Learn more about carbon and hydrogen and see how these atoms come together to form distinct molecules. Also, study the difference between saturated and unsaturated molecules.
Organic Molecules and Hydrocarbons
You may recall that atoms can be held together by covalent bonds, which are chemical bonds between atoms that share an electron pair. Organic molecules are molecules that contain carbon atoms, which are covalently bonded together. This category of molecules includes gasoline, sugar, proteins, and everything in between. Many of these molecules contain other types of atoms as well, but today we're going to focus on molecules that consist of the two most common atoms in organic chemistry: carbon and hydrogen.
Carbon and hydrogen can come together in different ways and can form many, many distinct molecules, which are collectively called hydrocarbons. Hydrocarbons are molecules that are made of hydrogen and carbon atoms.
The simplest organic molecule is methane, CH4, which contains one carbon atom and four hydrogen atoms. This molecule makes up much of what we call 'natural gas' that comes out of gas stoves in our houses. Methane is a saturated hydrocarbon. Saturated hydrocarbons are hydrocarbons that contain no rings and contain only single bonds between the different atoms. Methane is the simplest hydrocarbon because it contains only one carbon atom.
Methane is saturated because it has 2N + 2 hydrogen atoms
What about when it gets a little bit more complicated? How can we figure out whether a hydrocarbon is saturated? You can tell whether or not a hydrocarbon is saturated by using the saturation formula and the molecule's chemical formula. Saturated hydrocarbons have 2N + 2 hydrogen atoms, where N is the number of carbon atoms in the molecule. To put this formula into action, first we count the number of carbon atoms in the molecule (N in the formula). According to the saturation formula, we need 2N + 2 hydrogen atoms to make a saturated hydrocarbon. Methane contains one carbon atom, so for it to be saturated, it needs 2 * 1 (the number of carbon atoms in the molecule) + 2 hydrogen atoms, or 2 + 2 = 4 hydrogen atoms.
Alkanes, Alkenes and Alkynes
A hydrocarbon that contains no double bonds is called an alkane, or hydrocarbon containing only single bonds. Methane and ethane are both alkanes. For each element of unsaturation, which is a ring or double bond in a molecule, the total number of hydrogens in the hydrocarbon will decrease by two. Hydrocarbons that contain rings or double bonds are called unsaturated hydrocarbons, which are the opposite of saturated hydrocarbons. Alkanes - such as cyclohexane, which is an important precursor to nylon - that contain rings of carbon atoms are unsaturated.
Another example of a hydrocarbon is ethylene. Ethylene is an important building block in making plastic and also is involved in the ripening of fruit. Ethylene contains two carbon atoms and is an alkene. An alkene is a hydrocarbon that contains a carbon-carbon double bond. All alkenes by definition are unsaturated because of the double bonds. If we wanted to figure out how many hydrogen atoms ethylene has, we could just use the saturation formula. A saturated two-carbon hydrocarbon would have 2 * 2 (the number of carbons) + 2 hydrogen atoms to make 6 total hydrogen atoms. We know that ethylene has one element of unsaturation, which is the double bond, so we can figure out that ethylene has only four hydrogen atoms and its chemical formula is C2H4.
Alkenes are hydrocarbons that have carbon-carbon double bonds
It's also possible for hydrocarbons to contain carbon-carbon triple bonds; that is, two carbon atoms that share three electron pairs between them. These hydrocarbons that contain carbon-carbon triple bonds are called alkynes. Ethyne, which is also known as acetylene and is used in welding torches, is the simplest alkyne. It has two carbon atoms and contains two hydrogen atoms. As organic molecules get larger, their structures can get much more complex than just a simple chain.
Isomers and Aromatic Hydrocarbons
Another hydrocarbon you may be familiar with is butane, which is used in lighter fluid. Butane contains four carbon atoms and is a saturated hydrocarbon. We know that this molecule is saturated, so we can use the saturation formula to figure out how many hydrogen atoms there are in butane. Using the formula, we take 2 * 4, which is the number of carbons, and add 2 to it to get 10 total hydrogen atoms in the molecule.
This presents us with an interesting conundrum: how are we supposed to arrange these atoms? Because hydrogen atoms can have only one bond, we can assume that the carbon atoms will form the main part of the structure of this molecule. One way to arrange these carbon atoms is to put them in a straight chain and then add the hydrogen atoms so that all of the atoms have completed octets - that's four bonds for each carbon atom. Another way to arrange these atoms is to have a branched carbon chain; that is, a single carbon atom with more than two bonds to carbon. We can then fill in hydrogen atoms so that all of the atoms have completed octets. These molecules are isomers of one another; that is, they are molecules that have the same chemical formula but different structures. It turns out that the first one, the straight chain, is butane, while the second one is called 2-methylpropane.
Structure of the isomers butane and 2-methylpropane
There is one more important class of hydrocarbons, which are the aromatic hydrocarbons. These hydrocarbons that contain rings consisting of alternating single and double carbon-carbon bonds were initially called this because of their strong smells. Toluene, which is a very smelly component in nail polish, is one example of an aromatic hydrocarbon.
We've learned that organic molecules are molecules that contain carbon atoms, though they can also contain other atoms. We've learned that a large subclass of these molecules is made only of hydrogen and carbon. This group is called hydrocarbons. These molecules can be saturated and contain no rings and only single bonds, or they may be unsaturated and can contain either rings or multiple bonds. Rings and multiple bonds are also called elements of unsaturation.
We should remember that saturated hydrocarbons have two times the number of carbons (N) plus two hydrogen atoms in them (2N +2). Each element of unsaturation subtracts two hydrogens from the total number of hydrogens in the molecule. Hydrocarbons that contain no multiple bonds are called alkanes. Hydrocarbons with carbon-carbon double bonds are called alkenes. Hydrocarbons with carbon-carbon triple bonds are called alkynes. And hydrocarbons that contain rings of carbon atoms that are joined by alternating single and double bonds are called aromatic. Finally, hydrocarbons with more than three carbons can have isomers; that is, they can have the same chemical formula while having different arrangements of atoms within the molecules.
After watching this lesson, you should be able to:
- Define organic molecules
- Interpret how carbon and hydrogen can form different molecules together
- Compare and contrast saturated and unsaturated molecules