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Alkyl Halides (substitution reactions)
❏Alkyl halide (haloalkane) general formula is R-X. Where X is Cl, Br or I.
❏Involve substitution and elimination reactions. (two types of elimination and two
types of substitution)
❏Sp2 do not undergo substitution or elimination reactions normally.
❏Methyl Halide = Ch3-X. There are primary, secondary and Tertiary structures
❏C-F bonds are too strong for a reaction to occur.
❏Carbon attracts nucleophiles (species with lone pairs of electrons) because the Halogen
is EN and takes the electrons.
❏All reactions lead to elimination of halogen.
❏A nucleophile → any negative ion or neutral molecule that has lone-pair of electrons are
is attracted to a positive center. It is also a lewis base.
❏Alkyl halide is called the substrate because it is attacked by nucleophile. The Halogen
(X) is the leaving group.
❏Nucleophilic Reaction (SN2 Reaction)
❏Kinetic Studies (order of reactions) are important in the rate determining steps.
❏Optical purity = If the whole molecule is an R, then it contains no S.
❏Reaction is stereospecific (inversion of configuration → From R to S and vice
❏ Involves a single step → concerted reaction (bond making and breaking at the
same time). → Backside attack on sp3 Carbon.
❏Exothermic potential energy of SN2 reaction.
❏Substrate, LEaving group, Solvent and nucleophile affect reactivity of SN2
❏Reactivity; Methyl > 1 > 2 > 3 (collisions occurring are easier on methyl)
❏SN2 is senstitive to steric effect.
❏Strong base = strong nucleophile = poor leaving group.
❏Basicity increases from right to left: Ch3- > NH2 - > OH- > F-.
❏Negitive charged nucleophiles always stronger than conjugate acid. OH- > H2O.
❏When bulky groups present becomes a weaker nucleophile despite strong base.
❏The substrate needs to be weaker so that the nucleophile can kick it out.
❏Halides are good leaving groups, so are Sulonates.
❏Protic Solvent → Poses hydrogen attatched to electronegative elements. They
form hydrogen bonds with nucleophiles. (Water or Alcohol).
❏Aprotic Solvent → Polar solvent that does not possess hydrogen attached to
electronegative element. Include DMF, DMA, DMSO, Acetone, HMPA, and THF)
❏SN2 → Tranfer of charge.
❏Protic Versus Aprotic
❏Protic → solvents solvate anion nucleophilicity decreased. Energy is needed.
❏Aprotic → Solvents solvate cation leaving anion “naked”. Nucleophilicity of anion
❏Factors favouring SN2 Reactions