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Lecture 3

CHEM 212 Lecture Notes - Lecture 3: Steric Effects, Rate-Determining Step, Sn2 Reaction

Course Code
CHEM 212
Laura Pavelka

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Lecture 3:
SN1 and Sn2 Selectivity
How do we know which reaction will happen when?
Anything that favours lower energy pathways!
Factors that effect the rate of Sn1 Vs. SN2 reaction:
1. Structure of Substrate (certain substrates are better for sn1 and other for sn2)
2. Concentration / Nature of Nucleophile (just for Sn2)
3. Effect/ Nature of solvent (polar/nonpolar, protic/aprotic) intermolecular forces play a large
role in the reactivity
4. Nature of leaving group
Structure of substrate:
In SN2 the attacking and falling off of the Lg happens are the same time with a transition molecule
in the middle. For this less steric hinderence is favourable so that the ‘5 bonds’ can be formed. In
SN1 the LG first falls off and the Nu then attacks, this can allow the substrate to by 3 or also have
resonance. Sp2 or sp carbons are not involved in SN1 or SN2 either. Have to be alkyl.
Substrate: SN1- more hyper conjugation and/or resonance(good carbocation)| SN2 less steric
hindrance so Me>1>2(bad carbocation).
SN1 substrates: primary effect is stability of carbocation, the more stable the lower energy barrier of
the first and rate determining step. More hyper-conjugation makes it more stable as it makes the
carbocation feel less electron deficient than it actually is. While resonance structures are the best as
the electron is completely delocalized and the carbocation is actually less electron deficient. SN1 and
Halogenation both follow the same rule. The more stabilized the carbocation the faster the reaction
and the lesser the energy used.
Sn2 substrate: Primary effect is steric
hindrance around reacting site. We want
something less hindered so that the Nu
can get into the carbon while the LG is
falling of all at the same time. The worse
carbocation the better it is for SN2 and
bad for SN1. For 2 we need to look further, go to nucleophiles and other groups around details
because if we just look at second degree then it is good for both SN1 and SN2. Eg neopentyl is
primary but it is much slower than others.
Concentration of Nucleophile:
A nucleophile is any molecule/atom etc. which has a lone pair of electrons as a part of its octet
(neutral nucleophile) or it has an extra pair of electrons making it negatively charged (anionic
nucleophile) {lewis base}. Anionic is more reactive as compared to neutral.
This only matters for SN2. Usually the substrate gives away the answer but when it gets dicey we go
to the next factor that is Conc/nature of nucleophile.
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