Upload
mxr-3
View
239
Download
0
Embed Size (px)
Citation preview
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 1/55
6. Alkenes: Structure
and Reactivity
Based on McMurry‟s Organic Chemistry , 7th edition
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 2/55
2
Alkene - Hydrocarbon With
Carbon-Carbon Double Bond
Also called an olefin but alkene is better
Includes many naturally occurring materials
Flavors, fragrances, vitamins
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 3/55
3
Why this Chapter?
C-C double bonds are present in most
organic and biological molecules
To examine consequences of alkene
stereoisomerism
To focus on general alkene reaction:electrophilic addition
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 4/55
4
6.1 Industrial Preparation and
Use of Alkenes
Ethylene and propylene are the most
important organic chemicals produced
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 5/55
5
6.2 Calculating Degree of
Unsaturation Relates molecular formula to possible structures
Degree of unsaturation: number of multiple bonds or rings
Formula for a saturated acyclic compound is CnH2n+2
Each ring or multiple bond replaces 2 H's
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 6/55
Degrees of Unsaturations (DoU)…
Defined as the number of rings and/or
multiple bonds present in a molecule due to
the lost of 2 hydrogen atoms from the
saturated formula, CnH2n+2.
Also known as Index of Hydrogen Deficiency
(IHD)
1 ring 1 DoU
1 double bond 1 DoU
1 triple bond 2 DoU
6
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 7/55
DoU….
7
2
HC-)HC(
=DoU
unsn2+n2n
Where,
Huns = #H contain or suppose to contain in an
unsaturated compound
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 8/55
8
Example: C6H10
Saturated is
C6H14 Therefore 4 H's are not
present This has two
degrees of
unsaturation Two double bonds?
or triple bond?
or two rings
or ring and double bond
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 9/55
What happen if the compound have other atoms
together with H and C???
How do we calculate the DoU???
9
Degree of Unsaturation With Other
Elements….
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 10/55
1) Organohalogen compounds…
contain C, H and X, where X= halogen= F, Cl, Br or I
Halogen acts simply as a replacement for H in an
organic molecule. Huns = #H + #X
Eg: C4
H6
Br 2
, Huns
= #H + #X = 6 + 2 =8
10
Degree of Unsaturation With Other
Elements….
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 11/5511
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 12/5512
Degree of Unsaturation With
Other Elements
2) Organooxygen compounds…
contain C, H and O
Since O forms 2 bonds (-O-), its insertion or substraction
between C-C (C-O-C) and C-H (C-O-H) doesn‟t changethe number of H atoms, therefore doesn‟t affect the DoU.
Huns = #H
Eg: DoU for C5H8O = DoU for C5H8
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 13/5513
Organonitrogen compounds
3) Organonitrogen Compounds
Nitrogen has three bonds
So if it connects where H was, it adds a connection point
Subtract one H for equivalent degree of unsaturation in
hydrocarbon
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 14/5514
Summary - Degree of
Unsaturation
2
HC-)HC(=DoU
unsn2+n2n
Where,
For pure HC compound: Huns = #H
For organohalogen compound: Huns = #H + #X
For organooxygen compound: Huns = #H (ignore the #O)
For organonitrogen compound: Huns = #H - #N
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 15/5515
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 16/55
Naming of alkenes…
Step 1:
Name the parent carbon chain. The parent
carbon chain must be the longest carbon
chain that have at least 1 C=C double bondalong it. Name the compound accordingly
using suffix „ene‟.
16
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 17/55
Step 2:
Number the carbon atom in the chain.
Begin at the end nearer to the double bond
If the double bond is equidistant from both
ends, begin numbering at the end nearer to
the first branch point.
17
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 18/55
Step 3:
Write the full name of the alkene compound.
List the substituent alphabetically.
Indicate the position of double bond by numbering itbased on the number of the first alkene carbon
beside it. Place that number directly before the
parent name.
18
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 19/55
If more than 1 double bond present, indicate
the position of each and use one of the
suffixes –diene, triene, and so on.
19
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 20/55
but….
20
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 21/55
Naming Cycloalkenes…
Start numbering parent ring from the first carbon with
double bond.
The first substituent has as low a number as possible.
Not necessary to indicate the position of double bond inthe name because it is always between C1 and C2.
BUT, indicate the position of double bond if you have
more than 1 double bond.
21
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 22/5522
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 23/5523
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 24/5524
Many Alkenes Are Known by
Common Names
Alk b tit t
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 25/5525
Alkene as substituents….
•In some cases, a group containing an alkene
may need to be treated as a substituent.
•In these cases the substituent is named in a similar
fashion to simple alkyl substituents.
•The method is required when the alkene is not the priority group.
•The substituent is named in a similar way to the parent alkene.
•It is named based on the number of carbon atoms in the branch
plus the suffix -yl. i.e. alkenyl
•There are two common names that are widely used:
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 26/5526
6.4 Cis-Trans Isomerism in Alkenes
Carbon atoms in a double bond are sp2 -hybridized
Three equivalent orbitals at 120º separation in plane
Fourth orbital is atomic p orbital
Combination of electrons in two sp2
orbitals of twoatoms forms bond between them
Additive interaction of p orbitals creates a bonding
orbital
Subtractive interaction creates a anti-bonding orbital Occupied orbital prevents rotation about -bond
Rotation prevented by bond - high barrier, about
268 kJ/mole in ethylene
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 27/5527
Rotation of Bond Is Prohibitive
This prevents rotation about a carbon-carbondouble bond (unlike a carbon-carbon singlebond).
