Hydrocarbons – SS2 Chemistry Lesson Note (Third Term)

Organic chemistry is the chemistry of carbon compounds, except carbon (II) oxide, carbon (IV) oxide and trioxocarbonate (IV). Carbon forms numerous compounds because of catenation, the ability of its atoms to bond with one another and form long chains. Organic compounds are grouped into aliphatic and aromatic compounds.

Behavioural objectives

By the end of the lesson, students should be able to:

• define organic chemistry and catenation;
• state the features that make carbon unique and the characteristic features of organic compounds;
• classify organic compounds into aliphatic and aromatic compounds;
• define homologous series, alkyl group and functional group;
• distinguish between saturated and unsaturated compounds;
• distinguish between empirical, molecular and structural formulae.

What is organic chemistry?

Organic chemistry is the chemistry of carbon compounds except compounds such as carbon (II) oxide, carbon (IV) oxide and trioxocarbonate (IV). Carbon has the unique ability to form numerous organic compounds because it can catenate. Catenation is the ability of atoms of an element to form bonds between its atoms and produce a long chain structure.

All organic compounds contain carbon as the main element together with one or more other elements such as hydrogen, oxygen, chlorine, nitrogen and sulphur.

What is unique about carbon?

1. The valency of carbon: the electronic configuration of carbon is as follows:
   
   C = 6: 1s² 2s¹ 2p_x¹ 2p_y¹ 2p_z¹
   
   Carbon forms covalent bonds and after bond formation, it has neither a vacant orbital nor a lone pair of electrons. This makes many carbon compounds chemically stable.
2. The bond between carbon and hydrogen is non-polar. Thus, hydrogen atoms attached to carbon do not weaken carbon-carbon bonds.
3. The types of orbital hybridisation available to carbon: carbon can form three types of hybridisation, sp³, sp² and sp. Hence, it can form single, double or triple covalent bonds between its atoms.
4. A large amount of energy is required to break carbon single bonds.

What are the characteristic features of organic compounds?

1. Organic compounds are covalent.
2. They are non-polar substances and are insoluble in polar solvents.
3. They have low melting and boiling points.
4. They are highly flammable.
5. Their reactions are relatively slow compared to inorganic chemical reactions.

How are organic compounds classified?

Organic compounds are classified into aliphatic and aromatic compounds.

Aliphatic compounds

These are compounds whose molecules are composed of chains of carbon atoms. They can be:

• straight chain compounds, e.g. pentane;
• branched-chain compounds, e.g. 2-methyl butane.

Straight and branched chain aliphatic compounds exist as open chains and are called acyclic compounds. Aliphatic compounds which exist as closed chains are called cyclic compounds, e.g. cyclopropane.

Aromatic compounds

Benzene, C₆H₆, is a typical aromatic compound. Other aromatic compounds are derivatives of benzene, e.g. C₆H₅OH.

What terms are used in organic chemistry?

Homologous series

A homologous series is a family of organic compounds which follows a regular structural pattern and in which each successive member differs in its molecular formula by a -CH₂– group.

The simplest series of compounds in organic chemistry is the alkane series. The general molecular formula of the alkane series is C_nH_2n+2. It is the parent series from which every other series is obtained. Other homologous series include the alkenes, alkynes, alkanols, alkanoic acids, etc.

Characteristics of a homologous series

• All members conform to a general molecular formula, e.g. for alkanes, C_nH_2n+2.
• Each successive member differs in the molecular formula by a -CH₂– group.
• All members undergo similar chemical reactions.
• The physical properties of members change gradually along the series.
• All members are prepared by the same method.

Alkyl groups

Alkyl groups are all groups derived from the alkanes by the loss of a hydrogen atom. Alkyl groups have a general formula of C_nH_2n+1. They are named after the parent alkane by replacing the ending -ane with -yl. The alkyl groups derived from the first two members of the alkane series are:

Functional groups

The atom or group that replaces a hydrogen atom in an alkane determines the chemical properties of the compound formed. This substituent is called the functional group.

A functional group is an atom, a radical or a bond common to a homologous series and which determines the chemical properties of the series.

Examples of functional groups include the hydroxyl group -OH, the amino group -NH₂, the carboxyl group -COOH, and double and triple bonds.

The alkyl group in a compound determines the physical properties of the compound, while the functional group determines the chemical properties of the compound.

What are saturated and unsaturated compounds?

Saturated compounds are compounds containing atoms joined together by single covalent bonds. Alkanes are saturated compounds, e.g. ethane, C₂H₆.

Unsaturated compounds are compounds containing atoms joined together by double or triple bonds. Alkenes and alkynes are unsaturated compounds, e.g. ethene, C₂H₄.

What are the formulae of organic compounds?

Organic compounds are characterised by the following formulae:

1. Empirical formula is the simplest formula which indicates the component elements and the ratio of the combination of atoms in a compound.
2. Molecular formula is a chemical formula of a compound which indicates the actual number of atoms of each element in a compound.
3. Structural formula is a formula which indicates how atoms are arranged within the molecule of a substance.

The structural formula can be:

• open structural formula;
• condensed structural formula.

Evaluation

1. Define catenation.
2. State four characteristic features of organic compounds.
3. Define a homologous series and state the general molecular formula of the alkanes.
4. Distinguish between saturated and unsaturated compounds, giving one example of each.
5. Distinguish between empirical formula and molecular formula.

Assignment

Pick the correct option ONLY (questions 1 to 5).

1. An exceptionally large number of carbon compounds is essentially due to the ability of
   • (a) carbon to catenate liberally
   • (b) various groups to catenate
   • (c) nitrogen, hydrogen, phosphorus and the halogens to catenate with themselves
   • (d) hydrocarbons to dominate other groups.
2. Which of the following is not a characteristic feature of organic compounds?
   • (a) Covalent in nature
   • (b) They dissolve in all polar solvents
   • (c) Low melting and boiling points
   • (d) Highly flammable
3. Functional groups in organic compounds
   • (a) determine the chemical properties of the homologous series
   • (b) do not modify the other when they are more than one molecule
   • (c) have a general formula which may include the functional group
   • (d) are responsible for the physical properties.
4. Homologues have the same
   • (a) empirical formula
   • (b) structural formula
   • (c) general formula
   • (d) molecular formula
5. The four main classes of hydrocarbons are
   • (a) methane, ethene, ethyne and benzene
   • (b) ethane, ethene, ethyne and toluene
   • (c) cycloalkane, cycloalkene, alkynes and arenes
   • (d) alkanes, alkenes, alkynes and aromatics
6. Write the open chain structure of the following:
   
   i. CH₃C(CH₃)₂CH₂CH(CH₃)CH₂CH₃
   
   ii. (CH₃)₂CHCH₂CH(CH₂Cl)CH₃
   
   iii. CH₃C(Br)₂CH₂CH₃