2.3 – Functional Groups

What are Functional Groups?

A functional group is a group of atoms bonded together in a particular way that impact the molecule’s chemical behaviour. Molecules with the same functional group can typically undergo the same chemical reactions. The greater the number of functional groups, the greater the diversity of chemical reactions the molecule can undergo. Being able to recognize a molecule’s functional groups will help you to understand and predict its reactivity.

Examples of functional groups are shown in Figure 2.3.a.

A large organic molecule with the following functional groups circled: amide, ketone, arene, alcohol, ether and aryl halide.
Figure 2.3.a. Structure of 4–(4–(4–bromophenoxy)–3–(hydroxymethyl)phenyl)–N–isopropyl–3–oxopentanamide. The various functional groups of this molecule are circled in pink and named in blue.

 

Classes of Organic Compounds and Their Functional Groups

The table below summarizes the structures of various functional groups that you must be able to recognize. Areas highlighted in pink are the functional groups of the molecule.

 

Table 2.3.a. Common functional groups in organic chemistry.

Class General Structural Formula Example
Alkane An “R-H” A linear horizontal oriented zig-zag.
Alkene Two carbons doubly bound to each other, both in pink. A linear horizontal oriented zig-zag, with two lines between two points.
Alkyne Two carbons triply bound to each other, in pink. A linear horizontal oriented zig-zag, with three lines in a straight oriented portion.
Alcohol An “R-OH”, with the OH in pink.
Alkyl halide An “R-X”, with the X in pink.
Ether An “R-O-R”, with the O in pink.
Primary amine* An “R-N-H2”, with the N-H2 in pink.
Secondary amine* An “R-NH-R”, with the NH in pink.
Tertiary amine* An “R-N-R2", with the N in pink.
Thiol An “R-SH”, with the SH in pink.
Aldehyde A C doubly bound to O, and bound to R and H. The C, O and H are pink.
Ketone A C doubly bound to O, and bound to two R groups. The C and O are pink.
Carboxylic acid A C doubly bound to O, and bound to R and OH. The C, OH and O are pink.
Ester A C doubly bound to O, and bound to R and OR. The C, OR and O are pink.
Carboxylic acid anhydride A molecule with a carbon doubly bound to an O, singly bound to an R group, and singly bound to an O. This O is singly bound to another carbon, which is also bound to R and doubly bound to O. Everything except the R groups are in pink.
Acid halide A C doubly bound to O, and bound to R and X. The C, X and O are pink.
Primary amide* A C doubly bound to O, and bound to R and NH2. The C, NH2 and O are pink.
Secondary amide* A C doubly bound to O, and bound to R and NH-R. The C, NH-R and O are pink.
Tertiary amide* A C doubly bound to O, and bound to R and N-R2. The C, N-R2 and O are pink.
Arene An “Ar-R” A hexagonal benzene ring bound to an ethyl group.
Aryl halide An “Ar-X”. The X is in pink. A hexagonal benzene ring bound to a Br.
Phenol An “Ar-OH”. The OH is in pink. A hexagonal benzene ring bound to an OH group and a methyl group.

*For more information about how amines and amides can be designated as primary, secondary, and tertiary, see the Are You Wondering box at the end of this chapter.

(The full solution to this problem can be found in Chapter 5.1).

 

Primary, Secondary, Tertiary and Quaternary Designation

To predict a molecule’s reactivity (which we will see in Chapter 3), it is important to identify not only its functional groups but also the number of substituents bonded to a particular atom.

A saturated carbon is a carbon atom that is bonded to other atoms through single bonds only. Saturated carbon atoms can be designated as either methyl, 1°, 2°, 3° or 4° depending on how many other carbon atoms are bonded to it.

If a carbon atom is bonded to one carbon substituent, it is said to be primary.

Structure of a primary carbon. The carbon center is bound to 3 R groups and one carbon substituent.
Figure 2.3.b. Structure of a primary carbon. The central carbon atom highlighted in blue is bonded to one carbon substituent.

If a carbon atom is bonded to two carbon substituents, it is said to be secondary.

Structure of a secondary carbon. The carbon center is bound to 2 R groups and two carbon substituents.
Figure 2.3.c. Structure of a secondary carbon. The central carbon atom highlighted in blue is bonded to two carbon substituents.

If a carbon atom is bonded to three carbon substituents, it is said to be tertiary.

Structure of a tertiary carbon. The carbon center is bound to 1 R group and three carbon substituents.
Figure 2.3.d. Structure of a tertiary carbon. The central carbon atom highlighted in blue is bonded to three carbon substituents.

If a carbon atom is bonded to four carbon substituents, it is said to be quaternary.

Structure of a quaternary carbon. The carbon center is bound to 4 carbon substituents.
Figure 2.3.e. Structure of a quaternary carbon. The central carbon atom highlighted in blue is bonded to four carbon substituents .

This designation can also be applied to saturated alkyl halides, carbocations, and alcohols. The figure below shows examples of each.

