Organic Chemistry [H2 Chemistry]
Sub-Topics
Section Narrative
What is Organic Chemistry?
Organic chemistry is the study of the structure, properties, composition, reactions and preparation of carbon-containing compounds, which include not only hydrocarbons (compounds containing carbon and hydrogen only) but also compounds with any number of other elements, such as nitrogen, oxygen, halogens, phosphorus, silicon and sulfur, in addition to hydrogen. It is one of the major branches of chemistry.
Carbon, with a ground state electronic configuration of 1s2 2s2 2p2, has four valence electrons (Core Idea 1: Matter) and is able to form single, double and triple bonds with a variety of other atoms. Carbon is also unique among the elements in its unsurpassable ability to self-link into chains or rings of different sizes, giving rise to the millions of organic compounds known.
History of Organic Chemistry
Like other branches of Science, the development of Organic Chemistry occurred over a long period of time, during which the knowledge acquired through observation is organised and rationalised using different theories and models (see diagram below).
The history of organic chemistry can be traced back to antiquity when medicine men extracted chemicals from plants and animals to treat their tribe members. Science, let alone chemistry, was still unheard of then and they simply kept records of the useful properties and uses of things like willow bark, which was used as a painkiller. (It is now known that willow bark contains salicin, which can be converted to salicyclic acid and subsequently, acetylsalicyclic acid, the ingredient in aspirin.)
Organic chemistry was first made a branch of modern chemistry by Jöns Jakob Berzelius in the early 1800s. He classified chemical compounds into two main groups: those originating from living or once-living matter, which he termed organic, and those originating from mineral or non-living matter, which he termed inorganic. Berzelius, similar to most chemists of that era, believed that organic compounds could only come from living organisms mediated by some vital force, known as vitalism.
In 1828, Friedrich Wöhler, ironically a student of Berzelius’s, discovered that the organic compound urea could be made by heating ammonium cyanate, an inorganic salt. This observation demonstrated for the very first time that an organic compound could be synthesised from an inorganic source, which led eventually to the rejection of vitalism as a scientific theory.
Wöhler’s observation represents a milestone in the history of science for two reasons. First, it challenged the idea of vitalism. Second, this also marked the discovery of isomerism – the phenomenon of two or more different chemical structures (ammonium cyanate and urea) based on the same chemical formula (N2H4CO).
Chemists began their quest for ways to rationalise isomerism, which in turn led to theories pertaining to the structure and bonding of chemical compounds. By the 1860s, chemists like Kekulé and Couper separately published papers on relating the chemical formula of a compound to the ways the atoms are linked, that a carbon atom forms four bonds and carbon atoms can bond to one another. By the 1900s chemists like Lewis and Pauling developed models to understand the nature of the chemical bond, particularly the covalent bond for organic chemistry. The number of known organic compounds also increased exponentially throughout this time.
By the 20th century, organic chemistry has found applications in materials chemistry, pharmacology, chemical engineering and petro-chemistry, amongst many others. Millions of new substances were discovered or synthesised during this period, and today over 98% of all known compounds are organic.
Chemistry of the different functional groups.
The study of organic chemistry at the A-Level, as shown in the Organic Chemistry Map below, includes the chemistry of the different functional groups and understanding of the general principles of organic reaction mechanisms.
Functional groups are specific groups of atoms attached to a carbon backbone and they dictate the chemical and physical properties of molecules. These are also the sites of reactivity in organic compounds. For each of the functional groups, besides looking at the characteristic physical properties of the homologous series, synthetic methods to install the functional group in molecules will be examined. Characteristic reactions of the different functional groups will also be explored.
The reaction of one functional group lends itself to the construction of another functional group, and vice versa. The recognition of such functional group interconversion is invaluable to the study of organic chemistry. At the A-Level, most reactions involve the simple conversion of one functional group to another, which can usually be performed in either direction readily. For instance, secondary alcohols can be oxidised to give ketones, which can in turn be reduced back to secondary alcohols.
It should be noted that many of these reactions do not involve the formation or cleavage of carbon-carbon covalent bonds. Reactions that result in changes to the carbon backbone are important in organic synthesis as they lend themselves to the construction of varied carbon skeletons. Examples of such reactions are the oxidative cleavage of the carbon-carbon double bond, iodoform reaction and addition of HCN to carbonyl compounds.
Good knowledge and understanding of the reactions of functional groups can be applied to the synthesis of desired molecular architectures and the structural elucidation of unknown compounds.
Strengthening understanding of organic chemistry through emphasising mechanistic principles
The characteristic reaction of each functional group can be understood in terms of the nature of the reacting species and the type of reaction involved, which are related to the organic structure and bonding (Core Idea 2: Structure and Properties), as can be seen in the Organic Chemistry Map. Mechanistically, most reactions in organic chemistry are polar in nature (with the exception of those involving free radicals), entailing the flow of pair(s) of electrons from a nucleophilic/Lewis basic (electron-rich) site to an electrophilic/Lewis acidic (electron-poor) site.
