6.2. Functional Groups

Dinor Dhanabala; Sandra Fraley; Gordon Lake

Chapter 6: Introduction to Organic Chemistry

6.2. Functional Groups

Learning Objectives

By the end of this section, you will be able to:

Describe the role of functional groups in biological molecules

Functional groups are groups of atoms that occur within molecules and confer specific chemical properties to those molecules. They are found along the “carbon backbone” or of macromolecules. This carbon backbone is formed by chains and/or rings of carbon atoms with the occasional substitution of an element such as nitrogen or oxygen. Typical atoms found in functional groups are C, H, O, N, S, and P.

Each of the four types of macromolecules—proteins, lipids, carbohydrates, and nucleic acids—has its own characteristic set of functional groups that contributes greatly to its differing chemical properties and its function in living organisms.

A functional group can participate in specific chemical reactions. Figure 6.2.1. shows functional groups names, their structural formulas and related chemical characteristics. Molecule shape is crucial in organic chemistry so showing how atoms are arranged gives more information about their function. To inform the reader that a functional group is part of a larger organic molecule, R- is usually added, where R indicates the remainder of the organic molecule, and the line,–, represents the covalent bond that attaches ‘R’ to the functional group by a covalent bond.

Table shows the structure and properties of different functional groups. Hydroxyl groups, which consist of OH attached to a carbon chain, are polar. Methyl groups, which consist of three hydrogens attached to a carbon chain, are nonpolar. Carbonyl groups, which consist of an oxygen double bonded to a carbon in the middle of a hydrocarbon chain, are polar. Carboxyl groups, which consist of a carbon with a double bonded oxygen and an OH group attached to a carbon chain, are able to ionize, releasing H+ ions into solution. Carboxyl groups are considered acidic. Amino groups, which consist of two hydrogens attached to a nitrogen, are able to accept H+ ions from solution, forming H3+. Amino groups are considered basic. Phosphate groups consist of a phosphorous with one double bonded oxygen and two OH groups. Another oxygen forms a link from the phosphorous to a carbon chain. Both OH groups in phosphorous can lose a H+ ion, and phosphate groups are considered acidic. — **Figure 6.2.1:** These functional groups are in many different biological molecules. R, also known as R-group, is an abbreviation for any group in which a carbon or hydrogen atom is attached to the rest of the molecule. (credit: modification of Fig 2.27 from https://openstax.org/books/biology-2e/pages/2-3-carbon)

Methyl group

The methyl group is made up by a carbon atom attached to three hydrogen atoms. It is a derivative of methane, CH₄.

Hydroxyl group

These are different from hydroxide ions. Bases like NaOH and KOH ionizes into OH- ion and a cation in solution. While hydroxyl groups in organic molecules do not ionizes. These molecules (R-OH) are also referred to as alcohols.

Ethanol, CH₃CH₂OH, also called ethyl alcohol, is a particularly important alcohol for human use. Ethanol is the alcohol produced by some species of yeast (Figure 6.2.4.) that is found in wine, beer, and distilled drinks. It has long been prepared by humans harnessing the metabolic efforts of yeasts in fermenting various sugars:

This figure shows the reaction of glucose to produce ethanol and C O subscript 2. The reaction shows C subscript 6 H subscript 12 O subscript 6 ( a q ) arrow labeled “yeast” 2 C subscript 2 H subscript 5 O H (a q) plus 2 C O subscript 2 ( g ). The O H in ethanol is shown in red. — **Figure 6.2.4:** Fermentation of glucose by yeast.

Hydroxyl groups are also found in carbohydrates and water-soluble vitamins.

Carbonyl containing functional groups

Another class of organic molecules contains a carbon atom connected to an oxygen atom by a double bond, commonly called a carbonyl group. The trigonal planar carbon in the carbonyl group can attach to two other substituents leading to several subfamilies (aldehydes, ketones, carboxylic acids and esters) described in this section.

In an aldehyde, the carbonyl group is bonded to at least one hydrogen atom. In a ketone, the carbonyl group is bonded to two carbon atoms. In a carboxyl, the carbonyl is bonded to a hydroxyl group. The ester functional group resembles the carboxyl, differing only in the non-carbonyl oxygen: atom. Carboxyl’s second oxygen atom has H, forming a hydroxyl whereas ester’s second oxygen atom is covalently bonded to a carbon.

Aldehyde and ketone functional groups are the basis for distinguishing two classes of carbohydrates sugars, the aldose and ketose sugars, respectively. The carboxyl is present in the fatty acid components of the triglycerides (fats and oils) and in the amino acids of proteins. The ester bond is crucial the formation of complex nutrient lipids, the triglycerides.

