Organic Compounds

An alkane is a straight carbon chain with all single carbon–carbon bonds. As shown in Table 1, each carbon atom needs four bonds. Carbons at the end of the chain are bonded to one other carbon, so they will be bonded to three hydrogens. Carbons elsewhere in the chain will be bonded to two other carbon atoms, so they will be bonded to two hydrogens. See Figure 1.

The naming of alkanes proceeds as follows. Consider the molecule labeled “propane”. It is a three-carbon chain with only single bonds between the carbon atoms, hence it is an alkane. (Single bonds are indicated by a single line between the two atoms.) In Table P we see that three carbon atoms correspond to the prefix “prop” to which we add the “ane” ending; prop+ane = propane, and so the molecule is named appropriately.

Notice the entry for alkanes in Table Q. Under the heading “General Formula” we see the expression . For the molecules shown in Figure 1 we predict the chemical formulae as given in Table 2.

Count the hydrogen atoms on each molecule in Figure 1 and confirm that the general formula works.

Alkenes are similar to alkanes, but at least one pair of carbon atoms are double bonded. As a result there will be fewer bonds available for bonding to hydrogen. Because alkanes contain the greatest number of hydrogen atoms possible we say that they are saturated. Alkenes by contrast are unsaturated. You have probably heard about saturated and unsaturated fats and oils with regard to food. It refers to the absence or presence of double bonds between carbon atoms.

Examples of alkenes appear in Figure 4.

Two carbon atoms, according to Table P, indicate the prefix “eth”, and the double bond implies the ending “ene”; eth+ene = ethene. For longer carbon chains there is an added complication in that the we need to indicate the position of the double bond. Prop-1-ene indicates that the double bound is connected to the first carbon. We always begin counting from the end closest to the double bond, so with a three-carbon chain the double bond is always connected to the first carbon. For propene it really isn’t necessary to include the position, but for longer molecules it is.

Notice the entry for alkenes in Table Q. Under the heading “General Formula” we see the expression . For the molecules shown in Figure 4 we predict the chemical formulae as given in Table 3. (Notice that the formula does not apply to dienes.)

Count the hydrogen atoms on each molecule in Figure 4 and confirm that the general formula works.

An alkyne is like an alkane with one carbon–carbon triple bond. The example ethyne appears in Figure 8.

Two carbon atoms, according to Table P, indicate the prefix “eth”, and the triple bond implies the ending “yne”; eth+yne = ethyne. For longer carbon chains there is an added complication in that the we need to indicate the position of the triple bond.

Notice the entry for alkynes in Table Q. Under the heading “General Formula” we see the expression . For the molecules shown in Figure 8 we predict the chemical formulae as given in Table 4. (Notice that the formula only applies to chains with a single triple bond.)

Count the hydrogen atoms on each molecule in Figure 8 and confirm that the general formula works.

Halides refer to the Group 17 elements fluorine (), chlorine (), bromine () and iodine (); from Table R in the Reference Tables we see that these four halides have the prefixes fluoro, chloro, bromo and iodo, respectively. Some examples of molecules including halides appear in Figure 12.

Fluorinated and chlorinated hydrocarbons have many uses. Many are excellent solvents for use in the chemistry laboratory and the chemical industry. They can be very useful as cleaners. Some have been used as fire retardants, and others have been used in compressors for refrigeration or air conditioning. Unfortunately we have learned that they are very bad for the ozone layer, and many are carcinogenic. As a result usage of these types of chemicals has been significantly reduced.

You have probably heard about alcohols without understanding their chemical significance. The simplest alcohol is methanol which is also called wood alcohol because it can be obtained from wood. Methanol is very poisonous, and methanol ingestion causes blindness. Ethanol is the alcohol that is available in alcoholic beverages. You may have isopropyl alcohol in your medicine cabinet to use as a disinfectant. What all alcohols have in common is an group bonded to a carbon atom. See Figure 14.

Ethanol is named as follows. The prefix from Table P for a two-carbon chain is “eth” to which we add the proper ending; eth+anol = ethanol. There is an older naming scheme where the same alcohol is called ethyl alcohol. Similarly methanol is sometimes called methyl alcohol. In fact the proper IUPAC name for isopropyl alcohol is propan-2-ol, but without ambiguity it is sometimes written 2-propanol. The reason for placing the number right next to the -ol ending is because it is a better scheme for more complex molecules that may have multiple functional groups.

