Classwork Series and Exercises {Chemistry – SS2}: Introduction to Organic Chemistry

Chemistry, SS 2 Week: 1

Topic: Introduction to Organic Chemistry

Organic chemistry originally meant the chemistry of compounds obtainable from plants and animals – living organisms. Organic chemistry is that branch of chemistry that deals with the structure, properties, and reactions of compounds that contain carbon. In other word, organic chemistry is defined as the chemistry of carbon compounds.

Organic molecules = Molecules containing carbon.

Vitalism = Belief in a life force outside the jurisdiction of chemical/physical laws.

Early 19th century organic chemistry was built on a foundation of vitalism because organic chemists could not artificially synthesize organic compounds. It was believed that only living organisms could produce organic compounds.

Mechanism = Belief that all natural phenomena are governed by physical and chemical laws.

Pioneers of organic chemistry began to synthesize organic compounds from inorganic molecules. This helped shift mainstream biological thought from vitalism to mechanism.

For example, Friedrich Wohler synthesized urea in 1828; Hermann Kolbe synthesized acetic acid.

Organic compounds are made up of:

• The main element, carbon
• Hydrogen and oxygen which are usually present
• Elements such as nitrogen, the halogens, phosphorus, sulphur and some metals which are sometimes present.

Unique Nature of Carbon

The presence of numerous organic compounds is due to the following properties of carbon:

1. Catenation – the ability to form chains of atoms i.e. to combine with one another to form straight chains, branched chains or ring compounds.  This property is called catenation, and is fairly unique. 
2. The ease with which carbon combines with hydrogen, oxygen, nitrogen and the halogens.
3.  The ability to form multiple bonds i.e. single, double or triple covalent bonds.
4. The bond energy for carbon to carbon single bond is high.

Characteristics of Organic Compounds

1. They are covalent in nature.
2. They have low melting and boiling points because of the relatively weak intermolecular bonds within its compounds
3. Most organic compounds are non-polar and they cannot form bonds with water, unless the compounds consist of very electronegative elements like chlorine or groups like the hydroxyl group.
4. Many organic compounds are thermally unstable, decomposing into simpler molecules when heated to temperature above 500⁰C.
5. Most organic compounds are flammable and burn exothermically in a plenty supply of air to yield carbon (IV) oxide and water.
6. Reaction involving organic compounds tends to be much slower than the ionic reaction

Terms in Organic Chemistry

Hybridization

Hybridization is the combination or mixing of a given number of orbitals in an atom to carbon may form single, double and triple bonds. The hybridization of carbon involved in each of these bonds will be investigated.

Bonding in any element will take place with only the valence shell electrons. The valence shell electrons are found in the incomplete, outermost shell. By looking at the electron configuration, one is able to identify these valence electrons. The electron configuration of ground state (lowest energy state) carbon:

From the ground state electron configuration, one can see that carbon has four valence electrons, two in the 2s subshell and two in the 2p subshell. The 1s electrons are considered to be core electrons and are not available for bonding. There are two unpaired electrons in the 2p subshell, so if carbon were to hybridize from this ground state, it would be able to form at most two bonds. Recall that energy is released when bonds form, so it would be to carbon’s benefit to try to maximize the number of bonds it can form. For this reason, carbon will form an excited state by promoting one of its 2s electrons into its empty 2p orbital and hybridize from the excited state. By forming this excited state, carbon will be able to form four bonds.

Since both the 2s and the 2p subshells are half-filled, the excited state is relatively stable.

Example, let’s choose methane, CH₄. Draw the Lewis structure:

The Lewis structure shows four groups around the carbon atom. This means four hybrid orbitals have formed. In order to form four hybrid orbitals, four atomic orbitals have been mixed. The s orbital and all three p orbitals have been mixed, thus the hybridization is sp³

The four sp³ hybrid orbitals will arrange themselves in three dimensional spaces to get as far apart as possible (to minimize repulsion). The geometry that achieves this is tetrahedral geometry, where any bond angle is 109.5^o. A hydrogen 1s orbital will come in and overlap with the hybrid orbital to form a sigma bond (head-on overlap).

 Ethene, C₂H₄ . Draw the Lewis structure:

The Lewis structure shows three groups around each carbon atom. This means three hybrid orbitals have formed for each carbon. In order to form three hybrid orbitals, three atomic orbitals have been mixed. The s orbital and two of the p orbitals for each carbon have been mixed, thus the hybridization for each carbon is sp².

Let’s show this using the atomic orbitals of excited state carbon found in the valence shell:

The three sp² hybrid orbitals will arrange themselves in three dimensional spaces to get as far apart as possible. The geometry that achieves this is trigonal planar geometry, where the bond angle between the hybrid orbitals is 120^o. The unmixed pure p orbital will be perpendicular to this plane. Each carbon atom is sp², and trigonal planar. The head-on overlap of sp² orbitals forms a bond and the side by side overlap of pure p orbitals forms a pi bond between the carbon atoms. This accounts for the carbon-carbon double bond.

Acetylene (C₂H₂), draw the Lewis structure:

The Lewis structure shows two groups around each carbon atom. This means two hybrid orbitals have formed. In order to form two hybrid orbitals, two atomic orbitals have been mixed.

