Classification of hydrocarbons | Aliphatic vs. Aromatic, Saturated vs. Unsaturated hydrocarbons

Aliphatic hydrocarbons

1. Alkanes (Saturated Hydrocarbons)

•      Structural Isomerism
•      IUPAC (International Union of Pure and Applied Chemistry) nomenclature of alkanes
•      Properties of Alkanes
•      Reactions of Alkanes
  •      Alkyl Halides

2. Alkenes and Alkynes (Unsaturated Hydrocarbons)

•      Cis–Trans Isomerism
•      Reactions of Alkenes and Alkynes

Aromatic Hydrocarbons

•      Naming Aromatic Hydrocarbons
•      Properties of Aromatic Hydrocarbons
•      Reactions of Aromatic Hydrocarbons

Hydrocarbons – organic molecules which consist only of carbon atoms and hydrogen atoms connected by covalent bonds. 

There are two main classes of hydrocarbons - aliphatic and aromatic hydrocarbons.

The general term aliphatic hydrocarbon refers to a compound that has a structure based on straight or branched chains or rings of carbon atoms.

Aliphatic hydrocarbons are further divided into four families:

1. alkanes,
2. alkenes,
3. alkynes,
4. cyclic hydrocarbons (cycloalkanes, cycloalkenes, etc.).

 Types of hydrocarbons

Aliphatic hydrocarbons

Alkanes (Saturated Hydrocarbons)

A saturated hydrocarbon, also known as an alkane, is a hydrocarbon in which all bonds between the carbon atoms are single bonds.

The general chemical formula for an alkane is C_nH_2n+2. 

Alkanes in which the carbon atoms form long chains are called straight-chain alkanes.

Examples of the first ten alkanes

A cyclic alkane, or cycloalkane, is a hydrocarbon in which the carbon atoms form a closed loop instead of a chain.

The general formula of cyclic alkanes is C_nH_2n.

Structural Isomerism

Structural isomer is a compound that has the same molecular formula as another compound, but a different structure. 

Some hydrocarbons contain one or more hydrocarbon branches attached to the main structure of the molecule (alkyl group).

Alkyl groups are named with the prefix indicating the number of carbon atoms in the branch and a -yl suffix.

An alkyl group is a type of substituent group.

Substituent group - an atom or group that replaces a hydrogen atom in an organic compound.

IUPAC nomenclature of alkanes

The International Union of Pure and Applied Chemistry (IUPAC) has established a system for naming chemicals.

The names of the alkanes beyond butane are obtained by adding the suffix -ane to the Greek root for the number of carbon atoms.

For a branched hydrocarbon is used the longest continuous chain of carbon atoms to determine the root name for the hydrocarbon.

The length of a branch is indicated by the name of the alkyl group, and its location by numbering from the shortest end of the parent chain.

Cyclic hydrocarbons are named in a similar way. The name is based on the number of carbon atoms in the ring, with the prefix cyclo- added.

Properties of Alkanes

The bonds between carbon and hydrogen in alkanes are close to being non-polar.

Coupled with the fairly even arrangement of hydrogen atoms within alkane molecules, this causes the molecules to be non-polar.

Van der Waals forces are the main intermolecular force in hydrocarbon compounds.

These forces are very weak, so alkanes exhibit relatively low boiling and melting points.

The boiling points of alkanes are related to the length of the carbon chain: as the chain gets longer, the boiling point gets higher.

Fractional distillation (or fractionation) is used on an industrial scale in oil refineries.

Properties of the first ten straight-chain alkanes

Reactions of Alkanes

In general, alkanes are fairly unreactive.

This chemical inertness makes them valuable as lubricating materials and as the backbone for structural materials such as plastics.

Alkanes are used as fuels because their complete combustion releases a lot of energy, along with carbon dioxide and oxygen.

Alkyl Halides

An alkane that contains a halogen is called an alkyl halide. Alkyl halides may be formed by substitution reactions.

In a substitution reaction, one functional group in a chemical compound is replaced by another functional group.

Substituted methanes containing both chlorine and fluorine are called chlorofluorocarbons (CFCs) or Freons. Their general formula is CF_xCl_y.

These compounds are non-toxic and mostly unreactive.

Freons have been extensively used as coolant fluids in refrigerators and air conditioners.

Alkyl halides are named by writing the root of the halogen name first, with the suffix -o, followed by the name of the parent alkane.

The substituent groups are written in alphabetical order.

The halogens are much more electronegative than are carbon and hydrogen. This makes the molecule polar. The resulting polarity increases the strength of the intermolecular forces.

Alkenes and Alkynes (Unsaturated Hydrocarbons)

An unsaturated hydrocarbon - a hydrocarbon containing carbon–carbon double or triple bonds.

Alkene - an unsaturated hydrocarbon that contains at least one carbon–carbon double bond.

