Physical Properties and Functional Groups

Physical properties and functional groups

Compounds that contain very similar atoms can have very different properties depending on how those atoms are arranged. This is especially true when they have different functional groups. The table below shows some properties of different functional groups.

Tip:

Vanilla has a sweet smell. The smell of ethene and dimethyl ether cling in your nose though, and the smell of ether can even stay on your skin up to $\text{24}$ hours.

Functional group	Typical Smell	Example	Formula	Melting point (℃)	Boiling point (℃)	Phase (at 25℃)
alkane	odourless	ethane	$\text{C}_{2}\text{H}_{6}$	$-\text{183}$	$-\text{89}$	gas
alkene	sweet/musky	ethene	$\text{C}_{2}\text{H}_{4}$	$-\text{169.2}$	$-\text{103.7}$	gas
ether (-O-)	sweet	dimethyl ether	$\text{C}_{2}\text{H}_{6}\text{O}$	$-\text{141}$	$-\text{24}$	gas
haloalkane	almost odourless	chloro ethane	$\text{C}_{2}\text{H}_{5}\text{Cl}$	$-\text{139}$	$\text{12.3}$	gas
aldehyde	pungent fruity	ethanal	$\text{C}_{2}\text{H}_{4}\text{O}$	$-\text{123.4}$	$\text{20.2}$	gas
alcohol	sharp	ethanol	$\text{C}_{2}\text{H}_{6}\text{O}$	$-\text{114}$	$\text{78.4}$	liquid
ester	often fruity	methyl methanoate	$\text{C}_{2}\text{H}_{4}\text{O}_{2}$	$-\text{100}$	$\text{32}$	liquid
alkyne	odourless	ethyne	$\text{C}_{2}\text{H}_{2}$	$-\text{80.8}$	$-\text{84}$	gas
carboxylic acid	vinegar rancid butter	ethanoic acid	$\text{C}_{2}\text{H}_{4}\text{O}_{2}$	$\text{16.5}$	$\text{118.5}$	liquid

Table: Some properties of compounds with different functional groups.

Listed in the table are the common smells and other physical properties found for common functional groups. Only one representative example from each homologous series is provided. This does not mean that all compounds in that series have exactly the same properties. For example, short-chain and long-chain alkanes are generally odourless, while those with moderate chain length (approximately $\text{6}$ - $\text{12}$ carbon atoms) smell like petrol.

Some specific properties of a few functional groups will now be discussed in more detail.

Physical properties of the alcohols

Fact:

In humans, ethanol reduces the secretion of a hormone called antidiuretic hormone (ADH). The role of ADH is to control the amount of water that the body retains. When this hormone is not secreted in the right quantities, it can cause dehydration because too much water is lost from the body in the urine (ethanol is a diuretic). This is why people who drink too much alcohol can become dehydrated, and experience symptoms such as headaches, dry mouth, and lethargy. Part of the reason for the headaches is that dehydration causes the brain to shrink away from the skull slightly.

Fact:

Ethane has a relatively low solubility in water.

Because hydroxyl ($-\text{OH}$) groups can hydrogen bond, all three pentanol molecules have a greater solubility in water than ethane.

Ethanol is completely soluble in water in any amount. This is because it contains a hydroxyl group and has a much shorter non-polar chain length than pentanol.

pentanol	solubility ($\text{g.dm$^{-3}$}$)
ethane	$\text{0.057}$
1-pentanol	$\text{22}$
2-pentanol	$\text{45}$
3-pentanol	$\text{59}$
ethanol	soluble

The hydroxyl group ($-\text{OH}$) affects the solubility of the alcohols.

Definition: Solubility

Solubility is a measure of the ability of a substance (solid, liquid or gas) to dissolve in another substance. The amount of the substance than can dissolve is the measure of its solubility.

The hydroxyl group generally makes the alcohol molecule polar and therefore more likely to be soluble in water. However, the carbon chain resists solubility, so there are two opposing trends in the alcohols. Alcohols with shorter carbon chains are usually more soluble in water than those with longer carbon chains.