Specifically, the mechanisms of five of the general types of reactions will be discussed in detail, namely free-radical substitution (exemplified by alkanes), electrophilic addition (exemplified by alkenes), electrophilic aromatic substitution (exemplified by arenes), nucleophilic substitution (exemplified by halogenoalkanes) and nucleophilic addition (exemplified by carbonyl compounds), which can be understood on the basis of both kinetic and energetic considerations (Core Idea 3: Transformation).
Connections to themes of Environmental Sustainability and Materials
With the knowledge of Organic Chemistry, organic molecules that have specific structures and therefore, properties, can be designed and synthesised in the laboratory through careful control of the reagents and conditions used. An example of organic molecules whose properties have enabled their use in many applications as well as brought convenience and possibilities to our lives is plastics, a material that is part of a bigger class of organic compounds known as organic polymers.
Plastics are ubiquitous in our everyday life. They have the properties of being waterproof, lightweight, chemically inert, versatile, durable and are not easily biodegraded in the environment. In the food supply chain, the properties of plastics enable them to be used as food packaging which helps to extend shelf-life of food and which supports safe handling and distribution of food, thus contributing to minimising food wastage[1]. In the healthcare sector, plastics have contributed to improved hygiene such as in the form of sterile single-use materials like syringes and disposable face masks, amongst many other medical devices and uses[2]. Plastics are also components in electrical products such as mobile phones and laptops. Its lightweight property has also led to use of plastics in transportation, contributing to reduced carbon emissions[3].
However, plastic waste and their disposal, especially single-use plastics, has also led to growing environmental concerns such as pollution of oceans, posing a threat to marine lives and human health[4]. In addition, a major source of chemical feedstock to manufacture common plastics comes from crude oil, a finite resource[5].
Just as Chemistry has played a role in the development of plastics which improve our lives, Chemistry could play a part in mitigating the environmental consequences that arise from the use of plastics. These include designing biodegradable plastics, bio-based plastics or plastics that can be broken down at the end of its useful lifespan to recover its monomers to reuse. It can also include improving on the manufacturing processes to reduce materials and energy use, recycling plastics efficiently and ensuring safe degradation of plastic waste, such as using non-toxic reagents, amongst many other possibilities[6].
Organic Chemistry can collectively play a role in the design and synthesis of useful materials such as the plastics that we are familiar with, as well as alternative plastics that are easier to degrade, recycle or produce less toxic waste, thereby contributing to sustainability. For instance, the study of organic reaction mechanisms provides a foundation for understanding how polymers are formed and how they degrade in the natural environment. This can lead to the design and synthesis of plastics using suitable reagents and conditions to facilitate an environmentally friendly degradation method for plastics. This would also require a good understanding of organic reactions and the factors that influence reactivities of organic molecules, reactions, and mechanisms. How the properties of plastics can be controlled at the molecular level[7] also enables an appreciation of the role of structure in influencing the properties of matter (Core Idea 2: Structure and Properties).
[1] Dora, M., Iacovidou, E. (2019, June 27). Commentary: Why some plastic packaging is necessary. ChannelNewsAsia.
[2] Gibbens, S. (2019, October 4). Can medical care exist without plastic?. National Geographic. Retrieved 18 Oct 2020 from https://www.nationalgeographic.com/science/2019/10/can-medical-care-exist-without-plastic/
[3] Royal Society of Chemistry Materials Chemistry Division. (2019, April 24). Sustainable Plastics – the role of Chemistry. Retrieved October 18, 2020, from https://www.rsc.org/globalassets/04-campaigning-outreach/policy/environment-health-safety-policy/plastics-sustainability.pdf
[4] Parker, L. (2019, June 7). The world’s plastic pollution crisis explained. National Geographic. Retrieved 18 Oct 2020 from https://www.nationalgeographic.com/environment/habitats/plastic-pollution/
[5] Millet, H., Vangheluwe, P., Block, C., Sevenster, A., Garcia, L., Antonopoulos, R. (2018). The Nature of Plastics and Their Societal Usage. In Harrison, R.M., Hester, R.E (Eds.) Plastics and the Environment (pp. 1-20). Royal Society of Chemistry
[6] The Royal Society of Chemistry. (2020). Science to enable sustainable plastics. A white paper from the 8th Chemical Sciences and Society Summit. rsc.li/progressive-plastics-report
[7] Morrison, R. T., Boyd, R. N. (1987). Macromolecules. Polymers and Polymerisation. In Organic Chemistry (pp. 1233-1260). Allyn and Bacon, Inc.