eh in page 78 he wrote trans isomer is more stable than the cis
isomer due to reduced steric hindrance
but he didnt comment/write on either the cis or trans would be
the major product
so for the case of but-2-ene, is the trans isomer the major pdt?
due to this reason of more stable? anymore reasons?
for other cases might be different? different geometric
(cis-trans)isomers for different alkenes molecules have different
major/minor products for diff reasons?
First of all, be sure you don't confuse regiochemistry
(Zaitsev vs Hofmann) with stereochemistry (cis vs trans, or
E vs Z), and thermodynamic vs kinetic products.
You can go read up on regiochemistry and thermodynamic vs
kinetic products on your own if you like (don't ask me about
it).
In terms of stereochemistry, whether the cis/trans or E/Z
product is favored, depends on whether the elimination reaction
(eg. dehydrohalogenation) occurs via E1 or E2 mechanism (go read up
on the factors which influence E1 vs E2 on your own if you like,
don't ask me about it).
When E1 occurs, the major product is the thermodynamic or more
stable product (ie. trans geometic isomer).
When E2 occurs, there is only 1 product, which is determined
by the stereochemistry of the reactant undergoing the
elimination.
As Wikipedia states : "In the E2 mechanism, the two leaving
groups need to be antiperiplanar, because an antiperiplanar
transition state has staggered conformation with lower energy than
a synperiplanar transition state which is in eclipsed conformation
with higher energy ; consequently the reaction mechanism involving
staggered conformation is favored for E2 reactions."
Concordantly and consequently, the elimination product of the E2
mechanism is already pre-determined by the stereochemistry of the
reactant, and depending on case-by-case, will be *either* the cis
or trans geometric isomer *only*.
If you get a mixture of cis and trans products, it's either
because E1 occurred, or a mixture of E1 and E2 occurred.
Note that in reality (ie. at Uni level, in the chemical
industry), a mixture of E1, E2, SN1, SN2 all occurs simultaneously
and competitively, to generate a mixture of all possible
products.
FlyingGrenade, you're the one who opened this can of worms by
asking this question, your own fault blame yourself, don't expect
me to clean up the worms after you. You'll probably end up with
more questions than before, after reading this post above, but
don't ask me any more questions on these matters.
For the sensible pragmatic A level H2 Chem student, just focus
on your within H2 syllabus material and ignore whatever goes beyond
(ie. for the Singapore H2 Chem exam, don't try to bring in Zaitsev
or Hofmann or E1 or E2 or cis or trans or E or Z , unless
specifically asked by the exam question).
For the already-getting-A-grades-for-Prelims enthusiastic A
level student taking H3 or Olympiad Chem intending to study
Medicine or Chemistry in the Uni, you can go read up on your own
about these matters, and/or go ask your school teacher or private
tutor.
Moving on...