Table showing four varieties of carbonyl groups. Aldehydes have the carbon double bonded to an oxygen, a hydrogen, and a R chain. Ketones have the carbon double bonded to an oxygen and two R chains. Carboxyl groups have the carbon double bonded to an oxygen, a hydroxyl group, and a R chain. Esters have the carbon double bonded to an oxygen, one R chain, and an oxygen bonded to a second R chain. — **Figure 6.2.5:** Carbonyl functional groups

There are two structures drawn. The first structure is labeled C H subscript 3 C H O. It is also labeled, “An aldehyde,” and “ethanal (acetaldehyde).” This structure has a C atom to which 3 H atoms are bonded above, below, and to the left. In red to the right of this C atom, a C atom is attached which has an O atom double bonded above and an H atom bonded to the right. The O atom as two sets of electron dots. The second structure is labeled C H subscript 3 C O C H subscript 2 C H subscript 3. It is also labeled, “A ketone,” and “butanone.” This structure has a C atom to which 3 H atoms are bonded above, below, and to the left. To the right of this in red is a C atom to which an O atom is double bonded above. The O atom has two sets of electron dots. Attached to the right of this red C atom in black is a two carbon atom chain with H atoms attached above, below, and to the right. — **Figure 6.2.6:** Example of an aldehyde and a ketone

As text, an aldehyde group is represented as –CHO; a ketone is represented as –C(O)– or –CO–.

Examples

Formaldehyde, an aldehyde with the formula HCHO, is a colorless gas with a pungent and irritating odor. It is sold in an aqueous solution called formalin, which contains about 37% formaldehyde by weight. Formaldehyde causes coagulation of proteins, so it kills bacteria (and any other living organism) and stops many of the biological processes that cause tissue to decay. Thus, formaldehyde is used for preserving tissue specimens and embalming bodies. It is also used to sterilize soil or other materials. Formaldehyde is used in the manufacture of Bakelite, a hard plastic having high chemical and electrical resistance.

Dimethyl ketone, CH₃COCH₃, commonly called acetone, is the simplest ketone. It is made commercially by fermenting corn or molasses, or by oxidation of 2-propanol. Acetone is a colorless liquid. Among its many uses are as a solvent for lacquer (including fingernail polish), cellulose acetate, cellulose nitrate, acetylene, plastics, and varnishes; as a paint and varnish remover; and as a solvent in the manufacture of pharmaceuticals and chemicals.

Organic acids contain carboxyl groups. Fatty acids, amino acids and some vitamins contain a carboxyl group. Carboxylic acids are weak acids (see the chapter on acids and bases).

Acetic acid, CH₃COOH, constitutes 3–6% vinegar. Cider vinegar is produced by allowing apple juice to ferment without oxygen present. Yeast cells present in the juice carry out the fermentation reactions. The fermentation reactions change the sugar present in the juice to ethanol, then to acetic acid. Pure acetic acid has a penetrating odor and produces painful burns. It is an excellent solvent for many organic and some inorganic compounds, and it is essential in the production of cellulose acetate, a component of many synthetic fibers such as rayon.

Esters are produced by the reaction of acids with alcohols. The distinctive and attractive odors and flavors of many flowers, perfumes, and ripe fruits are due to the presence of one or more esters.

Connection to everyday life

For example, the ester ethyl acetate, CH₃CO₂CH₂CH₃, is formed when acetic acid reacts with ethanol (Figure 6.2.12.).

A chemical reaction is shown. On the left, a C H subscript 3 group bonded to a red C atom. The C atom forms a double bond with an O atom which is also in red. The C atom is also bonded to an O atom which is bonded to an H atom, also in red. A plus sign is shown, which is followed by H O C H subscript 2 C H subscript 3. The H O group is in red. Following a reaction arrow, a C H subscript 3 group is shown which is bonded to a red C atom with a double bonded O atom and a single bonded O. To the right of this single bonded O atom, a C H subscript 2 C H subscript 3 group is attached and shown in black. This structure is followed by a plus sign and H subscript 2 O. The O atoms in the first structure on the left and the structure following the reaction arrow have two pairs of electron dots. — **Figure 6.2.7.** Formation of an ester from carboxylic acid and alcohol.

Because esters do not have hydrogen bonds between molecules, they have lower vapor pressures than the alcohols and carboxylic acids from which they are derived.

