoxides
    Aldehydes and ketones
       Structure
       Nomenclature
          Aldehydes
          Ketones
       Properties
          Physical properties
          Tautomerism
       Synthesis
       Principal categories of reactions
          Oxidation-reduction reactions
          Nucleophilic addition
             Addition of noncarbon nucleophiles
             Addition of carbon nucleophiles
          Displacement at the alpha carbon
             Alpha halogenation
             Aldol reaction
          Conjugate addition
       Uses of aldehydes and ketones
          Formaldehyde
          Acetone
          Other carbonyl compounds of industrial use
    Carboxylic acids and their derivatives
       Nomenclature of carboxylic acids and their salts
       Properties of carboxylic acids
          Acidity
          Solubility
          Boiling point
          Odour
       Classes of carboxylic acids
          Saturated aliphatic acids
          Unsaturated aliphatic acids
          Aromatic acids
          Polycarboxylic acids
          Hydroxy and keto acids
          Amino acids
       Synthesis of carboxylic acids
          Hydrolysis of acid derivatives
          Oxidation
          Other synthetic methods
       Principal reactions of carboxylic acids
          Conversion to acid derivatives
          Reduction
          Other reactions
       Derivatives of carboxylic acids
          Carboxylic esters
             Nomenclature
             Synthesis
             Properties
          Reactions
             Lactones
             Polyesters
          Amides
             Nomenclature
             Synthesis
             Properties
             Reactions
             Polyamides
             Related compounds
          Acyl halides
             Nomenclature and synthesis
             Reactions
          Anhydrides
             Nomenclature and synthesis
             Reactions
          Nitriles
             Nomenclature
             Synthesis
             Reactions
    Amines
       Nomenclature
       Properties
          Physical properties
          Molecular shape and configuration
       Occurrence and sources
       Reactivity
          Addition
          Substitution
          Oxidation
          Elimination
       Uses
    Organic sulfur compounds
       The sulfur atom
       Analysis of organosulfur compounds
       Organic compounds of bivalent sulfur
          Thiols
             Preparation
             Reactions
          Sulfides
             Preparation
             Reactions
          Disulfides and polysulfides and their oxidized products
             Preparation
             Reactions
          Thiocarbonyl compounds
             Preparation
             Reactions
       Organic compounds of polyvalent sulfur
          Sulfoxides and sulfones
             Occurrence and preparation
             Reactions
          Other sulfinyl and sulfonyl compounds
          Sulfonium and oxosulfonium salts; sulfur ylides
          Sulfuranes: hypervalent organosulfur compounds
    Heterocyclic compounds
       General aspects of heterocyclic systems
          Comparison with carbocyclic compounds
          Nomenclature
       Major classes of heterocyclic compounds
          Three-membered rings
          Four-membered rings
          Five-membered rings with one heteroatom
          Six-membered rings with one heteroatom
          Five- and six-membered rings with two or more hetero-atoms
          Rings with seven or more members
          Rings with uncommon heteroatoms
             Selenium and tellurium
             Phosphorus, arsenic, and antimony
             Boron
    Organohalogen compounds
       Nomenclature
       Carbon-halogen bond strengths and reactivity
       Alkyl halides
          Structure and physical properties
          Natural occurrence
          Synthesis
          Reactions
             Nucleophilic substitution
             Elimination
             Preparation of Grignard reagents
       Vinylic halides
          Natural occurrence
          Preparation
             Dehydrohalogenation of a dihalide
             Addition of a hydrogen halide to an alkyne
          Reactions
       Aryl halides
          Natural occurrence
          Preparation
             Halogenation
             Diazonium salts
          Reactions
    Coloured compounds and dyes
       Historical developments
          Natural dyes
          Development of the synthetic dye industry
       Chromogens and colour
       General features of dyes and dyeing
          Fibre structure
          Fibre porosity
          Dye retention
             Fastness
             Attractive forces
          Dyeing techniques
             Vat dyeing
             Disperse dyeing
             Azo dyeing techniques
          Classifications of dyes
       Development of synthetic dyes
          Triphenylmethane dyes
          Anthraquinone dyes
          Xanthene and related dyes
          Azo dyes
          Phthalocyanine compounds
          Quinacridone compounds
          Reactive dyes
          Optical brighteners
          Food dyes
          Current dye-industry research
          Standardization tests and identification of dyes
    Carbohydrates
       Monosaccharides
       Disaccharides
       Polysaccharides
       Amino acids, peptides, and proteins
          Amino acids
             Structure
             Sources
          Peptides and proteins
             Structure
             Biosynthesis
             Laboratory synthesis
                Use of -halo acid chlorides
                The carbobenzoxy protecting group
                The Boc protecting group
                Solid-phase methods
    Lipids
       General description and structural features
       Triglycerides, phospholipids, and sphingolipids
          Fatty acids
          Triglycerides
             Structure
             Occurrence
             Selected examples
             Biosynthesis
             Synthesis
          Phospholipids
             Definition and structural features
             Occurrence
             Selected examples
             Biosynthesis
          Sphingolipids
             Definition
             Structural features
             Occurrence
             Selected examples
             Biosynthesis
             Biosynthesis of gangliosides
             Biosynthesis of long-chain glycosphingolipids
             Biosynthesis of lacto-phospholipids
       Prostaglandins and related compounds: the eicosanoids
          Structures of prostaglandins
          Biosynthesis
          The action of anti-inflammatory drugs on eicosanoids
    Isoprenoids
       Chemistry of isoprenoids
          Structural features
          Classification
          Tail-to-tail coupling
          Natural sources
          Biological functions
          Uses
          Isolation and identification
          Purification
          Analysis
          Determination of structure
          Biosynthesis
       Comparative survey of isoprenoid compounds
          Monoterpenes
          Sesquiterpenes
          Diterpenes
          Triterpenes
          Tetraterpenes
          Polyterpenes
    Steroids
       Historical background
       Chemistry of steroids
          Steroid numbering system and nomenclature
          Methods of isolation
          Determination of structure and methods of analysis
          Total synthesis of steroids
          Partial synthesis of steroids
       Natural distribution and functions
          Sterols and bile acids
          Sex hormones
          Adrenal hormones
          Steroids of lower organisms
       Biosynthesis and metabolism of steroids
          Cholesterol
          Steroid hormones
          Steroid metabolism in plants
       Structural relationships of the principal categories of steroids
          Sterols
          Bile acids and alcohols
          Estrogens
          Progestational steroids (gestogens, progestins)
          Androgens
          Adrenal cortical hormones
          Ecdysones
          Cardiac glycosides and aglycones
          Toad poisons
          Sapogenins and saponins
       Pharmacological actions of steroids
          Androgens and anabolic steroids
          Antiandrogens
          Synthetic estrogens and gestogens: steroid contraceptives
          Antiestrogens
          Adrenal hormones
          Cardiotonic steroids
          The general biological significance of steroids
Bibliography
    General works
    Inorganic compounds
       Boranes and carboranes
       Coordination compounds
       Organometallic compounds
    Organic compounds
       Hydrocarbons
       Alcohols, phenols, and ethers
       Aldehydes and ketones
       Carboxylic acids and their derivatives
       Amines
       Organic sulfur compounds
       Heterocyclic compounds
       Organohalogen compounds
       Coloured compounds and dyes
       Carbohydrates
       Amino acids, peptides, and proteins
       Lipids

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