You’ve probably heard of the word “organic compound” (or even “inorganic compound”) thrown around when you are first exposed to chemistry. In this guide, we’ll define an organic compound and understand why it has contributed to a vast subfield within chemistry (aka organic chemistry). In fact, we'll dive into the basics of organic compounds, explore the structure and names of hydrocarbons, understand their different classes, and look at how they are used in everyday life.

A question you may have is “why should I care?” This subject is key to fields like biochemistry, medicine, environmental science, and engineering, which is especially relevant if you see yourself in any of these disciplines in future studies. Let's get started! 🧪

📚 Basics of Organic Chemistry

Let’s start with the broader field in question: organic chemistry. Organic chemistry is all about the study of carbon-containing compounds… but what’s so special about carbons, anyway?

Take a look below at $C_{10}$ polyyne, a molecule that demonstrate some of carbon’s unique properties: having the ability to form single and triple bonds and possessing a ring-like structure. Indeed, carbon's unique ability to form four strong covalent bonds makes it incredibly versatile—it can bond with other carbon atoms forming long chains or rings and also with elements like hydrogen, oxygen, nitrogen, sulfur, and halogens.

Polynne molecule.

Image courtesy of American Chemical Society

Moving forward, we define organic compounds as compounds having carbon atoms bonded to other carbon atoms or to other elements. It’s important to realize that there are a gajillion different examples of organic compounds out there; to make life easier for us, let’s divide organic compounds into a couple types you might want to familiarize yourself with. 😌

Key Types of Organic Compounds

As you read, don’t worry about the functional group (e.g. hydroxyl, carbonyl) as much as we’ll talk about this in the next study guide.

1. Hydrocarbons only have carbon and hydrogen atoms.

Image courtesy of Britannica

1. Alcohols have one or more hydroxyl (-OH) groups.

Image courtesy of Alamy

1. Ketones have a carbonyl group (C=O) within the chain.
2. Aldehydes have a carbonyl group at the end of the carbon chain.
3. Carboxylic acids have a carboxyl group (-COOH).

Image courtesy of Ontario Open Library

Again, no need to learn the reaction, but pay attention to where the C=O (or -COOH) lies between the three organic molecules.

1. Amines feature an amino group (-NH2).

Image courtesy of American Chemical Society

Image courtesy of American Chemical Society

Functional Groups

For now, what you need to know is that functional groups are specific groups of atoms within molecules that determine the characteristics and chemical reactivity of those molecules.

Image courtesy of GuyHowto

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🧪 Structure & Nomenclature of Hydrocarbons

Structural Formulas vs. Molecular Formulas

• The structural formula shows how atoms are connected in a molecule (e.g., ball-and-stick or line model of $C_2H_4$ shows us which Cs are attached to which Hs)
• The molecular formula tells us which atoms are present and in what quantity (e.g., $C_2H_4$ has 2 carbon atoms and 2 hydrogen atoms).

Image courtesy of Inspired Pencil

IUPAC Nomenclature

Nomenclature is a fancy word for a system to name things! In the case of organic compounds, the International Union of Pure and Applied Chemistry (IUPAC) provides systematic ways to name compounds:

To illustrate, we have an example:

Image courtesy of Kwantlen Polytechnic University

Following the 4-step process:

1. The longest continuous chain is the horizontal chain of six carbons (6 = hex- prefix). Since we only see single bonds, we’re dealing with an alkane (-ane suffix).
2. See blue numbers in image: since the right-to-left configuration is closer to the substituent than left-to-right, we start numbering from the right!
3. The substituent in the 3rd carbon has two C groups (CH3 and CH2), which corresponds to an ethyl group (2 = eth- prefix).
4. Altogether, we get 3-ethylhexane!

Isomerism in Hydrocarbons

Another aspect of organic chemistry that makes it beautiful in nature is the existence of isomers. Isomers have identical molecular formulas but different structures. When talking about isomers, there are two main types:

Structural isomers have different connectivity between atoms.

Image courtesy of ThoughtCo

Image courtesy of Shinken

Geometric isomers have the same connectivity but differ in spatial arrangement around a double bond (cis-trans isomerism).

Image courtesy of ChemTalk

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🔍 Alkanes, Alkenes, Alkynes & Aromatic Hydrocarbons

While it’s true that we can name hydrocarbons, how do we distinguish between them? We can do so by the way their carbon atoms bond! While you’re not expected to know why, bonding differences lead to differences in the overall geometry and how the orbitals hybridize. 🤝

In essence, alkanes have single C-C bonds throughout. If the molecule has at least one double C=C bond, it’s an alkene; if it has at least one triple bond (C≡C), then it’s an alkyne. Let’s dive deeper into the properties of each!

