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Alkenes: Structure and Reactivity^ Alkenes
, also called ______, are hydrocarbons containing
one or more double bonds.
olefins
Naming the alkenes:^ Rule 1
: Find the longest chain containing the double bond and name the Rule
1: Find the longest chain containing the double bond and name thecompound by replacing the alkane suffix –ane with –ene. Rule 2: Number carbons in the chain so that double bond carbons have the
lowest possible numbers; if the double bond is equidistant from twoends, begin at the end nearer the first substituent.
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Nomenclature of Alkenes
vinylic carbons
Hex-2-ene
2-Methylhex-3-ene
allylic carbons
vinyl
2 Eth l
t
1
2 - Methyl
buta
- 1 3-
diene
_____ group
gro p vinyl allyl
2 -Ethylpent-1-ene
2-Methylbuta-1,3-diene- diene
and -
triene
Cycloalkenes:
allyl _____ group
Factoids of Alkenes
Polarization of Alkenes
m.p. -
o C b.p. 4
o C
m.p. -
o C b.p. 1
o C
Th^
b^
i^
i^
i^
l^
Thi^
l^ d
h^
ibili^
f^ i^
d^ 8-
The barrier to rotation is very large. This leads to the possibility of
cis^
and
trans
isomers ________, stable, separable compounds with different properties.
E ,
Z^
Nomenclature
Sometimes “
cis ” and “
trans
” are ambiguous;
in such cases we resort to the more formal
cis^
or^ trans
Z
Steps to assign
E^ and
Z^ descriptors
in such cases we resort to the more formal E , Z
nomenclature.
Z
Steps to assign
E^ and
Z^ descriptors
- For each C atom, assign substituent priority•^ Z^
if highest priority substituents on same side of double bond
-^ E^
if highest priority substituents on opposite side of double bond
hi^ h
l
higher
lower
lower
higher
higher
higher
lower
lower
g
lower
lower
Stability of Alkenes
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more hyperconjugation
Heat of Hydrogenation: a Measure of Stability
o ∆ H
Preparation of Alkenes
follows Zaitsev’s rule
Elimination Reactions:
An E2 reaction that generatesthe thermodynamically lessfavored isomer is said tofollow the
Hofmann rule follow the
Hofmann
rule.
+ H
Dehydration of Alcohols:
+ H
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Reactivity in dehydration:o^1
o< 2 < 3
o^ alcohols
Reactions of Alkenes: The C=C Double
Bond as a NucleophileBond as a Nucleophile
A pi bond can donate electronsp bo d ca
do a e e ec
o s
This atom has ____________from
(^2) sp to^ sp
3
re-hybridized
The brackets denote
This carbon now has an empty
The
brackets denote an ___________ alongthe reaction pathway
intermediate
p orbital; its incomplete octet(6 electrons) makes it electrondeficient
Product Distribution and
the Reaction Coordinate Diagramthe Reaction Coordinate Diagram
For this step the TS
≠^ is
For^
this step, the TS
is
late. Thus, we orderedthe energies of the TS
≠s
according to the energiesaccording to the energiesof the intermediates.
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Electrophilic Addition: Alkyl Halide Preparation^ •
The reaction is successful with HCl and with HI as well as HBr• HI is generated from KI and phosphoric acid• HI is generated from KI and phosphoric acid Predict the relative reactivity of the following alkenes towardelectrophilic addition of HX and give your explanations.
(CH
)C=CH 3 2
2
CH
CH=CHCH 3
3
CH
CH=CH 3
2
CH
=CH 2
2
(CH
)C=CH 3 2
CH 2
CH=CHCH 3
CH 3
CH=CH 3
CH 2
=CH 2
2
Rearrangement of Carbocations
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Electrophilic Hydration of Alkenes
Alkene hydration and alcohol dehydration are equilibrium processes.
mechanism^ Acid Catalyzed Equilibration of Alkenes
y^
q
catalytic^ H
+
catalytic H
+^
more stable; major componentin isomeric product mixture
Electrophilic Addition of Halogen to Alkenes
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a racemic mixture (
± )
Stereochemistry Elucidates the Mechanism
- Br
a carbocation wouldbe trigonal planar
O t t
ti fi d f
ll^ t
Octet satisfied for all atoms
Bromonium Trapped by Other Nucleophiles
( ± )
Br^2 CH^ OH^3
( ± ) ( ± )
similarly followingMarkovnikov rule
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Regioselectivity in Halonium Ion Opening
Carbene and Carbenenoid Reactions
bicyclo[4.1.0]heptane
cis -diethylcyclopropane
Simmons-Smith reagent
: (iodomethyl)zinc iodide
“^ b
id
” (^
b^
lik^
b t^
)
“ carbenoid
”^ (a carbene-like substance)
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92%
Reduction of Alkenes: Catalytic Hydrogenation
syn-
addition
1
diastereoselective diastereoselective
Oxidation and Reduction in Organic
ChemistryChemistry
Reduction
increase electron density on carbon by:
Reduction
increase electron density on carbon by:— forming: C–H— or breaking: C–O
C–N
C–X
Oxidation
decrease electron density on carbon by:— forming: C–O
C–N
C–X
— or breaking: C–H— or breaking: C–H
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an epoxide
Epoxide Synthesis electrophilic
oxidation by
peroxycarboxylic acid:
via halohydrin:
Diol Formation: Epoxide Ring-Opening
vicinal
anti dihydroxylation
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Vicinal
Syn
Dihydroxylation of Alkenes
Not very useful forsynthesis: easily over-oxidized into ketones
cold, dilute(w/ NaOH)
oxidized into ketonesand/or carboxylic acids.
Use of OsO
or NaHSO
3
Use
of OsO
+ OsO+^ OsO
3
3
8-
a^ syn
vicinal diol
OsO
: expensive and toxic 4
Oxidative Cleavage: Ozonolysis
( or^ CH
SCH 3
) 3
mechanism:
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More Oxidative Cleavages to Carbonyl
CompoundsCompounds
KMnO
4
+ H
4
HIO
4
or NaHSO
3
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