Organic Chemistry

The Fundamentals of Organic Chemistry ..

Finally, show the orbital overlap. The C-H bonds are formed from overlap of two carbon sp2

orbitals with the two hydrogen 1s orbitals. This leaves one carbon sp2 orbital and one carbon p

orbital for forming the double bond. The carbon sp2 orbital and one of the oxygen sp2 orbitals

overlap to form a sigma bond. The pi bond is formed from overlap of the carbon p orbital and

the oxygen p orbital. Last, place the two oxygen lone pairs into the remaining unoccupied sp2

hybridized orbitals on oxygen as shown earlier.

This problem is admittedly pretty difficult. The first step is assigning the hybridizations of each

of the atoms. The outer carbons are sp2 hybridized, and the inner carbon is sp hybridized

Next, show all the valence orbitals on each of the atoms. The tricky part is lining up the orbitals

from the middle carbon to the outer carbons so the orbitals can overlap to form one double

bond each. Each double bond consists of a sigma bond and a pi bond. Therefore, each of the

carbon-carbon sigma bonds must consist of an sp2

-sp orbital overlap. Pi bonds are formed from

the p orbital overlaps. Therefore, you have to line up the p orbitals so it’s possible for the

orbitals to overlap with the central carbon.

Finally, show the orbital overlap. First, the C-H bonds are formed from the overlap between the

outer carbon sp2 orbitals and the hydrogen 1s orbitals. The sigma bonds in the two double

bonds are formed in both cases from the overlap between the central carbon sp orbital and the

two outer carbon sp2 orbitals. The pi bonds are then formed from the overlap of the two p

orbitals on the central carbon and the lone p orbitals on the outer carbons.

An interesting outcome of this orbital diagram is that the orbital diagram predicts that the two

hydrogens on the left will be coming into and out of the plane of the paper, while the two hydrogens on the right will be going up and down in the plane of the paper. As a matter of fact, this

turns out to be the geometry found experimentally. Chalk one up to orbital diagrams!

The language of chemistry isn’t a spoken language or a written language but a language of

pictures. Lewis structures are the pictorial words of the organic chemist, much like

hieroglyphics were the pictorial words of the ancient Egyptians. Organic chemists currently

use a number of different methods for drawing structures. You may already be familiar with

the full Lewis structure (if not, see Chapter 1), but organic chemists often like to abbreviate

Lewis structures by using simpler drawings to make speaking the language of organic chemistry faster and easier, much like you abbreviate words when text messaging your friends.

Two abbreviations to Lewis structures that you should become familiar with are the condensed structure and the line-bond structure, because you see these two structural abbreviations again and again throughout organic chemistry. This chapter familiarizes you with

drawing and interpreting these structural abbreviations (condensed and line-bond structures) and helps you understand what the structural abbreviations mean. Before you get

down to the dirty business of drawing structures, you practice determining formal charges

and the number of lone pairs on atoms in a structure, two skills that are essential to mastering organic structures.

Assigning Formal Charges

The following equation shows a down-’n’-dirty method of calculating the formal charge on an

atom. The dots are the non-bonding electrons assigned to an atom, and the sticks are the

total number of bonds attached to an atom (a single bond counts as one stick, a double bond

counts as two sticks, a triple, three):

Formal charge of an atom = number of valence electrons – dots – sticks