Download Chapter 7: Structure and Synthesis of Alkenes

Chapter 7:
Structure and Synthesis of
Alkenes
Ú Introduction (7-1)
Ú Alkenes are hydrocarbons with C=C in their
structure. They are also known as “olefins”.
Ú We have already covered the basics of alkenes
nomenclature, but here are a few important
groups with common names that you can use as
well.
313
Orbital Description of the Alkene Double Bond (7-2)
Ú We already covered this in the first 2 chapters. The
carbon of the alkene group are sp2 hybridized,
leaving a p orbital available to form a π bond
Ú Cis/trans isomerism conversion is not possible
314
Element of Unsaturation (7-3)
Ú Alkenes are said to be “unsaturated”
because they react with hydrogen to give the
corresponding alkane (a saturated
compound). Alkenes are said to have one
element of unsaturation.
315
Ú The element of unsaturation of a
compound can be calculated using the
following formula:
Index = # C - # H - # halogen + # N +1
2
2
2
316
Practice Questions
Ú Question:
What is the unsaturation index for:
C15H13ClO
317
Ú How many element of unsaturation are
present in podophyllotoxin?
318
Nomenclature of cis-trans Isomers (7-5)
Ú We have seen before that cis and trans
alkenes were possible. Those were called
geometric isomers.
Ú However, sometimes this notation cannot
produce an unambiguous name.
319
Ú In these cases where the substituents are all
different, use the E/Z system. This system can be
used all the time, replacing the cis/trans notation.
– E: entgagen (opposite)
– Z: zusammen (together)
This system follows the same principles studied
in Chapter 5 for the Cahn-Ingold-Prelog
notation of chiral carbons.
320
Ú Procedure:
A) assign the substituent a priority for
each carbon of the double bond as: #1
and #2.
– #1 is the highest priority and is
determined by the atomic number (or
molecular weight) of the atom directly
attached to the C=C carbon
B) if a priority cannot be determined
using the atom directly attached to the
C of C=C, go to the next atom until a
priority can be determined.
321
Ú Looking at both carbons, determine if
the substituents labeled (1) on each
carbon of the double bond are on the
same side or opposite sides of the
double bond
– If both (1) are on the same side: notation is
Z
– If both (1) are on opposite sides: notation is
E
322
Practice Questions
Ú Draw and label the E and Z isomers for each of
the following:
323
Ú What are the configurations of the double
bonds in the following compounds?
324
Ú Draw the structure of (Z)-3-isopropyl-2-
heptene.
325
Ú Stability of Alkenes (7-7)
The stability of alkenes can be
summarized by the Zaitsev’s Rule:
– More highly substituted double bonds
are usually more stable
– In a reaction: chemical reactions
producing alkenes will tend to produce
the most substituted C=C
326
327
Ú Practice Question
Classify the following alkenes in order of
increasing stability.
A
B
C
D
328
Ú Cycloalkenes stability is related to the
ring strain involved:
C5 or larger are stable
C3 and C4 are less stable
Small rings (C4 to C7) are usually
“cis” (more stable)
329
Ú For larger ring (C8 and larger) it is
possible to have “trans” geometry
H
H
H
H
cis
trans
Trans cyclodecene is almost as stable as the cis isomer
330
Physical Properties (7-8)
Bp and Mp: increases with molecular weights. But
branching tends to decrease it (similar to alkanes).
– Relatively non-polar
331
Synthesis of Alkenes by Eliminations
of Alkyl Halides (7-9)
Dehydrohalogenation
Is the elimination of a hydrogen an halogen from
an alkyl halide: Mechanism: E2
Base: abstract a proton from the carbon adjacent to
C-X (needs strong base, HO-, CH3O-)
X-: leaves at the same time
332
Ø Must have an anti-coplanar arrangement
in order for the reaction to take place
Ø Mechanism: concerted (bonds are formed
and broken at the same time)
X
H
C
-
C
B
H
X
333
Works best with 3o halides and hindered secondary
halides.
C
Br
NaOH
If other types of halides (unhindered 2o and 1o) are
used, it is best to use a very bulky base. This helps
in avoiding substitution reactions.
334
Ú Example:
any of the bases from previous slide would give
this product
Br
Et3N
Difficulty:
Ø With bulky bases and sterically hindered alkyl
halides, it is not unusual to get the Hoffman
product instead of the Zaitsev’s product.
Zaitsev’s product: most substituted C=C
Hoffman product: least substituted C=C
335
Ú Because:
Abstraction of the most hindered proton is
not possible
336
Ú Since the reaction is concerted, and an
anticoplanar arrangement of the proton and
leaving group is necessary, there are
stereochemical consequences to the reaction.
E
337
Elimination from Substituted
Cyclohexanes
Ú E2 elimination can take place from either the most stable
or least stable chair conformation of the molecule. The
requirement to follow is an anticoplanar arrangement of
H and X in axial positions.
338
339
Ú Therefore, the most substituted alkene may
not be produced in these reactions, and the
most stable chair conformation may not be the
one leading to the product.
this H cannot participate in the
elimination since it is equatorial
Cl
H
Cl
more stable
H
Base
E2 conditions
340
Practice Question
Ú What it the major organic product of the E2 elimination
of trans-1-chloro-2-methylcyclohexane.
341
Dehalogenation of vicinal dibromides
Usually performed by reduction with
iodide ion or Zn/acetic acid.
-
I
Br
acetone
Br
Zn
Br
+ ZnBr2
Br
Acetic acid is not involved, it is there simply to clean
the surface of the metal.
342
Ú Need anticoplanar arrangement for the 2
bromines…or the molecule must be able to
reach this conformation for the reaction to
proceed.
Br
H
Ph
NaI
Ph
Ph C C
acetone
H
Br
H
H
Ph
Ú What is the product of this reaction?
H
Ph
Br
Ph
Br
NaI
H
343
Practice Question
Ú What is the product of this reaction?
Br
NaI
Br
344
Alkene Synthesis by Dehydration of Alcohols
(7-10)
Dehydration simply means: removal of water
Mechanism is E1 (elimination 1) and will
produce a carbocation… therefore,
rearrangement to the most stable carbocation is
possible.
345
Mechanism of Dehydration of Alcohols
346
Practice Questions
Ú Complete the following reactions carried out
under acidic conditions and heat.
H2SO4
OH
OH
H2SO4
OH
H3PO4
347
Ú For the last reaction from the last question
(giving the blue product), show how the
reaction takes place by drawing a detailed
mechanism.
348
Study Problems
7-33
Label each structure as Z, E or neither.
349
7-44
Predict the major product of the acidcatalyzed dehydration of the following
alcohols.
2-pentanol
1-methylcyclopentanol
2,2-dimethyl-1-propanol
350
7-45
Propose a detailed mechanism for the
following reaction.
Br
CH3OH
heat
O
351
7-45
Propose a mechanism for the following
reaction.
352