A table of designations of functional groups. The rows, in order, are labelled alkyl halide, carbocation and alcohol. In the first row, the molecule under the methyl column is an orange carbon bound to 3 H’s and Br. Under the primary column is an orange carbon bound to an orange carbon, 2 H’s and Br. Under the secondary column is an orange carbon bound to 2 orange carbons, 1 H and Br. Under the tertiary column is an orange carbon bound to 3 orange carbons and Br. In the second row, all carbon centers have a formal positive charge. The molecule under the methyl column is a carbon with a bound to three H’s. Under the primary column is an orange carbon bound to 2 H’s and one alkyl group. Under the secondary column is an orange carbon bound to 1 H and two alkyl groups. Under the tertiary column is an orange carbon bound to three orange carbons. In the third row, all carbons centers are bound to OH. The molecule under methyl column is bound to 3 H’s, under the primary column it is bound to 2 H’s and one alkyl group, under the secondary column it is bound to 1 H and two alkyl groups, and under the tertiary column it is bound to 3 alkyl groups, the alkyl groups being in orange.
Figure 2.3.f. Methyl, primary, secondary and tertiary designations for alkyl halides, carbocations and alcohols.

Note that when carbon is bonded to a heteroatom (like a halogen or an oxygen shown in the examples above), there is no quaternary designation. This is because carbon cannot have more than 4 bonds.

Are You Wondering? Primary, Secondary, and Tertiary Centres

The designation of primary, secondary, and tertiary also applies to the nitrogen centre in amines and amides (Figure 2.3.g). To determine whether the amine or amide is primary, secondary, or tertiary, count the number of carbon-containing substituents bonded to the nitrogen centre. If there is one carbon-containing substituent, then it is a primary amine. If there are two carbon-containing substituents, then it is a secondary amine. If there are three carbon-containing substituents, then it is a tertiary amine.

An image showing the distinction between primary, secondary and tertiary amides.
Designation of primary, secondary, and tertiary for amines (top) and amides (bottom).

 

(The full solution to this problem can be found in Chapter 5.1).

 

Key Takeaways

  • A functional group is defined as two or more bonded atoms which affect a molecules reactivity or chemical properties.
    • The same functional group on two different molecules will exhibit similar reactivity
  • Carbon bonded to other atoms with single bonds only is called a saturated carbon.
  • Carbon can be designated primary, secondary, tertiary or quaternary, depending on how many other carbons it is bound to. Carbon can also be referred to as methane if it is not bound to any other carbons.
    • Primary carbon: a carbon bound to one other carbon.
    • Secondary carbon: a carbon bound to two other carbons.
    • Tertiary carbon: a carbon bound to three other carbons.
    • Quaternary carbon: a carbon bound to four other carbons.
  • This characterization of carbons can be applied to any other carbon bound to functional groups such as alcohols, alkyl halides, etc.

Key terms in this chapter:

Key term Definition
Heteroatom An atom other than carbon, such as oxygen, nitrogen, and sulfur.

Diversity in Chemistry: Slayton Evans Jr.

A portrait of Slayton Evans Jr.

Although a wide variety of the most important functional groups have been introduced in this chapter, this does not even cover half of the known groups. Phosphorus, P, is a common heteroatom found in biology that makes up a number of functional groups, such as the phosphate group.Slayton Evans Jr., a professor at the University of North Carolina, was a trailblazer in organophosphorus chemistry, resulting in a deeper understanding of their properties and aiding in the development of novel methods to synthesize them for pharmaceutical use. Raised in Mississippi, Evans lived in a segregated public housing project with his family and became enamoured by science after he received a chemistry lab toy set and miniature microscope.

After his postdoctoral fellowship, Evans joined the University of North Carolina as the first African American chemistry professor at the university in 200 years, and quickly became successful due to his excellent work and dedication to teaching. Evans was also known as an outstanding mentor who advocated for recruiting underprivileged and students from underrepresented groups, closely guiding many of his undergraduate and graduate students. More information on Evans can be found on his page at UNC. Below shows a snapshot of Evans’ academic tree, with connections to other famous scientists such as Linus Carl Pauling, Kenichi Fukui, and Anna Krylov.

An academic family tree connecting numerous significant figures in science, including Krylov, Evans, Pauling, Fukui, and Lewis. These scientists go back to a common ancestor of Wilhelm Ostwald, who worked as a scientist in the late 19th century. These academic trees depict the relationship between scientists and their mentors and students, showcasing the lineage of academic knowledge and development of scientific ideas and transfer of knowledge through a long period. The evolution of science starting from Pauling’s basic ideas of neutrons can expand and evolve into something much more complicated, such as Krylov’s spin-flip theory related to quantum mechanics. Information regarding academic family trees is public and can be easily accessed online at Academic Tree.

 

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Organic Chemistry and Chemical Biology for the Students by the Students! (and the Profs...) Copyright © 2023 by Emma Abreu; Anumta Amir; Anthony Chibba; Jim Ghoshdastidar; Sharonna Greenberg; Angela Liang; Layla Vulgan; and Shuoyang Wang is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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