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Organic Chemistry - Carbon

Learning organic chemistry means learning a lot about carbon. Carbon has one pair of electrons in the 2s subshell and two unpaired electrons in the 2p subshell. If it were not for hybridization, carbon would be able to form only two covalent bonds but because of hybridization, it can form four covalent bonds. When carbon hybridizes it takes an electron out of its 2s subshell and puts it unpaired into a p subshell. The carbon atom ends up with four new hybrid orbitals and there is an unpaired electron in each. Because the four hybrid orbitals came originally from 1s orbital and 3p orbitals, we call this kind of hybridization sp3 hybridization. 

Carbon can form a carbon-carbon double bond and you need to know a little about how this bond is formed. When a carbon-carbon double bond is formed this is what happens; two carbon atoms undergo hybridization but instead of forming four sp orbitals they form three sp orbitals and leave 1p orbital as a pure p orbital. Then, in order to form two bonds between them, each carbon atom donates one electron from one of its sp2 orbitals and each carbon atom donates one electron from its pure p orbital. Those two bonds together make the double bond. The bond between the sp2 orbitals is called a sigma bond. The bond between the pure p orbitals is called a pie bond. Double bonds are shorter than single bonds but stronger. Triple bonds are even shorter and stronger. The double and triple bonds do not rotate around their axis but a single bond can. 

A covalent bond can be polar, which means simply that it sort of has a positive end and a negative end because its shared pair of electrons are not being shared equally. Think of a molecule of CH4. In this molecule, hydrogen is less electronegative than carbon. So, for each of the four CH bonds, carbon is a little negative and hydrogen is a little positive. Notice that the molecule as a whole has no net positive or negative end because of symmetry. The center of positive charge is right in the middle at the carbon and the center of negative charge is also right in the middle at the carbon. Now think about the CH3Cl molecule. The hydrogens in this molecule are less electronegative than carbon. So for each of the three C-H bonds, carbon is a little negative and hydrogen is a little positive. Chlorine is more electronegative than carbon. So, in the C-Cl bond, carbon is a little positive and chlorine is a little a negative. 

In this molecule, the center of positive charge is toward the bottom of the molecule and the center of negative charge is toward the top. That means this molecule is polar. If in a whole molecule the center of positive charge and the center of negative charge are in different places, then that molecule is polar. Another way of saying that a molecule is polar is to say that the molecule is a dipole. When a molecule is a dipole, then it has something called a dipole moment. You will not have to know exactly what a dipole moment means. Remember that water is a dipole. You might think from the formula H2O that water would not be a dipole since it has two hydrogen atoms attach to one oxygen atom but water is a polar molecule that is because the molecule is bent due to hybridization and does not form a straight line if the water molecule formed a straight line like the CO2 molecule, then it would not be a dipole.