Alcohols tend to have higher boiling points than the hydrocarbons because of the strong hydrogen bond between hydrogen atoms of one hydroxyl group and the oxygen atom of another hydroxyl group.

Physical properties of haloalkanes

There can be more than one halogen substituted for a hydrogen atom on one haloalkane. The more halogens are substituted the less volatile the haloalkane becomes. Take for example the haloalkane series shown in the figure below.

A series of haloalkanes with increasing numbers of chlorine atoms:

(a) chloromethane, (b) dichloromethane, (c) trichloromethane and (d) tetracholormethane.

Definition: Volatility

The tendency of molecules at the surface of a compound to enter the gas phase. The more volatile a compound the more likely this is.

For every extra chlorine atom on the original methane molecule the volatility of the compound decreases. This can be seen by the increase in both the melting and boiling point (see table below) as one goes from chloromethane through to tetrachloromethane. The more halogen atoms in the compound the stronger the intermolecular forces are, which leads to higher melting and boiling points.

Common Name	Number Cl atoms	Melting point (℃)	Boiling point (℃)
chloromethane	$\text{1}$	$-\text{97.4}$	$-\text{24.2}$
dichloromethane	$\text{2}$	$-\text{96.7}$	$\text{39.6}$
trichloromethane	$\text{3}$	$-\text{63.5}$	$\text{61.2}$
tetrachloromethane	$\text{4}$	$-\text{22.9}$	$\text{76.7}$

Table: Melting and boiling points of haloalkanes with increasing numbers of chlorine atoms.

Properties of carbonyl compounds

Carboxylic acids are weak acids, in other words they only dissociate partially. Why does the carboxyl group have acidic properties? In the carboxyl group, the hydrogen tends to separate itself (dissociate) from the oxygen atom. In other words, the carboxyl group becomes a source of positively-charged hydrogen ions ($\text{H}^{+}$). This is shown in the figure below.

The dissociation of a carboxylic acid.

The carboxylic acid functional group is soluble in water. However, as the number of carbon atoms in the attached carbon chain increases the solubility decreases. This is discussed in greater detail in the next section.

Remember that carboxylic acids form hydrogen bonding dimers (the formation is called dimerisation) as shown in the figure below.

Ethanoic acid forming a carboxylic acid hydrogen bonding dimer.

The ability of a molecule to hydrogen bond leads to increased melting and boiling points when compared to a similar molecule that is unable to hydrogen bond. Similarly, the ability of a molecule to form a hydrogen bonding dimer leads to increased melting and boiling points when compared to similar molecules that can only form one hydrogen bond (see table below).

Molecule	Hydrogen bonds per molecule	Melting point (℃)	Boiling point (℃)
ethane	0	-183	-89
ethanol	1	-114	$\text{78.4}$
ethanoic acid	2	$\text{16.5}$	$\text{118.5}$

Table: The melting and boiling points of similar organic compounds that can form different numbers of hydrogen bonds.

Physical properties of ketones

Hydrogen bonds are stronger than the van der Waals forces found in ketones. Therefore compounds with functional groups that can form hydrogen bonds are more likely to be soluble in water. This applies to aldehydes as well as to ketones.

The hydrogen bond between water and a ketone (propanone).

Common Name	Number Cl atoms	Melting point (℃)	Boiling point (℃)
chloromethane	\(\text{1}\)	\(-\text{97.4}\)	\(-\text{24.2}\)
dichloromethane	\(\text{2}\)	\(-\text{96.7}\)	\(\text{39.6}\)
trichloromethane	\(\text{3}\)	\(-\text{63.5}\)	\(\text{61.2}\)
tetrachloromethane	\(\text{4}\)	\(-\text{22.9}\)	\(\text{76.7}\)

pentanol	solubility (\(\text{g.dm$^{-3}$}\))
ethane	\(\text{0.057}\)
1-pentanol	\(\text{22}\)
2-pentanol	\(\text{45}\)
3-pentanol	\(\text{59}\)
ethanol	soluble

Physical Properties and Functional Groups