Image courtesy of Innunco

Alkanes

• Saturated hydrocarbons with single bonds only.
• Nonpolar molecules; relatively low reactivity.
• Reactions include combustion (exothermic reaction with O2) and substitution reactions where an H atom is replaced by another atom/group.

Alkenes

• Unsaturated hydrocarbons containing at least one C-C double bond.
• Exhibit cis-trans isomerism due to restricted rotation around double bonds.
• Undergo addition reactions such as hydrogenation (adding H2) or hydration (adding H2O).

Alkynes

• Unsaturated hydrocarbons that contain at least one triple bond between two carbons leading to linear structures.
• More reactive than alkenes owing to their triple bond; they undergo similar addition reactions but may require different catalysts.

Aromatic Hydrocarbons

In addition to categorizing based on the type of C-C bond, we also have aromatic compounds, ringed structures that are stabilized by the electron density shared between the carbons in the ring (you can think of them as a mini-community sharing communal electrons!)

Benzene is a classic aromatic compound; it is a six-membered ring stabilized by delocalized electrons over $\pi$ orbitals. We oftentimes depict benzene by alternating single/double bonds or a circle within a hexagon shape for benzene ring representation.

Image courtesy of Wikimedia Commons

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🌍 Real-world Applications & Implications

Why do we care so much about organic compounds (especially hydrocarbons)? At first, it may seem abstract to think that Cs and Hs play a significant role in the energy and healthcare industry, for example, but they actually serve as building blocks for the materials and processes within these industries!

• Fuels: Hydrocarbon fuels like gasoline power our vehicles while natural gas heats homes because these substances release significant amounts of energy when combusted. 🔥
• Environmental Impact: Burning fossil fuels emits CO₂ contributing to global warming plus other pollutants that can lead to smog formation affecting air quality negatively. 😷
• Petrochemical Industry: It's not all about fuel! From plastics to pharmaceuticals—hydrocarbon derivatives play critical roles in manufacturing various goods essential for modern life. 💊

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✏️ Practice Questions

Now that you’ve read a comprehensive review of organic compounds, let’s try out some practice questions to put what you’ve learned so far to the test!

(1) Draw structural formulas for all possible isomers of pentane.

Of course, you’re not expected to know the exact names of every pentane isomers. The important idea to recognize is that pentane has the prefix “pent-,” which suggests that there are five carbons (think pentagon = 5 sides!).

Since we’re dealing with an alkane, the chemical formula is $C_nH_{2n+2}$, so there are 2(5) + 2 = 12 Hs. The rest of the problem is a matter of being creative with drawing and rearranging the carbon atoms. Try it out yourself, then compare with the image below to see if you got one, two, or all three isomers!

Image courtesy of Quora

(2) Name this molecule according to IUPAC rules: CH₃CH₂CH(CH₃)CH₂CH₃

• Draw out the structure! Remember: Cs are connected to each other, and they are flanked by Hs on the outside. Brackets = branches

Image courtesy of Kathy_Wellman22 (Quizlet)

• Let’s start by identifying our main chain: the horizontal line of five carbons (pent-). We see that the molecule has C-C single bonds throughout, which indicates that it’s an alkane.
• Moving on to our substituent, we have one CH3 group, which suggests a methyl group (meth- prefix).
• In this case, it doesn’t matter which direction you start counting your main carbon chain as the substituent is in the middle (3rd carbon from either side). Our final molecule, therefore, is 3-methylpentane!

(3) Why does 1-butene have lower boiling points than butane even though both have four carbon atoms?

This is definitely more of a conceptual question!

When looking at alkanes vs. alkenes, we noted that alkanes are more saturated than alkenes in terms of having the maximum number of hydrogen atoms for each carbon atom. This means that butane, our alkane, will have more electrons than 1-butene, our alkene, and thus more forces stabilizing the structure. In other words, we’d have to put in more energy (heat) to break all the bonds in butane overall compared to 1-butene even though 1-butene has a double bond.

Because of these results, the melting and boiling point of alkenes are a few degrees lower than those of alkanes in general!

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⭐ Conclusion

Amazing job so far! Do you feel more confident doing organic chemistry? Remember that learning organic chemistry can be like learning a new language; indeed, practice will make you fluent. Good luck and have fun! 😁

Image courtesy of Quora