Alkyl halides are organic compounds in which a halogen atom is bonded to an sp3 carbon of an alkyl group. They have the general form R-X, where R is an alkyl group and X is F, Cl, Br, or I. These molecules are important because the carbon-halogen bond is polar and often reactive, making alkyl halides useful starting materials in organic synthesis.
They appear in reactions that build alcohols, ethers, amines, alkenes, and many other compounds.
The halogen pulls electron density away from carbon, giving the carbon a partial positive charge and making it vulnerable to attack by nucleophiles. In substitution reactions, a nucleophile replaces the halide, while in elimination reactions, a base removes a hydrogen and the halide leaves to form an alkene. Reactivity depends on the structure of the alkyl group, the leaving group ability of the halide, the strength of the nucleophile or base, and the solvent.
Understanding alkyl halides helps students predict whether SN1, SN2, E1, or E2 pathways are most likely.
Key Facts
- General structure: R-X, where R = alkyl group and X = F, Cl, Br, or I.
- The C-X bond is polar because halogens are more electronegative than carbon: Cδ+ - Xδ-.
- Leaving group ability generally increases down the halogen group: I- > Br- > Cl- >> F-.
- SN2 rate law: rate = k[alkyl halide][nucleophile].
- SN1 rate law: rate = k[alkyl halide].
- Alkyl halides are classified by the carbon bonded to X: methyl, primary, secondary, or tertiary.
Vocabulary
- Alkyl halide
- An organic compound with a halogen atom bonded to an sp3 carbon in an alkyl group.
- Leaving group
- An atom or group that departs with a pair of electrons during a substitution or elimination reaction.
- Nucleophile
- An electron-rich species that donates an electron pair to form a bond with an electron-poor atom.
- Substitution reaction
- A reaction in which one atom or group on a molecule is replaced by another atom or group.
- Elimination reaction
- A reaction in which atoms or groups are removed from adjacent carbons to form a double bond.
Common Mistakes to Avoid
- Calling every molecule with a halogen an alkyl halide is wrong because the halogen must be bonded to an sp3 alkyl carbon, not an aromatic ring or vinylic carbon.
- Ignoring the leaving group trend is wrong because C-I and C-Br bonds usually react more readily than C-Cl, while alkyl fluorides are often poor substrates.
- Assuming tertiary alkyl halides do SN2 reactions easily is wrong because bulky groups block backside attack by the nucleophile.
- Forgetting to identify the beta hydrogen in elimination is wrong because an alkene can form only when a base removes a hydrogen from a carbon adjacent to the carbon bearing the halogen.
Practice Questions
- 1 Classify each alkyl halide as methyl, primary, secondary, or tertiary: CH3Br, CH3CH2Cl, (CH3)2CHBr, and (CH3)3CCl.
- 2 For the SN2 reaction CH3CH2Br + OH- -> CH3CH2OH + Br-, if [CH3CH2Br] doubles and [OH-] triples, by what factor does the reaction rate change?
- 3 A student predicts that 2-bromopropane reacts with a strong base to give only substitution. Explain why elimination can compete and what structural feature of the substrate allows it.