An organic compound is one that is composed of carbon and hydrogen. It may also contain atoms of other elements, such as oxygen, nitrogen, sulfur, and chlorine.
Basic organic compunds contain only carbon and hydrogen, and they are called hydrocarbons. The name substituted hydrocarbon is used when other elements are also present.
Carbon atoms have the unique property of being able to bond to each other and form long "chains" of carbons, or to form circular "rings" of carbons. That, and the fact that the addition of one atom of a different element makes an entirely new compound is the reason that there are millions of organic compounds in the world.
The simplest hydrocarbon is methane, which is made of one carbon atom surrounded by four hydrogen atoms. The work of Pauling showed that the four hydrogens are in a tetrahedral arrangement around the carbon.
The next simplest hydrocarbons are those whose carbons are joined in line one to another. They are called aliphatic hydrocarbons. The first group is that of the alkanes, and the name of each of these compounds ends in "-ane". The alkanes have the generic formula of CnH2n+2. All of the carbons have single covalent bonds between them. They are also called saturated hydrocarbons, which means that all of the carbons have the maximum number of bonds (4).
The first ten alkanes should be memorized, because their names are the basis for most of the naming of the rest of the organic compounds.
Molecular Formula Name Structural Formula CH4 methane C2H6 ethane C3H8 propane C4H10 butane C5H12 pentane C6H14 hexane C7H16 heptane C8H18 octane C9H20 nonane C10H22 decane
The next group of hydrocarbons are called alkenes, and the difference between them and the alkanes is the presence of a double covalent bond in the carbon chain. They have the generic formula CnH2n and all of the names end in "-ene". They represent the simplest of the unsaturated hydrocarbons. All of the formulas for the alkanes shown above (starting with ethane) would be written with two less hydrogens and the name would end in -ene. For example, ethane would become ethene and its formula would be C2H4.
Sometimes the chains can be circled (like the wagon train circling up for the night) and the end carbons join together. This brings about the loss of two hydrogen atoms. A common example is that of the six-carbon hexane forming the six-carbon cyclohexane which has the formula C6H12. Notice that there are two hydrogens less than in hexane. Notice, also, the use of the prefix "cyclo-" in front of the parent name, hexane. This gives a nice picture of a circle of six carbons joined together. There are two forms that the molecule can take. Pictured here is the "boat" form. The other form is the "chair" form where the molecule can be imagined as having the shape of a reclining chair. A sample of cyclohexane is composed of a 50-50 mixture of each form.
The other main group of hydrocarbons is that of the aromatic hydrocarbons. They contain six carbons arranged in a "ring" and contain three double covalent bonds between every other carbon-carbon pair. The basic aromatic hydrocarbon is benzene, C6H6. Notice that the molecule is planar, with all bond angles equal to 120 degrees. Two benzene rings can join together and form naphthalene, C10H8. Both of these compounds serve as the basis for thousands of other very important compounds.
Substituted HydrocarbonsAtoms of other elements can be joined to the carbons in place of one or more hydrogens. Oxygen, nitrogen, sulfur and the halogens are the most common atoms that replace hydrogens. The resulting compound is called a substituted hydrocarbon. Sometimes a combination of two of these other elements will be found in place of hydrogens. These other elements give rise to what are called functional groups. The presence of different functional groups causes the substituted hydrocarbon to be one of several classes of organic compounds.
Probably the most common of these functional groups is the "-OH" group, which is known as the hydroxyl group. It is NOT the hydroxide ion, OH1-, as it does not have a charge. The dash in front of the OH stands for a single covalent bond, which is what will be formed between the oxygen and a carbon atom. An aliphatic hydrocarbon that has one hydroxyl group attached to a carbon is called an alcohol. The symplest alcohol is methyl alcohol, or methanol. Notice that the one word name, methanol, ends in -ol and the root is methan-. You should be able to figure out that the formula would be that of methane, but with an -OH in place of one of the hydrogens. It doesn't matter where you put the hydroxyl group in methanol.
The molecular formula is usually written as CH3OH, because it gives more of a picture of the actual structure than does CH4O. This way of writing the formula becomes more important as the number of carbons increases. Take the case of ethyl alcohol, or ethanol. The parent compound is ethane, so the formula is C2H6O, but C2H5OH gives specific information that the compound is an alcohol and not any other compound.
Things become a bit more complex starting with the three carboned propane. The hydroxyl group can be attached to either end carbon (same compound in either case) or to the middle compound, which produces a slightly different compound. The first case represents the compound n-(for normal) propanol or 1-propanol. The use of the number tells to which postion the hydroxyl is attached. The second carbon placement of the hydroxyl group gives rise to the name 2-propanol, or, using an older system of naming compounds, isopropanol, or isopropyl alcohol. This is probably the type of rubbing alcohol in the medicine cabinet in your homes. The commercial isopropyl alcohol is usually a 70% solution in water. The molecular formula is C3H8O, with C3H7OH showing that the compound is an alcohol, and the even more specific formulas, CH3CH2CH2OH showing 1-propanol, and CH3CHOHCH3 showing 2-propanol.
Notice that in two examples described thus far there have been two different compounds for each of two molecular formulas. The ethanol and dimethyl ether were both C2H 6O and the two alcohols were each C3H8O. Both cases are examples of isomers.
Let's look at the definition of an isomer: two or more different compounds, each with the same molecular formula. Notice the use of the prefix, "iso-". You have already seen on several occassions that this prefix means "same". At this point, you should stop and use the Merck Index, or CRC handbook, to look up these four compounds and compare them--the ethanol with dimethyl ether and the 1-propanol with the 2-propanol. Determine for yourself how the change in placement of a single atom of oxygen can drastically change the nature of the compound.