The general chemical formula for an alkene is C_nH₂.

Alkyne - an unsaturated hydrocarbon that contains at least one carbon–carbon triple bond.

The general chemical formula for an alkyne is C_nH_n.

Cis–Trans Isomerism

Stereoisomers have the same kind and number of atoms bonded in the same order but have different arrangements in space.

Stereoisomers cannot be changed from one to another by simple rotation. Bonds would have to be broken and re-formed.

Stereoisomers are distinct compounds with different properties, such as different melting points.

Cis isomer has matching alkyl groups located on the same side of the double bond.

Trans isomer  has the groups located on opposite sides of the double bond.

Reactions of Alkenes and Alkynes

One common type of reaction that alkenes and alkynes undergo is an addition reaction.

In an addition reaction, two molecules react to form one.

The multiple bonds in alkenes and alkynes enable the organic molecules to react with hydrogen.

The addition of hydrogen is a type of addition reaction known as hydrogenation, and results in the hydrocarbon becoming saturated.

Addition reactions may also occur with: 

• halogens,
• hydrogen halides such as hydrogen chloride,
• water.

In a halogenation reaction, a halogen such as bromine or chlorine reacts with an alkene or alkyne.

The halogenation of an alkene produces an alkyl halide.

The halogenation of an alkyne produces a halogenated alkene or, if excess halogen is present, an alkyl halide.

A hydrogen halide such as hydrogen chloride or hydrogen bromide may also react with an alkene or alkyne. This reaction is called a hydrohalogenation reaction. The resulting compound includes both the halogen and the hydrogen atoms and may be a halogenated alkene or an alkyl halide.

In a hydration reaction, water reacts with an unsaturated hydrocarbon. This reaction produces a type of organic compound that contains a hydroxyl group (- OH). This compound is called an alcohol.

Markovnikov’s rule - the rule for predicting the products of addition reactions:

when a hydrogen halide or water is added to an alkene, the hydrogen atom generally bonds to the carbon atom within the double bond that already has more hydrogen atoms bonded to it.

Markovnikov’s rule applies to hydrohalogenation of both straight-chain and cyclic hydrocarbons. It also applies to hydration reactions.

Summary - alkanes vs. alkenes vs. alkynes

Aromatic Hydrocarbons

An aromatic hydrocarbon is an unsaturated hydrocarbon that has a ring structure and a bonding arrangement that causes it to be chemically stable.

Benzene, C₆H₆, is a flat 6-carbon ring with a hydrogen atom bonded to each carbon atom.

The structural diagram often shows benzene as having three double bonds alternating with three single bonds.

Measurements of the bond length between carbon atoms, however, have shown that all six bond lengths are equal.

If benzene actually had three double bonds, then those bonds would be shorter than the other three.

To indicate that all six bonds are identical, the structure is shown as alternating between two arrangements of double bonds or as a hexagon with a circle inside.

The hexagon with the circle indicates that the electrons in the bonds are shared equally between all 6 carbon atoms.

Naming Aromatic Hydrocarbons

There are two conventions for naming compounds that include a benzene ring.

Which one we use depends on the structure of the other part of the compound.

We use the first convention when naming aromatic compounds that have noncarbon substituents or a small alkyl group.

In this case, the benzene ring is generally considered to be the parent molecule.

The attached functional groups are named as substituents to benzene.

The various isomers of disubstituted benzene rings are often named using an alternative system. The names ortho, meta and para (prefixes o, m or p) identify the relative positions of two groups on the ring.

Ortho means the two groups are on the 1,2 positions,

Meta means they are on the 1,3 positions and

Para means they are on the 1,4 positions.

If a single group is attached to a benzene ring, we put the name of the group before the root -benzene.

If two or more substituents are bonded to the benzene ring, we number the carbon atoms of the benzene ring starting with the first substituent (alphabetically) and continue numbering in the direction of the next closest substituent.

In the second naming convention, the benzene ring is considered to be a substituent on a hydrocarbon chain.

A benzene ring that has lost 1 hydrogen atom is called a phenyl group. 

Some derivatives of benzene have common names that do not follow the IUPAC system. 

Aniline is also known as aminobenzene and toluene is also known as methylbenzene.

Properties of Aromatic Hydrocarbons

Many aromatic hydrocarbons are liquids at room temperature, while others are crystalline solids.

Their symmetrical structure causes them to be non-polar unless they contain an electronegative substituent, so they are generally insoluble in water.

Reactions of Aromatic Hydrocarbons

Its unusual bonding makes the benzene ring behave quite differently from an unsaturated hydrocarbon.

Each bond in benzene is identical and they are much more stable than a carbon–carbon double bond.

Substitution reactions are characteristic of saturated hydrocarbons, and addition reactions are characteristic of unsaturated ones.

Benzene reacts more like a saturated hydrocarbon because of the specialized bonding in the benzene ring.