Functional groups
Because there are so many different chemical compounds it is impossible to learn all of their specific properties. That is why chemical compounds are grouped into ‘families’ with similar properties. The part of a molecule responsible for similar physical and chemical properties in a family is called a functional group.
A selection of common functional groups and families is given in the following paragraphs.
The family or functional group names are also used in the USCG cargo compatibility chart (page 38). This chart indicates which chemicals could react with each other. Names corresponding with the names in this chart are underlined.
Hydrocarbons
Hydrocarbons are compounds that contain only hydrogen (H) and carbon (C). These structures are the basis for most organic compounds.
All hydrocarbons are flammable, but their flashpoints vary. Hydrocarbons are not soluble in water.
There are two subgroups of hydrocarbons, based on the types of bonds between the carbon atoms.
Saturated hydrocarbons
A hydrocarbon that has only carbon-carbon single bonds is called a saturated hydrocarbon. The names alkane or paraffin can also be used.
In these hydrocarbons the full binding capacity of the carbon atoms is used, with each carbon atom having four links to other atoms). Saturated hydrocarbons generally have a lower density than water, and are not soluble in water.
Saturated hydrocarbons are not considered to have a functional group since they do not react. The name paraffin originate from the Latin parum affinis, “little activity”.
Table 4 Nomenclature of alkanes
| Alkane | Cyclo-alkane | |||
| C | Name | Formula | Name | Formula |
| 1 | Methane | CH4 | - | - |
| 2 | Ethane | C2H6 | - | - |
| 3 | Propane | C2H6 | Cyclopropane | C3H6 |
| 4 | Butane | C4H10 | Cyclobutane | C4H8 |
| 5 | Pentane | C5H12 | Cyclopentane | C5H10 |
| 6 | Hexane | C6H14 | Cyclohexane | C6H12 |
| 7 | Heptane | C7H16 | Cycloheptane | C7H14 |
| 8 | Octane | C8H18 | Cyclooctane | C8H16 |
| 9 | Nonane | C9H20 | Cyclononane | C9H18 |
| 10 | Decane | C10H22 | Cyclodecane | C10H20 |
| n | CnH2n+2 | CnH2n | ||
Alkanes are hydrocarbons with the carbon atoms arranged in a linear way. The number of carbon atoms determines the length of the chain, the number of hydrogen atoms increase in a linear way. Each carbon atom bonds at least two hydrogen atoms, both carbon atoms at the end of the chain bond one extra hydrogen atom. From C1 to C4 are gases, C5 to C16 are liquids and above C16 are solids at ambient temperature.
Alkanes with more than 3 carbon atoms can form ring structures. These compounds are called cyclo-alkanes.
Unsaturated hydrocarbons
In unsaturated hydrocarbons, there are less hydrogen atoms than the carbon atoms of the compound can bind. The excess bonding capacity is used to form double or even triple bonds between the carbon atoms.
Note: When hydrocarbons become more complex, drawing their molecule structure becomes tedious. In such cases skeletal formulas are used. Basic principles of skeletal formulas are:
- Each corner and each end of a line segment represents a Carbon atom.
- Double (or triple) bonds are indicated by adding a line parallel the existing line.
- Hydrogen atoms can be added intuitively based on the fact that a carbon atom has 4 bonding opportunities.
- All atoms other than Carbon and Hydrogen will be shown explicitly by using their chemical symbol. (for instance Cl for Chlorine)
When functional groups are discussed, it is also possible that only the functional group is shown, and a symbol ‘R’ is used to represent the base structure.
Unsaturated hydrocarbons are more reactive than saturated hydrocarbons because as this double bond is more reactive .
An alkene (or olefin) has a carbon-carbon double bond. Alkenes are relatively stable compounds, but are more reactive than alkanes due to the presence of the carbon-carbon double bond.
Alkynes contain triple bonds that are more reactive than the double bond of the alkenes.
Unsaturated hydrocarbons can also form ring structures called cycloalkenes. There is a specific group of cycloalkenes called aromatic hydrocarbons. Aromatic hydrocarbons are rings of unsaturated hydrocarbons with a specific configuration of double bonds (in most cases alternating single and double bonds). Most common aromatic hydrocarbons are based on benzene rings (Figure 14 ). Aromatic hydrocarbons represent a specific class because they are more stable than theoretically would be expected. Groups attached to the outside of the aromatic ring, can however be manipulated as they would be expected to.
BTEX is an acronym that stands for benzene, toluene, ethylbenzene and xylenes. These substances are known to have harmful effects on the central nervous system,, subject to exposure concentration.
