Written By: Elena Duran Chao
What is organic chemistry?
Organic chemistry, as its name suggests, deals with the chemistry of all organic compounds. Now, this begs the question: What are organic compounds? All carbon containing compounds are organic, which prompts the question of how they form bonds. Well, a single carbon atom is able to form a maximum of 4 bonds with other elements, typically with hydrogen, oxygen and nitrogen. However, compounds comprised solely of carbon and hydrogen are given a different name: hydrocarbons. As you can see, the specific terms in this topic have very creative names!
What are the main types of organic compounds?
Alkanes
The primary group that you need to know about for IGCSE Chemistry is the alkanes. These compounds are constituted only of hydrocarbons and adhere to a general formula of CnH2n+2, such as methane (CH4), ethane (C2H6), propane (C3H8) and butane (C4H10). You may notice the alternation in prefixes for each organic compound. This variation in prefix indicates the number of carbon atoms there are in each organic compound: meth- means 1 carbon; eth- means 2 carbons; prop- means 3 carbons; but- means 4 carbons. Of course, the list can go on and on, but for the IGCSE Chemistry test, the syllabus confines your knowledge to the first four. As the number of carbon compounds in this group increases, the boiling point of the molecule also increases. Since they only have single-bonded carbon atoms, they are considered as saturated compounds.
Alkenes
The second significant group that you need to familiarize yourself with in IGCSE Chemistry is the alkenes, another subset of hydrocarbons. They are composed of all the organic compounds with the general formula of CnH2n, starting with ethene (C2H4), propene (C3H6), and butene (C4H8). You may have noticed the absence of the first one on the list: this is because methene doesn’t exist. You see, the alkene group is considered to be unsaturated hydrocarbons, meaning that they are comprised of one double bonded carbon molecule. As the prefix of meth- suggests, it only contains 1 carbon atom, hence not being able to form a double bond with another atom.
But there’s more to it that meets the eye. Initially, everything seems quite fundamental. However, specific hydrocarbons come in different variations. Specifically in the alkenes group, butene has 2 versions: but-1-ene and but-2-ene. When inspected carefully, there are slight differences in the displayed formula (the diagram of all the particles connected with bonds). In but-1-ene, the double bonded carbon molecule will be the first and second one from the left. Nevertheless, in but-2-ene, the double bonded carbon molecule will appear in the second and third carbon atoms from the left. Looking at the structural diagrams, you may wonder why there’s no but-3-ene, with the double bonded carbon molecule between the third and fourth carbon atoms. This is because but-3-ene is the same as but-1-ene, but flipped from left to right. Not that hard, right?
Alcohols
The third major group that you need to know in order to pass the IGCSE Chemistry exam is the alcohols, which are not hydrocarbons as they have an oxygen in their configuration. They have a general formula of CnH2n+1OH, including methanol (CH3OH), ethanol (C2H5OH), propanol (C3H7OH) and butanol (C4H9OH). Noticeably, there’s an OH in every organic compound that’s identified as an alcohol, and it is precisely what makes the compound an alcohol. That’s why we call the OH part of the configuration “the functional group of alcohol”. A “functional group” is defined as the part of the structure that shows which group the organic compound belongs to (in this case, OH belongs to alcohols).
Moreover, as we talked about before, variations exist within organic compounds. In the group of alcohols, there are 2 versions of propanol (propan-1-ol and propan-2-ol), and 2 versions of butanol (butan-1-ol and butan-2-ol). However, the “1” and “2” in each of the versions no longer symbolize where the double bonded carbon molecule is placed. Seeing as alcohol is a saturated compound, they do not have double bonds. However, they do have an OH connected to a carbon atom. Therefore, the “1” and “2” in each version of propanol and butanol symbolizes which carbon has been bonded to the OH.
Carboxylic acids
This is the fourth main group of organic compounds that you need to know about. These carboxylic acids are generally considered to be weak acids, meaning that they only dissociate or ionize partially in solution. The functional group of a carboxylic acid is an oxygen atom double bonded to a carbon atom, which in turn is single bonded to an OH molecule, forming an “L” shape when drawn out. Since they are still considered to be acids, albeit a weak one, they are also defined as proton donors, per the Brønsted–Lowry definition. Hence, they create a reversible reaction where the carboxylic acid can be separated into an ion of the acid, plus a hydrogen ion.