The atomic orbitals of excited state carbon found in the valence shell:

The two sp hybrid orbitals arrange themselves in three dimensional spaces to get as far apart as possible. The geometry which achieves is linear geometry with a bond angle of 180^o. The two pure p orbitals which were not mixed are perpendicular to each other. The triple bond consists of one sigma bond and two pi bonds. The geometry around each carbon is linear with a bond angle of 180^o.

Homologous Series

A Homologous Series is a family of organic chemical compounds which follows a regular structural pattern, and whose structures differ only by the number of CH₂ units in the main carbon chain.

The simplest example of a homologous series in organic chemistry is that of alkanes. Alkanes consist of carbon and hydrogen atoms only, in proportions according to the general formula: C_nH_2n+2

Where the letter n represents the number of carbon atoms in each molecule of the compound. Hence the first 10 molecules in the homologous series of linear alkanes may be listed as follows (below, right):

Example of the Homologous Series of Alkanes, Structure: C_nH_2n+2

Name of AlkaneNumber Carbon atoms   Molecular Formula  

Methane1   C H₄   

Ethane2   C₂H₆

Propane   3    C₃H₈

Butane     4    C₄H₁₀

 The basic molecular structure of all members of the series takes the same form. The difference between members of the homologous series depends on the value of n, which represents the number of carbon atoms in the chain.

The homologous series of alkanes is the simplest and probably most often cited example but there are many other homologous series of organic compounds.

Name of Series	General chemical formula*
Alkanes	CnH2n+2
Alkenes	CnH2n
Alkynes	CnH2n-2
Haloalkanes	CnH2n+1X
Alkanol (alcohols)	CnH2n+1OH or CnH2n+2O
Alkanal (Aldehydes)	CnH2nO
Alkanone (Ketone)  Alkanoic (CarboxylicAcids)	CnH2nO CnH2nO2
Alkanoate (Ester) Amines	CnH2nO2 CnH2n+1NH2 or CnH2n+3N
Amides	CnH2n-1ONH2 or CnH2n+1ON
Nitriles	CnH2n-3N

Properties of Homologous Series

1. All members of the series have a general formula
2. The successive members differ in formula by a constant amount –CH2- or by relative molecular mass of 14
3. They have their melting point, boiling point and density increasing steadily with increasing relative molecular mass
4. They have the same general methods of preparation.

Alkyl Group

Alkyl group includes all groups derived from the alkanes by the loss of a hydrogen atom. For example, a methyl group (CH₃) is a fragment of a methane molecule (CH₄). The –yl- ending means “a fragment of an alkane formed by removing a hydrogen”.

Alkane	Formula	Alkyl group	Formula
methane	CH4	methyl group	-CH3
Ethane	CH3CH3	ethyl group	-CH2CH3
propane	CH3CH2CH3	propyl group	-CH2CH2CH3
Butane	CH3CH2CH2CH3	butyl group	-CH2CH2CH2CH3

 Functional Groups

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

The reactions and reactivity of organic compounds are often determined by the functional groups attached to the carbon chain of which they are a part. It is therefore necessary to be able to recognize and remember information about the most common and important functional groups in order to understand and succeed at organic chemistry.

Saturated and Unsaturated Hydrocarbons

Saturated Carbon Hydrocarbon

Compounds of carbon and hydrogen whose adjacent carbon atoms contain only one (carbon-carbon) bond are known as saturated hydrocarbons. Their carbon-hydrogen bonds are also single covalent bonds. They are called saturated compounds because all the four bonds of carbon are fully utilized and no more hydrogen or other atoms can attach to it. Thus, they can undergo only substitution reactions. They are also representative of open-chain aliphatic hydrocarbons. These saturated hydrocarbons are called as alkanes.

Unsaturated Carbon Compounds
Compounds of carbon and hydrogen that contain one double covalent bond between carbon atoms (carbon=carbon) or a triple covalent bond between carbon atoms () are called unsaturated hydrocarbons. In these molecules, since all the bonds of carbon are not fully utilized by hydrogen atoms, more of these can be attached to them. Thus, they undergo addition reactions (add on hydrogen) as they have two or more hydrogen atoms less than the saturated hydrocarbons (alkanes).

Unsaturated hydrocarbons can be divided into ‘alkenes’ and ‘alkynes’ depending on the presence of double or triple bonds respectively.
Differences between properties of saturated and unsaturated compounds:

Saturated Organic Compounds	Unsaturated Organic Compounds
These organic compounds contain single carbon-carbon covalent bond.	These organic compounds contain at least one double or triple covalent bond.
Due to the presence of all single covalent bonds, these compounds are less reactive.	Due to the presence of double and triple bonds, these compounds are more reactive.
Saturated compounds undergo substitution reactions. Example:	Unsaturated compounds undergo addition reactions. Example:
The number of hydrogen atoms is more when compared to its corresponding unsaturated hydrocarbon.	The number of hydrogen atoms is less when compared to its corresponding unsaturated hydrocarbon.

Formulae of Organic Compound

1. The molecular formula is the actual ratio of atoms to one another in a molecule.
2. A structural formula represents a structure and emphasizes the bond connection between atoms. 
3. A condensed formula is a simplification of the structural formula.
4. A line formula is a simplified representation of a structural formula in which many of the C-H bonds are not shown. In a line formula, the carbons are understood to exist a vertices of each of the angles and the number of  hydrogens necessary are also understood, though not written.
   

Alkane

Molecular Formula

Structural Formula

Condensed Formula

Line

 

Alkane

Molecular Formula

Structural Formula

Condensed Formula

Line