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Topic 4 Part I Summary: "Haloalkanes"

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Chemistry 1027A/B
Mel Usselman

Key Concepts In Organic Chemistry: Haloalkanes Part I (Usselman Version) -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. 3 2 -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). 3 -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
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