2.2: Functional Groups

Functional groups are a group of atoms with characteristic properties, which when linked to the carbon skeleton of a molecule, alter the properties of that molecule. For example, the presence of certain functional groups on a molecule will make them hydrophilic, whereas others will make them hydrophobic. These functional groups are an indispensable part of organic chemistry and important components of biological molecules, such as carbohydrates, proteins, lipids, and nucleic acids. Each functional group is a unique arrangement of atoms and is assigned a name for the combination of atoms that make up the group.

Hydrocarbons
Strong, stable bonds between carbon atoms produce complex molecules containing chains, branches, and rings. The chemistry of these compounds is called organic chemistry. Hydrocarbons are organic compounds composed of only carbon and hydrogen. The alkanes are saturated hydrocarbons—that is, hydrocarbons that contain only single bonds. Alkenes contain one or more carbon-carbon double bonds. Alkynes contain one or more carbon-carbon triple bonds. Aromatic hydrocarbons contain ring structures. They are also known as aryl groups.

Alcohols and Ethers
Many organic compounds that are not hydrocarbons can be thought of as derivatives of hydrocarbons. A hydrocarbon derivative can be formed by replacing one or more hydrogen atoms of a hydrocarbon with a functional group, which contains at least one atom of an element other than carbon or hydrogen. The properties of hydrocarbon derivatives are determined largely by the functional group. The –OH group, known as a hydroxyl group, is the functional group of an alcohol. The –R–O–R– group is the functional group of an ether.

Aldehydes, Ketones, Carboxylic Acids, and Esters
Functional groups related to the carbonyl group include the –CHO group of an aldehyde, the –CO– group of a ketone, the –COOH group of a carboxylic acid, and the –COOR group of an ester. The carbonyl group, a carbon-oxygen double bond, is the key structure in these classes of organic molecules: Aldehydes contain at least one hydrogen atom attached to the carbonyl carbon atom, ketones contain two carbon groups attached to the carbonyl carbon atom, carboxylic acids contain a hydroxyl group attached to the carbonyl carbon atom, and esters contain an oxygen atom attached to another carbon group connected to the carbonyl carbon atom. All of these compounds contain oxidized carbon atoms relative to the carbon atom of an alcohol group.

Amines
The addition of nitrogen into an organic framework leads to two families of molecules. Compounds containing a nitrogen atom bonded in a hydrocarbon framework are classified as amines.

This text is adapted from Openstax, Chemistry 2e, Section 20.1: Hydrocarbons, Openstax, Chemistry 2e, Section 20.2: Alcohols and Ethers, Openstax, Chemistry 2e, Section 20.3: Aldehydes, Ketones, Carboxylic Acids, and Esters, and Openstax, Chemistry 2e, Section 20.4: Amines and Amides.

Functional groups are covalently bonded sets of atoms that affect the properties of a molecule.

Molecules containing the same functional groups often exhibit similar properties and undergo similar reactions.

Naming functional groups allows for the quick identification of the atoms in a molecule, which is useful in organic chemistry and biochemistry.  

Hydrocarbons are the basis of organic molecules and contain varying numbers of carbon and hydrogen atoms. These include alkanes, alkenes, and alkynes. Carbon atoms in alkanes are linked together by single bonds, whereas alkenes and alkynes contain double or triple bonds, respectively.

Hydrocarbons can form closed rings. If the ring contains only single bonds, it is called a cycloalkane. If the carbons are linked together by alternating single and double bonds, it is called an aromatic ring or an aryl group.

One of the simplest non-carbon containing groups is the hydroxyl group which contains an oxygen atom and a hydrogen atom. It is an important constituent of several compounds, such as methanol, water, and carbohydrates.

Another category of functional groups is the carbonyl groups, which contain a carbon and an oxygen linked together by a double bond. Carbonyl containing groups include functional groups such as aldehydes, ketones, esters, and carboxyl groups.

Aldehydes have hydrogen and a variable or R group attached to a carbonyl, while ketones have two variable groups. R groups often represent hydrocarbons but can be other groups or atoms. 

Aldehydes and ketones are key functional groups in carbohydrates

Esters consist of a carbonyl group flanked by a R group and an oxygen, while carboxylic acids, or carboxyl groups, are flanked by an R group and an alcohol. Lipids contain esters, and amino acids contain carboxylic acids.

Amino acids also have another common functional group called an amine or amino group. An amine has a nitrogen with a lone pair linked to a combination of hydrogens and R groups. Amino acids are the building blocks of proteins. 

A phosphate group is a phosphorus bonded to four oxygens and is commonly found in DNA, the genetic material of all living organisms. It can also be attached to proteins to alter their function.

Functional group – Group of atoms in a molecule that cause characteristic chemical reactions. Hydrocarbon – Organic compound with only carbon and hydrogen atoms in its structure. Hydroxyl group – Functional group with an oxygen bonded to hydrogen (–OH); found in alcohols. Carbonyl group – Functional group with a carbon double bonded to oxygen (C=O); in aldehydes/ketones. Amino group – Functional group with nitrogen bonded to one or more hydrogens (–NH₂); defines amines.

Define functional groups – Explain how specific atom clusters affect structure, reactivity, and polarity in organic compounds. (e.g., hydroxyl) Differentiate hydrocarbons from their derivatives – Compare pure carbon-hydrogen chains with modified groups that change chemical behavior. (e.g., alkene) Identify common functional groups – Recognize recurring structures in molecules and link them to chemical properties and interactions. (e.g., carbonyl) Explain how functional groups impact solubility, acidity, and reactivity – Understand how these groups alter molecule roles in synthesis and biological systems. Apply knowledge of functional groups to macromolecules – Link structure and behavior in proteins, lipids, carbohydrates, and nucleic acids.

Questions that this Functional groups this video will help you answer: What are functional groups in organic chemistry? How do functional groups affect the properties of organic compounds? What are the different types of functional groups found in organic molecules?

Students – Understand how functional groups influence structure, reactivity, and function in organic molecules. Educators – Provides a structured framework to teach functional group types, behavior, and relevance in class. Researchers – Gain a precise refresher on functional groups to support synthesis and molecular analysis in labs. Science Enthusiasts – Offer insights into the diverse roles functional groups play in organic chemistry and biology.