CHEM261 Lecture Notes - Lecture 5: Concerted Reaction, Hexamethylphosphoramide, Activation Energy

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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
versa)
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
Reactions.
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
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
is increased.
Factors favouring SN2 Reactions
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