Alkenes: Structure & Properties Alkane (acyclic): CnH2n+ - Oneonta

16 Pages·1999·0.06 MB·English

by  Bruce Knauer

Alkenes: Structure & Properties
Alkane (acyclic): C H > saturated.
n 2n+2
Alkene (acyclic): C H > unsaturated.
n 2n
eg ethylene (IUPAC: ethene), C H : H C=CH
2 4 2 2
The carbon-carbon double bond is the distinguishing
feature of alkenes.
s
It is formed between two sp2 carbons. One bond is ,
formed by head to head overlap of sp2 orbitals; the
p
second is , formed by overlap of parallel 2p orbitals.
p
The bond has two halves, one above and the other
below the plane defined by the two carbons and the four
other atoms to which they are attached.
.
The bond angles around the sp2 carbons are 120o.
s
-bond requires ca. 90 kcal/mole to break.
p
-bond requires ca. 65 kcal/mole to break.
p
Owing to the shape of the -bond and its strength, there
is not free rotation around a double bond as there is
around most single bonds.
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The butenes: C H
4 8
CH -CH -CH=CH : 1-butene,
3 2 2
(CH ) C=CH : 2-methylpropene
3 2 2
CH -CH=CH-CH : 2-butene
3 3
. .
l(o(ok
Appear to be 3 isomers, but again!
There are two isomeric 2-butenes, for 4 butene isomers in
all.
H C CH H C H
3 3 3
C C C C
H H H CH
3
cis-2-butene trans-2-butene
The cis/trans isomers are configurational (not
conformational) stereoisomers and are sometimes called
geometric isomers.
These isomers have different properties:
cis- bp = 4oC, mp = -139oC;
trans- bp = 1oC, mp = -106oC.
The potential for geometric isomerism will exist in alkenes
unless at least one of the carbons joined by the double
bond carries two identical groups.
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If one of the carbons is part of a ring to which the double
bond is exocyclic, the carbon is considered to carry two
identical groups if, starting at the trigonal carbon,
traversing the ring in the clockwise direction is the same
as traversing it in a counterclockwise direction; otherwise
this carbon is considered to carry two different groups.
In the case below, the left doubly-bonded carbon clearly
holds two different groups. The right doubly-bonded
carbon is considered to hold two groups because in
traversing the ring in opposite directions the ring-groups
appear in different order. Consequently, this compound
has a geometric isomer which is shown at the bottom of
the page.
CH CH
3 3
H C H C
3 1 3 3
C C 2 C C 2
3 1
H H
1 = CHCH , 1 = CH ,
3 2
2 = CH , 2 = CH ,
2 2
3 = CH 3 = CHCH
2 3 CH
3
H
The geometric isomer is C C
H C
3
3
On the other hand, the compound shown below does not
have a geometric isomer because in traversing the ring on
the right we find that the ring-groups are in the same
order whether we go clockwise or counterclockwise.
H C H C
3 3 3
1
C C 2 CH C C 2 CH
3 1 3
3
H H
1 = CH , 1 = CH ,
2 2
2 = CHCH , 2 = CHCH ,
3 3
3 = CH 3 = CH
2 2
There is no geometric isomerism here.
E, Z System for Naming Geometric Isomers —
Learn how to do this. It is covered on pgs. 83-86 in
McMurry, “Fundamentals of Organic Chemistry,” 4th
edition.
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Classification of Reactions – by:
1) Functional group
2) Kind
a) Addition: A + B > C
b) Elimination: A > B + C
c) Substitution: A-B + C-D > A-C + B-D
d) Rearrangement: A > B,
where B is a constitutional isomer of A
3) Mechanism
a) General Type
b) Specific Details
General Mechanism Type
Two common reaction types –
± Polar
[ Free Radical
Polar – most important.
Bonds are formed heterogenically: an electron rich moiety
(molecule or part of a
molecule) called a nucleophile
X Y
donates a pair of electrons to
nucleophile electrophile
an electron poor moiety, an
electrophile. Basically, a
nucleophile is a Lewis base; an electrophile is a Lewis
acid.
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Bonds are broken heterolytically: one part of a molecule
accepts both electrons in bond as it is broken.
When a single bond is broken the departing
X Y
moiety is called the leaving group.
leaving
group
Examples of polar reactions:
This bond will be broken
O H
heterolytically
H C C C H + O H
3
H
This bond will be
formed heterogenically
O H O H
H C C C H C C C + H O H
3 3
H H
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When C is site of attack,
Lewis base = nucleophile
Lewis acid = electrophile
This bond will be
Nucleophile
formed heterogenically
O H
H C C C + H C
3
Electrophile
H O
p
This bond will be
broken heterolytically
O H H
H C C C C
3
H O
(Free) Radical –
Reaction involves a (free) radical --- a chemical species
with an unpaired electron.
Bonds form homogenically: radical and moiety with which
it reacts each provide one electron.
Bonds break homolytically: atom on each side of bond
takes one electron from bond.
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Example: Polymerization of Ethylene to Form
Polyethylene
This is a chain reaction. The product of each step, a free
radical, is a reactant in a subsequent step.
moves one electron
Initiation- C O O C
O O moves a pair of electrons
heat or
hn homolytic
C O O C
O O
These are free radicals: each has an unpaired electron.
Propagation steps- homogenic
H H
C O + C C C O CH CH
2 2
H
O H O
homolytic
H H
homogenic
C O CH CH + C C
2 2
H
O H
homolytic C O CH CH CH CH
2 2 2 2
O
H C CH
2 2
C O (CH2CH2)n CH2 CH2
over and over
O
homogenic
R
Termination- R CH + H C R R CH CH R
2 2 2 2
homogenic
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Reaction Equilibrium
K = e-D G°/RT = 1/eD G°/RT,
eq
where, R = 1.99 cal/degree-mole
T = temperature in oK
D
G° = the standard Gibbs free energy change
= G° - G°
products reactants
[For the moment let’s not worry about exactly what “Gibbs
free energy” is. We’ll get back to this. For the moment
let’s just agree that both reactants and products contain
some kind of energy that we call Gibbs free energy.]
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D D
If Go = 0, K = 1. The larger Go is, the smaller K is;
D
the smaller or more negative Go is, the larger K is.
(If Go > Go , K < 1,
products reactants
reactants favored at equilibrium;
if Go < Go , K > 1,
products reactants
products favored at equilibrium.)
A + B C + D
G
Equilibrium
Mixture
A + B
G
C + D
Equilibrium
Mixture
C + D
A + B
G
Equilibrium
Mixture
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