Hydroxyl group: Alcohols, Phenols, Hydroxides
The –OH group in a hydrocarbon is called an alcohol. If the hydrocarbon is aromatic, it is called a phenol. An –OH group connected to an inorganic compound is called a hydroxide group.
Alcohols are either flammable or combustible. Their solubility depends on the length of the carbon chain: Ethanol (C-2) is fully miscible with water; cetylalcohol (C-16) is immiscible with water. Alcohols have higher boiling points than alkanes of similar molecular weight.
Inorganiccompounds containing hydroxide are common bases, meaning they have a high pH value. Most hydroxides are insoluble in water, but the most common hydroxide (lye) is very soluble in water.
The name glycol or diol refers to an organic compound with two hydroxyl groups. Their properties are similar to the alcohols.
Cresols are a specific group of phenols (see Figure 18 ), these compounds consist of a benzene ring with both a hydroxyl group and a methyl group (CH3).
Table 5 exampes of alcohols and glycols
| Family | Compound name | Application |
| Alcohol | Methanol | Solvent |
| Phenol | Propofol | Antiseptic |
| Hydroxide | Sodium hydroxide (lye) | Cleaning agent |
| Glycol | Ethylene glycol | Antifreeze |
| Cresol | Ortho cresol | Solvent |
Ethers
Ethers consist of an oxygen atom, with a hydrocarbon group on both sides. Only the atom oxygen is the functional group here.
Ethers in general are of low chemical reactivity, but they are more reactive than alkanes. Ethers have relatively low boiling points.
The family glycol ethers consists of the ethers based on ethylene glycol. Glycol ethers are known to be good solvents, and are therefore used for this purpose.
Epoxides or alkylene oxides are cyclic ethers with 3 atoms in the ring. Because the ring is very small, the atoms are strained, making epoxides more reactive than normal ethers.
Epichlorohydrin is a specific epoxide with a chloride atom attached. It is highly reactive and used in the production of other chemical substances. Epichlorohydrin is irritant and toxic as well as carcinogenic depending on concentration.
Table 6 examples of ethers
| Family | Compound name | Application |
| Ether | Diethyl ether | Anaesthetic |
| Glycol ethers | 2-Methoxyethanol
(Ethylene glycol monomethyl ether) |
solvent |
| Epoxide | Epichlorohydrin | Production of
epoxy resins |
Carbonyl group: aldehydes, ketones
This functional group consists of a carbon atom with a double bonded oxygen atom (C=O). If the carbon atom is bonded to at least one hydrogen atom, the compound is called an aldehyde. If the carbon atom is bonded to two other carbon atoms, the compound is called a ketone.
Ketones and aldhydes have higher boiling points than the corresponding hydrocarbons. The smaller molecules of this group (low molecular weight) are soluble in water.
Acetone is a commonly known ketone used in nail polish remover and paint thinner while
Formaldehyde is a common aldehyde used in the production of polymers.
Acids
Acids are defined as sour tasting substances that can react with bases, metals and carbonates. Solutions of acids have pH’s lower than 7.
Acids may be either organic or inorganic.
Organic acids are hydrocarbons with the functional group -COOH. Most organic acids are weak acid, the degree of corrosivity decreasing with increasing chain length.
A mineral or inorganic acid is an acid derived from one or more inorganic compounds. Mineral acids tend to be very soluble in water and insoluble in organic solvents. Commonly used mineral acids are sulphuric acid and nitric acid because of their corrosive and oxidative properties. In the USCG chart, mineral acids other than the two mentioned here are referred to as non-oxidizing mineral acids.
Bases
Bases are the chemical opposites of acids. When acids and bases react with each other, they neutralise each other in forming salts. Solutions of bases have pH’s higher than 7.
Caustics are particularly corrosive bases. Caustic soda or sodium hydroxide (NaOH) is a common example of a caustic substance.
Esters
Esters are formed by the reaction of an acid with an alcohol.
Esters have characteristic fruity smells. The smells of fruits are the result of natural occurring esters.
The boiling points of esters are lower than those of acids with similar molecular weight.
Ethyl acetate is a common ester with many uses. Because of its pleasant fruity smell it is used in perfumes and artificial flavours. As a solvent it is used to decaffeinate coffee beans.
Nitrogen: Amines, imines, nitriles, cyanides
Nitrogen atoms can from single, double or triple bonds to a carbon atom.
Organic compounds with a single bonded nitrogen atom are called amides. Organic compounds with carbon-nitrogen double and triple bonds are imines and nitriles, respectively.
Boiling points of amides are higher than the corresponding alkanes and lower than the corresponding alcohols. Small amines are soluble in water; the solubility decreases with increasing molecular weight. Amines are slightly basic.
