1. Topic 2: Alkenes: Structure
Alkenes are hydrocarbons that contain a carbon-carbon double
bond. The word olefin is used as a synonym for alkene, but the term
alkene is preferred. Alkenes occur abundantly in nature. Ethylene, for
example, is a plant hormone that induces ripening in fruit. Life would be
impossible without such alkenes as β-carotene, which contains 11 double
bonds. β-carotene is the orange pigment responsible for the color of
carrots, serves as a valuable source of vitamin A and is thought to offer
some protection against certain types of cancer.
β-carotene
Additionally, many alkenes serve as the precursors to many industrially
important compounds. Approximately 26 million tons of ethylene
(CH2=CH2) and 14 million tons of propylene (CH3CH=CH2) are
produced each year in the U.S. for use in the synthesis of many
important substances.

2. I. Calculating a Molecule’s Degree of Unsaturation
UNSATURATED:
Refers to molecules that contain less than the max. number
of H atoms
Remember that alkanes are saturated, meaning that they
contain the max. number of H atoms
DEGREE OF UNSATURATION:
Each ring and/or double bond corresponds to a loss of 2 H atoms (from the alkane formula CnH2n+2) and = 1° unsaturation
Ex.
1° unsaturation
2° unsaturation

3. [ (2 x # of C + # of N ─ # of halogens ─ # of H ) + 2 ] ÷ 2
The degrees of unsaturation for a molecule can be calculated from the following formula:
[ (2 x # of C # of N ─ # of halogens ─ # of H ) ] ÷ 2
Keep in mind that this formula will tell you how many degrees of
unsaturation are present in a molecule but it will not tell you whether the
unsaturation is due to a multiple bond or a ring.

4. III. Cis – Trans Isomerism in Alkenes
* Because rotation around a double C-C bond is impossible
without breaking the bond, cis-trans isomerism occurs in
alkenes just as it does for cyclic compounds.
CIS ISOMERS:
Substituents are on the same side
Ex cis-2-butene
TRANS ISOMERS:
Substituents are on the opposite side
Ex trans-2-butene

5. If one of the double bonds is attached to two identical groups,
cis- trans isomerism cannot occur.
These molecules are identical: These molecules are not identical:
They are not cis-trans isomers They are cis-trans isomers

6. IV. Naming Alkanes
* Cis-trains isomerism is not limited to disubstituted alkenes.
However, when more than two substituents are present, such
as in trisubstituted (3 substituents other than H) or
tetrasubstituted (4 substituents other than H), the cis-trans
naming system fails. A more general method for describing
double bond geometry, called the E, Z system, is used.
CAHN, INGOLD, and PRELOG :
The three scientists who proposed
the sequence rules for the E, Z
naming system (circa 1966)

7. Z E CAHN ─ INGOLD ─ PRELOG SEQUENCE RULES
The rules are used to assign high and low priority to the groups attached
to each carbon.
These sequence rules, named after the scientists who proposed them,
consider each of the carbon atoms in the double bond separately.
If the high priority groups are on the same side of the double bond, the
designation Z is used (from the German word zusammen meaning
“together”).
If the high priority groups are on the opposite side of the double bond,
the designation E is used (from the German word entgegen meaning
“opposite”).
Z means the groups are on “ze zame zide”
Z E

8. Br > Cl > O > N > C > H
Rule 1: Considering each of the carbon atoms of the double bond separ-
ately, identify the two atoms directly attached and rank them
according to atomic number.
Br > Cl > O > N > C > H
low
high
low
low
high
low
high
high
(E)-2-chloro-2-butene
(Z)-2-chloro-2-butene

9. Rule 2: If a decision can’t be reached by ranking the 1st attached atom,
then look at the 2nd, 3rd, etc.
low
high
low
high
low
high

10. Rule 3: Multiple-bonded atoms are equivalent to the same number of
single-bonded atoms.
is equivalent to

11. V. Alkene Stability
* Although cis-trans interconversion of alkene isomers does
not occur spontaneously without breaking the pi bond, it can
be made to happen by treating the alkene with a strong acid
catalyst. When this is done, many time it is found that one
isomer is preferred over another, suggesting that some
isomers are more stable than others. H+
Cis (24%) Trans (76%)

12. STERIC STRAIN:
The strain imposed on a molecule when two groups try to
occupy the same space bulkier groups = more strain
HYDROGENATION REACTION:
Reaction in which an alkene or alkyne reacts w/ H2 in the presence of a Pt or Pd catalyst to form an alkane
Ex.
HEAT OF HYDROGENATION:
Amount of heat released (measured as ∆H = change in enthalpy) during a hydrogenation reaction
The more heat released upon hydrogenation, the more
unstable (more steric strain) the original alkene was

13. Ex.
Cis Isomer Trans Isomer
∆H = -120 kJ ∆H = -115 kJ
*less stable *more stable