Two different kinds of isomers are represented here:
- functional isomer
- structural isomer
The hydroxyl group is written -OH, whereas the way to show that the oxygen is bonded in-between two carbons is -O-. These represent two different finctional groups, thus two different classes of organic compounds are represented. The case of the two alcohols, however, is a result of a structural difference in the placement of the same functional group.
Now, back to alcohols:
You should now be able to turn the remaining seven alkanes into a normal alcohol (hydroxyl on the end carbon) and write their names correctly. Notice, as the chain gets longer (has more carbons) the number of possible sites for the hydroxyl to bond also increases, thus increasing the total number of possible alcohols.
There are a few important alcohols that have more than one hydroxyl group. One of the most widely used this that of ethylene glycol, the main ingredient in automobile anitfreeze. Can you figure out the formula?
Based on what you've learned so far, doesn't the name written this way give you a good picture of what the compound looks like?
- First, the "ethyl" means it contains two carbons, just like ethane.
- Second, the "ene" means that it is short two hydrogens, but instead of a double bond between the two carbons, two hydroxyl groups are present, one per carbon.
- Notice the "ol" ending. This tells you that it is an alcohol. The scientific name, or IUPAC name is 1,2-ethanediol.
A second important alcohol with more than one hydroxyl group is glycerol, also called glycerine. Its a three carbon alcohol that contains three hydroxyl groups, one to each carbon. Its IUPAC name of 1,2,3-propanetriol should give you a better picture. It is used in soaps and lotions, and also to make nitroglycerine.
The generic formula for an alcohol is shown by R-OH, where R stands for whatever the -OH is bonded.
An ether results when there is an oxygen atom between two carbons in the chain. The simplest is dimethyl ether, which has the same molecular formula as ethanol. The way the formula is written to show the ether, rather than the alcohol, is CH3OCH3. There are two other ethers of interest, ethylmethyl ether and diethyl ether. You should be able to write the formulas for each of these. Ethers are represented as R-O-R', where R and R' can be the same or different hydrocarbon units.
Notice: when the parent hydrocarbon, such as methane or ethane has something else attached in place of one or more hydrogens, the ending is often changed from -ane to -yl.
A new class of substituted hydrocarbons arises when an oxygen atom is double bonded to the carbon at the end of the chain. In this case there are two less hydrogen atoms, so instead of three end hydrogens, there is the C=O and only one hydrogen. The simplest aldehyde is one that you have probably heard of, formaldehyde, CH2O. Its IUPAC name is methanal. Notice the -al ending as opposed to the -ol ending that alcohols have. Be careful about paying attention to the endings. They identify the class to which the compound belongs. These compounds show the generic formula, H-R=O.
A different class of organic compounds results if the C=O occurs somewhere along the chain other than on the end carbon. The simplest ketone has three carbons. Why not start with two carbons? It has the common name acetone, and is in most fingernail polishes and removers. It is sometimes called dimethyl ketone, but is more properly called propanone. Break apart the name to see how the name propanone gives a better picture of the compounds formula than does acetone.
- First of all, the propan- indicates that the parent hydrocarbon is propane, and thus has three carbons.
- Second, the ending -one goes along with the ending of the name of the class of compounds to which it belongs, ketones.
How would you write the formula for butanone or hexanone? Would you need to include a number as part of the name? The generic formula for ketones is R-C=O(-R').
The organic acids are also called carboxcylic acids. They have a more complex functional group, and if you look at it, you can see both the C=O of the aldehyde and the -OH of the alcohol. The organic, or carboxyl group, is -C=O(-OH), often written as COOH, or even CO2H. Organic acids may have more than one carboxyl group. The simplest organic acid is methanoic acid, CHOOH, or formic acid, to use the older name. Ants inject formic acid into their victim whem they bite them.
methanoic acid (formic acid)
The next in line is, of course, ethanoic acid, CH3COOH. What is the common name of this acid? Notice the ending, -oic, to the IUPAC name, and -ic, to the common name. There are some very important organic acids, and one of the most important is ascorbic acid, better known as Vitamin C. Look up the formulas for ascorbic acid, as well citric, tartaric, and oxalic acids. The generic formula is R-(COOH)x
ethanoic acid (acetic acid)
Why are these compounds acids? Well, they must be able to produce at least one hydrogen ion when they are put into solution, since that is the general definition for an acid. Which hydrogens, and how many hydrogen ions do they prooduce? Even though these organic acids may contain quite a few hydrogen atoms in the molecule, only select hydrogens are able to be "ionized" or turned into hydrogen ions. These "select" hydrogens are those in the carboxyl group (-COOH) The presence of one or more of these groups, therefore, causes the compound to belong to the organic acids.
These compounds have the generic formula, R-COO-R'. Notice that this formula is similar to that of the organic acid, but now the H of the -COOH has been replaced by a hydrocarbon group. The ending of the name of an ester is -ate, such as in ethyl ethannoate. Let's look at it's formula. The ethyl is the name of R' and the acetate represents R.
The generic formula for amines is R-NH2, where one hydrogen has been replaced by an amino group (-NH2). The simplest amine is methyl amine, where the "R" group is methyl. This kind of amine is called a primary amine. There can also be secondary and tertiary amines, with the generic formulas R2-NH and R3-N, respectively with a second, and a third hydrogen replaced with an "R" group. The "R" groups can all be the same, or they can be different.