Different groups of amines can be distinguished in terms of reactivity:
Aliphatic amines are amines with ‘normal’ hydrocarbon groups attached to it. If the hydrocarbon group also has a hydroxyl group attached to it, the amine is called an alkanol amine. Aromatic amines have an aromatic base structure with an amine group. Alkanol amines and aromatic amines are less alkaline than aliphatic amines.
Inorganic nitriles are called cyanides. The difference between cyanides and nitriles is mainly that the cyanide group in nitriles is tightly bound while inorganic cyanides can release a cyanide ion. Cyanides are deadly poisonous.
Table 7 examples of amines
| Family | Compound name | Application |
| Amine | Chlorpheniramine | Antihistamine |
| Nitrile | Acetonitrile | Solvent |
| Cyanide | Sodium Cyanide | Gold purification |
Isocyanates
Isocyanates have the functional group R-N=C=O.
They are known to be very reactive and can be harmful to living tissue. Exposure to isocyanates and to their vapours should be avoided because they are toxic and irritant,, subject to exposure concentration.
Methyl isocyanate is of industrial significance in being used in the manufacture of pesticides.
Amides
Amides are substances with an amino group NHx bonded to a carbonyl carbon atom (the C in C=O).
In contrast with amines, amides are not basic. Most amides are solid at room temperature and have high boiling points and high melting points.
Amides are the base structure for nylons and polyamides (Aramid, Twaron, Kevlar).
Nitro compounds
Nitro compounds contain the nitro functional group NO2.
Most nitro compounds are highly explosive. This increases with impurities.
Frequently transported nitro compounds are nitrobenzene and nitrophenol. They are used in various chemical processes.
Nitrates, Nitrites
Nitrate and nitrite ions are NO3 and NO2 respectively. They can either be attached to an organic base structure, or to an inorganic substance.
Organic nitrates and nitrites mainly exhibit the physical properties of their base structures.
Inorganic nitrates and nitrites are both toxic in water, with the latter being most toxic though again subject to exposure concentrations.
Table 8 examples of nitrates and nitrites
| Family | Compound name | Application |
| Inorganic nitrate | Ammonium nitrate | Fertilizer |
| Inorganic nitrite | Sodium nitrite | Food preservative |
| Organic nitrate | Nitroglycerin | Medicine |
| Organic nitrite | Amyl nitrite | Medicine |
Acid anhydrides
Acid anhydrides or organic anhydrides are substances with the following functional group:
Most commonly the C=O groups are derived from the same organic acid. Acid anhydrides can be symmetrical (same acid) or unsymmetrical (differing acides).
Acetic anhydride is based on two acetic acid groups. This anhydride is used in the production of esters.
Acrylates
Acrylates are the salts and esters of acrylic acid. Acrylates contain vinyl groups (C=C), directly attached the carbonyl carbon (the C in C=O).
Acrylates are very reactive because of the double bond. As a result they can easily form polymers.
Methyl methacrylate is used in the production of Perspex (polymethylmethacrlyate)
Halides
Chlorine (Cl), Bromine (Br), Iodine (I) and Fluorine (F) are halogens. When bonded to an organic compound, they are called halides or halogenated hydrocarbons the bonding being covalent.
However, when attached to a metallic element e.g. sodium, they are called salts, the bonding being ionic.
Salts are, in different degrees, soluble in water and generally have a low hazardous classification.
The properties of organic halides generally resemble those of their parent hydrocarbon, with their boiling points being higher with increasing molecular weight.
The vinyl halides are a sub-group in which the halogen is attached directly to a double carbon bond.
Table 9 examples of halides, salts, vinyl halides
| Family | Compound name | Application |
| Halide | Dichloromethane | Solvent |
| Salt | Sodium chloride | Table salt |
| Vinyl halide | Vinyl chloride | Production of PVC |
Allyls
An allyl group is a hydrocarbon group with the structural formula shown below.
Because the allylic C bonds are more reactive than normal C bonds allyls are themselves more reactive. This is the reason why allyls are intermediates in many chemical processes.
Substitued allyls have a functional group attached, for example, a halide or hydroxyl group.
An example of a substituted allyl is allyl alcohol. This compound is used in the production of glycerol.
Peroxides
Peroxides are compounds containing an oxygen-oxygen single bond. They can be either organic or inorganic.
Both organic and inorganic peroxides are very reactive, as they are an abundant source of oxygen.
Organic peroxides are used as catalysts in the production of polymers because they can start a polymerization reaction in materials that have unsaturated chemical bonds. Inorganic peroxides are powerful oxidizers.
Barium peroxide is inorganic peroxide; it is used in pyrotechnics and tracer ammunition.
Methyl Ethyl Ketone Peroxide (MEKP) is organic peroxide commonly known for its use in glass reinforced plastics production.
