Organic and biochemistry
Alkene
Assitance Lecturer Amjad Ahmed Jumaa
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Alkenes
Alkenes family has the general formula CnH2n, which contain less
hydrogen, carbon for carbon than alkanes, and contain carbon-carbon
double bond.
since alkenes evidently contain less than the maximum quantity of
hydrogen they are referred to as unsaturated hydrocarbons.
The simplest member of the alkenes family is ethylene, C2H4.
In this compound the carbon atoms are joined by a double bond.
The carbon-carbon double bond is the distinguishing feature of the
alkenes structure:
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To start, then, we connect the carbon atoms by a covalent bond, and then
attach two hydrogen atoms to each carbon atom. At this stage we find that each
carbon atom possesses only six electrons in its valence shell, instead of the
requiredeight, and thai the entire molecule needs an additional pair, of electrons
if it is to beneutral. We can solve both these problems by assuming that the
carbon atomscan share two pairs of electrons. To describe this sharing of two
pairs of electrons,we say that the carbon atoms are joined by a double bond. The
carbon-carbondouble bond is the distinguishing feature of the alkene structure.
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Quantum mechanics gives a more detailed picture of ethylene and the
carboncarbon double bond. To form bonds with three other atoms,
carbon makes use of three equivalent hybrid orbitals: sp2 orbitals, formed
by the mixing of one s and two p orbitals.
repulsion among orbitals gives four tetrahedral bonds, so it gives three
trigonal bonds. If we arrange the two carbons and four hydrogens of
ethylene to permit maximum overlap of orbitals, we obtain the structure
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If we arrange the two carbons and four hydrogens of ethylene to permit
maximum overlap of orbitals, we obtain the structure:
carbon atom lies at the center of a triangle, at whose corners are located
the two hydrogen atoms and the other carbon atom. Every bend angle is
120. Although distributed differently about the carbon nucleus, these bonds
individually are very similar to the bonds in ethane, being cylindrically
symmetrical about a line joining the nuclei, and are given the same
designation: a bond(sigma bond).
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The molecule is not yet complete, however. In forming the sp2 orbitals,
each carbon atom has used only two of its three p orbitals. The remaining
p orbital consists of two equal lobes, one lying above and the other lying
below the plane of the three sp2 orbitals:
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•The carbon-carbon "double bond" is thus made up of a strong a bond and
a weak -n bond. The total bond energy of 163 kcal is greater than that of the
carbon carbon single bond of ethane (88 kcal). Since the carbon atoms are
held more tightly together, the CC distance in ethylene is less than the CC
distance in ethane; that is to say, the carbon-carbon double bond is shorter
than the carboncarbon single bond.
This quantum mechanical structure of ethylene is verified by direct evidence.
Electron diffraction and spectroscopic studies show ethylene to be a flat
molecule, with bond angles very close to 120. The CC distance is 1.34 A as
compared with the CC distance of 1.53 A in ethane.
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Propylene
The next member of the alkene family is propylene, C3H6 . In view of its
great similarity to ethylene, it seems reasonable to assume that this
compound also contains a carbon-carbon double bond. Starting with
two carbons joined by a double bond, and attaching the other atoms
according to our rule of one bond per hydrogen and four bonds per
carbon, we arrive at the structure:
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