There are nine structures represented in this figure. The first is labeled, “raspberry,” and, “iso-butyl formate.” It shows an H atom with a line going up and to the right which then goes down and to the right. It goes up and to the right again and down and to the right and up and to the right. At the first peak is a double bond to an O atom. At the first trough is an O atom. At the second trough, there is a line going straight down. The second is labeled, “apple,” and, “butyl acetate.” There is a line that goes up and to the right, down and to the right, up and to the right, and down and to the right. At the second peak is a double bond to an O atom. At the end, on the right is O C H subscript 3. The third is labeled, “pineapple,” and, “ethyl butyrate.” It is a line that goes up and to the right, down and to the right, up and to the right, down and to the right, up and to the right, and down and to the right. At the second peak is a double bond to an O atom and at the second trough is an O atom. The fourth is labeled, “rum,” and “propyl isobutyrate.” It shows a line that goes down and to the right, up and to the right, down and to the right, up and to the right, down and to the right and up and to the right. The first complete peak has a double bond to an O atom and the second trough has an O atom. The fifth is labeled, “peach,” and “benzyl acetate.” It shows a line that goes up and to the right, down and to the right, up and to the right and down and to the right. This line connects to a hexagon with a circle inside it. The first peak has a double bond to an O atom and the first trough has an O atom. The sixth is labeled, “orange,” and, “octyl acetate.” It shows a line that goes up and to the right and down and to the right and up and to the right and down and to the right and up and to the right and down and to the right and up and to the right and down and to the right and up and to the right and down and to the right. The first peak has a double bond to an O atom and the first complete trough has and an O atom. The seventh is labeled, “wintergreen,” and “methyl salicylate.” It shows a hexagon with a circle inside of it. On the right, is a bond down and to the right to an O H group. On the right is a bond to a line that goes up and to the right and down and two the right and up and to the right. At the first peak is a double bond to an O atom, the next trough shows and O atom and at the end of the line is a C H subscript 3 group. The eighth is labeled, “honey,” and “methyl phenylacetate.” It shows a hexagon with a circle inside of it. It shows it connecting to a line on the right that goes down and to the right then up and to the right and down and to the right and up and to the right. At the first peak that is not part of the hexagon is a double bond to an O atom. At the last trough is an O atom. The ninth is labeled, “strawberry,” and “ethyl methylphenylglycidate.” This shows a hexagon with a circle inside of it. On the right, it connects to a line that goes up and to the right and down and to the right and up and to the right and down and to the right and up and to the right and down and to the right. At the first peak is a line that extends above and below. Below, it connects to an O atom. At the next trough, the line extends down and to the left to the same O atom. At the next peak is a double bond to an O atom and at the next trough is an O atom. — **Figure 6.2.8:** Examples of esthers.

Amino group

An amino group contain a nitrogen bonded to two hydrogen atoms, are able to accept H+ ions from solution, forming NH₃+. Amino groups are considered basic. Amino acids, monomers of protein, and urea posse amino groups.

The amino acid alanine with the central carbon atom bonded to a hydrogen atom, an amino group, a carboxyl group, and a methyl group for the side chain or R group. — **Figure 6.2.10:** Amino acid, Alanine

A carbon double bonded to oxygen and two amino (NH2) groups. — **Figure 6.2.11:** Urea molecule.

Phosphate group

A phosphate group contains a phosphorous atom in the center with 2 hydroxyl (-OH) groups, an oxygen with a double bond and another oxygen with a single bond. The rest of the organic molecule is attached to the single bonded oxygen. Both the OH groups attached to phosphorous can lose H+ ions and are considered acidic. Phosphates are found in nucleic acids, phospholipids and high energy molecules, such as adenosine triphosphate (ATP).

A monomer of nucleic acids is a nucleotide (figure 6.2.13.(c)) and each nucleotide is made of a nitrogenous base, a pentose sugar and a phosphate group.

Diagram a shows DNA as a double helix composed of the nitrogenous bases adenine, thymine, guanine, and cytosine paired up along a sugar-phosphate backbone. The helix has labeled 3 prime and 5 prime directions or ends. In diagram b, the molecular level arrangement of the bases connected by hydrogen bonding within the sugar-phosphate backbone is shown. Adenine is shown with hydrogen bonding to thymine and similarly the linkage via hydrogen bonding between cytosine and guanine is shown. Again, 3 prime and 5 prime directional information is provided. In diagram c, the bonding between a nitrogenous base, sugar, and phosphate is shown. — **Figure 6.2.13:** **(a)** double-helix, **(b)** four nucleotides of two strands of DNA interacting with each other, **(c)** a nucleotide.

Section summary

Functional groups are groups of atoms that confer specific properties to hydrocarbon (or substituted hydrocarbon) chains or rings that define their overall chemical characteristics and function.
The –OH group is the functional group of an alcohol. Alcohols do not ionize in water.
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.
Amino groups contain a nitrogen with two hydrogens attached. They can accept protons and hence are considered bases.
Phosphate group is made of -PO₂(OH)₂. They can release protons and are considered acids.

Glossary

alcohol: organic compound with a hydroxyl group (–OH) bonded to a carbon atom

aldehyde: organic compound containing a carbonyl group bonded to two hydrogen atoms or a hydrogen atom and a carbon substituent

carbonyl group: carbon atom double bonded to an oxygen atom

carboxylic acid: organic compound containing a carbonyl group with an attached hydroxyl group

ester: organic compound containing a carbonyl group with an attached oxygen atom that is bonded to a carbon substituent

ketone: organic compound containing a carbonyl group with two carbon substituents attached to it

functional group: group of atoms that provides or imparts a specific function to a carbon skeleton

License and attributions:

Biology, Second edition, 2018, Clark, M.A. et al. License: CC BY 4.0. Located at https://openstax.org/books/biology-2e/pages/2-3-carbon
Chemistry: Atoms first, Second edition, 2019, Flowers, P. et al. License: CC BY 4.0. Located at https://openstax.org/books/chemistry-atoms-first-2e/pages/21-introduction
Some illustrations by Lake, G., Fraley, S. License: CC BY-NC 4.0

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BIO130: Introduction to Physiology Copyright © 2024 by Dinor Dhanabala; Sandra Fraley; and Gordon Lake is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.