Alkenes 1

University of Babylon – College of Pharmacy

Department of Pharmaceutical Chemistry

Level: first class, second semester
Title of course, Organic Chemistry

Lecture n. 6 - Alkenes 1. 20-3/2018

Structure And Preparation

5.1 Unsaturated hydrocarbons

In our discussion of the alkanes we mentioned briefly another family of hydrocarbons, the alkenes, which contain less hydrogen, carbon for carbon, than the alkanes, and which can be converted into alkanes by addition of hydrogen.
The alkenes were further described as being obtained from alkanes by loss of hydrogen in the cracking process.
Since alkenes evidently contain less than the maximum quantity of hydrogen, they arc referred to as unsaturated hydrocarbons.
This unsaturation can be satisfied by reagents other than hydrogen and gives rise to the characteristic chemical properties of alkenes.

5.2 Structure of ethylene.

The carbon-carbon double bond.
The simplest member of the alkene family is ethylene, C2H4 .
In view of theready conversion of ethylene into ethane, we can reasonably expect certain structural similarities between the two compounds.
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 required
eight, and thai the entire molecule needs an additional pair,of electrons if it is to be
neutral.
We can solve both these problems by assuming that the carbon atoms can 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-carbon double bond is the distinguishing feature of the alkene structure.

5.3 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…

5.4 Hybridization and orbital size

The carbon-carbon double bond in alkenes is shorter than the carbon-carbon
single bond in alkanes because four electrons bind more tightly than two.
But, in addition, certain other bonds in alkenes are significantly shorter than their counterparts in alkanes: for example, the C H distance is 1.103 A in ethylene compared with 1.112 A in ethane. To account for this and other differences in bond length,we must consider differences in hybridization of carbon.
The carbon-hydrogen bonds of ethylene are single bonds just as in,say,ethane, but they are formed by overlap of sp2 orbitals of carbon,instead of sp3 orbitals as in ethane. Now, compared with an sp3 orbital, an sp
2 orbital has less p character and more s character. A p orbital extends some distance
from the nucleus; an s orbital, on the other hand, lies close about the nucleus.
As the s character of a hybrid orbital increases, the effective size of the orbital decreases and, with it, the length of the bond to a given second atom.
Thus an sp2-s carbon-hydrogen bond should be shorter than an sp3-s carbon-hydrogen bond.

5.6 Geometric isomerism

Since the isomeric 2-butenes differ from one another only in the way the atoms are oriented in space (but are like one another with respect to which atoms are attached to which other atoms), they belong to the general class we have called
Stereoisomers (Sec.4.1).
They are not, however, mirror images of each other,and hence are not enantiomers. As we have already said, stereoisomers that are not
mirror images of each other are called diascereomers.
The particular kind of diastereomers that owe their existence to hindered rotation about double bonds are called geometric isomers.
The isomeric 2-butenes,then, are diastereomers, and more specifically, geometric isomers