Esters
This is the fifth and final group of organic compounds that you need to know before entering the exam. They are esters, and typically have the characteristics of being fragrant. They are made through the reaction between carboxylic acids and alcohols in order to produce an ester and water as the waste product. Let’s take an example:
Methanoic acid (carboxylic acid) + Ethanol (alcohol) → Ethylmethanoate (ester) + Water
As you can probably see, esters have a really long name, but as we recognized earlier in the article, in this topic, names of different compounds are basically a simplified definition of the term. The first half of the name in the ester, “ethyl”, indicates the alcohol used in the reaction. The second half of the name, “methanoate”, indicates the carboxylic acid used in the reaction. It’s that simple!
You may be wondering why water was produced as a waste product. This is because during the reaction between the carboxylic acid and the alcohol, an H (hydrogen) was taken from the carboxylic acid, and an OH (hydroxide) was taken from the alcohol to form H2O (a.k.a. water). This type of reaction is usually called a condensation reaction, as it forms water.
Homologous series
Each of the previously mentioned main groups of organic compounds constitutes its own homologous series, which is defined by a family of compounds with the same functional group. Here is a list of all the functional groups in each of the main types of organic compounds discussed above:
Alkanes: single bonds all throughout (only containing C and H)
Alkenes: 1 double bonded carbon molecule
Alcohols: O-H molecule
Carboxylic acids: O double bonded with C, which in turn is single bonded to an O-H molecule
Esters: O double bonded with C, which in turn is single bonded to an O-C molecule
What are the different formulas used to draw organic compounds?
There exist four key types of formulas essential for sketching an organic compound: the general formula, the molecular formula, the displayed formula, and the structural formula. Now, what are the differences between each of them? And why are there so many different types of drawing methods for the same compound?
General formula
The general formula is not a drawing but a tool to use when calculating the number of each element in a compound. The simplified definition is: it’s the formula based on the amount of carbon there is in the compound. Here are the following general formulas for alkanes, alkenes and alcohols:
Alkanes: CnH2n+2
Alkenes: CnH2n
Alcohols: CnH2n+1OH
Molecular formula
The molecular formula, in its simplest explanation is the formula with the number of each atom in the compound written down. As you can see in the general formula, there is an “n” in the subscript of some atoms, and this “n” is used as a substitute for the real number. Here are some examples:
Methane: CH4
Ethene: C2H4
Propanol: C3H7OH
Displayed formula
The displayed formula is a drawn out diagram with all the bonds between the atoms clearly shown, as well as the atoms themselves. Here’s an example of methane, drawn out in a displayed formula:
Structural formula
The structural formula, not drawn out, is slightly different to the molecular formula. This may be the hardest formula to know between all 4. This is because it incorporates not only the knowledge of the molecular formula, but also of the displayed formula. Structural formulas are made so that the organic compound may be separated into sections, but still written out in a similar fashion to the molecular formula. This may be confusing at first, but let’s use an example to explain it better.
In this diagram, this is propene.
The displayed formula is shown with the black letters and bonds. However, the structural formula is shown with the red boxes around each section of the propene. By separating them into sections, it may be easier to write down the formula and to understand which organic compound it represents and the order in which each atom goes. Hence, because of this displayed formula of propene, the structural formula for it is: CH2CHCH3, as per the red boxes.
Structural isomers
Structural isomers are defined as compounds with the same molecular formula, but the atoms are connected differently. For example, C4H10 is butane, as we have previously established in the article. However, it may also be 2-methylpropane. This is because when the atoms are connected differently (like when the carbon drops down to be under another carbon rather than next to it), they also become different organic compounds and have different chemical properties. Before, we talked about different versions of the same compound, such as propan-1-ol and propan-2-ol. Well, the technical term for these versions are called structural isomers of propanol! Therefore, it is useful to know that although the molecular formula does provide us with information about the number of atoms in the compound, it still gives insufficient information about the structure.
Conclusion
There are 5 main types of organic compounds that you need to understand as the basis of understanding organic chemistry at an IGCSE level: Alkanes, alkenes, alcohols, carboxylic acids, and esters. While learning about organic chemistry, it is imperative to know how to draw the different formulas that you may be asked to do in the exam. There are 4 types of formulas to know: the general formula, the molecular formula, the displayed formula, and the structural formula, all of which were discussed in the article. Now, this is just part 1 of learning about the organic chemistry topic on the IGCSE Chemistry test. Be sure to look out for part 2, where we will go into more detail about the various reactions involving the organic compounds learned in this article!
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