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Isomerism

Organic Chemistry: Key Concepts in Introductory Topics, Alkanes, Alkenes & Arenes

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Previously, i have shared a lot of key concepts regarding Organic Chemistry based on the GCE A-Level H2 Chemistry syllabus (code 9729) in Singapore, which is suitable for JC1 and JC2 A-Level students.

Today, i am going to do a consolidation of the key concepts for the following sub-topics in H2 Organic Chemistry:

  1. Introductory Topics (Types of Formulae, Isomerism)
  2. Alkanes
  3. Alkenes
  4. Arenes

For those in JC1 (J1), this few topics in Organic Chemistry will be tested in the upcoming Promotional Examination (Promos Exam).

You can click individual links below to learn more about each concept.

You can also watch some of my Chemistry YouTube Videos on Organic Chemistry which is consolidated at the end of this blog post.

Here we go ………..

H2 Organic Chemistry Notes & Tips

1. Introductory Topics

Organic Chemistry: Introductory Topics

Organic Chemistry: Types of Formulae

Organic Chemistry: Isomerism in Organic Compounds

Organic Chemistry: Constitutional (Structural) Isomerism

Organic Chemistry: Cis-Trans (Geometric) Isomerism

Organic Chemistry: Optical Isomerism

Organic Chemistry: Optical Activity & Plane-Polarised Light… Continue reading ...

Organic Chemistry: Constitutional (Structural) Isomerism

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First of all, re-cap that isomerism can be broadly classified into 2 types:

  • Constitutional isomerism, in which atoms are linked together in different ways
  • Stereoisomerism, in which atoms have the same connectivity but different arrangements in space.

Today, we will be talking about constitutional isomerism.

Constitutional isomers have the same molecular formula but different structural formulae. This simply means that the atoms are linked together in different ways. They are also known as structural isomers.

There are three types of constitutional isomerism:

  1. Positional Isomerism
  2. Chain Isomerism
  3. Functional Group Isomerism

1. Positional Isomerism

Positional isomers are molecules with the same molecular formula and same functional group but differ in the position of the functional group.

Positional isomers are also commonly known as compounds having the same substituents at different positions on the same carbon skeleton.

Positional isomers have similar chemical properties but different physical properties.

Examples of positional isomers:

Source: Socratic
Source: Socratic

2. Chain Isomerism

Chain isomers refers to organic compounds with the same molecular formula and same functional group, but different carbon skeleton.

Chain isomers have similar chemical properties but different physical properties.

Examples of chain isomers:

Source: Chemguide.co.uk

3. Functional Group Isomerism

… Continue reading ...

Organic Chemistry: Special (Unusual) Cases of Chirality and Achirality

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Chirality of Molecule

First and foremost, let’s recap on the meaning of chirality in an organic molecule.

We have learned that there must be at least one chiral centre in the molecule for it to be chiral and exhibits enantiomerism (optical isomerism). Chiral centre is a tetrahedral atom (usually carbon) that has four different substituent groups on it. This gives rise to enantiomers. Due to the chiral centre, there is an absence of a internal plane of symmetry in the molecule. A internal plane of symmetry is any plane cutting through the molecule such that one side is a perfect reflection of the other. For molecules with an internal plane of symmetry, we say that they are achiral and will not exhibit enantiomerism.

Three quick questions for you to think through before we move on:

  • Must a molecule with chiral centre(s) be always chiral?
  • Must a molecule without a plane of symmetry be always chiral?
  • Must a molecule have a chiral centre in order to be chiral?

There are 3 special (unusual) cases of chirality and achirality which JC students taking A-Level H2 Chemistry syllabus should take note, namely:

  1. Meso compounds
  2. Some molecules without a plane of symmetry can
… Continue reading ...

Organic Chemistry: Meso Compounds and Maximum Possible Numbers of Enantiomers

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Meso Compounds

Meso compounds are achiral molecules that possess multiple chiral centres. The reason for them being achiral is because they possess an internal plane of symmetry (also known as centre of symmetry). This internal plane of symmetry cause their mirror images to be superimposable.

Meso compound is therefore optically inactive. The optical activity of one half of the meso molecule cancels out the optical activity of the other half of it.

Maximum Possible Number of Enantiomers (Optical Isomers)

Maximum possible number of enantiomers = 2n whereby n is the number of chiral centres in the molecule

Note that the above formula gives the maximum possible number of enantiomers!

To see whether it also gives you the actual number of enantiomers, you need to draw out all the possible isomers and then check whether there are any identical isomers. The identical isomer is known as the meso compound which is optically inactive and will not be considered as an enantiomer (optical isomer).

Let’s take a look at an example to have a clearer picture on finding the actual number of enantiomers.

Example: 2,3-dichlorobutane.

Number of chiral centres in molecule, n = 2

Maximum possible … Continue reading ...

Organic Chemistry: Optical Activity & Plane-Polarised Light

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In previous post, we mentioned that enantiomers (also known as optical isomers) rotate plane-polarised light in opposite directions and are said to exhibit optical activity. So how do we know if an organic molecule rotate plane-polarised light? We use an instrument known as a polarimeter.

A polarimeter is a scientific instrument used to measure the angle of rotation caused by passing polarised light through an optically active compound.

Some chemical substances are optically active, and polarized (uni-directional) light will rotate either to the left (counter-clockwise) or right (clockwise) when passed through these compounds. The amount by which the light is rotated is known as the angle of rotation.

Today, we shall take a closer look at:

  • Meaning of Optical Activity
  • Plane-Polarised Light
  • How a Polarimeter Works

Meaning of Optical Activity

All molecules, whether chiral or achiral, are able to rotate plane-polarised light due to interaction with the electron cloud of the molecule. For a particular molecule, the extent and direction of rotation depends on the orientation of the molecule in relation to the plane-polarised light.

For achiral molecules (compounds not exhibiting enantiomerism), each of these rotations are cancelled out due to the symmetry of the molecules… Continue reading ...