Key Concepts In Organic Chemistry: Haloalkanes
-The general formula of saturated mono-halo acyclic alkanes is C H n 2n + 1X Count the X as an H for the
units of unsaturation calculation. The names of the given halo functional groups are as follows: F =
fluoro, Cl = chloro, Br = bromo, I = Iodo.
-It is essential to recognize the type if alkyl halide. A primary alkyl halide (1°), RCHX, defines the carbon
bearing the halogen as bonded to only one carbon group. A secondary alkyl halide (2°), R CH X, defines 2 2
the carbon bearing the halogen as bonded to two carbon groups. A tertiary alkyl halide (3°), R CX, 3
defines the carbon bearing the halogen as bonded to three carbon groups.
-An sp C-X bond is polar. Sp C-X bonds are much less polar due to the electrons being closer.
-The major reaction of haloalkanes is nucleophilic substitution, in which the halogen atom is replaced by
a nucleophilic group. The halogen atom is just one representative of a range of groups that can be
displaced by a nucleophile; these groups are generally called leaving groups (LG).
-Leaving groups are all electronegative groups that create a partial +ve charge on an sp C atom; Nu
groups all bear an electron pair. They consist of neutral or negatively-charged species. Nucleophilic
substitution is the major reaction type for saturated compounds nearing polar covalent bonds. It can
occur by two quite different mechanisms, known as: S 1 (unimoleNular nucleophilic substitution) and S 2 N
(bimolecular nucleophilic substitution).
-The S N reaction mechanism is a one-step mechanism that allow us to predict and explain many things
going on between the two molecules. The mechanistic step (RDS) is bimolecular and the reaction will be
a second order one that depends on the initial concentration of both reagents. The stereochemical
inversion at the reaction center: Nu attacks the substrate [R-LG] directly opposite from the leaving group
(termed backside attack). In terms of the change in R, S configuration labels, sometimes the label of the
stereocentre remains the same even though inversion has occurred.
-The steric bulk around the reaction centre will slow the reaction: the rate of an S 2 dependsNcritically on
the structure of the R-LG molecule. This is summarized as follows: CH -LG > 1°C-L3 > 2°C-LG >>> 3°C-LG
-Aprotic solvents (ones lacking a positively-charged H atom) make the reaction go faster because the
nucleophile is not surrounded by extensive H bonding. The better the Nu