Structure and Synthesis of Alkenes
Alkenes (olefins) are hydrocarbons which have carbon–carbon double bonds. H
C C
H

H
H

A double bond is a σ bond and a π bond.
Double bond B.D.E. σ bond B.D.E.

= 146 kcal/mol
= 83 kcal/mol

Therefore π B.D.E. must = 63 kcal/mol.
A π bond is weaker than a σ bond. π Bonds are more reactive than σ bonds, and π bonds are considered to be a functional group.
Structure of Alkenes
In ethylene (ethene) each carbon is bonded to 3 other atoms, with zero nonbonding electrons => sp2 hybridization.

The C-H bonds are formed by overlap of sp2 orbitals from the Carbon overlapping with 1s orbital from the Hydrogen.
(The C-H bonds in ethane are sp3/1s overlapping orbitals. The ethene C-H bonds contain more s character than the ethane C-H bonds and are therefore shorter and stronger).

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H

1.33A
H

H

H

H

HH

1.08A

1.09A

HH
H
1.54A

sp2 are 1/3 s, whereas sp3 are 1/4 s in character. (s orbitals are closer to the nucleus and lower in energy).
The carbon-carbon bond in ethene is shorter and stronger than in ethane partly because of the sp2-sp2 overlap being stronger than sp3-sp3, but especially because of the extra π bond in ethene.
Double Bond
The unhybridized p orbitals on each carbon align parallel to form a π bond.

The π bond prevents rotation about the C=C bond because it would force the p orbitals into a non overlapping formation (i.e. break the π bond).
The π bond exists with half its electron density above the σ bond, and half below. Elements of Unsaturation
Alkanes are said to be saturated since they have they maximum number of bonds to hydrogen.
An alkene is unsaturated.

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Any π system or a ring system reduces the maximum number of hydrogens a molecule can have.
An element of unsaturation relates to 2 missing hydrogens from the saturated formula (CnH2n+2).
Consider alkane, alkenes and cycles:
H2
C
H3C CH2CH3 propane C3H8 saturated H2C CHCH3 propene C3H6 one element of unsaturation

H2C

CH2

cyclopropane
C3H6
one element of unsaturation

Heteroatom complications
Heteroatoms are considered anything other than C or H.
Halogens
These simply substitute for hydrogens in the molecular formula.
Therefore just like C2H6 is saturated, so is C2H4F2.
Oxygen
CH3-CH3 is saturated (C2H6)
CH3-O-CH3 is also saturated (C2H6O)
An oxygen can be added without requiring any additional hydrogens, so ignore the number of oxygens when calculating elements of Unsaturation.
Nitrogen
Nitrogen is trivalent, and when it replaces a C in a chain it requires only one hydrogen (-NH- vs. -CH2- ), so nitrogens count as half a carbon.
Thus C4H9N is equivalent to C4.5H9. (i.e. one element of Unsaturation).

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Nomenclature of Alkenes
Simple alkenes are named like alkanes (root from the longest carbon chain), but the –ane suffix is replaced by-ene.
H2C CH2 ethene H2C CHCH3 propene cyclohexene

When the chain is longer than 3 carbons, number the atoms such that the double bond is given the lowest number (i.e. start at the end nearest the double bond).
H
H2C C CH2CH3
1-butene

H H
H3C C C CH2CH3
2-pentene

Compounds with 2 double bonds are called dienes, 3 double bonds are trienes, etc.

H H
H2C C C CH2
1,3-butadiene 1,3,5,7-cyclooctatetraene

For branches, each alkyl group is given a number, but the double bond is still given preference when numbering the chain.
H H
H3C C C CH2
CH3
3-methyl-1-butene

When alkenes are substituents, they are termed alkenyl groups, and may be named systematically.
CH2

H
H2C C CH2

methylene 2-propenyl group group (allyl group)

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Geometric isomers
The rigidity of a π bond gives rise to geometric isomers.
When similar groups (not H’s) are bound to the same side of the double
bond