# Lecture 7: Alkyl Halides: Structure and Properties
## Organic Chemistry I
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## Learning Objectives
By the end of this lecture, students will be able to:
1. Classify alkyl halides as primary, secondary, or tertiary
2. Name alkyl halides using IUPAC and common nomenclature
3. Describe the physical properties of alkyl halides and explain their trends
4. Identify the C-X bond as the key reactive site due to bond polarity
5. Explain bond strengths and bond lengths of C-X bonds
6. Understand the concept of leaving groups and their importance in substitution and elimination reactions
7. Describe methods of preparation of alkyl halides
8. Recognize organometallic compounds as related derivatives
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## Lecture Content
### I. Structure and Classification of Alkyl Halides
Alkyl halides, also known as haloalkanes, are organic compounds that contain a carbon-halogen bond (C-X), where X can be fluorine, chlorine, bromine, or iodine. Their general formula is R-X, where R represents an alkyl group. These compounds are classified based on the degree of substitution of the carbon bearing the halogen. A primary (1 degree) alkyl halide has the halogen on a carbon bonded to one other carbon, as in CH3CH2Br. A secondary (2 degree) alkyl halide has the halogen on a carbon bonded to two other carbons, as in (CH3)2CHBr. A tertiary (3 degree) alkyl halide has the halogen on a carbon bonded to three other carbons, as in (CH3)3CBr. A methyl halide, such as CH3Cl, is a special case.
Beyond these standard classifications, several important subtypes exist. Vinyl halides have the halogen bonded to a carbon participating in a C=C double bond and are generally unreactive in SN1 and SN2 reactions. Aryl halides bear the halogen on an aromatic ring and are similarly unreactive under standard substitution conditions. Allylic halides, with the halogen on the carbon adjacent to a double bond, and benzylic halides, with the halogen adjacent to an aromatic ring, are both notably reactive.
### II. Nomenclature of Alkyl Halides
In the IUPAC system, alkyl halides are named by treating the halogen as a substituent on the parent alkane chain, using the prefixes fluoro-, chloro-, bromo-, and iodo-. The chain is numbered to give the halogen the lowest possible locant, and when multiple halogens are present, the prefixes di-, tri-, and tetra- are used, with all substituents listed alphabetically. Examples include 2-bromopentane, 1-chloro-3-methylbutane, and 1,2-dichloroethane.
Common names, which are still widely used in practice, consist of the alkyl group name followed by the halide name: methyl chloride for CH3Cl, ethyl bromide for CH3CH2Br, isopropyl iodide for (CH3)2CHI, and tert-butyl chloride for (CH3)3CCl. Several compounds have special names that are universally recognized: chloroform (CHCl3), carbon tetrachloride (CCl4), methylene chloride or dichloromethane (CH2Cl2), and the Freons, which are chlorofluorocarbons such as CF2Cl2 (Freon-12).
### III. Physical Properties of Alkyl Halides
The boiling points of alkyl halides are higher than those of the corresponding alkanes because of their greater molecular weight and stronger London dispersion forces. Boiling points increase as one descends the halogen group (R-F < R-Cl < R-Br < R-I) and as the carbon chain lengthens, while branching decreases boiling points by reducing surface area. The methyl halide series illustrates the trend clearly: CH3F boils at -78 degrees C, CH3Cl at -24 degrees C, CH3Br at 4 degrees C, and CH3I at 42 degrees C.
Regarding density, monochloro and monofluoro alkanes are generally less dense than water, while polyhalogenated compounds and most bromo and iodo alkanes are denser than water. Chloroform, dichloromethane, and carbon tetrachloride all have densities well above 1.0 g/mL. Alkyl halides are insoluble in water due to their nonpolar carbon-hydrogen framework but dissolve readily in organic solvents. In fact, several halogenated compounds serve as common laboratory solvents.
The C-X bonds are polar because halogens are more electronegative than carbon, creating dipole moments whose magnitude depends on both the halogen and the molecular geometry. The C-F bond is the most polar, though it has a relatively small dipole moment due to its short bond length, while C-Cl and C-Br bonds have significant dipole moments.
Panel A: A table showing physical properties of methyl halides (CH3F, CH3Cl, CH3Br, CH3I) including boiling point, bond length, bond dissociation energy, and dipole moment, with trends indicated by arrows. Panel B: Electrostatic potential maps of CH3F, CH3Cl, CH3Br, and CH3I showing the distribution of electron density, with the halogen end colored red (electron-rich) and the carbon/hydrogen end colored blue (electron-poor). The increasing size of the halogen atom is visible across the series.
### IV. The Carbon-Halogen Bond
The polarity of the C-X bond, with carbon bearing a partial positive charge and the halogen bearing a partial negative charge, makes the carbon electrophilic and therefore susceptible to nucleophilic attack. This polarity is the fundamental basis for the reactivity of alkyl halides.