The basic structure of an ether is shown in Figure 17.

An example of an ether appears in Figure 18.
In this case the and groups of Figure 17 are replaced by the ethyl and methyl alkyl groups as discussed in Section 3.3.1.

An aldehyde has an oxygen atom doubly bonded to a carbon atom at the end of a carbon chain. See Figure 21.

A carbon atom doubly bonded to an oxygen atom is known as a carbonyl group, and its placement at the end of a chain makes it an aldehyde.
To name an aldehyde begin with the name of the alkane with a carbon chain of the same length, drop the“e” at the end of the name and add “al”. For example, butane-e+al = butanal.

Probably you have heard of formaldehyde which is used to preserve dead tissue. The proper IUPAC name is methanal. Note that one never needs to specify the position of the aldehyde group because it is always at the end.

The ketone functional group is similar to the aldehyde functional group except the double bonded oxygen atom is bonded to a carbon atom that is not at the end of the chain. See Figure 23.

Recall that a carbon atom doubly bonded to an oxygen atom is known as a carbonyl group; its placement away from the end of a chain makes it a ketone.
Aldehydes and ketones are classified separately because, despite many similarities, some of their chemical properties are quite different. To name a ketone start with the name of the related alkane, drop the “e” and add “one”. For example, propane-e+one = propanone.

The carboxyl group has a carbon atom double-bonded to an oxygen atom and also bonded to an alcohol group. Do not confuse the carboxyl group with the carbonyl group. See Figure 25.

The carboxyl group always occurs at the end of a carbon chain because, with four bonds per carbon atom, there are simply not enough bonds for it to bond to two other carbon atoms. To name an organic acid start with the name of an alkane, drop the “e” and add “oic acid”. For example, propane-e+oic acid = propanoic acid. While a carboxyl group may look like the combination of an aldehyde and an alcohol, it introduces all new chemical behaviors.

Methanoic acid is also known as formic acid. The name derives from the Latin word formica because this chemical was first obtained by distilling a certain type of ant with a stinging bite. Ethanoic acid is more commonly known as acetic acid; vinegar is a primarily acetic acid and water.

Do not confuse esters described in this section with ethers described in Section 3.6. The ester functional group is shown in Figure 27.

Perhaps an ester is most easily understood from how they are synthesized in the chemistry laboratory. See Figure 28.

An ester is formed by dehydration synthesis with an organic acid and an alcohol. An alcohol and an organic acid combine to form an ester and water. An example of an ester is shown in Figure 29.

Propyl acetate (propyl ethanoate) is formed from 1-propanol and acetic (ethanoic) acid. The group from the organic acid combines with a hydrogen from the alcohol to form a molecule of , and the carbon from the organic acid connects to the oxygen from the alcohol. The result is like an ester with a carbonyl group attached to the oxygen atom.

The amine functional group introduces nitrogen into organic chemistry. See Figure 32.

If all three groups, , and , are replaced with hydrogen atoms this is just ammonia, . Oftentimes the hydrogen atoms are replaced with alkly groups.

The amide functional group is shown in Figure 35.

The amide functional group has a carbon atom double-bonded to an oxygen atom and single-bonded to a nitrogen atom. An amide is created by a dehydration synthesis with an organic acid and an amine. Notice that the amide functional group looks like an ester with a nitrogen in place of an oxygen. (Compare Figure 35 with Figure 27.) The dehydration synthesis of an amide is shown in Figure 36.

A hydrogen atom previously bonded to the nitrogen in the amine combines with from the carboxyl group to form a molecule of . The result is a carbonyl group attached to a nitrogen.

Examples of amides are shown in Figure 37.

N-methyl acetamide is formed from the dehydration synthesis of acetic acid with methyl amine. N-methyl propanamide is formed from the dehydration synthesis of methyl amine with propanoic acid. Amides are named as follows. Take the name of the organic acid and replace the “oic” ending with the “amide” ending. Any groups that were attached to the amine are listed out front with the ”N-” prefix to specify that they are attached to the nitrogen. Note that we could also have alkyl groups with numeric prefixes if they were connected to the primary carbon chain from the organic acid.