In the previous lessons you learned that the properties of an organic molecule depend on the molecule’s structure. Recall that double and triple bonds between carbon atoms affect the properties of hydrocarbons. This is why double and triple bonds are also considered to be functional groups.
Properties of Alkanes, Alkenes, and Alkynes
Alkanes, alkenes, and alkynes are all nonpolar molecules and therefore have low solubility in water. They also have relatively low melting and boiling points.
Alkynes are more reactive than alkenes, which in turn are more reactive than alkanes.
The branches of branched hydrocarbons are also often referred to as functional groups. This is because the branching arrangement dramatically changes its structure, which will therefore change its properties.
Properties of Branched-Chain Hydrocarbons
Going from “branched” to “highly branched,” molecules become more compact, the melting point of a molecule increases, and the boiling point decreases.
The properties of an organic compound are largely determined by the functional groups it contains. By learning the properties associated with a given functional group, you can predict the properties of an organic compound. This is similar to how knowing an element’s chemical family (given by its group number) allows you to predict its properties.
The five new functional groups you have been introduced to in this lesson all add a new element to hydrocarbons: oxygen. Oxygen has 6 valence electrons, and therefore is capable of forming two covalent bonds. This allows oxygen atoms to form bonds with two additional atoms, or one other atom in a double bond.
Consider the parent hydrocarbon chain: Propane
| Propane, C3H8 | |
|---|---|
|
Properties colorless, odorless gas nonpolar low solubility in water melting point: -187.7 °C boiling point: -42 °C |
Click the name of each of the functional groups below to learn how the addition of a functional group to a parent hydrocarbon can change the properties of the molecule.
Alcohols are organic compounds in which the hydroxyl functional group (-OH) is bound to a carbon atom.
The structure of an alcohol is similar to that of water, as it has a bent shape. Like water, alcohols are polar, containing an unsymmetrical distribution of charge between the oxygen and hydrogen atoms. Since alcohols are able to hydrogen bond, their boiling points are higher than those of their parent molecules.
| Propanol, C3H7O | |
|---|---|
Straight, linear hydrocarbon chain of 3 carbon atoms. The carbon atom at the end of the chain is single bonded to a hydroxyl (OH-) group. |
Properties: colorless liquid; mild odor polar soluble in water melting point: -126 °C boiling point: 98 °C |
When a carbonyl functional group is placed within a molecule, it is known as a ketone.
Due to the carbonyl group, ketones are polar.
Ketones are often more reactive than alcohols and carboxylic acids of comparable molecular weights.
| Propanone, C3H6O | |
|---|---|
|
Properties colorless liquid; pungent floral odor polar soluble in water melting point: -94 °C boiling point: 56 °C |
Aldehydes differ from ketones in that the carbonyl is placed at the end of the carbon skeleton rather than between two carbon atoms of the backbone.
Ketones and aldehydes can both be readily reduced to alcohols, usually in the presence of a strong reducing agent. In the presence of strong oxidizing agents, they can be oxidized to carboxylic acids.
| Propanal, C3H6O | |
|---|---|
Straight, linear hydrocarbon chain of 3 carbon atoms. The carbon atom at the end of the chain is double bonded to an oxygen atom and single bonded to a hydrogen atom. |
Properties colorless liquid; pungent, fruity odor polar soluble in water melting point: -81 °C boiling point: 46 °C |
Esters are characterized by a carbon bound to three other atoms: a single bond to a carbon or hydrogen, a double bond to an oxygen, and a single bond to an oxygen. The singly bound oxygen is bound to another carbon.
Esters participate in hydrogen bonds as hydrogen bond acceptors, but they cannot act as hydrogen bond donors--unlike their parent alcohols and carboxylic acids. This ability to participate in hydrogen bonding allows some water-solubility.
| Ethyl formate, C3H6O2 | |
|---|---|
A carbon atom at one end of the chain is double bonded to an oxygen and then bonded to another oxygen. That oxygen is then bonded to a chain of two carbon atoms. |
Properties colorless liquid; fruity odor polar soluble in water melting point: -80 °C boiling point: 54 °C |
A carboxyl group (COOH) is a functional group consisting of a carbonyl group (C=O) with a hydroxyl group (O-H) attached to the same carbon atom. Carboxyl groups have the formula -C(=O)OH, usually written as -COOH or CO2H.
Carboxylic acids act as both hydrogen bond acceptors, due to the carbonyl group, and hydrogen bond donors, due to the hydroxyl group. As a result, they often participate in hydrogen bonding. This gives them increased stability as well as higher boiling points relative to the acid in aqueous solution. Carboxylic acids are polar molecules; they tend to be soluble in water.
| Propionic acid, C3H6O2 | |
|---|---|
Straight, linear hydrocarbon chain of 3 carbon atoms. The carbon atom at the end of the chain is double bonded to an oxygen atom and single bonded to a hydroxyl (OH-) group. |
Properties Colorless, oily liquid; rancid odor polar soluble in water melting point: -20.5 °C boiling point: 141 °C |
Question
These five functional groups all have one thing in common: they add the element oxygen to organic molecules. How does this change affect the properties of organic molecules?
Typically, molecules with these functional groups are more polar (and more likely to be soluble in water), have higher melting and boiling points, and have more distinct odors compared to a parent hydrocarbon of similar size.