Bond dissociation energies decrease as halogen size increases: the C-F bond is strongest at approximately 485 kJ/mol, followed by C-Cl at approximately 350 kJ/mol, C-Br at approximately 295 kJ/mol, and C-I at approximately 240 kJ/mol. The same trend is reflected in bond lengths: C-F at 1.39 angstroms is the shortest, increasing through C-Cl (1.78), C-Br (1.93), to C-I (2.14 angstroms). Weaker C-X bonds are broken more easily, which explains why alkyl iodides react fastest and alkyl fluorides react slowest (or not at all) in substitution and elimination reactions. In each of these reactions, the halogen departs as a leaving group.
### V. Leaving Groups
A leaving group is the atom or group that departs with a pair of electrons during a reaction. In alkyl halides, the halide ion serves as the leaving group, and its ability to stabilize the resulting negative charge determines how effectively it departs. Good leaving groups are weak bases: the halide leaving group ability follows the order I- > Br- > Cl- >> F-, which parallels the decreasing basicity (I- is the weakest base) and decreasing C-X bond strength.
Beyond halides, other important leaving groups include water (from protonated alcohols) and the sulfonate esters: tosylate (OTs), mesylate (OMs), and triflate (OTf). These sulfonates are excellent leaving groups that effectively convert a hydroxyl group, which is inherently a poor leaving group, into a readily displaceable one. Poor leaving groups are strong bases such as HO-, RO-, NH2-, H-, and carbanions (R-), which cannot be directly displaced without prior activation.
### VI. Preparation of Alkyl Halides
Alkyl halides can be prepared from alcohols through several pathways. Treatment with a hydrogen halide (HX) works best for tertiary alcohols with HBr or HI. Thionyl chloride (SOCl2) converts alcohols to alkyl chlorides under mild conditions. Phosphorus tribromide (PBr3) is well suited for converting primary and secondary alcohols to alkyl bromides. Tosylation of the alcohol followed by nucleophilic substitution provides yet another route.
Radical halogenation of alkanes (R-H + X2 with heat or UV light) produces alkyl halides but suffers from selectivity issues that often yield mixtures of products, as discussed in detail in Lecture 16. Alkyl halides can also be prepared from alkenes through addition of HX (Markovnikov addition) or addition of X2 (dihalide formation), reactions covered in Lectures 13-14. The Finkelstein reaction, in which an alkyl chloride is treated with sodium iodide in acetone (R-Cl + NaI yielding R-I + NaCl), is driven by the precipitation of sodium chloride from the acetone solution, shifting the equilibrium via Le Chatelier's principle.
### VII. Organometallic Compounds
Organometallic compounds contain a carbon-metal bond and are closely related to alkyl halides, often being prepared directly from them. Grignard reagents (RMgX) are formed by reacting an alkyl halide with magnesium metal in an ether solvent such as diethyl ether or THF. In the resulting reagent, the carbon bears a partial negative charge, which is a polarity reversal compared to the original alkyl halide. This makes Grignard reagents powerful nucleophiles that are used extensively for forming new C-C bonds.
Organolithium reagents (RLi) are prepared by reacting alkyl halides with lithium metal and are even more reactive than Grignard reagents. Gilman reagents, or lithium dialkylcuprates (R2CuLi), are prepared from organolithium reagents and copper(I) iodide and are particularly useful for conjugate addition and coupling reactions. All organometallic reagents react violently with water and must be handled under strictly anhydrous conditions.
Panel A: A reaction scheme showing the preparation of a Grignard reagent from bromobenzene: PhBr + Mg (in Et2O) yielding PhMgBr. The electrostatic polarity is shown with delta- on C and delta+ on Mg, contrasted with the original C-Br bond showing delta+ on C and delta- on Br. Panel B: A summary diagram of alkyl halide transformations showing R-X at the center with arrows pointing to products: R-OH (substitution with OH-), R-Nu (SN2 with nucleophile), R-MgX (Grignard formation), alkene (elimination), and R-R' (coupling with organocuprate). Each arrow is labeled with the reagent required.
### VIII. Environmental and Biological Importance
Chlorofluorocarbons (CFCs) are extremely stable compounds due to their strong C-F and C-Cl bonds, but this stability allows them to reach the stratosphere intact, where ultraviolet light breaks the C-Cl bonds to generate chlorine radicals that catalytically destroy ozone. The Montreal Protocol of 1987 banned most CFCs, and they have been largely replaced by HFCs and HCFCs that are less damaging to the ozone layer.
Nature also produces organohalogen compounds. Marine organisms synthesize many such compounds, and chloromethane is released by seaweed and fungi. The thyroid hormones T3 and T4 contain iodine as a key structural element. In medicine, halogenated compounds have served as anesthetics, from the historical use of halothane (CF3CHBrCl) to modern fluorinated agents like sevoflurane and desflurane. Halogenated pesticides and herbicides, including DDT and lindane, have been used historically but raise environmental concerns due to their persistence, a direct consequence of the strength of C-X bonds.
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