Organic Chemistry I – Structure and Nomenclature A. Review of Hydrocarbon 1. Alkanes

Alkanes is basically the single-bonded between carbon atoms, so alkanes is called saturated hydrocarbon. It is called saturated hydrocarbon because there are no more atoms could be added to the bonding between carbon-carbon atoms. The normal formula of alkanes is CnH(2n+2) and if it has branches, the properties would be different even they share the same molecular formula.

Could you give the reasons why this property could happen? (Hint: the definition of boiling point) 2. Alkenes and alkynes

If alkanes is the saturated hydrocarbon, so alkenes and alkynes is the unsaturated hydrocarbons. The main characteristic that distinguishes between alkenes and alkynes is the bonding between carbon-carbon atoms. In alkenes, it has double bond in carbon-carbon atoms and the normal formula would be CnH2n. In the other side, alkynes has triple bond in carbon-carbon atoms and the normal CnH(2n-2). Therefore, the properties of these three hydrocarbons would be different since the structures and the formulas are different. B. The structure of hydrocarbons 1. Alkanes In alkanes because they are single bonded molecule, so the hybridisation of the carbon atoms is sp3 which forms the tetrahedral shape. For example, in ethane there are 2 carbon atoms are bonded together by single bond which is formed by an orbital of sp3 in each carbon atom. The overlap between those carbon atoms would form a single bond which is called sigma (σ) bond. The other orbital of sp3 would bind the hydrogen atoms as shown in figure below. Therefore, in longer chain of alkanes it would form an un-planar structure.

2. Alkenes

If in alkanes the atomic orbital of s and p are hybridised to form sp3 orbital, so in alkenes there are only one s orbital and two p sub-orbitals are hybridised, and one p sub orbital are left un-hybridised. Therefore the mixing between those orbitals is called sp2 orbital which gives trigonal planar structure. Firstly, a sigma bond is formed in the same way as alkanes. However, in alkenes there is a pair p orbital un-hybridised and it would overlap to form a pi (π) bond. Since pi bond (overlap to form the electron cloud) is not overlap in the same way as sigma bond, so energetically pi bond is weaker than sigma bond. However, it gives an extra bond to alkenes to give higher bonding energy between carbon atoms compare to alkanes carbon atom. The effect of the existence of pi bond is the restricted rotation which alkanes do not have the restriction. The restricted rotation causes alkenes have geometrical isomerism which is called cis-trans isomerism. 3. Alkynes

The triple bond in alkynes is formed by an s orbital and a p orbital hybridised and two p orbitals are un-hybridised. The un-hybridised p orbitals would form 2 pi bonds and a sigma bond in the same way like in alkenes. From this hybridisation the structure of alkynes would be linear with bond angle is 180o. 4. The carbon-carbon bond length

Due to hybridisation, the bond between hydrocarbons is different. The pi bonding would create a shorter length of bonding due to its overlapping that requires shorter distance than sigma bond. C. How to draw the organic molecules? Basically the organic molecule would represent by 4 ways to draw it. 1. Displayed formula

All the bonding with all atoms in the molecules is drawn. 2. Skeletal formula

In this diagram, is represented by a line for each bond, so for alkenes there is a pair of line to represent the double bond and as well three line for alkynes.. The bonding between carbon and hydrogen are not drawn, but carbon with the other atom is drawn. 3. Structural formula

No lines are drawn. 4. The 3-D formula

The line represents the bonding is in the plane, the bold line are out of the line and the dashed line are in the plane.