Mumbai University (Sybsc) .organic chemistry (USCH301) (SEM III ) Alkyl Halides Mumbai University (Sybsc) .organic chemistry (USCH301) (SEM III )

Alkyl Halides R-X (X = F, Cl, Br, I) Classification of alkyl halides according to the class of the carbon that the halogen is attached to. RCH2-X R2CH-X R3C-X 1o 2o 3o

NAMING ALKYL HALIDES Find longest chain, name it as parent chain (Contains double or triple bond if present) Number from end nearest any substituent (alkyl or halogen)

NAMING IF TWO HALIDES OR ALKYL ARE EQUALLY DISTANT FROM ENDS OF CHAIN Begin at the end nearer the substituent whose name comes first in the alphabet

STRUCTURE OF ALKYL HALIDES C-X bond is longer as you go down periodic table C-X bond is weaker as you go down periodic table C-X bond is polarized with partial positive charge on carbon and partial negative charge on halogen

PREPARING ALKYL HALIDES FROM ALKENES Alkyl halide from addition of HCl, HBr, HI to alkenes to give Markovnikov product

General Features of Nucleophilic Substitution Three components are necessary in any substitution reaction.

To draw any nucleophilic substitution product: Find the sp3 hybridized carbon with the leaving group. Identify the nucleophile, the species with a lone pair or  bond. Substitute the nucleophile for the leaving group and assign charges (if necessary) to any atom that is involved in bond breaking or bond formation.

The Nucleophile Nucleophiles and bases are structurally similar: both have a lone pair or a  bond. They differ in what they attack.

Mechanisms of Nucleophilic Substitution In a nucleophilic substitution: But what is the order of bond making and bond breaking? In theory, there are three possibilities. [1] Bond making and bond breaking occur at the same time. In this scenario, the mechanism is comprised of one step. In such a bimolecular reaction, the rate depends upon the concentration of both reactants, that is, the rate equation is second order.

Consider reaction [2] below: Kinetic data show that the rate of reaction [2] depends on the concentration of only the alkyl halide. This suggests a two-step mechanism in which the rate-determining step involves the alkyl halide only. This is an example of an SN1 (substitution nucleophilic unimolecular) mechanism.

The mechanism of an SN2 reaction would be drawn as follows. Note the curved arrow notation that is used to show the flow of electrons.

An energy diagram for the SN2 reaction:

Stereochemistry All SN2 reactions proceed with backside attack of the nucleophile, resulting in inversion of configuration at a stereogenic center. Stereochemistry of the SN2 reaction

Methyl and 1° alkyl halides undergo SN2 reactions with ease. 2° Alkyl halides react more slowly. 3° Alkyl halides do not undergo SN2 reactions. This order of reactivity can be explained by steric effects. Steric hindrance caused by bulky R groups makes nucleophilic attack from the backside more difficult, slowing the reaction rate.

The nature of the solvent is a fourth factor. Polar protic solvents like H2O and ROH favor SN1 reactions because the ionic intermediates (both cations and anions) are stabilized by solvation. Polar aprotic solvents favor SN2 reactions because nucleophiles are not well solvated, and therefore, are more nucleophilic.