Creates possible alternative structures
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 28/5528
The presence of a carbon-carbon double bond cancreate two possiblestructures
c is isomer - two similar groups on same side of the double bond
t rans isomer - similar groups on oppositesides
Each carbon must havetwo different groups for these isomers to occur
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 29/55
29
Cis, Trans Isomers Require That End
Groups Must Differ in Pairs
180°rotation superposes
Bottom pair cannot be superposed without breaking C=C
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 30/55
30
6.5 Sequence Rules: The E,Z
Designation
Neither compound is clearly “cis” or “trans”
Substituents on C1 are different than those on C2
We need to define “similarity” in a precise way to
distinguish the two stereoisomers
Cis, trans nomenclature only works for disubstituted
double bonds
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 31/55
31
E,Z Stereochemical Nomenclature
Priority rules of Cahn,
Ingold, and Prelog
Compare where higher
priority groups are with
respect to bond anddesignate as prefix
E -entgegen , opposite
sides
Z - zusammen ,together on the same
side
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 32/55
32
Ranking Priorities: Cahn-Ingold-
Prelog Rules
RULE 1
Must rank atoms that are connected at comparison point
Higher atomic number gets higher priority
Br > Cl > S > P > O > N > C > H
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 33/55
33
RULE 2
If atomic numbers are the same, compare at nextconnection point at same distance
Compare until something has higher atomic number
Do not combine – always compare
Extended Comparison
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 34/55
34
RULE 3
Substituent is drawn with connections shown and no
double or triple bonds
Added atoms are valued with 0 ligands themselves
Dealing With Multiple Bonds:
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 35/55
35
6.6 Stability of Alkenes
Cis alkenes are less stable than trans alkenes
Compare heat given off on hydrogenation: Ho
Less stable isomer is higher in energy
And gives off more heat
tetrasubstituted > trisubstituted > disubstituted > monosusbtituted hyperconjugation stabilizes
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 36/55
36
Comparing Stabilities of Alkenes
Evaluate heat given off when C=C is converted to C-C
More stable alkene gives off less heat
trans-Butene generates 5 kJ less heat than cis-butene
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 37/55
37
Hyperconjugation
Electrons in neighboring filled orbital stabilize vacant
antibonding orbital – net positive interaction
Alkyl groups are better than H
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 38/55
38
6.7 Electrophilic Addition of
Alkenes
General reaction
mechanism:
electrophilic addition
Attack of electrophile
(such as HBr) on bondof alkene
Produces carbocation
and bromide ion
Carbocation is an
electrophile, reacting
with nucleophilic bromide
ion
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 39/55
What is electrophile???
39
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 40/55
40
Two step process
First transition state is high energy point
Electrophilic Addition Energy Path
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 41/55
41
Electrophilic Addition for
preparations
The reaction is successful with HCl and with HI as well as
HBr
HI is generated from KI and phosphoric acid
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 42/55
42
6.8 Orientation of Electrophilic
Addition: Markovnikov‟s Rule
In an unsymmetricalalkene, HX reagents canadd in two different ways,but one way may bepreferred over the other
If one orientationpredominates, the reactionis regiospecific
Markovnikov observed inthe 19th century that in theaddition of HX to alkene,the H attaches to the
carbon with the most H‟sand X attaches to the other end (to the one with themost alkyl substituents)
This is Markovnikov’s
rule
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 43/55
43
Example of Markovnikov‟s Rule
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 44/55
44
Markovnikov‟s Rule (restated)
More highly substituted carbocation forms as intermediate
rather than less highly substituted one
Tertiary cations and associated transition states are more
stable than primary cations
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 45/55
45
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 46/55
46
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 47/55
47
6.9 Carbocation Structure and
Stability
Carbocations are planar and the tricoordinate carbon issurrounded by only 6 electrons in sp2 orbitals
The fourth orbital on carbon is a vacant p-orbital
The stability of the carbocation (measured by energy
needed to form it from R-X) is increased by the presenceof alkyl substituents
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 48/55
48
I d ti t bili ti f ti
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 49/55
49
Inductive stabilization of cation
species
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 50/55
50
6.10 The Hammond Postulate
If carbocation intermediate is more stable than another,
why is the reaction through the more stable one faster?
The relative stability of the intermediate is related to an
equilibrium constant (Gº)
The relative stability of the transition state (whichdescribes the size of the rate constant) is the activation
energy (G‡)
The transition state is transient and cannot be
examined
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 51/55
51
Transition State Structures
A transition state is the highest energy species in areaction step
By definition, its structure is not stable enough to exist for one vibration
But the structure controls the rate of reaction So we need to be able to guess about its properties in an
informed way
We classify them in general ways and look for trends inreactivity – the conclusions are in the Hammond Postulate
E i ti f th H d
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 52/55
52
Examination of the Hammond
Postulate
A transition state
should be similar to
an intermediate
that is close in
energy
Sequential states
on a reaction path
that are close in
energy are likely tobe close in
structure - G. S.
Hammond
C ti R ti d th
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 53/55
53
Competing Reactions and the
Hammond Postulate
Normal Expectation: Faster reaction gives more stable
intermediate
Intermediate resembles transition state
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 54/55
54
6.11 Mechanism of Electrophilic Addition:
Rearrangements of Carbocations
Carbocations undergo
structural rearrangements
following set patterns
1,2-H and 1,2-alkyl shifts
occur Goes to give more stable
carbocation
Can go through less stable
ions as intermediates
H dride shifts in biological
7/27/2019 Chapter6 Alkenes Structure & Reactivity
http://slidepdf.com/reader/full/chapter6-alkenes-structure-reactivity 55/55
Hydride shifts in biological
molecules